**1. Introduction**

Wheat (*Triticum aestivum* L.) is the most cultivated crop on Earth [1, 2] being a key cereal for global food security. Wheat provides calories to 85% of the world population (contributing of 5 to 57% of daily consumed calories, depending of the country) and proteins to more than 82% of the world population (contributing of 6 to 60% of daily calories intake, depending of the country) [3]. Historically (1961–2009) the increase in the world production of wheat occurred primarily due to the increase in productivity which supplied the increase in the demand for this cereal [4]. As the global population continues to increase, the world demand for wheat is predicted to continue raising [2, 4], being forecast that in order to feed the world population in 2050 it will be necessary to almost double the current wheat

production [5]. The challenges to achieving this production target include abiotic (drought, heat and salinity) and biotic (insects, pathogens and weeds) stresses that can be enhanced by climate changes [5].

Pathogens are among the main threats to high yield of wheat and a threatening to food security. Wheat is affected by many pathogens, however their occurrence and yield loss, estimated for each disease, vary from country to country and season to season. The main aboveground wheat diseases worldwide are rusts (*Puccinia* spp.), septoria nodorum blotch (*Parastagonospora nodorum*), septoria tritici blotch (*Zymoseptoria tritici*), tan spot (*Pyrenophora tritici-repentis*), fusarium head blight (*Fusarium graminearum* species complex), powdery mildew (*Blumeria graminis* f. sp. *tritici*) and wheat blast (*Magnaporthe oryzae* Pathotype Triticum).

The control of these diseases is carried out preferably through resistant cultivars. However, for some of the diseases, there are no cultivars with sufficient resistance to contain the damage in yield or the resistance is ephemeral, especially when governed by race-specific resistance genes, due to the rapid evolution of the pathogen [6–9]. As a result, the use of fungicides is common in wheat crops, but it raises the cost of production and it does not always give the expected control for some diseases [10, 11]; furthermore there is risk of development of resistance in the fungal to fungicides [12].

In this scenario, silicon (Si) become as an attractive alternative to be included in the management of wheat crop. Silicon is a mineral element considered benefic to plants, however in many soils its concentration available for plants is low [13]. In these soils, fertilization with Si sources has shown positive results. Numerous studies demonstrate the beneficial effect of Si in relieving abiotic stresses and in the control of biotic diseases on Si-accumulator plants (reviewed by [14–17]). This chapter presents the current knowledge on Si up take by the wheat plant, its effects on grain productivity and wheat technological quality, physiological aspects, and biochemical and histological defenses enhanced by the element, on several wheat-pathogen interaction.

## **2. Silicon wheat absorption**

The knowledge of Si absorption has been studied in different plants such as monocotyledonous and dicotyledonous species providing evidence to explain the process [18–21]. Initially was believed that the transpiration was the main factor determining Si uptake in plants. New evidence confirmed that the Si absorption and accumulation could be explained by the active transport mechanisms inherent to the roots and the shoots.

In wheat, the first evidences of active transport mechanisms come with studies showing that approximately 90% of the Si absorbed by the plant was transferred to the shoots, maintaining the roots in a relatively low-Si status [22, 23]. Later, Mayland et al. [24] reported that the amount of Si accumulated by the wheat plant was higher than expected to occur only via transpiration providing data to support the classification of wheat as a Si accumulator (accumulating Si in concentration up to 20 g kg-1 of dry weight). Advancing, Rafi and Epstein [25] reported that Si is rapidly absorbed by wheat plants from solution containing Si at 0.5 mM, a concentration near of that of the element in soil solutions, and the uptake rate were similar between plants 'preloaded' with Si and plants grown previously in solutions without Si addition. Further studies demonstrated that Si uptake by wheat is under metabolic control due to the absorption of Si show a concentration dependence obeying Michaelis–Menten kinetics and it is affected by metabolic inhibitors (dinitrophenol and potassium cyanide) [26]. Later, Montpetit et al. [21] cloned and functional characterized the *TaLsi1*, a wheat Si transporter gene, which is an ortholog of *OsLsi1*

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

from rice. The genes *TaLsi1* and *OsLsi1* belong to Nod26-like intrinsic proteins (NIPs) III subgroup of the aquaporin membrane protein family.

Thus, the Si absorption is facilitated by specific NIPs with a distinct selectivity that facilitate the passive transport of water and/or small uncharged solutes such as monosilicic acid [Si(OH)4] [27]. According to Ma and Yamaji [19] specific NIPs as *Lsi1* (Si influx transporter) facilitates the passive transport of Si across the plasma membrane from the environment (external solution) to the plant cell in the form of [Si(OH)4], and efflux transporters known as *Lsi2* mediate the loading of Si into the xylem to facilitate root-to-shoot translocation, which, in turn, moves Si to the aerial parts where it is deposit as amorphous Si (SiO2). According to the authors, these Si transporters are localized to the plasma membrane, but, in different plant species, show different expression patterns and tissue or cellular localizations that are associated with different levels of Si accumulation [19]. In this context, the molecular characterization and phylogeny of the Si permeable channel, *TaLsi1*, which is expressed only in the roots and independent from Si concentrations, can explain the Si absorption by wheat plants [14, 21].

The concentration of Si on wheat tissue varies according to the soil and cultivar. For example, a study conducted in two locations (Abed and Sejet, Denmark) showed that Si concentration in the wheat straw ranged from 11.3 g kg-1 to 23.4 g kg-1 of dry weight. The study performed with 20 genotypes, showed that on average wheat grown in Abed contained 25% more Si than wheat grown in Sejet, which as attributed to variation in edaphic factors such as soil pH or silicate mineral composition which affect the Si availability to plants [28]. In regarding to wheat genotypes, the difference between the lowest and highest Si concentration was 75% at Abed and 44% at Sejet reflecting differences in the ability of roots to take up Si from the soil solution [28]. Carter et al. [29] and Ranjbar et al. [30] also observed difference among wheat cultivars regarding Si concentration in the shoot. Ranjbar et al. [30] also showed that there is a relationship between shoot Si concentration and Si acquisition efficiency. These studies clearly indicated that accumulation of Si in the wheat shoot is variable among cultivars which may influenced by the cultivar ability to absorption and also by the availability of Si in the soil.

After uptake by roots, Si follow the transpiration flow and it is accumulated beneath cuticle forming a double layer Si-cuticle, associated to cell wall and in Si-accumulating cells [31]. Furthermore, it was been previously reported that the highest Si concentration was present in major transpiration parts of the plants followed by the other parts of the plants [32]. In wheat, the highest silicified cells were present in leaf blade followed by the awn, leaf sheath, lemma, rachilla and stem, thus leaf blade contains the highest Si concentration [33]. In agreement, another study showed that the accumulation of Si was highest in vegetative tissue (leaf blades > leaf sheaths > stem) and lowest in grain followed by roots, increasing with increasing stomata density in the tissues [34]. In awns, the number of silicified cells was linearly correlated to Si concentration in dry weight which suggests cellular control over silicification [35]. Using scanning electron microscopic, authors found a continuous silica layer under the cuticle, extended silicification in the epidermis cell wall and in sclerenchyma cells near the vascular bundles, but not in the stomata, suggesting that an active process directs the soluble Si away from the water evaporation stream [35]. On the leaves, X-ray microanalysis revealed that Si was deposited in a linear pattern that corresponded to the silica cells, being greater the amounts of Si in the linear areas of silica cells from plants grown in soil supplied with silicate fertilizer [36, 37]. Another study showed that Si was predominantly deposited in the epidermis cells of the leaves and their cell walls [38].

As wheat is a Si-accumulating species, it may remove considerable amount of Si if straw is removed from the field. In this context, a study considering long-term cultivated field analyzed the impact of Si accumulating plants on the biogeochemical cycle of Si and indicated that the concentration of amorphous silica is lower in cultivated soils compared to natural ecosystems, due to the amorphous Si pool decreases with time particularly in surface soil, contrary to natural ecosystems [39]. For instance, an estimation of shoot Si uptake by wheat based on 10-year average of harvested area, production level, reported biomass/harvested portion ratio and shoot Si content in United States indicated that the annual shoot Si uptake of wheat is 2.144,278 tons and 108 kg ha−1 [40]. In this sense, in crop systems in which the straw is removed from the field, the available Si in soils do not sustain high Si concentrations not only for wheat but also other crops in the long term [39]. Under this condition, it is clear the concern on the Si reduction from field pointing out the necessity of Si sources as fertilizers and eventually the management of the wheat straw to obtain the benefits of Si to wheat plants.

#### **2.1 Silicon fertilization**

Orthosilicic is the second most abundant element in the earths crust and plays a number of important roles in the plants. The silicic acid is present in the soil as an uncharged monomeric molecule below pH 9 [19]; their concentration in soil varying between 0.1 to 0.6 mM [41]. In the past 20 years, the scientific documentation on the benefits of Si to crops has helped establish Si fertilization as an agronomic practice in many agricultural lands worldwide [40]. Thus, it is recognized that Si fertilization confers benefits to wheat crop.

In this context, the most common Si fertilizers are wollastonite and slag (calcium silicate). In the case of wollastonite which is a natural calcium silicate [42], that contains higher fractions of easily soluble Si compared to slags [40]. It is considered to be the most efficient Si fertilizer for soil application due to that it can release the largest amount of plant available Si (2.31–3.6%) into soil solution [43, 44]; however, its use is often limited because of its relatively high cost [45]. Calcium silicate slags are by-products of the metallurgical smelting process, contain varying percentages of Si [46], and have been observed positive effects on correcting soil acidity [47], plant growth and alleviation of stresses [48–50]. Other commonly used Si fertilizers are sodium metasilicate and potassium silicate. These Si fertilizers have been found very helpful in improving growth parameters in biotic, drought and salt stress in wheat [38, 51].

In the case of pyrolitic fine silica particles, sodium metasilicate or silica gel is used for agricultural purposes. In soil, wheat plants grown under identical growing conditions, the efficiency of the Si compounds to increase the Si concentration on the plants increased in the order sodium metasilicate > silica gel >pyrolitic fine silica particles and seemed to correlate with the ease of formation of orthosilicic acid from these compounds [38]. For instance, the application of liquid and powder silicate fertilizers in the soil contributed similarly to the concentration of Si to the soil solution and doubled the Si concentration on wheat tissue [52]. Furthermore, Si uptake by wheat plant as well as its growth is significantly affected by the type of Si pool in the soil and factors controlling its solubility [53].

On the other hand, foliar application, mainly as sodium metasilicate and potassium silicate, is cause of debate due to the major portion of the Si uptake come from to the roots; however, some effects under biotic and abiotic stress have been observed (see below).

The demand of Si fertilizer due to the necessity in different agricultural environments allow the introduction and application of nano-Si fertilizer with some kind of efficient. The nano-Si is high bioavailability as smaller particle size that can be rapidly and completely form to absorb by plants and form a thick silicated layer on

leaf surface [54]. In this case a study showed that addition of potassium silicate or nano-Si fertilizer in a Calcaric Cambisols increased the concentration of Si in wheat tissue, mainly in the shoot, but there is a significant relationship between the Si level/source and wheat cultivars [30].
