The map positions were integrated into IRGSP map according to marker information. Detail information can be found

**Table 4.** Summary of minor blast *R* genes, donors, map position, tightly linked DNA markers, and associated blast

**(cM) ># Markers Name of pathogenic Strains Ref.**

**(cM) ># DNA Markers Avirulent race/isolate Ref.**

[50]

**Figure 3.** Location of cloned and mapped *R* genes on rice chromosomes. The locations of *R* genes have been integrat‐ ed into IRGSP map according to marker information, then the map was built using Mapmaker software. Centimorgan was used to measure the map positions showing in the right column of the choromosome. The underlined words indi‐ cate either SSR or RFLP markers (see additional resources: http://www.shigen.nig.ac.jp/rice/oryzabaseV4/insd/detail/ 3554).


\* This R gene shares the same name with another R gene. # The map positions were integrated into IRGSP map according to marker information. Detail information can be found on http://rgp.dna.affrc.go.jp/E/IR.

**Table 5.** Summary of cloned *R* genes, map position, closely linked DNA markers, and their expression.

## **3. Structure and function of blast** *R* **genes**

**Chr.**

*R* **gene cloned**

2 *Pib* Engkatek, Tohoku IL9, Teqing, Tjinam, BL1

6 *Pi9 O.minuta,*

75-1-127

11 *Pik* To-To, Kusabue,

11 *Pikm* Hokushi,Tsuyuake , IRBLkm-Ts

> Kaybonnet, Lemont, Lebonnet

Tsukinohikari, St NO. 1

11 *Pikh* Tetep, K3,

11 *Pb1* Modan,

12 *Pita* Tetep, Katy, Teqing

Kanto 51, K60, Chugoku 31, Shin 2-1, K2, K3, , Minehikari, GA 20

**Donor and cultivar or line carrying the gene**

204 Rice - Germplasm, Genetics and Improvement

1 *Pi37* St. No1 136.1 RM302,

9 *Pi5* Tetep, RIL 260 31.3-33.0 76B14r,

**Map position**

6 *Pid2* Digu (I) 65.8 Single Receptor

**(cM) ># Markers**

RM212

6 *Pid3* Digu 65.2-65.8 Single NBS-LRR Q 737

40N23r

k8824, k3951, k39512

k6441

to marker information. Detail information can be found on http://rgp.dna.affrc.go.jp/E/IR.

**Table 5.** Summary of cloned *R* genes, map position, closely linked DNA markers, and their expression.

119.9-120.3 k8823,

115.1-117.0 k2167,

**Locus**

1 *Pit* K59, Tjahaja 12.2 T256 Multiple CC-NBS-LRR Repressible [71]

1 *Pish* Shin 2, Norin 22 148.7-154.8 Multiple CC-NBS-LRR Constitutive [73]

4 *pi21* Owarihatamochi 58.6 P702D03 Multiple NBS-LRR Cytoplasm [75]

6 *Pi2* C101A51 58.7 Multiple NBS-LRR R 838 S Constitutive [78] 6 *Piz-t* TKM, Toride 1 58.7 zt56591 Multiple NBS-LRR S 839 R Constitutive [79]

6 *Pi25\** Gumei 2 63.2-64.6 Multiple CC-NBS-LRR [80] 8 *Pi36* Q61, Kasalath 21.6-25.2 CRG3 Single NBS-LRR S 590 D Constitutive [81]

11 *Pi1* LAC23, C101LAC 112.1-117.9 Multiple NBS-LRR [83]

11 *Pikp* Tetep K60 119.9-120.3 k3957 Multiple CC-NBS-LRR Constitutive [87]

11 *Pi54rh O. rhizomatis* 119.9-120.3 Multiple CC-NBS-LRR Extracellular Inducible [89] 11 *Pia* Aichi Asahi 36.0 Yca72 Multiple NBS-LRR [90]

\* This R gene shares the same name with another R gene. # The map positions were integrated into IRGSP map according

85.7-91.4 Single CC-NBS-LRR Age-

**structure Protein type**

**Subcellular localization**

247 M

Stop

Multiple CC-NBS-LRR Constitutive [84], [85]

Multiple NBS-LRR Constitutive [86]

Membrane I 441 M Constitutive [76]

Multiple NBS-LRR Cytoplasm V 239 A, I

154.1 RM208 Multiple NBS-LRR Inducible [74]

58.7 Multiple NBS-LRR Constitutive [77]

101.9 RM224 Multiple NBS-LRR Inducible [88]

50.4 Single NBS-LRR Cytoplasm A 918 S Constitutive [92]

Multiple CC-NBS-LRR Cytoplasm Pi5-1 is

kinase

**FNPs Expression Ref.**

Constitutive [72]

[79]

[82]

inducible, Pi5-2 is constitutive

dependent

[91]

Among the mapped *R* genes (Table 3 and 4), 22 genes including 20 major and 2 minor *R* genes (*Pb1* and *pi21*) have been molecularly characterized (Table 5). Noticeably, *Pid2*, *Pid3*, *Pi36*, *Pb1*, and *Pita* are single copy genes; while others are members of small gene families. A total of eight *R* genes have been identified on chromosome 11, with six at the *Pik* locus; six *R* genes on chromosome 6, four of which are at the *Piz* locus. Most cloned blast *R* genes are adequate in providing complete resistance to strains of *M. oryzae* that contain the corresponding avirulence genes. Interestingly, two different members of each of *Pi5*, *Pik*, *Pikp*, *Pikm*, and *Pia* are required for complete resistance to some avirulent races.

Similar to other plant *R* genes, all cloned blast *R* genes to date encode predicted proteins with centrally located nucleotide binding sites (NBS) and leucine rich repeat (LRR) at the carboxyl terminus (Figure 4), with the exception of *Pid2* and *pi21* encoding a B-lectin kinase protein and a proline containing protein, respectively. Plant NBS-LRR proteins can be divided into two subgroups based on whether they contain a Toll-interleukin receptor (TIR)-like domain (TIR-

**Figure 4.** Structure of all cloned *R* genes. The light green bar represents the length of the *R* genes. The highlighted bars represent the different domains of the *R* genes.

NBS-LRR) or a putative coiled-coil (CC) structure (CC-NBS-LRR) in their amino-terminal region. The rice genome has 500 NBS-LRR gene families, and most of them belong to the CC-NBS-LRR family. The NBS domain contains kinase 1a (p-loop), kinase 2 and 3a (RNBS-B) motif, which presumably bind to ATP and trigger downstream signal transduction; whereas, the LRR is predicted to recognize pathogen effectors, either directly or indirectly. Other noticeable protein domains of plant *R* proteins were also summarized in Figure 4.

The observed structural similarities of blast R proteins might imply that their predicted conserved regions are associated with functional roles in triggering resistance to *M. oryzae*. Cloned blast *R* genes can be separated into two clades, I and II (Figure 5). Clade I consists of all NBS-LRR genes and clade II contains both NBS-LRR and non-NBS-LRR gene, such as *Pid2* and *pi21.* Among them, *Pi1-5, Pik-1, Pikp-1*, and *Pikm1-TS* on chromosome 11 share substantial homology to the *Pi9* locus on Chromosome 6; whereas, *Pi1-6, Pik-2, Pikp-2*, and *Pikm2-T* are more similar to *Pid2* and *pi21*, which are not NBS-LRR *R* genes. Homologous sequences of blast *R* genes can be found in the diverse germplasm of cultivated species including domesticated landrace varieties and wild relatives of rice. These observations suggest that genetics of rice immunity is ancient and may have been evolved during speciation and domestication.

**Figure 5.** Phylogenetic tree of all cloned blast *R* genes. The tree was constructed using protein sequences by software Mega 5.0 NJ method.

## **4.** *R* **gene-mediated signaling transduction pathways**

NBS-LRR) or a putative coiled-coil (CC) structure (CC-NBS-LRR) in their amino-terminal region. The rice genome has 500 NBS-LRR gene families, and most of them belong to the CC-NBS-LRR family. The NBS domain contains kinase 1a (p-loop), kinase 2 and 3a (RNBS-B) motif, which presumably bind to ATP and trigger downstream signal transduction; whereas, the LRR is predicted to recognize pathogen effectors, either directly or indirectly. Other noticeable

The observed structural similarities of blast R proteins might imply that their predicted conserved regions are associated with functional roles in triggering resistance to *M. oryzae*. Cloned blast *R* genes can be separated into two clades, I and II (Figure 5). Clade I consists of all NBS-LRR genes and clade II contains both NBS-LRR and non-NBS-LRR gene, such as *Pid2* and *pi21.* Among them, *Pi1-5, Pik-1, Pikp-1*, and *Pikm1-TS* on chromosome 11 share substantial homology to the *Pi9* locus on Chromosome 6; whereas, *Pi1-6, Pik-2, Pikp-2*, and *Pikm2-T* are more similar to *Pid2* and *pi21*, which are not NBS-LRR *R* genes. Homologous sequences of blast *R* genes can be found in the diverse germplasm of cultivated species including domesticated landrace varieties and wild relatives of rice. These observations suggest that genetics of rice immunity is ancient and may have been evolved during speciation and domestication.

**Figure 5.** Phylogenetic tree of all cloned blast *R* genes. The tree was constructed using protein sequences by software

Mega 5.0 NJ method.

protein domains of plant *R* proteins were also summarized in Figure 4.

206 Rice - Germplasm, Genetics and Improvement

It is now commonly accepted that products of *R* genes in plants can specifically recognize avirulence genes from the pathogen directly or indirectly to initiate innate immunity system responses. Direct recognition of the putative product of the avirulence gene, *AVR-Pita1* by the *Pita* protein was first reported in 2000 [93]. Over a decade later, in 2012, another blast *R* protein, *Pik*, with similar structure to *Pita*, was found to directly recognize the corresponding avirulence gene *AvrPik* [94]. Direct interactions between other blast *R* and avirulence genes have not been reported; suggesting that indirect interactions may be responsible in triggering effective signal transduction pathways. Other plant genes involved in signal transduction have been investi‐ gated by the use of *R* proteins as bait in the yeast two-hybrid system (Y2H). While Y2H is a highly effective tool, it is limited in indentifying immediate plant components of *R* proteins. Molecular basis of blast *R* gene-mediated signaling has been a subject of intensive investigation worldwide. Abundant genes that may be involved in *R* gene-mediated signaling have been identified with DNA microarray [95, 96], and most of them were pathogenicity related genes. Genetic analysis using mutagenesis has been another commonly used alternative to identify downstream components. However, most mutants identified, thus far, are lesion mimic mutants [97]. A major effort to identify *Pita* mediated signaling was accomplished by treating 20,000 Katy seeds with *Pita/Pita2 /Piks* using fast neutrons, ethyl methyl sulfate (EMS), and gamma irradiation [98]. A total of 142 rice seedlings, with altered disease reactions, were identified from independent M2. The susceptibility of M2 individuals was verified in subse‐ quent generations, and 20 of them were confirmed to be derived from Katy using 20 diagnostic single sequence repeat (SSR) markers. Consequently, the *Ptr(t)* gene in rice was identified to be essential for *Pita* mediated signal transudation [99]. Molecular cloning of *Ptr(t)* will shed light on the interaction mechanism of *Pita* and *Ptr(t)*, and subsequent plant genes involved in defense responses.

## **5. The management of blast disease-marker assisted selection**

Blast disease has been effectively managed by a combination of fungicides and *R* genes integrated into diverse cultural practices. These include seed treatment with fungicide; preventive application of fungicide before heading; crop rotation; balanced application of fertilizers with nitrogen, potassium, and phosphate; and maintaining a sufficient water level during tillering and flowering stages. However, the most effective way to manage rice blast is by the utilization of resistant cultivars due to its environmental and economic sustainability. Incorporating major blast *R* genes have been traditionally accomplished by classical breeding methods and can be accelerated by the use of marker assisted selection (MAS) [100]. MAS has become a practical tool in cultivar improvement by selecting important traits at the early growth stages based on DNA markers, thus breeders can screen for resistance without having to maintain pathogen culture. MAS is efficient and consistent in the field and greenhouse [101]. MAS is also reliable in dealing with traits whose phenotype is affected by the environment. To date, 99 blast *R* genes have been mapped with closely linked DNA markers; and some of them can be used for MAS. DNA markers were also developed from portions of cloned *R* genes, such as *Pi-ta* and *Pi-b*, for their introduction into elite rice cultivars using MAS. Markers for *Pita*, one of the most important *R* genes for blast in the United States, were developed [102]; while, linked markers for 4 blast *R* genes (*Pik*, *Pib*, *Pita2*, and *Pii*) are effective against eight to ten races of *M. oryzae* were identified [103]. Using MAS, *R* genes like *Pi1*, *Pi5*, *Piz-5*, and *Pita* have been established in different rice genotypes [82, 100, 104, 105]

## **6. Future prospects**

Blast disease is a moving target where the fungus can rapidly adapt to the host. The major difficulty in controlling rice blast is the durability of genetic resistance. Rice cultivars contain‐ ing only a single *R* gene to a specific pathogen race often become susceptible over time due to the emergence of new virulent races. In theory, *R* genes can be found in rice germplasm in different rice production areas. Stacking *R* genes with overlapped resistance spectra can lead to long lasting resistance. Knowledge of genetic identity of contemporary *M. oryzae* is crucial for precise deployment of rice cultivars with different *R* genes [104]. Effective blast manage‐ ment also requires unprecedented international cooperation. IRRI and research institutions worldwide have been coordinating their resources for both genotyping using next generation of DNA sequencing and phenotyping at different geographic locations. The knowledge gained by this massive collaborative effort ought to lead to more effective methods to reduce crop loss due to blast disease worldwide.

## **Acknowledgements**

We thank the Arkansas Rice Research and Promotion Board, and the US National Science Foundation (Plant Research Program no. 0701745), Natural Science Foundation of China (Program no. 31000847), Zhejiang Natural Science Foundation (Program no. Y3100577), and Qianjiang Talents Project supported by Science Technology Department of Zhejiang Province (Program no. 2011R10038) for their partial financial supports. USDA is an equal opportunity provider and employer.

## **Author details**

Xueyan Wang1 , Seonghee Lee2 , Jichun Wang3 , Jianbing Ma4 , Tracy Bianco5 and Yulin Jia5\*

\*Address all correspondence to: yulin.jia@ars.usda.gov

1 China Jiliang University, Hangzhou, China

2 Noble Foundation, Oklahoma, USA

3 Jilin Academy of Agricultural Sciences, Changchun, China

4 University of Arkansas Rice Research and Extension Center, Stuttgart, Arkansas, USA

5 USDA ARS Dale Bumpers National Rice Research Center, Stuttgart, Arkansas, USA

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them can be used for MAS. DNA markers were also developed from portions of cloned *R* genes, such as *Pi-ta* and *Pi-b*, for their introduction into elite rice cultivars using MAS. Markers for *Pita*, one of the most important *R* genes for blast in the United States, were developed [102]; while, linked markers for 4 blast *R* genes (*Pik*, *Pib*, *Pita2*, and *Pii*) are effective against eight to ten races of *M. oryzae* were identified [103]. Using MAS, *R* genes like *Pi1*, *Pi5*, *Piz-5*, and *Pita*

Blast disease is a moving target where the fungus can rapidly adapt to the host. The major difficulty in controlling rice blast is the durability of genetic resistance. Rice cultivars contain‐ ing only a single *R* gene to a specific pathogen race often become susceptible over time due to the emergence of new virulent races. In theory, *R* genes can be found in rice germplasm in different rice production areas. Stacking *R* genes with overlapped resistance spectra can lead to long lasting resistance. Knowledge of genetic identity of contemporary *M. oryzae* is crucial for precise deployment of rice cultivars with different *R* genes [104]. Effective blast manage‐ ment also requires unprecedented international cooperation. IRRI and research institutions worldwide have been coordinating their resources for both genotyping using next generation of DNA sequencing and phenotyping at different geographic locations. The knowledge gained by this massive collaborative effort ought to lead to more effective methods to reduce crop loss

We thank the Arkansas Rice Research and Promotion Board, and the US National Science Foundation (Plant Research Program no. 0701745), Natural Science Foundation of China (Program no. 31000847), Zhejiang Natural Science Foundation (Program no. Y3100577), and Qianjiang Talents Project supported by Science Technology Department of Zhejiang Province (Program no. 2011R10038) for their partial financial supports. USDA is an equal opportunity

, Jianbing Ma4

, Tracy Bianco5

and Yulin Jia5\*

, Jichun Wang3

have been established in different rice genotypes [82, 100, 104, 105]

**6. Future prospects**

208 Rice - Germplasm, Genetics and Improvement

due to blast disease worldwide.

**Acknowledgements**

provider and employer.

, Seonghee Lee2

1 China Jiliang University, Hangzhou, China

2 Noble Foundation, Oklahoma, USA

\*Address all correspondence to: yulin.jia@ars.usda.gov

**Author details**

Xueyan Wang1


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## **Rice Straighthead Disease – Prevention, Germplasm, Gene Mapping and DNA Markers for Breeding**

Wengui Yan, Karen Moldenhauer, Wei Zhou, Haizheng Xiong and Bihu Huang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/56829

## **1. Introduction**

Straighthead is a physiological disorder of rice that results in sterile florets with distorted lemma and palea, and in extreme cases, the panicles or heads do not form at all (Atkins, 1974). As a result, heads remain upright at maturity due to lack of grain development: hence, the name 'straighthead'. The diseased panicles may not emerge from the flag leaf sheath when the disease is severe. Either the lemma or palea or both may be lacking, even if they are present they are distorted and crescent-shaped, particularly in long grain cultivars, forming a charac‐ teristic symptom of straighthead called 'parrot beak' (Rasamivelona et al., 1995). Other symptoms include unusually vigorous dark green leaves in mature plants and strikingly abnormal root systems with large, shallow roots with few branches and root hairs (Atkins, 1974; Bollich et al., 1989).

**Figure 1.** Straighthead symptoms in rice field of the United States (US) (left and middle) and Argentina (right).

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

LSD 0.05

1663 for comparing cultivars within a

water

Straighthead can cause a complete loss of grain yield in rice when sever (Fig. 1). In a study conducted by Wilson et al. (2001), grain yield reduction due to straighthead was up to 94% for a popular cultivar Cocodrie (Table 1). Yan et al. (2005) concluded that US cultivar Cocodore, Mars, Kaybonnet and Bengal were highly susceptible to straighthead, indicated by a yield reduction from 80% for Bengal to 96% for Mars in a study conducted in 1999 and 2000 (Fig. 2). Similarly, in a study conducted in 2001, Cocodrie and Mars suffered a yield reduction of 97% and 95%, respectively from straighthead (Table 2). Cocodrie, Cypress and Wells were grown on 73% of rice hectares in the southern US in 2001 (RTWG, 2002). The susceptibility of these widely grown cultivars to straighthead represents a potentially serious threat to southern US rice production, especially for Arkansas where about 50% of the US rice is produced (Wilson et al., 2010a). Therefore, the prevention of straighthead is not only an important target in the DD50 Computerized Rice Management Program http://dd50.uaex.edu/dd50Logon.asp (Slaton, 2001), but also is reminded to rice growers each year when the time of its prevention is getting close by Cooperative Extension Agents http://www.uaex.edu (Wilson et al., 2010b; 2010c).


management \*Yield loss (%) for each cultivar was calculated by: [(Drain and dry yield – Continuous flood yield) / Drain and dry yield] x 100.

**Table 1.** Grain yield (kg/ha) of rice cultivars affected by straighthead disease under different water managements at the Rice Research and Extension Center, University of Arkansas near Stuttgart during 1999 (Wilson et al., 2001).

Rice Straighthead Disease – Prevention, Germplasm, Gene Mapping and DNA Markers for Breeding http://dx.doi.org/10.5772/56829 221

**Figure 2.** Comparison of grain yield between straighthead affected (MSMA Treated) and un-affected (MSMA Untreat‐ ed) cultivars at the Dale Bumpers National Rice Research Center near Stuttgart, Arkansas in 1999 and 2000, where straighthead was induced by soil incorporation of 6.7 kg of monosodium methanearsonate (MSMA) per hectare (Yan et al., 2005).

## **2. Global threat from straighthead and its causal factors**

Straighthead can cause a complete loss of grain yield in rice when sever (Fig. 1). In a study conducted by Wilson et al. (2001), grain yield reduction due to straighthead was up to 94% for a popular cultivar Cocodrie (Table 1). Yan et al. (2005) concluded that US cultivar Cocodore, Mars, Kaybonnet and Bengal were highly susceptible to straighthead, indicated by a yield reduction from 80% for Bengal to 96% for Mars in a study conducted in 1999 and 2000 (Fig. 2). Similarly, in a study conducted in 2001, Cocodrie and Mars suffered a yield reduction of 97% and 95%, respectively from straighthead (Table 2). Cocodrie, Cypress and Wells were grown on 73% of rice hectares in the southern US in 2001 (RTWG, 2002). The susceptibility of these widely grown cultivars to straighthead represents a potentially serious threat to southern US rice production, especially for Arkansas where about 50% of the US rice is produced (Wilson et al., 2010a). Therefore, the prevention of straighthead is not only an important target in the DD50 Computerized Rice Management Program http://dd50.uaex.edu/dd50Logon.asp (Slaton, 2001), but also is reminded to rice growers each year when the time of its prevention is getting close by Cooperative Extension Agents http://www.uaex.edu (Wilson et al., 2010b;

> **10 days delay flood**

**20 days delay flood**

**Yield loss %\***

2010c).

**Cultivar**

LSD 0.05

LSD 0.05

x 100.

**Continuous flood**

220 Rice - Germplasm, Genetics and Improvement

2923 for comparing water managements within a cultivar

1663 for comparing cultivars within a

water management **Drain and dry**

Bengal 1210 5695 3629 4435 79 Cocodrie 353 6048 1361 1865 94 Cypress 3427 6250 6602 6300 45 Drew 4032 6905 5292 6451 42 Jefferson 5695 6854 6653 6048 17 Madison 3478 6149 4536 4990 43 Priscilla 5594 7510 7358 5443 26 Wells 5695 7913 6250 7459 28

\*Yield loss (%) for each cultivar was calculated by: [(Drain and dry yield – Continuous flood yield) / Drain and dry yield]

**Table 1.** Grain yield (kg/ha) of rice cultivars affected by straighthead disease under different water managements at the Rice Research and Extension Center, University of Arkansas near Stuttgart during 1999 (Wilson et al., 2001).

Straighthead was first reported to dramatically affect grain yield in the US by Hewitt (1912). In the early 1900s, Collier (1912) estimated that approximately 20% of the US rice acreage suffered significant yield reductions by 12 to 15% due to straighthead. Afterwards, straight‐ head researches were published in Japan (Iwamoto, 1969), Portugal (called 'branca') (Cunha and Baptista, 1958), Australia (Dunn et al., 2006), Thailand (Weerapat, 1979), and Argentina (Yan et al., 2010a) (Fig. 1).

No pathogen has been identified to be associated with straighthead, so it is regarded as a physiological disease. The occurrence and severity of straighthead have been associated with soil organic matter (Editor's Note, 1946), low pH and low free iron (Baba and Harada, 1954), thiol compounds (Iwamoto, 1969), sandy to silt loam soil textures (Rasamivelona et al., 1995; Slaton et al., 2000), continuous flooding (Wilson et al., 2001), high soil As (Gilmour and Wells, 1980), N fertilization (Dilday et al., 1984; Dunn et al., 2006), and soil Cu availability (Ricardo and Cunha, 1968). A recent work suggested possible roles of magnesium but not As in naturally-occurring straighthead by chemical analyses of rice plant (node, internode, stem, leaf and root) and seed (brown and milled seed and hull) (Belefant-Miller and Beaty, 2007). Soil aeration is believed to speed the decay of soil organic matter (Editor's Note, 1946) and help oxidize arsenic (As) into arsenate, which is biologically inactive (Marin et al., 1992). Arsenic is toxic to many plant species including snap bean (*Phaseolus vulgaris* L.) (Sachs and Michael, 1971), soybean (*Glycine max* L.), potato (*Solanumtuberosum*L.), cotton (*Gossypiumhirsutum* L.), and rice (Baker et al., 1976).

In a straighthead study conducted by Yan et al. (2008) using resistant and susceptible cultivars in 2004 and 2005, minerals in flag leaves of heading panicles were measured because the susceptible cultivars could not produce seeds and direct measurement on seeds is not feasible. Straighthead was correlated negatively with grain yield (r=-0.89), plant height (r=-0.60) and flag leaf contents of Ca (r=-0.51), Mn (r=-0.31) and S (r=-0.26) and positively with days to head (r=0.63). Leaf Ca was associated positively with grain yield (r=0.60), leaf Mn (r=0.81), Fe (r=0.42), S (r=0.40) and Cu (r=0.38) and negatively with days to 50% heading (r=-0.64). The increased Mn in the flag leaves was associated with the increased leaf Ca (r=0.81), Fe (r=0.49), Cu (r=0.48), S (r=0.40) and As (r=0.29), but with the decreased days to 50% head (r=-0.56). Flag leaf S concentration was correlated positively with plant height (r=0.37), grain yield (r=0.35) and leaf P (r=0.59), K (r=0.49) and Mn (r=0.40) and negatively with days to head (r=-0.64) and leaf Na (r=-0.41) and Zn (r=-0.41). Leaf As concentration was correlated with the leaf Cu (r=0.65), Na (r=0.58), Fe (r=0.51) and Mn (r=0.29), but negatively with leaf K (r=-0.49) and B (r=-0.42). However, the exact causal factors of naturally occurring straighthead are still unknown.

## **3. Methods for straighthead evaluation and prevention**

## **3.1. Evaluation methods for straighthead**

Because the symptoms of As injury are similar to straighthead of rice, incorporation of As in a form of monosodium methanearsonate (MSMA) has become the common and only practice for evaluating rice susceptibility to straighthead in research and breeding programs up to present (Horton et al., 1983; Frans et al., 1988; Wilson et al., 2001; Dunn et al., 2006; Pan et al., 2012).

A special field has been designated for straighthead research and breeding with MSMA amendment for more than 20 years (Somenahally et al., 2011) at the University of Arkansas, Division of Agriculture, Rice Research and Extension Center (RREC) jointly located with the United States Department of Agriculture (USDA), Agricultural Research Service (ARS), Dale Bumpers National Rice Research Center near Stuttgart, Arkansas. Usually, the MSMA as a solution in a spray volume of 85 L ha-1 at a rate of 6.7 kg MSMA ha-1 is directly applied to the soil surface with a calibrate CO2-backpack sprayer and incorporated into the soil before planting the seeds (Yan et al., 2008).

At maturity of growth stage R9 (Counce et al., 2000), straighthead is visually rated in the center of a plot based on floret fertility or sterility and panicle emergence from the flag leaf sheath. The rating scale ranged from 1 to 9, 1 = no apparent sterility (more than 80% grains developed) and 100% of the panicles completely emerged; 2 = 71 to 80% of the grains developed and 96 to 100% of the panicles completely emerged; 3 = 61 to 70% of the grains developed and 91 to 95% of the panicles completely emerged; 4 = 41 to 60% of the grains developed and 85 to 90% of the panicles completely emerged; 5 = 21 to 40% of the grains developed and 75 to 80% of the panicles completely emerged (at this stage distorted and parrot-beak grains initially appear); 6 = 11 to 20% of the grains developed and 65 to 70% of the panicles completely emerged; 7 = 0 to 10% of the grains developed and most of the panicles emerged but remained totally erect; 8 = no grains developed and 0 to 10% of the panicles emerged from the flag leaf sheath but erect; and 9 = short stunted plants with no panicle emergence. Indicated by Table 2, at rate 1 straighthead, cultivars have either no numerical reduction of yield or slightly numerical reductions which are far from statistical significance (p>0.60). The yield reduction is not statistically significant at the rate 4 or below, but highly significant (p<0.0001) at the rate 7 with a reduction of 95% or above.


† PI: Plant Introduction number in the U.S. germplasm system.

‡ Subspecies: I = *indica* and J = *japonica.*

susceptible cultivars could not produce seeds and direct measurement on seeds is not feasible. Straighthead was correlated negatively with grain yield (r=-0.89), plant height (r=-0.60) and flag leaf contents of Ca (r=-0.51), Mn (r=-0.31) and S (r=-0.26) and positively with days to head (r=0.63). Leaf Ca was associated positively with grain yield (r=0.60), leaf Mn (r=0.81), Fe (r=0.42), S (r=0.40) and Cu (r=0.38) and negatively with days to 50% heading (r=-0.64). The increased Mn in the flag leaves was associated with the increased leaf Ca (r=0.81), Fe (r=0.49), Cu (r=0.48), S (r=0.40) and As (r=0.29), but with the decreased days to 50% head (r=-0.56). Flag leaf S concentration was correlated positively with plant height (r=0.37), grain yield (r=0.35) and leaf P (r=0.59), K (r=0.49) and Mn (r=0.40) and negatively with days to head (r=-0.64) and leaf Na (r=-0.41) and Zn (r=-0.41). Leaf As concentration was correlated with the leaf Cu (r=0.65), Na (r=0.58), Fe (r=0.51) and Mn (r=0.29), but negatively with leaf K (r=-0.49) and B (r=-0.42). However, the exact causal factors of naturally occurring straighthead are still unknown.

Because the symptoms of As injury are similar to straighthead of rice, incorporation of As in a form of monosodium methanearsonate (MSMA) has become the common and only practice for evaluating rice susceptibility to straighthead in research and breeding programs up to present (Horton et al., 1983; Frans et al., 1988; Wilson et al., 2001; Dunn et al., 2006; Pan et al.,

A special field has been designated for straighthead research and breeding with MSMA amendment for more than 20 years (Somenahally et al., 2011) at the University of Arkansas, Division of Agriculture, Rice Research and Extension Center (RREC) jointly located with the United States Department of Agriculture (USDA), Agricultural Research Service (ARS), Dale Bumpers National Rice Research Center near Stuttgart, Arkansas. Usually, the MSMA as a solution in a spray volume of 85 L ha-1 at a rate of 6.7 kg MSMA ha-1 is directly applied to the soil surface with a calibrate CO2-backpack sprayer and incorporated into the soil before

At maturity of growth stage R9 (Counce et al., 2000), straighthead is visually rated in the center of a plot based on floret fertility or sterility and panicle emergence from the flag leaf sheath. The rating scale ranged from 1 to 9, 1 = no apparent sterility (more than 80% grains developed) and 100% of the panicles completely emerged; 2 = 71 to 80% of the grains developed and 96 to 100% of the panicles completely emerged; 3 = 61 to 70% of the grains developed and 91 to 95% of the panicles completely emerged; 4 = 41 to 60% of the grains developed and 85 to 90% of the panicles completely emerged; 5 = 21 to 40% of the grains developed and 75 to 80% of the panicles completely emerged (at this stage distorted and parrot-beak grains initially appear); 6 = 11 to 20% of the grains developed and 65 to 70% of the panicles completely emerged; 7 = 0 to 10% of the grains developed and most of the panicles emerged but remained totally erect; 8 = no grains developed and 0 to 10% of the panicles emerged from the flag leaf sheath but erect; and 9 = short stunted plants with no panicle emergence. Indicated by Table 2, at rate 1

**3. Methods for straighthead evaluation and prevention**

**3.1. Evaluation methods for straighthead**

222 Rice - Germplasm, Genetics and Improvement

planting the seeds (Yan et al., 2008).

2012).

§ No MSMA (monosodium methanearsonate) was applied as check conditions.

Stralghthead (SH) raling 1-9: 1 as normal and 9 as the worst SH.
