**5. Defense responses of wheat activated against pathogens in the presence of Silicon**

Several researches have demonstrated the potential of Si in increasing the resistance of wheat against a range of pathogens. Several diseases were reduced on wheat plants supplied with Si through roots or foliar and the mechanism of defense

#### *Silicon Use in the Integrated Disease Management of Wheat: Current Knowledge DOI: http://dx.doi.org/10.5772/intechopen.95285*

studied. The role of Si on wheat-pathogen interactions is related to its action to increase the plant's defense against to the stressor agent [85].

For blast, in which the pathogen infection leads to increase in the production of reactive oxygen species (ROS) and damage to cell membranes [86], in Si-supplied plants occurred lower concentrations of hydrogen peroxide(H2O2) and malondialdehyde indicating, therefore, that the ROS generation and cellular damage were greatly limited [87]. According to authors, the activities of enzymes superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX) and glutathione-S-transferase (GST) were higher in the leaves of the plants not supplied with Si, while in leaves from Si-supplied plants the glutathione metabolism seemed to play a role in such defense because glutathione reductase activity was increased. In line with this hypothesis, the higher expression levels of the defense-related genes pathogenesis-related 1, chitinase (CHI), POX and phenylalanine ammonia-lyase (PAL), as well as the higher activities of CHI and POX at intermediate and advanced stages of *M. oryzae* infection, respectively, associated to an increase on the concentration of ligninthioglycolic acid derivatives was reported contributing to defense against blast in Si-supplied plants [37, 88]. Cytological and histochemical analysis revealed that in Si-supplied plants the pathogen hyphae were restricted to in the invaded cells, delaying the colonization of the neighboring cells and consequently reducing the progress of the disease [68]. In another study, *M. oryzae* colonization was constrained in the cells on the leaves of Si-supplied plants in association with intense deposition of phenolic-like compounds (flavonoids) [36]. Phenolic-like material was also detected in the parenchyma cells of spikes, and scanning electron micrographs showed that fungal hyphae were scarcely observed in the epidermis, parenchyma and collenchyma cells indicating that these tissues were less colonized by fungal in comparison to the plants not supplied with Si [88].

In the wheat–*B. graminis* f. sp. *tritici* pathosystem, Si increased the resistance to fungus infection by specific defense reactions including papilla formation, production of callose, fungitoxic phenolic compounds and Si deposition at the site of infection [74]. The release of glycosilated phenolics along the cell wall and in association with the compromised haustoria was associated to the degradation of *B. graminis* haustoria [74, 89]. These defense responses potentiated by Si resulted in growth restriction to 10% of epidermal cells and poorly development of haustoria contrasting to leaves of wheat plants not treated with Si that had abundant hyphae of *B. graminis* on epidermal surface and typical haustoria formation in 90% of epidermal cells [74]. The study performed by Rémus-Borel et al. [89] verified that necrotic zones were not detected on *B. graminis* infected tissue, indicating that the response to infection potentiated by Si was not associated with a hypersensitive response, but the newly produced compounds of phenolic-like material that were associated with degraded *B. graminis* haustoria and collapsed conidial chains which interfered with pathogen development. In addition, biochemical defense response *to B. graminis* infection was reported to be associated to production of phytoalexins linked to metabolism of aconitate, which limited the diseases development [90]. A transcriptomic analysis revealed that wheat plants reacted to inoculation with *B. graminis* by an upregulation of many genes linked to stress and metabolic processes and a downregulation of genes linked to photosynthesis, but in Si-supplied plants the disease development is reduced fact that is translated into a nearly perfect reversal of genes regulated by the effect of *B. graminis* [91]. Another study revealed that *B. graminis* development established a close relationship with the antioxidant response of wheat plants [75]. According authors, the activity of SOD, CAT and APX decreases as Si doses increases indicating a relationship between the applied doses of Si and decrease in *B. graminis* infection due to the reduction of basal antioxidant enzyme

activity and ROS. Thus, the decrease of antioxidant enzymes influenced by Si could generate ROS status for more efficient responses of defense to *B. graminis* [75].

On the spot blotch, Si-supply to wheat plants caused a reduction in the rate of infection of *B. sorokiniana* in wheat epidermal cells, due to the physical barrier formed by the cuticle-Si double layer [79]. According to authors, this physical barrier may have reduced the diffusion of lytic enzymes and selective non-host toxins released by the pathogen on the leaf surface, as shown by the reduction of the wax layer degradation. However, even evident the potential of Si accumulated in the plant tissue (cell wall, beneath the cuticle and cell cumulating Si) in inhibiting or delaying the pathogen infection process, this deposition is not homogeneous in the epidermal tissue, which allows the formation of successful infection sites. At this infection sites, others defense potentiated by Si played an important role. Indeed, the increase in the activity of the enzyme POX and increase on the concentration of ligninthioglycolic acid derivatives were related as defense mechanisms, triggered by Si, in the wheat - *B. sorokiana* pathosystem [76].

For tan spot, Si-supply to wheat plants increased biochemical defense mechanisms and histo-cytological defense responses [80, 81]. The most prominent responses from Si-supplied plants were: the accumulation of H2O2 in the epidermal cells that occurred early, more intensely and in more epidermal cells, mainly at the beginning of pathogenesis; the alteration of enzyme activities such as SOD, CAT, POX, CHI and PAL; and the accumulation of phenylpropanoid derivatives at the infection site [80, 81]. Together, these defense responses restricted the spread of the pathogen and the damage caused in the plant tissues resulting in a reduction in cell death at *P. tritici-repentis* infection sites [80]. In regarding to the fast and greater accumulation of H2O2 in the epidermal cells of the Si-supplied plants is important highlight that the accumulation of H2O2 is known to be a mechanism of pathogen attack inducing cell death through *P. tritici-repentis* toxins [92]. However, the early (<12 hours after inoculation) accumulation of H2O2 in the epidermal cell of the Si-supplied plants of moderately resistant cultivar, compared to late accumulation (>24 after inoculation) in the mesophyll and epidermal cells of the non supplied plants, indicated that H2O2 was a defense mechanism. This inference is because accumulation of H2O2 occurred before pathogen penetration into the leaf tissue and was related to lower infection efficiency (the ratio between the number of conidia on the leaf surface and the number of lesions formed). Furthermore, on the Si-supplied plants, early fluorescence in epidermal cells, in neighboring cells and in the cell in which *P. tritici-repentis* attempted to penetrate, indicated that phenylpropanoid derivative accumulation were also contributing to disease resistance [81].

#### **6. Physiological effects of silicon in wheat under pathogen stress**

The photosynthesis is the major physiological process in plants; therefore, if plants are infected by pathogens some process in their physiology can be negatively affected. The pathogen infection can be responsible to decrease photosynthesis at different levels [93], modification or damage of the photosynthetic apparatus [94] and interfering with normal source-sink relationships in plants [95, 96].

In this sense exist a general consensus that Si improves the plant resistance to various biotic and abiotic stresses. Thus, the effect of Si on plant physiology it has been observed mainly when plant is under some kind of stress. For example, under biotic stress imposed by *B. graminis* f. sp. *tritici*, an analysis of around 55,000 transcripts indicated that around 3000 genes were differentially expressed on

#### *Silicon Use in the Integrated Disease Management of Wheat: Current Knowledge DOI: http://dx.doi.org/10.5772/intechopen.95285*

pathogen-inoculated plants, but a nearly perfect reversal in the transcript profile of downregulated stress-related genes occurred when Si was supplied [91]. This result indicated that Si rather than being involved directly in the regulation of gene expression, prevented or attenuated the effects on transcription imposed by pathogen [91]. Furthermore, several studies revealed that wheat plants supplied with Si when challenged by pathogens showed lower affectation and/or ameliorative on photosynthetic process as assessed via measurements of the leaf gas exchange and the chlorophyll *a* (Chl*a*) fluorescence kinetics.

In this regard, some studies showed that concentration of photosynthetic pigments and structural and functional damage of chloroplasts produce alterations on photochemical machinery with losses in the amount of chlorophylls and carotenoids, as a result it has been observed decreased values for the net photosynthesis rate [93]. In a study on wheat-*Magnaporthe oryzae* interaction, in Si-supplied plants occurred a maintaining the concentration of photosynthetic pigments such as total chlorophyll, violanxanthin + antheraxanthin + zeaxanthin, *β*-carotene and *α*-carotene which helped to maintain the structural and functional viability of the photosynthetic machinery minimizing, therefore, lipid peroxidation and the production of ROS to ensure the integrity of the leaf cells [97]. In the same pathosystem, photosynthetic performance was studied in Si-supplied plants which showed higher values for net photosynthesis rate coupled with improved photochemistry associated to Chl*a* fluorescence parameters, and also increased concentrations of total chlorophylls [66, 98]. Also, Si-supplied plant showed less functional damage to the photosystem II (PSII) without reductions in the values of maximum quantum quenching, photochemical yield of PSII and electron transport rate, but higher values for quenching non-photochemical [97].

Likewise, the impairment caused by blast on the photosynthetic process, primarily related to the *F*v/*F*m parameter, on wheat leaves, was in lesser extent on the plants sprayed with potassium silicate [69]. Furthermore, authors did not detect any significant alteration on the gas exchange and Chl*a* fluorescence parameter for plants sprayed three times (every 96 h interval) as the potassium silicate rates increased from 2.5 to 12.5 g L−1 indicating that potassium silicate do not cause perturbation to the photosynthetic machinery of wheat plants.

In addition, the pathogen infection usually leads to the development of symptoms that result in a decrease on the photoassimilates production [99], resulting in low performance of photochemical reactions associated to PSII, that mainly influence the reduction in CO2 assimilation [93] producing alteration in some parameters of leaf gas exchange. In this way, alterations with diffusional limitations and significant losses both in the electron transport rate and biochemical capacity for carboxylation associated with losses in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity have been measured on the leaves of wheat plants infected with *M. oryzae* [87]. In this pathosystem, Aucique Perez [66] observed that these dysfunctions could largely be avoided in the presence of Si, which might directly be associated with lower blast symptoms on Si-supplied plants; in those plants net CO2 assimilation rate, stomatal conductance to water vapor, and transpiration rate were significantly higher, showing that Si improving gas exchange performance. Furthermore, Araujo et al. [95] measured several parameters of Chl*a* fluorescence, sugars (glucose, fructose and sucrose) and starch concentration, the activities of enzymes acid invertase and sucrose phosphate synthase in leaves and spike of wheat challenged by *M. oryzae* showing evidences of the beneficial effects of Si in improving the source-sink relationship on infected leaves and spikes by preserving the alteration in assimilates production and partitioning during the grains filling process.

Overall, in all the experiments the authors agreed that the effect of Si on photosynthesis process is major in plants challenged by the pathogen. Indeed, transcriptomic studies performed on several plant species submitted to different types of biotic stress showed a reduction in transcript levels of genes related to photochemistry, Calvin cycle and the synthesis of chlorophylls [100]. Observations in non-inoculated plants, in general, does no showed significant difference between the non-supplied and Si-supplied plants for the values of leaf gas exchanges, photochemical parameters associated with Chl*a* fluorescence, soluble sugars and some enzymes of sucrose metabolism [95]. These findings are in line with previous study in which was not found any differences on the photosynthetic activity of rice plants with and without Si supply [46]. In this context, Coskun et al. [14] pointing out to the question of Si's role in the absence of stress having little or no effect, however remains a contentious issue. Probably Si is indirectly involved in the nutrition of the plant and it is undeniable that Si prevents or mitigates the strains imposed by stress, and this, ultimately, is reflected in improvements in plant growth, function and metabolic activity.
