The effect of abiotic stress on plant resistance to insect herbivores
Beek, Bart van
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Plants in the field are often subject to a combination of abiotic and biotic stress. During the combinatory stress plant phytohormonesignaling cross-communicatesin order to optimizedevelopment. The resulting hormone crosstalk leads to adaptation divergent from the individualstresses. For this reason, abioticstress can influenceplant resistance towards insect herbivores.During insect herbivore feeding, plant defense responsesare initiatedfollowingrecognition of D/HAMPs by PRRs. Jasmonic acid (JA) together with abscisicacid (ABA) form the core signaling pathway in plant resistance towardsinsect herbivores. Elevated JA and ABA levels lead to increased expression of MYC, MYB, WRKYtranscription factors resulting in accumulation of defensive compounds. In addition, salicylic acid (SA) isinvolved in the defense against piercing insects and insect egg deposition. However, elevated SA levels antagonize JA signaling leading to increasedinsect susceptibility. Furthermore, elevated ethylene (ET)levelsinhibitthe expression of MYC2leading to increasedsusceptibility to insects. Both heat stress and drought result in enhanced JA and ABAbiosynthesis and signaling. For this reason, both abiotic stresses synergistically increaseglucosinolate and proteinase inhibitorsaccumulation resulting inincreaseddefenseagainst insect herbivores. In addition, UV-B radiating can also increase the accumulation of defensive compounds in a JA-dependentmanner. In contrast, submergence leads to the suppressionof JA biosynthesis. Moreover,SA-mediated WRK22 expression during flooding further suppressesJA signaling.In addition, the accumulation of ET during submergence inhibits MYC2 expressionhampering defensive responses. Similarly,shading leads to a reduction of active JA levels due to sulfation. The reduced JA levels are found to result in a decreased accumulation of defensive compounds. In this manner, both submergence and shading increase plant susceptibilitytowards insect herbivores.