JAR1-mediated JA-Ile accumulation: a mechanism towards drought stress resistance in <em>Arabidopsis thaliana</em>

Abstract

In the present work, the regulatory capacity of biologically active jasmonate, jasmonyl-isoleucine (JA-Ile), under normal and progressive drought stress conditions throughout a plant’s life-cycle was investigated. To alter endogenous JA-Ile levels, two different plant lines were used: i) the T-DNA insertion line <em>jar1-11</em>, which contains significantly reduced amount of JA-Ile and ii) complementary to this, a T-DNA insertion line (JAR1-OE) expressing <em>JAR1.1-YFP</em> under the control of the 35S promoter, which results in JAR1 overexpression and enhanced endogenous JA-Ile levels. This line was newly developed within this work. Both lines displayed difference in growth and stress resistance compared to the wild type and each other. Under normal growth conditions <em>jar1-11</em> plants displayed a larger rosette with narrower leaf blades, while JAR1-OE plants had stunted growth with lateral leaves. And while JAR1-OE was late in flowering, a reciprocal trend was observed in <em>jar1-11</em>. Furthermore, <em>jar1-11</em> plants were more susceptible to drought stress, while JAR1-OE plants were highly resistant. In line with the difference in JAR1, hormone analysis revealed increased accumulation of JA-Ile in JAR1-OE under drought, while <em>jar1-11</em> accumulated JA that could not be converted to JA-Ile. In addition, the homeostasis of some precursors and highly abundant catabolic products of JA and JA-Ile were differentially affected in these lines. Global gene expression analysis by RNA-seq revealed a reprogramming of the jasmonate signaling pathway with a positive feedback upregulation in JAR1-OE under drought stress. By contrast, in <em>jar1-11</em> the biosynthesis of jasmonates was inhibited. Positive feedback in JAR1-OE helps plants to acquire pre-stress tolerance with positive stomatal regulation, anti-oxidant activity and modulation of ABA biosynthesis. This ultimately helps the plants in coping with subsequent drought stress through regulation of the photosynthetic machinery and other biological processes. Furthermore, calmodulin-like protein 12 (CML12) was identified as a potential target of jasmonate signaling. Intriguingly, CML12 behaves differentially at the transcriptional and translational levels to the presence or absence of JAR1 or endogenously added JA-Ile supporting a potential cross-talk between jasmonate and Ca²⁺-signaling. Finally, the transcription factor AtMYB2 was found to be a regulator of jasmonate signaling as it could control the accumulation of JA and JA-Ile under normal growth as well as drought stress conditions.