**5. Molecular basis of disease resistance in wheat**

Wheat is an allopolyploid, means a polyploid species that resulted from interspecific or intergeneric hybridization of two or more genomes from different species. Polyploidy, a common form of plant evolution, is associated with promoting the genetic diversity that facilitates adaptation to a range of environments. Because wheat is a global crop, it is under continuous exposure to a large variety of parasite species and strains, many of which have the ability to move around the globe. Long-term co-evolution between plants and their pathogens has equipped plants with a sophisticated multi-layered immune system to guard themselves against pest and pathogens [28]. Specificity between pathogenic variants (races) and plant genotypes (cultivars) follows gene for gene-for-gene interactions, whose outcome is conditioned by alleles of a gene regulating resistance (*R* gene) in plant and alleles of its corresponding gene regulating avirulence (*Avr* gene) in pathogen [29]. The plant immune system is typically described in terms of two components: pattern triggered immunity (PTI) which is activated by recognition of microbial or pathogen-associated molecular patterns (MAMPs or PAMPs) and effector-triggered immunity (ETI) involving gene for gene kind of resistance [30, 31]. ETI is often based on the recognition of cytosolic effectors by immune receptors with a conserved nucleotide-binding domain (NBARC) and a leucine-rich repeat domain (LRR) also called NLRs. This type of resistance is usually associated with a hypersensitive response (HR) localized to infection sites. To date, only a handful of these biotic stress resistance genes have been isolated and cloned in wheat (*T. aestivum*). Donors of the *R* genes are genetically diverse, including species in the primary gene pool (*Triticum* spp.), secondary gene pool (e.g. *T. timopheevii*), and tertiary gene pool (e.g. *Aegilops*, *Secale*, and *Thinopyrum*).

## **5.1 NBS: LRR proteins - basis of race-specific/seedling/all stage resistance (ASR)**

A few resistance genes have been cloned for race-specific resistance in wheat so far, which belong to a conserved gene family encoding NBS-LRR (Nucleotide binding site-leucine-rich repeat) proteins, also known as R-proteins (NLR) [30]. For example, powdery mildew genes, *Pm3* and *Pm8* and leaf rust resistance genes *Lr10* and *Lr21*. These R-proteins impart complete but race specific resistance. NBS-LRR proteins are a conserved class of immune receptors that directly or indirectly recognize pathogen-specific effector proteins. These proteins are secreted by pathogens into the host cell to suppress defense response and to establish infection. Recognition of effectors by NBS–LRR proteins triggers a signaling cascade resulting in a strong resistance response called hypersensitive reaction (HR) [31]. HR eventually leads to death of the infected host cell by this means preventing further spread of the pathogen [32]. Since this type of disease resistance depends on the recognition of specific pathogen effectors, even point mutations within effector genes or their loss can disrupt recognition by the corresponding NBS–LRR protein. Such mutations in pathogen effectors result in the emergence of new virulent pathogen races and breakdown of disease resistance. Mutated pathogen spores that avoid recognition by the corresponding R gene will have a huge selective advantage facilitating their rapid multiplication. Dispersal of fungal pathogens by wind over long distances adds to the quick spread of newly evolved virulent pathogen strains. Ug99, for instance, spread out from Kenya to South Africa and the Near East in less than a decade.

So far, only 31 genes have been cloned (**Table 2**) for biotic stress resistance (30 for disease resistance) from bread wheat and its wild relatives. Among these, most of the genes impart race specific resistance to the plant. These R-genes encode proteins with an NBS-LRR domain with a coiled-coil (CC) domain. This type of gene typically shows a greater degree of variation in LRR-encoding sequences [60, 61]. This is consistent with the idea that the LRR-encoding sequence is important for target specificity [61, 62]. The sequence variation in NBS-encoding region can also play significant role in specificity. For powdery mildew resistance, *Pm3* locus encodes seven alleles (*Pm3a*–*Pm3g*) providing resistance to different races of *Blumeria graminis* f. sp. *tritici* [63]. Sequence analysis indicated that the *Pm3* alleles evolved either by gene conversion/recombination or by single point mutations within the NBS and LRR regions [61].

#### **5.2 Transporter proteins: basis of durable/adult plant resistance (APR)**

Due to rapid pathogen evolution, R gene resistance is often not durable. One strategy to increase the longevity of disease resistance in wheat cultivars is to pyramid several R genes in one cultivar. To overcome such resistance gene stacks, simultaneous mutations in several effector genes would be required in one single pathogen spore. Race-non-specific resistance is supposed to be more durable when deployed in agriculture. Such kind of resistance mechanism sometimes may also be effective against multiple pathogens. These are normally quantitative traits conferring partial resistance that is able to slow down disease development. For example *Lr34*, *Yr36*, and *Pm21*. *Lr34* confers non-specific, partial, and slow rusting resistance, and has been deployed worldwide, maintaining its effectiveness in agriculture for decades. Due to its role in conferring resistance to pathogens other than leaf rust, it is also known as *Yr18, Pm38, Sr57* and *Bdv1* for resistance to stripe rust, powdery mildew, stem rust, and barley yellow dwarf virus, respectively [64]. The successful cloning of *Lr34, Yr36, and Lr67* revealed these APRs encode an ABC transporter, a kinase-START protein, and a hexose transporter, respectively (**Table 2**). They appear to each have their own resistance mechanism, function


*Breeding Wheat for Biotic Stress Resistance: Achievements, Challenges and Prospects DOI: http://dx.doi.org/10.5772/intechopen.97359*

*1 ABC- ATP binding cassette.*

*2 START- Steroidogenic acute regulatory protein-related lipid transfer domain.*

*3 PFT- Pore-forming toxin.*

#### **Table 2.**

*List of major cloned resistance genes in wheat for different biotic stresses.*

constitutively and often increase the basal level of resistance of the host, which is different from the recognition based NLRs.
