Have plants learned to forget stress? This review proposes benefits to forgetfulness and finds key roles for RNA turnover.

Improving the responsiveness, acclimation, and memory of plants to abiotic stress holds substantive potential for improving agriculture. An unresolved question is the involvement of chromatin marks in the memory of agriculturally relevant stresses. Such potential has spurred numerous investigations yielding both promising and conflicting results. Consequently, it remains unclear to what extent robust stress-induced DNA methylation variation can underpin stress memory. Using a slow-onset water deprivation treatment in Arabidopsis (Arabidopsis thaliana), we investigated the malleability of the DNA methylome to drought stress within a generation and under repeated drought stress over five successive generations. While drought-associated epi-alleles in the methylome were detected within a generation, they did not correlate with drought-responsive gene expression. Six traits were analyzed for transgenerational stress memory, and the descendants of drought-stressed lineages showed one case of memory in the form of increased seed dormancy, and that persisted one generation removed from stress. With respect to transgenerational drought stress, there were negligible conserved differentially methylated regions in drought-exposed lineages compared with unstressed lineages. Instead, the majority of observed variation was tied to stochastic or preexisting differences in the epigenome occurring at repetitive regions of the Arabidopsis genome. Furthermore, the experience of repeated drought stress was not observed to influence transgenerational epi-allele accumulation. Our findings demonstrate that, while transgenerational memory is observed in one of six traits examined, they are not associated with causative changes in the DNA methylome, which appears relatively impervious to drought stress.

Organelle-nuclear retrograde signaling regulates gene expression, but its roles in specialized cells and integration with hormonal signaling remain enigmatic. Here we show that the SAL1-PAP (3′-phosphoadenosine 5′- phosphate) retrograde pathway interacts with abscisic acid (ABA) signaling to regulate stomatal closure and seed germination in Arabidopsis. Genetically or exogenously manipulating PAP bypasses the canonical signaling components ABA Insensitive 1 (ABI1) and Open Stomata 1 (OST1); priming an alternative pathway that restores ABA-responsive gene expression, ROS bursts, ion channel function, stomatal closure and drought tolerance in ost1-2. PAP also inhibits wild type and abi1-1 seed germination by enhancing ABA sensitivity. PAP-XRN signaling interacts with ABA, ROS and Ca²⁺; up-regulating multiple ABA signaling components, including lowly-expressed Calcium Dependent Protein Kinases (CDPKs) capable of activating the anion channel SLAC1. Thus, PAP exhibits many secondary messenger attributes and exemplifies how retrograde signals can have broader roles in hormone signaling, allowing chloroplasts to fine-tune physiological responses.

Abiotic stress and recovery transcriptomes reveal extremely short RNA half-lives, changes to cotranslational decay, and recovery-specific networks, consistent w

Uncoupling high light signal perception from distal signal reception shows that 1O2 EXECUTER-dependent retrograde signaling is required for initiation of system