**6. Insect resistance in wheat**

#### **6.1 Resistance categories**

Responses which govern insect resistance in plants can be classified into three categories. Tolerance can be defined as the response of plant which allows the plant to survive insect damage with low or no damage to the yield. Tolerance is generally governed by a complex set of genetic traits. Tolerance does not affect the overall survival of insects thereby poses no selection pressure. Tolerance has been reported in a number of crops [65, 66]. The non-preference of a plant by insect pest or antixenosis is another mechanism used by plants against insects. Generally, antixenosis is manifested by some morphological or chemical factors which hinder feeding of the pest and sometimes rejection as host. Antibiosis, the third category, can be defined as the condition when pest health and reproduction are negatively affected by the resistant plant. Most of the resistance observed in field (up to 90%) is due to antibiosis.

#### **6.2 Resistance mechanisms**

Over the due course of evolution traits for direct and indirect defense mechanisms against insect attacks have developed in plants. The classification of these mechanisms has been further done as direct mechanisms and indirect mechanisms. Structural barriers constitute the direct defenses. Tissue toughness, glandular and non-glandular trichomes and plant pubescence are included in these types of defenses. Allelochemicals in plant tissues are also included in direct defenses. These exhibit toxic, anti-feedant, and repellent effects on the attacking arthropods. The digestive enzyme inhibitors, cyanogenic glycosides, glucosinolates, lectins, glucosinolates, terpenoids and alkaloids are involved in this [67, 68]. An extensive review of constitutive & induced morphological & chemical plant defenses has been done [65, 66, 69, 70]. These defenses mediate antixenosis & antibiosis. Volatile organic compounds constitute the indirect defenses. The plants which are damaged by pest arthropod release these compounds. These compounds lead to attraction of arthropod predators & parasitoids or the ones that cause repelling of oviposition of pest arthropods [71]. The specific plant indirect defense responses are represented by herbivore associated molecular patterns (HAMPs). These are the responses to the specific herbivore derived elicitors. This occurs in the in oral or ovipositor secretions. These facilitate indirect defenses against herbivores [72]. The widely researched HAMPs are the insect fatty acid plant amino acid conjugates. These are obtained from the lepidopterous larvae [71, 73].

#### **6.3 Constitutive and induced resistance genes**

The arthropod selects host plant tissue substrate based on well-coordinated interactions occurring within evolutionarily conserved protein(s) which are encoded by attacking arthropod & responding host plant. The arthropod successfully manipulates the host plant as a result of suitable arthropod-plant interactions. When there is incompatibility in the arthropod-plant interaction, the arthropod does not succeed resulting in the survival of the plants attacked [74]. The plant and

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

fungal endophytic genes are expressed in both the interactions. These are expressed constitutively or via induced defense responses. These occur following herbivory and find involvement in arthropod resistance [75, 76].

Under field conditions, resistance has been explained more clearly by the effects which are controlled by the constitutive genes. This is concluded based on the limited research done till date. The effects owing to the induced gene expression do not contribute much in this [77, 78]. The generation of reactive oxygen species and the signal cascades which involve salicylic acid (SA), jasmonic acid (JA), abscisic acid, ethylene and gibberellic acid occurs in plants as a response to arthropod herbivory. Direct and indirect defense proteins are resulted by the downstream production [79–82]. The aphid bacterial endosymbionts could also lead to defense signals [83]. Jasmonic acid based transcriptomes are elicited by the plant tissue damage caused by arthropods with chewing mouthparts. On the contrary, arthropods with piercing-sucking mouthparts induce the jasmonic acid- salicylic acid-based transcriptomes [71]. Recent documentation has been done of the jasmonic acid- salicylic acid signaling induced by both types of herbivory and jasmonic acid- salicylic acid cross talk [68, 74, 84, 85]. The expression of several plant genes which are produced in the initial responses to arthropod herbivory are controlled by the JA, 12- oxophytodienoic acid, and jasmonoyl-amino acid conjugates (which are governed by zinc finger protein expressed in inflorescence meristem) repressor proteins [86]. Several defense allelochemicals are produced by the defense response gene upregulation. This occurs via JA and some other pathways [69]. Scanty information is available regarding the arthropod induced expression of the plant metabolism genes. There are very few evidences indicating the down regulation of few of these genes. This is reported to occur in the beginning just after the arthropod herbivory sets in and later on upregulated in the ensuing days [84, 87].

The identification of arthropod pest elicitors of resistance genes is yet to done. An undefined elicitor protein of *Diuraphis noxia* is recognized by the wheat plant receptors. *D. noxia* is recognized by plant-signaling gene products feeding in incompatible interactions [88]. Secondary metabolites possessing the Hydroxamic acids (Hx) (1,4-benzoxazin-3-ones) group, find involvement in the resistance of certain cereals against bacteria, fungi and several insects including aphids [89]. In the seed, Hydroxamic acids (Hx) are absent. This increases after germination. The young seedlings exhibit the concentration peak [90]. This is basically located in the mesophyll protoplasts, the vascular bundles [91] and in the sieve elements [92]. In the mature plants, the Hx levels decline after the seedling stage. Even then, the young tissue still exhibits a high concentration of Hx [90]. In the plants, the Hx compounds occur as 2-β-O-D-glucopyranosides [90]. When the tissue is injured, these are enzymatically hydrolyzed by endo-β-glucosides to DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3- one) [92]. DIMBOA is the main Hx aglucone in the wheat extracts. It leads to antibiosis, decreased performance, feeding deterrence and reduced reproduction in aphids [93].

An enhancement in the overall activity of several enzymes was observed. All the enzymes such as superoxide dismutase, phenylalanine ammonia lyase glutathione reductase, and polyphenol oxidase have a major role in the defense of plants towards the feeding of aphid [94]. An early defense strategy is mounted by the Hessian fly-resistant *Ae. tauschii*. The production of anti-feedant proteins (lectins), secondary metabolites and ROS radicals is involved in this strategy. These successfully counter the larval extra oral salivary plant cell degrading proteases, lead to fortification of the cell wall and prevention of the Hessian fly larvae from establishing permanent feeding sites [95].

There are different types of carbohydrate binding proteins known as lectins which are present in tissues of plants. Resistance building potential is possessed by these lectins for wheat against insects. To tackle HF, the identification of

genes leading to production of this type of lectins seems a potential method. The genes include Hfr-2 called as HF destructor. This is expressed in the leaf sheaths of the resistance genotypes [96] On similar lines, the mannose binding lectins serve as storage protein and accumulate in the phloem sap. This might act against HF. Anti-insect properties are possessed by these lectins. This is attributed to the accumulation of lectin in the midgut of insects, killing them instantly. Another defensive mechanism present in resistant varieties of wheat is the production of Wci-1 mRNAs and Hfr-1. This occurs in response to the attack of HF larvae. The Hfr-1 gene is known as the defender gene against HF. It has the ability to control crop from severe attack [97]. The identification of arthropod pest elicitors of resistance genes is yet to done. An undefined elicitor protein of *Diuraphis noxia* is recognized by the wheat plant receptors. *D. noxia* is recognized by plant-signaling gene products feeding in incompatible interactions [88]. An Avirulence (*Avr*) gene is there on the parasites side. This encodes one of the several effector proteins that the parasite applies to the plant to help in colonization. A Resistance (R) gene is there on the plant's side. It mediates a surveillance system which detects the Avr protein. The detection is done either directly or indirectly. It triggers effector-triggered plant immunity. The arthropods are responsible for a significant proportion of plant biotic stress but even then they have not been integrated into important models of plant immunity that arise from plant pathology. The absence of molecular evidence for arthropod Avr effectors has been a limiting factor. This evidence was discovered in a plant pathogen around thirty years back. Now, there is evidence for arthropods with the cloning of the Hessian fly's vH13 *Avr* gene. Resistance against RWA is supposed to be induced by gene-for-gene model. The resistant gene produces a protein in this mechanism. This protein contains nucleotide binding site-leucine rich repeat (NBSLRR) domain [98, 99]. Firstly, this NBSLRR domain recognizes and then interacts with cognate Avr protein which is produced by the respective insect [100]. It has been reported that another domain (serine/threonineprotein kinases: STKs) is produced by *Dn* genes. This confers resistance against the RWA [101].
