A mutant lacking six abscisic acid (ABA) receptors and ABA-mediated activation of SnRK2.2/2.3/2.6 kinases shows an extreme ABA-insensitive phenotype, even thoug

Experiments with Arabidopsis mutants and sensitive and tolerant pairs in several other species have elucidated the molecular basis of plant ozone sensitivity and ozone lesion development. They have indicated an important role for hormonal signalling in determining the outcome of ozone challenge at the cellular level. The reactive oxygen species (ROS) from ozone degradation can cause either direct necrotic damage or induce the process of programmed cell death. Perception of ozone or ROS from its degradation in the apoplast activates several signal transduction pathways that regulate the responses of the cells to the increased oxidative load. Plant hormones salicylic acid, jasmonic acid, ethylene and abscisic acid are involved in determining the duration and extent of ozone‐induced cell death and its propagation. Salicylic acid is required for the programmed cell death, ethylene promotes endogenous ROS formation and lesion propagation, and jasmonic acid is involved in limiting the lesion spreading. Abscisic acid is most likely involved through the regulation of stomata and thus is expected to affect lesion initiation. The roles and interactions of perception of ozone, the immediate downstream responses, hormone biosynthesis and signalling during ozone lesion initiation and formation are reviewed.

Summary: WRKY transcription factors (TFs) have been mainly associated with plant defense, but recent studies have suggested additional roles in the regulation of other physiological processes. Here, we explored the possible contribution of two related group III WRKY TFs, WRKY70 and WRKY54, to osmotic stress tolerance. These TFs are positive regulators of plant defense, and co‐operate as negative regulators of salicylic acid (SA) biosynthesis and senescence. We employed single and double mutants of wrky54 and wrky70, as well as a WRKY70 overexpressor line, to explore the role of these TFs in osmotic stress (polyethylene glycol) responses. Their effect on gene expression was characterized by microarrays and verified by quantitative PCR. Stomatal phenotypes were assessed by water retention and stomatal conductance measurements. The wrky54wrky70 double mutants exhibited clearly enhanced tolerance to osmotic stress. However, gene expression analysis showed reduced induction of osmotic stress‐responsive genes in addition to reduced accumulation of the osmoprotectant proline. By contrast, the enhanced tolerance was correlated with improved water retention and enhanced stomatal closure. These findings demonstrate that WRKY70 and WRKY54 co‐operate as negative regulators of stomatal closure and, consequently, osmotic stress tolerance in Arabidopsis, suggesting that they have an important role, not only in plant defense, but also in abiotic stress signaling.