**2. Rice biotechnology for abiotic stress tolerance**

Rice is an economically important crop and serious efforts are needed to increase its production. The production of rice can be increased in various ways like use of the available agricultural land at its maximum, utilization of marginal lands for growing the rice crop and using the lands effected by various abiotic stresses (e.g. drought stress, heat stress, cold stress, salt stress, UV radiation stress and metal toxicity) by growing the transgenic rice cultivars that are developed with the help of biotechnology. This is necessary because the agricultural land is decreasing and demand for food is increasing with the increasing world population and it is becoming very difficult to meet the food demands of this exponentially growing population with the same efficiency of the present day cultivars of rice [4].

The production of rice is greatly influenced by various abiotic stresses and it can be compensated by the various genes and regulatory networks present in different stress tolerant cultivars of rice and these genes and networks can be taken from the other plant species that can be used for developing new cultivars of rice (**Figure 1**). Abiotic stresses can induce the expression of many genes. The complex transcriptional networks are responsible for regulating the induction of stress tolerant genes. The important genes that are involved in transcriptional networks are studied by molecular techniques and these genes are useful in developing abiotic stress tolerant transgenic rice [5]. So, various biotechnology methods are used to develop rice cultivars that can combat with abiotic stresses (**Table 1**).

#### **Figure 1.**

*Abiotic stresses (e.g. drought, cold, heat, salinity, UV radiations, heavy metals) follow a same mechanism to survive and retain their growth and development. It has four main steps that are (a) avoidance: avoiding the contact with stresses, (b) escape: changing the life-cycle, (c) recovery: vegetative growth potency and (d) tolerance: nullifying the impacts of stresses.*

**5**

**2.1 Drought stress**

*Some transgenes inserted for improving the abiotic stresses.*

**Table 1.**

*Introductory Chapter: Recent Advances in Rice Biotechnology for Abiotic Stress Tolerance*

**S. No. Transgene(s) Improved Trait(s) References**

1. *OsHsp101, AtHsp101* Heat stress tolerance enhanced [6] 2. *OsMAPK44* Drought and salinity stress tolerance enhanced [7] 3. *OsPIP1;3* Cold stress tolerance ability enhanced [8] 4. *Choline mono-oxygenase* Heat and salinity stress tolerance enhanced [9] 5. *AVP1, SsNHX1* Improved ROS and salinity stress tolerance [10]

tolerance ability

8. *SUB1A* Enhanced submergence tolerance [13]

10. *STAR1, STAR2* Enhanced tolerance to Aluminum toxicity [15] 11. *ZFP245* Improved ROS, cold and drought stress tolerance [16] 12. *P5CSF129A* Salinity stress tolerance enhanced [17] 13. *Isoflavone reductase* Salinity stress tolerance enhanced [18]

15. *OsMAPK2* Tolerance to phosphate deficiency [20]

17. *OsLEA3-2* Salinity and drought tolerance capacity enhanced [22] 18. *OsMYB55* Heat stress tolerance enhanced [23] 19. *PCK, PPDK* Improved ROS stress tolerance [24] 20. *OsETOL1* Enhanced submergence stress tolerance [25] 21. *OsMYB48-1* Salinity and drought tolerance capacity enhanced [26] 22. *VrDREB2A* Increased tolerance to salinity and drought stress [27] 23. *TaMYB3R1* Enhanced drought and salt stress tolerance [28] 24. *CaPUB1* Enhanced cold stress tolerance [29]

[11]

[12]

[14]

[19]

[21]

[30]

6. *OsSBPase* Improved photosynthetic efficiency and heat

7. *HvCBF4* Salinity, drought and cold tolerance capacity enhanced

9. *OsDREB1F* Salinity, drought and cold tolerance capacity enhanced

14. *OsHMA3* Enhanced drought and submergence stress tolerance

16. *OsNAC5* Salinity, drought and cold tolerance capacity enhanced

25. *OsLEA4* Enhanced drought, salt and heavy metal stress tolerance

26. *OsNAC2* Enhanced drought and salt stress tolerance [31] 27. *OsGS* Improved ROS and drought stress tolerance [32] 28. *TsPIP1;1* Enhanced salinity stress tolerance [33] 29. *RhMYB96* Enhanced salt tolerance [34]

Drought is affecting agricultural land worldwide from few past years. Many molecular, physiological and metabolic changes occur in plants due to drought stress that damages their growth and development [35]. During drought stress, plants respond variously and express change in their physiology and morphology.

*DOI: http://dx.doi.org/10.5772/intechopen.94036*


*Introductory Chapter: Recent Advances in Rice Biotechnology for Abiotic Stress Tolerance DOI: http://dx.doi.org/10.5772/intechopen.94036*

#### **Table 1.**

*Recent Advances in Rice Research*

either old plant breeding techniques or new targeted genome editing techniques like zinc finger proteins (ZFNs), transcription activator-like nucleases (TALENs) and clustered regularly interspaced short palindromic repeats associated with Cas9 protein (CRISPR-Cas9) to develop cultivar of rice for improved agronomical traits.

Rice is an economically important crop and serious efforts are needed to increase its production. The production of rice can be increased in various ways like use of the available agricultural land at its maximum, utilization of marginal lands for growing the rice crop and using the lands effected by various abiotic stresses (e.g. drought stress, heat stress, cold stress, salt stress, UV radiation stress and metal toxicity) by growing the transgenic rice cultivars that are developed with the help of biotechnology. This is necessary because the agricultural land is decreasing and demand for food is increasing with the increasing world population and it is becoming very difficult to meet the food demands of this exponentially growing population with the same efficiency of the present day cultivars of rice [4].

The production of rice is greatly influenced by various abiotic stresses and it can be compensated by the various genes and regulatory networks present in different stress tolerant cultivars of rice and these genes and networks can be taken from the other plant species that can be used for developing new cultivars of rice (**Figure 1**). Abiotic stresses can induce the expression of many genes. The complex transcriptional networks are responsible for regulating the induction of stress tolerant genes. The important genes that are involved in transcriptional networks are studied by molecular techniques and these genes are useful in developing abiotic stress tolerant transgenic rice [5]. So, various biotechnology methods are used to develop rice cultivars that can combat with abiotic stresses

*Abiotic stresses (e.g. drought, cold, heat, salinity, UV radiations, heavy metals) follow a same mechanism to survive and retain their growth and development. It has four main steps that are (a) avoidance: avoiding the contact with stresses, (b) escape: changing the life-cycle, (c) recovery: vegetative growth* 

*potency and (d) tolerance: nullifying the impacts of stresses.*

**2. Rice biotechnology for abiotic stress tolerance**

**4**

**Figure 1.**

(**Table 1**).

*Some transgenes inserted for improving the abiotic stresses.*

#### **2.1 Drought stress**

Drought is affecting agricultural land worldwide from few past years. Many molecular, physiological and metabolic changes occur in plants due to drought stress that damages their growth and development [35]. During drought stress, plants respond variously and express change in their physiology and morphology. Rice drought resistance is achieved by four procedures that are (a) avoidance: avoiding the contact with stress, (b) escape: changing life-cycle, (c) recovery: vegetative growth potency and (d) tolerance: nullifying the impacts of stress. These procedures are fulfilled by variable mechanisms like reduced leaf area, leaf rolling, senescence of older leaves, increased root proliferation, dense root system, scavenging reactive oxygen species (ROS), early flowering, osmotic adjustment, stomatal closure that minimize the water loss, changes in elasticity of cell wall and maximum uptake of deep water allow plants to survive during extended periods of drought and even allow them to reproduce in limited water supply by maintaining physiological activities [36]. Osmotic potential is kept lower inside the plant cells than outside the cells by an important mechanism known as osmotic adjustment. So, it allows the plants to retain its turgidity and prevents the water loss. Drought stress tolerance in plants can be achieved by the accumulation of inorganic and organic substances like proline, potassium ions, glucose and sucrose.

This mechanism participates in osmotic adjustment and maintains the turgor pressure. Plants exposed to drought stress produce high level of ROS that are extremely toxic and damages the DNA, carbohydrates, proteins and lipid. In drought tolerant plants, non-enzymatic antioxidants and antioxidant enzymes are produced that protect plants from the deleterious effects of ROS. Currently, the production system of rice depends upon the excessive supply of water and therefore, it is more susceptible to drought stress. The drought stress is the most significant restrictive factor in the production of rice and it is becoming a very severe problem. The varieties of rice favored by farmers in the tropical and subtropical regions are vulnerable to drought stress [35]. Rice is a significant economic crop of Asia that is mainly cultivated in lowland regions, where agriculture depends on the seasonal rainfall. Thus, rice crop in these areas is very susceptible to drought stress but this problem can be rectified by introducing some drought tolerant genes in the economically important cultivars of rice. For example, jasmine rice or 'Khao Dawk Mali 105' (KDML105) is a world-famous cultivar of rice exported from Thailand, had suffered from drought stress because of limited irrigation and unpredictable rainfalls. Drought tolerance can be achieved after keeping genetic background of KDML105 conserved by chromosome-segment substitution lines (CSSL) which have drought-tolerant quantitative-trait loci (DT-QTL) obtained from the back crossing between drought tolerant donor IR58586-F2-CA-143 (DH212) and KDML105. For more understanding, the drought tolerance related physiological responses consider another CSSL named as CSSL1-16. This line has properties like high proline, high water status, good membrane stability and great osmotic adjustment. Furthermore, it can recommence growth after recovery from stress. So, it can be used as a potential candidate for developing further drought tolerant cultivars while keeping their own genetic background preserved [37].

Another example of producing drought tolerant rice plants is introduction of pea DNA Helicase-47 (PDH47) from the plant *Pisum sativum* by using a constitutive promoter of cauliflower mosaic virus 35S- CaMV had been introduced in the ASD16 a cultivar of Indica rice via *Agrobacterium tumefaciens*. The transcripts of PDH47 are upregulated during drought stress that is correlated with increased water status, hydrogen peroxide accumulation and proline accumulation. It also regulates many stress responsive genes present endogenously during drought stress in transgenic rice [38]. There are many more genes that are expressed during the drought stress in different plants and are also potential candidates for producing transgenic rice that are drought tolerant.

**7**

*Introductory Chapter: Recent Advances in Rice Biotechnology for Abiotic Stress Tolerance*

Heat stress is a major limiting factor in the production of crops across the world because of global warming. A negative correlation is present between the increased temperature and crop yield especially in case of rice, wheat, barley and maize [39]. The heat stress can damage rice plants severely by inhibiting the metabolic activities, seed setting, plant growth and pollen fertility; therefore production of rice is reduced [40]. Excessive heat can also decrease the photosynthetic ability of plant, reduce water use efficiency, seed weight and grain mass and shorten the leaf area. Heat stress can damage both at vegetative and reproductive stage from sprouting to maturity. But, flowering and booting are the two more critical stages that can cause complete sterility in rice cultivars [41]. Heat tolerance belongs to plants that can lessen the effects of stress and give enough economic yields even at high temperature. Like other plant species, rice also has variations in germplasm to combat with heat stress. Tolerance is achieved by adjusting different molecular, morphological and physiological traits in rice cultivars. High temperature enhances the expression of stress tolerating genes and increases the metabolite production which is beneficial for achieving stress tolerance in plants [42]. During heat stress plants adopt multiple mechanisms like avoidance, survival and escape. These mechanisms impose avoidance for short term and develop resistance for long term survival. At the cellular level, effects of stress can be neutralized by certain factors and mechanisms; such as transcriptional control, antioxidant defense, osmolytes, late embryogenesis abundant (LEA) proteins and the factors that participate in signaling cascades. In hot environments, yield is reduced because of early maturity comes under the domain of avoidance strategies while suffering from heat

Several morphological markers like long anthers, high pollen fertility, long basal pores and large basal dehiscence can be used for determining heat tolerant rice. As well as grain yield, number of spikelet, weight of thousand grains and seed setting percentage can be used for screening rice germplasm against high temperature stress; a great reduction is found in all these factors during heat stress. So, these parameters can be used to screen heat tolerant rice. Opening of the rice spikelet in early morning can be another beneficial criteria for the selection of heat tolerant rice cultivars [41]. A thermo-tolerant wild cultivar known as *Oryza meridionalis* can maintain high photosynthesis rate at high temperature. This high rate of photosynthesis is due to good stability and activity of Rubisco and it can be used as a significant physiological marker to determine the heat tolerance in rice [43]. The chlorophyll content and electrolyte leakage from leaves and roots can be increased during heat stress; it can also be used as a marker to analyze the heat tolerance. The fluid content of the membrane, lipid molecule present in membranes, cooling effect of transpiration and leaf position are some other beneficial physiological marker to

Plant breeding efficacy has been increased by using marker assisted selection (MAS). Numerous molecular markers are used in MAS, but, single nucleotide polymorphism (SNPs) and simple sequence repeats (SSRs) are extensively used. This procedure is used to gather the information about the genes involved in providing resistance against biotic and abiotic stresses [44]. Still, there are very limited numbers of rice cultivars that are engineered to tolerate heat stress. Because the SNPs are present more than the SSRs in the rice genome, so SNPs are more prone to use for developing heat tolerance. A few SNPs related with heat tolerance have been known. Due to the very complex nature of this trait, each marker can contribute very less towards the variance. Therefore, several markers that are linked

*DOI: http://dx.doi.org/10.5772/intechopen.94036*

**2.2 Heat stress**

stress [41].

determine the heat tolerance in rice.

*Introductory Chapter: Recent Advances in Rice Biotechnology for Abiotic Stress Tolerance DOI: http://dx.doi.org/10.5772/intechopen.94036*

#### **2.2 Heat stress**

*Recent Advances in Rice Research*

glucose and sucrose.

background preserved [37].

are drought tolerant.

Rice drought resistance is achieved by four procedures that are (a) avoidance: avoiding the contact with stress, (b) escape: changing life-cycle, (c) recovery: vegetative growth potency and (d) tolerance: nullifying the impacts of stress. These procedures are fulfilled by variable mechanisms like reduced leaf area, leaf rolling, senescence of older leaves, increased root proliferation, dense root system, scavenging reactive oxygen species (ROS), early flowering, osmotic adjustment, stomatal closure that minimize the water loss, changes in elasticity of cell wall and maximum uptake of deep water allow plants to survive during extended periods of drought and even allow them to reproduce in limited water supply by maintaining physiological activities [36]. Osmotic potential is kept lower inside the plant cells than outside the cells by an important mechanism known as osmotic adjustment. So, it allows the plants to retain its turgidity and prevents the water loss. Drought stress tolerance in plants can be achieved by the accumulation of inorganic and organic substances like proline, potassium ions,

This mechanism participates in osmotic adjustment and maintains the turgor pressure. Plants exposed to drought stress produce high level of ROS that are extremely toxic and damages the DNA, carbohydrates, proteins and lipid. In drought tolerant plants, non-enzymatic antioxidants and antioxidant enzymes are produced that protect plants from the deleterious effects of ROS. Currently, the production system of rice depends upon the excessive supply of water and therefore, it is more susceptible to drought stress. The drought stress is the most significant restrictive factor in the production of rice and it is becoming a very severe problem. The varieties of rice favored by farmers in the tropical and subtropical regions are vulnerable to drought stress [35]. Rice is a significant economic crop of Asia that is mainly cultivated in lowland regions, where agriculture depends on the seasonal rainfall. Thus, rice crop in these areas is very susceptible to drought stress but this problem can be rectified by introducing some drought tolerant genes in the economically important cultivars of rice. For example, jasmine rice or 'Khao Dawk Mali 105' (KDML105) is a world-famous cultivar of rice exported from Thailand, had suffered from drought stress because of limited irrigation and unpredictable rainfalls. Drought tolerance can be achieved after keeping genetic background of KDML105 conserved by chromosome-segment substitution lines (CSSL) which have drought-tolerant quantitative-trait loci (DT-QTL) obtained from the back crossing between drought tolerant donor IR58586-F2-CA-143 (DH212) and KDML105. For more understanding, the drought tolerance related physiological responses consider another CSSL named as CSSL1-16. This line has properties like high proline, high water status, good membrane stability and great osmotic adjustment. Furthermore, it can recommence growth after recovery from stress. So, it can be used as a potential candidate for developing further drought tolerant cultivars while keeping their own genetic

Another example of producing drought tolerant rice plants is introduction of pea DNA Helicase-47 (PDH47) from the plant *Pisum sativum* by using a constitutive promoter of cauliflower mosaic virus 35S- CaMV had been introduced in the ASD16 a cultivar of Indica rice via *Agrobacterium tumefaciens*. The transcripts of PDH47 are upregulated during drought stress that is correlated with increased water status, hydrogen peroxide accumulation and proline accumulation. It also regulates many stress responsive genes present endogenously during drought stress in transgenic rice [38]. There are many more genes that are expressed during the drought stress in different plants and are also potential candidates for producing transgenic rice that

**6**

Heat stress is a major limiting factor in the production of crops across the world because of global warming. A negative correlation is present between the increased temperature and crop yield especially in case of rice, wheat, barley and maize [39]. The heat stress can damage rice plants severely by inhibiting the metabolic activities, seed setting, plant growth and pollen fertility; therefore production of rice is reduced [40]. Excessive heat can also decrease the photosynthetic ability of plant, reduce water use efficiency, seed weight and grain mass and shorten the leaf area. Heat stress can damage both at vegetative and reproductive stage from sprouting to maturity. But, flowering and booting are the two more critical stages that can cause complete sterility in rice cultivars [41]. Heat tolerance belongs to plants that can lessen the effects of stress and give enough economic yields even at high temperature. Like other plant species, rice also has variations in germplasm to combat with heat stress. Tolerance is achieved by adjusting different molecular, morphological and physiological traits in rice cultivars. High temperature enhances the expression of stress tolerating genes and increases the metabolite production which is beneficial for achieving stress tolerance in plants [42]. During heat stress plants adopt multiple mechanisms like avoidance, survival and escape. These mechanisms impose avoidance for short term and develop resistance for long term survival. At the cellular level, effects of stress can be neutralized by certain factors and mechanisms; such as transcriptional control, antioxidant defense, osmolytes, late embryogenesis abundant (LEA) proteins and the factors that participate in signaling cascades. In hot environments, yield is reduced because of early maturity comes under the domain of avoidance strategies while suffering from heat stress [41].

Several morphological markers like long anthers, high pollen fertility, long basal pores and large basal dehiscence can be used for determining heat tolerant rice. As well as grain yield, number of spikelet, weight of thousand grains and seed setting percentage can be used for screening rice germplasm against high temperature stress; a great reduction is found in all these factors during heat stress. So, these parameters can be used to screen heat tolerant rice. Opening of the rice spikelet in early morning can be another beneficial criteria for the selection of heat tolerant rice cultivars [41]. A thermo-tolerant wild cultivar known as *Oryza meridionalis* can maintain high photosynthesis rate at high temperature. This high rate of photosynthesis is due to good stability and activity of Rubisco and it can be used as a significant physiological marker to determine the heat tolerance in rice [43]. The chlorophyll content and electrolyte leakage from leaves and roots can be increased during heat stress; it can also be used as a marker to analyze the heat tolerance. The fluid content of the membrane, lipid molecule present in membranes, cooling effect of transpiration and leaf position are some other beneficial physiological marker to determine the heat tolerance in rice.

Plant breeding efficacy has been increased by using marker assisted selection (MAS). Numerous molecular markers are used in MAS, but, single nucleotide polymorphism (SNPs) and simple sequence repeats (SSRs) are extensively used. This procedure is used to gather the information about the genes involved in providing resistance against biotic and abiotic stresses [44]. Still, there are very limited numbers of rice cultivars that are engineered to tolerate heat stress. Because the SNPs are present more than the SSRs in the rice genome, so SNPs are more prone to use for developing heat tolerance. A few SNPs related with heat tolerance have been known. Due to the very complex nature of this trait, each marker can contribute very less towards the variance. Therefore, several markers that are linked with various quantitative trait loci (QTLs) are used to improve a cultivar for heat tolerance [45]. Many genes are responsible for heat tolerance at various stages; for example, ZFP is a gene that is related with heat tolerance at seedling stage [46] and OsWRKY11 is another gene that is responsible for heat tolerance in rice [47]. The heat tolerant genes are needed to be identified in rice cultivars and the genes from the other plant species can also be transferred to the rice cultivars to improve the heat tolerance. Other approaches like use of mutagens and a recent technology like CRISPR-Cas9 should be applied for further development of heat tolerant rice cultivars.

#### **2.3 Cold stress**

The cold stress is a significant environmental factor that affects the development and growth of rice crop. At the stage of seedling development, sudden decrease in temperature can influence the development of chlorophyll [48]. The damage of rice seedling due to cold stress ultimately decreases the grain yield. So, the cold stress is a major limitation which can be overcome by using cold tolerant rice varieties [49]. Because rice crop is evolved in tropical region, so it has limited adaptability to chilling stress. Improving rice varieties to make them cold tolerant, enable the cultivation of rice in northerly latitudes. Different signal transduction pathways and genetic networks are involved in controlling chilling tolerance [50]. In japonica rice, chilling tolerance is achieved by interactions between rice G-protein α-subunit 1 (RGA1) and chilling tolerance divergence 1 (COLD1) followed by the calcium signaling initiated in the response of downstream network of stress response belongs to C repeat binding factor (CBF) that is a transcription factor [51]. Still, there is limited information available about the stress response and adaptation. The plant show response to abnormal surrounding temperature by changing their gene expression and adapt the suitable architecture due to the developmental plasticity. In SAM (shoot apical meristem) intrinsic signals can be disrupted by cold stress and stress tolerance is enhanced by setting the dormancy cycling at the SAM [52]. For the survival mechanism against cold stress, protection of niche forms of root stem cell is a sacrifice [53]. The differentiated cells are properly organized, and meristematic activity is maintained by this rehabilitated development in response to the cold temperature. During cold stress, many specific genes like OsMYB3R-2 are activated by the various transcription factors to maintain the mitotic cell and their cold tolerance. The survival and growth are enhanced by keeping maintained the cellular activity and cell behavior during and after the cold stress.

The formation of axillary bud following by its outgrowth in axils of primordia of leaf controls the shoot branching. The axillary bud initiation is controlled by MOC1 (Monoclum 1) and LAX 1 (Lax Panicle 1). The signaling pathways involved in the axillary bud outgrowth formation and biosynthesis of strigolactone are important for tillering in rice crop and are controlled by the dwarf genes, that are dwarf 53 (D53), dwarf 27 (D27), dwarf 17 (D17), dwarf 14 (D14), dwarf 10 (D10) and dwarf 3 (D3) [54]. In rice the outgrowth of axillary buds is repressed by the OsTB1 that acts in the downstream region of the dwarf genes. The interaction between the OsMADS57 and OsTB1 is responsible for reducing the inhibitory effect on the D14, a gene responsible for the organogenesis of rice tillers by producing a receptor for the hormone strigolactone [55], and it enables D14 to regulate the initiation of axillary bud [56]. The plant endogenous conditions and environmental situations control the development of axillary buds that are basically indeterminate structures [57]. To adapt the cold environment a network has been identified having core gene OsMADS57. The rice tiller growth is maintained by the overexpression of OsMADS57 during cold stress. OsMADS57 directly binds with the promoter of the

**9**

tolerant varieties [61].

**2.4 Salt stress**

*Introductory Chapter: Recent Advances in Rice Biotechnology for Abiotic Stress Tolerance*

play a significant role in adapting the cold environment [60].

The progress in developing the genetically modified plants by introducing and overexpressing the novel genes appears to be a very good practically possible option to speed up the process of breeding in plants. Instinctively, the faster way of inserting the genes with beneficial traits is genetic engineering instead of molecular or conventional breeding; and it would be the only possible option if the gene of interest belongs to some other species of plants, distant relatives, or from any non-plant source. The utilization of gene knockout strategies and genomic approaches are developing to enhance the efforts to measure thoroughly and make it easy to completely understand the complicated quantitative traits like tolerance to different temperature fluctuations and extremes. Many relevant genes have been identified by using molecular and genetic techniques to understand the plant response for the cold stress and these genes can be used further for developing cold

Rice crop is susceptible to salt stress and 1/3 of total world agricultural land is affected with it. The salinity of both water and soil has negative impact on the production of rice. The increasing level of sodium ions in the agricultural lands is becoming a severe threat for the agriculture worldwide. The plants suffer an osmotic stress due to the accumulation of salts at the outer side of the roots and suffer an ionic stress due to the accumulation of salts at the inner side of the plants [4]. The increase in food supply must be equal to the rate of increasing population and this demand can only be satisfied via utilizing all the available resources of land at their maximum potential. So, it is also necessary to use the saline soils at their fullest production potential. Different methods like agronomic adjustments, reclamation and different biological amendments are carried out in combination for increasing the production of saline soils. The use of salinity tolerant genetically improved crop varieties is the most suitable option for the sustainable crop

OsWRKY94 gene and enhances its expression in response to chilling stress while retarding its expression during normal temperature. Moreover, during normal temperature and cold stress, OsTB1 directly target and suppress the OsWRKY94 gene and under the cold stress OsMADS57 is responsible for promoting the transcription of D14 and suppress the tillering while during normal conditions expression of D14 was repressed to enhance the tillering. It shows that OsTB1 and OsMADS57 equally contribute to develop the cold tolerance in rice by targeting OsWRKY94 [52]. So, they can be used as potential candidates for developing cold tolerant rice cultivars. Cold tolerance can also be achieved by the biogenesis of chloroplast in rice crop. In plant development and growth, chloroplast plays an important role and its development is affected by the chilling temperature. The genes and regulators that are involved in the biogenesis of chloroplast are identified and characterized. The mutant of WSL5 (white strip leaf 5) in rice has been characterized. The white stripped leaves are developed by this mutant during early leaf development stage and show albino phenotype during chilling stress. A unique chloroplasttargeted-pentatricopeptide repeat protein is encoded by the WSL5; the molecular and genetic analysis has revealed it. The RNA sequence analysis revealed that in the mutant, expression of the nuclear-encoded-photosynthetic genes was significantly repressed and the genes responsible for the chloroplast formation were also changed significantly. WSL5 gene is required for the development of chloroplast during cold stress [58]. A transcription factor OsMYB3R-2 from rice having a DNA binding domain is involved in enhancing the cold tolerance [59]. Another transcription factor MYBS3 of rice having a DNA binding repeat also

*DOI: http://dx.doi.org/10.5772/intechopen.94036*

#### *Introductory Chapter: Recent Advances in Rice Biotechnology for Abiotic Stress Tolerance DOI: http://dx.doi.org/10.5772/intechopen.94036*

OsWRKY94 gene and enhances its expression in response to chilling stress while retarding its expression during normal temperature. Moreover, during normal temperature and cold stress, OsTB1 directly target and suppress the OsWRKY94 gene and under the cold stress OsMADS57 is responsible for promoting the transcription of D14 and suppress the tillering while during normal conditions expression of D14 was repressed to enhance the tillering. It shows that OsTB1 and OsMADS57 equally contribute to develop the cold tolerance in rice by targeting OsWRKY94 [52]. So, they can be used as potential candidates for developing cold tolerant rice cultivars.

Cold tolerance can also be achieved by the biogenesis of chloroplast in rice crop. In plant development and growth, chloroplast plays an important role and its development is affected by the chilling temperature. The genes and regulators that are involved in the biogenesis of chloroplast are identified and characterized. The mutant of WSL5 (white strip leaf 5) in rice has been characterized. The white stripped leaves are developed by this mutant during early leaf development stage and show albino phenotype during chilling stress. A unique chloroplasttargeted-pentatricopeptide repeat protein is encoded by the WSL5; the molecular and genetic analysis has revealed it. The RNA sequence analysis revealed that in the mutant, expression of the nuclear-encoded-photosynthetic genes was significantly repressed and the genes responsible for the chloroplast formation were also changed significantly. WSL5 gene is required for the development of chloroplast during cold stress [58]. A transcription factor OsMYB3R-2 from rice having a DNA binding domain is involved in enhancing the cold tolerance [59]. Another transcription factor MYBS3 of rice having a DNA binding repeat also play a significant role in adapting the cold environment [60].

The progress in developing the genetically modified plants by introducing and overexpressing the novel genes appears to be a very good practically possible option to speed up the process of breeding in plants. Instinctively, the faster way of inserting the genes with beneficial traits is genetic engineering instead of molecular or conventional breeding; and it would be the only possible option if the gene of interest belongs to some other species of plants, distant relatives, or from any non-plant source. The utilization of gene knockout strategies and genomic approaches are developing to enhance the efforts to measure thoroughly and make it easy to completely understand the complicated quantitative traits like tolerance to different temperature fluctuations and extremes. Many relevant genes have been identified by using molecular and genetic techniques to understand the plant response for the cold stress and these genes can be used further for developing cold tolerant varieties [61].

#### **2.4 Salt stress**

*Recent Advances in Rice Research*

cultivars.

**2.3 Cold stress**

with various quantitative trait loci (QTLs) are used to improve a cultivar for heat tolerance [45]. Many genes are responsible for heat tolerance at various stages; for example, ZFP is a gene that is related with heat tolerance at seedling stage [46] and OsWRKY11 is another gene that is responsible for heat tolerance in rice [47]. The heat tolerant genes are needed to be identified in rice cultivars and the genes from the other plant species can also be transferred to the rice cultivars to improve the heat tolerance. Other approaches like use of mutagens and a recent technology like CRISPR-Cas9 should be applied for further development of heat tolerant rice

The cold stress is a significant environmental factor that affects the development and growth of rice crop. At the stage of seedling development, sudden decrease in temperature can influence the development of chlorophyll [48]. The damage of rice seedling due to cold stress ultimately decreases the grain yield. So, the cold stress is a major limitation which can be overcome by using cold tolerant rice varieties [49]. Because rice crop is evolved in tropical region, so it has limited adaptability to chilling stress. Improving rice varieties to make them cold tolerant, enable the cultivation of rice in northerly latitudes. Different signal transduction pathways and genetic networks are involved in controlling chilling tolerance [50]. In japonica rice, chilling tolerance is achieved by interactions between rice G-protein α-subunit 1 (RGA1) and chilling tolerance divergence 1 (COLD1) followed by the calcium signaling initiated in the response of downstream network of stress response belongs to C repeat binding factor (CBF) that is a transcription factor [51]. Still, there is limited information available about the stress response and adaptation. The plant show response to abnormal surrounding temperature by changing their gene expression and adapt the suitable architecture due to the developmental plasticity. In SAM (shoot apical meristem) intrinsic signals can be disrupted by cold stress and stress tolerance is enhanced by setting the dormancy cycling at the SAM [52]. For the survival mechanism against cold stress, protection of niche forms of root stem cell is a sacrifice [53]. The differentiated cells are properly organized, and meristematic activity is maintained by this rehabilitated development in response to the cold temperature. During cold stress, many specific genes like OsMYB3R-2 are activated by the various transcription factors to maintain the mitotic cell and their cold tolerance. The survival and growth are enhanced by keeping maintained the

cellular activity and cell behavior during and after the cold stress.

The formation of axillary bud following by its outgrowth in axils of primordia of leaf controls the shoot branching. The axillary bud initiation is controlled by MOC1 (Monoclum 1) and LAX 1 (Lax Panicle 1). The signaling pathways involved in the axillary bud outgrowth formation and biosynthesis of strigolactone are important for tillering in rice crop and are controlled by the dwarf genes, that are dwarf 53 (D53), dwarf 27 (D27), dwarf 17 (D17), dwarf 14 (D14), dwarf 10 (D10) and dwarf 3 (D3) [54]. In rice the outgrowth of axillary buds is repressed by the OsTB1 that acts in the downstream region of the dwarf genes. The interaction between the OsMADS57 and OsTB1 is responsible for reducing the inhibitory effect on the D14, a gene responsible for the organogenesis of rice tillers by producing a receptor for the hormone strigolactone [55], and it enables D14 to regulate the initiation of axillary bud [56]. The plant endogenous conditions and environmental situations control the development of axillary buds that are basically indeterminate structures [57]. To adapt the cold environment a network has been identified having core gene OsMADS57. The rice tiller growth is maintained by the overexpression of OsMADS57 during cold stress. OsMADS57 directly binds with the promoter of the

**8**

Rice crop is susceptible to salt stress and 1/3 of total world agricultural land is affected with it. The salinity of both water and soil has negative impact on the production of rice. The increasing level of sodium ions in the agricultural lands is becoming a severe threat for the agriculture worldwide. The plants suffer an osmotic stress due to the accumulation of salts at the outer side of the roots and suffer an ionic stress due to the accumulation of salts at the inner side of the plants [4]. The increase in food supply must be equal to the rate of increasing population and this demand can only be satisfied via utilizing all the available resources of land at their maximum potential. So, it is also necessary to use the saline soils at their fullest production potential. Different methods like agronomic adjustments, reclamation and different biological amendments are carried out in combination for increasing the production of saline soils. The use of salinity tolerant genetically improved crop varieties is the most suitable option for the sustainable crop

production in these areas [62]. The genetic diversity of crops regarding salinity tolerance must be evaluated for developing the salt tolerant crop varieties. The molecular mapping approaches have made it possible to identify the genomic regions responsible for the salt tolerance and assessment of the genetic diversity of different crops and varieties is becoming very easy [63]. The chromosomal regions (QTLs) responsible for the tolerance of salt stress in rice crop can be identified through various molecular mapping approaches.

The salt stress badly effects the physiological, morphological and biochemical features of the rice. It has the negative impact on plant height, shoot dry weight, total tillers, total dry matter and root dry weight. The various physiological attributes that are affected by the salt stress are senescence, calcium ion uptake, sodium ion uptake potassium ion uptake, total cations uptake, osmotic potential, transcription efficiency and relative growth rate [64]. The biochemical features of rice that are effected by the salt stress are proline content, anthocyanins, peroxidase (POX) activity, calcium content, sodium content, potassium content, chlorophyll content and hydrogen peroxide content [4]. Various levels of salinity tolerance are observed at the whole plant level and leaves of rice [65]. Likewise, the rice plant behavior against the salinity stress may vary at reproductive and vegetative phase and this may not be related to the net relative tolerance of the plant. It is mandatory to know the stage that is more susceptible to the salt stress because it is important for comparing the performance of different cultivars during stress. The process of photosynthesis is necessary for the good vegetative and reproductive growth. In the leaf tissues, increased sodium concentration adversely affects the essential cellular metabolism and net photosynthesis. In the process of photosynthesis, chlorophyll content is significantly important but during salinity stress, there is no relationship found between the photosynthesis and chlorophyll content because net photosynthesis rate is decreased by the sodium ion concentration which does not have any connection with the chlorophyll content. It shows disturbance in some other cellular processes of photosynthesis due to salinity stress. The carbon dioxide fixation and stomatal aperture are affected by the sodium ion accumulation in the leaf at the same time, so it can be a reason for the decrease in photosynthesis during salt stress [4]. Different mechanisms have been evolved in rice plant to cope with salt stress conditions. An example of this type of mechanism is compartmenting of salts within the plant.

Numerous genes and QTLs are activated during the salt stress, which could be determined by the different molecular mapping approaches. Many types of molecular marker are present to identify different QTLs. SNP (Single nucleotide polymorphism), SSLP (simple sequence length polymorphism), RFLP (restriction fragment length polymorphism), AFLP (Amplified fragment length polymorphism), STS (sequence tagged sites) and SSRs (simple sequence repeats) are various markers of DNA that are used for genotyping in the studies of molecular mapping [66]. For example: the QTLs of pollen fertility, sodium ion concentration and calcium, sodium and potassium accumulation have been identified in F2 population of rice by using SSR marker [67]. The QTLs for yield related traits and morphological traits are identified by F2 population of rice as plant material and SSR as DNA marker [66] and the QTLs of potassium and sodium uptake for increasing the salinity tolerance in rice are determined by the AFLP, RFLP and SSR [68]. QTLs of salt stress responsive genes have been identified by the SNP in the rice crop [69]. The rice germplasm and these identified QTLs are found salt stress tolerant and are useful for three main reasons: (1) salt tolerance can be understood by the molecular genetics in rice, (2) rice germplasm that is tolerant to salt stress can be introduced to make them salt tolerant and (3) for the screening of rice germplasm, identified QTLs are used against salinity stress [4]. Moreover, new

**11**

*Introductory Chapter: Recent Advances in Rice Biotechnology for Abiotic Stress Tolerance*

genes responsible for the salt tolerance can be identified and incorporated in the

The ultraviolet (UV) radiations are present in the region of solar electromagnetic spectrum that has the wavelength (λ) from 200 nm to 400 nm. The UV radiations have shorter wavelength as compared to photosynthetically active radiation that has the wavelength ranges from 700 nm to 400 nm. The UV radiations are composed of three different types that are UV-A, UV-B and UV-C. The UV-C radiations have a very smaller wavelength that is 200-280 nm and it emits photons with high energy which are absorbed totally by ozone layer and are not be able to reach the Earth surface [70]. The UV-A radiations ranges from 315 nm to 415 nm and are more constant. The UV-A radiations causes a little harm to the plants, so the major source of damage are UV-B radiations that have wavelength between 280 and 315 nm [71]. UV-B can cause mutations, reduction in photosynthetic activity, reduction in chlorophyll content, lower electron transfer rate, damage genetic material, decrease the biomass, reduce leaf size, lessen leaf number and eventually decrease the plant productivity. At the cell level, it initiates the oxidative stress through enhancing the level of ROS that ultimately damages the DNA, lipids and proteins; so, the integrity and functionality of cell membrane and enzymes is compromised. The light for photosynthesis can be maximally captured in higher plant by the exposure with UV-B. It is important for increasing the secondary metabolites, enzymatic and non-enzymatic antioxidants, bioactive compounds and cyto-solutes (sugars, glycine, betane, proline) for the survival of plants. The adverse effects of the radiation can be mitigated by the growth regulators present in the plants. The specific signaling pathways are present in plants that are involved in regulating protective gene expression responses against UV-B are vital for survival of plants in sunlight. The identification of the genes responsible for the UV-B radiation, survival is neces-

sary to develop the crop varieties resistant to UV radiation stress [72].

Many solutes are present in the rhizosphere required for the growth and development of plants. Plants uptake these solutes by roots and distribute them in the whole plant body. The successful plant life is ensured by up taking the other components with water from the rhizospheric soil by roots. The developmental plasticity and physiological activity in the plant roots is carried out by the water uptake with soluble elements. The distribution and uptake of these inorganic materials inside the plants is an intrinsic property of energy and material fluidity. In plant cells, a plethora of physiological and structural functions is supported by these essential ions but if these essential ions are present in the non-physiological concentrations, they can turn out as limiting factors. The cellular homeostasis is affected by their availability to plants, Inequalities in comparative abundance of these elements in soil and their rate of uptake. The defense system and adaption of plants is dependent on the developmental and physiological changes triggered by ion toxicity; but it can cause the permanent damage to the plant. In the rhizospheric soil, the ions of heavy metals are also present that can be absorbed by the roots with the water and nutrients and can be incorporated into the plant tissues. The zinc, iron, manganese, copper, aluminum, chromium, cadmium, cobalt, lead, arsenic, nickel and molyb-

The concentration of metal ions is excessive in polluted areas and plants growing in those areas suffer from metal toxicity. Some soils have high level of heavy metals

*DOI: http://dx.doi.org/10.5772/intechopen.94036*

rice cultivars to make them salt tolerant.

**2.5 UV radiation stress**

**2.6 Heavy metal toxicity**

denum are some toxic metals for plants [73].

genes responsible for the salt tolerance can be identified and incorporated in the rice cultivars to make them salt tolerant.
