Morphological and chemical components of resistance to pod borer, Helicoverpa armigera in wild relatives of pigeonpea

Abstract

Host plant resistance is an important component for minimizing the losses due to the pod borer, Helicoverpa armigera, which is the most devastating pest of pigeonpea. An understanding of different morphological and biochemical components of resistance is essential for developing strategies to breed for resistance to insect pests. Therefore, we studied the morphological and biochemical components associated with expression of resistance to H. armigera in wild relatives of pigeonpea to identify accessions with a diverse combination of characteristics associated with resistance to this pest. Among the wild relatives, oviposition non-preference was an important component of resistance in Cajanus scarabaeoides, while heavy egg-laying was recorded on C. cajanifolius (ICPW 28) and Rhynchosia bracteata (ICPW 214). Accessions belonging to R. aurea, C. scarabaeoides, C. sericeus, C. acutifolius, and Flemingia bracteata showed high levels of resistance to H. armigera, while C. cajanifolius was as susceptible as the susceptible check, ICPL 87. Glandular trichomes (type A) on the calyxes and pods were associated with susceptibility to H. armigera, while the non-glandular trichomes (trichome type C and D) were associated with resistance to this insect. Expression of resistance to H. armigera was also associated with low amounts of sugars and high amounts of tannins and polyphenols.Accessions of wild relatives of pigeonpea with non-glandular trichomes (type C and D) or low densities of glandular trichomes (type A), and high amounts of polyphenols and tannins may be used in wide hybridization to develop pigeonpea cultivars with resistance to H. armigera.

Mechanisms of plant defense against insect herbivores

“…The first line of plant defense against insect pests is the erection of a physical barrier either through the formation of a waxy cuticle, 9,16 and/or the development of spines, setae, and trichomes. 18,19 Structural defenses includes morphological and anatomical traits that confer a fitness advantage to the plant by directly deterring the herbivores from feeding, 16 and range from prominent protrubances on a plant to microscopic changes in cell wall thickness as a result of lignification and suberization. 9,19 Structural traits such as spines and thorns (spinescence), trichomes (pubescence), toughened or hardened leaves (sclerophylly), incorporation of granular minerals into plant tissues, and divaricated branching (shoots with wiry stems produced at wide axillary angles) play a leading role in plant protection against herbivory.…”

“…Among the secondary metabolites, plant phenols constitute one of the most common and widespread group of defensive compounds, which play a major role in HPR against herbivores, including insects. [4][5][6]18 Phenols act as a defensive mechanism not only against herbivores, but also against microorganisms and competing plants. Qualitative and quantitative alterations in phenols and elevation in activities of oxidative enzyme in response to insect attack is a general phenomenon.…”

“…9 Glandular trichomes secrete secondary metabolites including flavonoids, terpenoids, and alkaloids that can be poisonous, repellent, or trap insects and other organisms, thus forming a combination of structural and chemical defense. 9,18 Induction of trichomes in response to insect damage has been reported in many plants. 22 This increase in trichome density in response to damage can only be observed in leaves developing during or subsequent to insect attack, since the density of trichomes of existing leaves does not change.…”

“…Tannins have a strong deleterious effect on phytophagous insects and affect the insect growth and development by binding to the proteins, reduce nutrient absorption efficiency, and cause midgut lesions. 18,43,44 Tannins are astringent (mouth puckering) bitter polyphenols and act as feeding deterrents to many insect pests. They precipitate proteins nonspecifically (including the digestive enzymes of herbivores), by hydrogen bonding or covalent bonding of protein-NH 2 groups.…”

Role of trichomes on leaves and pods for imparting resistance in chickpea [Cicer arientinum (L.)] genotypes against Helicoverpa armigera (Hübner)

“…Shabbir et al (2014) reported genotypes which had higher pod trichome density and pod wall thickness were more resistant. Sharma et al (2009) at Andhra Pradesh also reported that glandular trichomes (type A) on the calyxes and pods were associated with the susceptibility to H. armigera, while the non-glandular trichomes were associated with resistance to this insect. Veeranna and Hussain (1997) observed that with increased density of non-glandular trichomes on pods, the damage due to pod feeding insects was reduced in pigeon pea.…”