Publikationen - Molekulare Signalverarbeitung

Salicylic acid (SA) has been proposed to antagonize jasmonic acid (JA) biosynthesis and signaling. We report, however, that in salicylate hydroxylase-expressing tobacco (Nicotiana tabacum) plants, where SA levels were reduced, JA levels were not elevated during a hypersensitive response elicited by Pseudomonas syringae pv phaseolicola. The effects of cotreatment with various concentrations of SA and JA were assessed in tobacco and Arabidopsis (Arabidopsis thaliana). These suggested that there was a transient synergistic enhancement in the expression of genes associated with either JA (PDF1.2 [defensin] and Thi1.2 [thionin]) or SA (PR1 [PR1a-β-glucuronidase in tobacco]) signaling when both signals were applied at low (typically 10–100 μm) concentrations. Antagonism was observed at more prolonged treatment times or at higher concentrations. Similar results were also observed when adding the JA precursor, α-linolenic acid with SA. Synergic effects on gene expression and plant stress were NPR1- and COI1-dependent, SA- and JA-signaling components, respectively. Electrolyte leakage and Evans blue staining indicated that application of higher concentrations of SA + JA induced plant stress or death and elicited the generation of apoplastic reactive oxygen species. This was indicated by enhancement of hydrogen peroxide-responsive AoPR10-β-glucuronidase expression, suppression of plant stress/death using catalase, and direct hydrogen peroxide measurements. Our data suggests that the outcomes of JA-SA interactions could be tailored to pathogen/pest attack by the relative concentration of each hormone.

BarJey leaf segments treated with jasmonate respond with the synthesis of specific proseins, referred to as jasmonate‐induced proteins (JIPs). Application of abscisic acid (ABAl also induced JIP synthesis (Weidhase et al. 1987). In this study the effects of inhibitors on sorbitol‐induced increases of endogenous jasmonates and ABA were investigated. The promotion of jasmonates by sorbitol was inhibited by the growth retardant tetcyclacis at concentrations as low as 1 ftM. In parallel with the decrease of jasmonates, JIP gene expression was reduced as reflected by a decline in the level of a 23‐kDa protein UIP‐23) and mRNAs of JIP‐6 and JIP‐23. 12‐Oxo‐phytodienoic acid, an inlermediale in the lipoxygenase (LOX) pathway leading to jasmonic acid was able to overcome the inhibition by tetcyclacis and increases both the endogenous jasmonate content and transcript accumulation. This suggests that tetcyclacis acts upstream of 12‐oxo‐phytodienoic acid and in keeping with this proposal, an increase in relative LOX activity was detected after tetcyclacis treatment. Although tetcyclacis was shown to inhibit the degradation of ABA to phaseic acid, its effect on jasmonate synthesis is much more pronounced.