Terrestrial plants are exposed to complex stresses of high salt-induced abscisic acid (ABA) and submergence-induced hypoxia when seawater floods fields. Many studies have investigated plant responses to individual stress conditions, but not so much for coupled or sequentially imposed stresses. We examined molecular regulatory mechanisms of gene expression underlying the cellular responses involved in crosstalk between salt and hypoxia stresses. Salt/ABA- and AtMYC2-dependent induction of a synthetic ABA-responsive element and the native RD22 promoters were utilized in our cell-based functional assays. Such promoter-based reporter induction was largely inhibited by hypoxia and hypoxia-inducible AKIN10 activity. Biochemical analyses showed that AKIN10 negatively modulates AtMYC2 protein accumulation via proteasome activity upon AKIN10 kinase activity-dependent protein modification. Further genetic analysis using transgenic plants expressing AKIN10 provided evidence that AKIN10 activity undermined AtMYC2-dependent salt tolerance. Our findings unravel a novel molecular interaction between the key signalling constituents leading crosstalk between salt and hypoxia stresses in Arabidopsis thaliana under the detrimental condition of submergence in saltwater.

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High-temperature stress often leads to differential RNA splicing, thus accumulating different types and/or amounts of mature mRNAs in eukaryotic cells. However, regulatory mechanisms underlying plant precursor mRNA (pre-mRNA) splicing in the environmental stress conditions remain elusive. Herein, we describe that a U5-snRNP-interacting protein homolog STABILIZED1 (STA1) has pre-mRNA splicing activity for heat-inducible transcripts including HEAT STRESS TRANSCRIPTION FACTORs and various HEAT SHOCK PROTEINs for the establishment of heat stress tolerance in Arabidopsis (Arabidopsis thaliana). Our cell-based splicing reporter assay demonstrated STA1 acts on pre-mRNA splicing for specific subsets of stressrelated genes. Cellular reconstitution of heat-inducible transcription cascades supported the view that STA1-dependent premRNA splicing plays a role in DREB2A-dependent HSFA3 expression for heat-responsive gene expression. Further genetic analysis with a loss-of-function mutant sta1-1, STA1-expressing transgenic plants in Col background, and STA1-expressing transgenic plants in the sta1-1 background verified that STA1 is essential in expression of necessary genes including HSFA3 for two-step heat stress tolerance in plants. However, constitutive overexpression of the cDNA version of HSFA3 in the sta1-1 background is unable to execute plant heat stress tolerance in sta1-1. Consistently our global target analysis of STA1 showed that its splicing activity modulates a rather broad range of gene expression in response to heat treatment. The findings of this study reveal that heat-inducible STA1 activity for pre-mRNA splicing serves as a molecular regulatory mechanism underlying the plant stress tolerance to high-temperature stress.The splicing of precursor mRNA (pre-mRNA) is a necessary step for intron-containing gene expression in eukaryotic cells to produce mature transcripts for protein translation (Wahl et al., 2009). This process is highly ordered and tightly controlled by multisubunit spliceosome activity to mix and match introns and exons of pre-mRNAs. The high molecular weight spliceosome complex comprises small nuclear ribonucleoprotein particles (snRNPs) called U1, U2, U4/U6, and U5 snRNPs. For splicing of pre-mRNA introns, U1 snRNP recognizes the 59-splicing site, and U2 snRNP binds to the adenosine at the branch point of introns with the assistance of U2 auxiliary factors. U4/U6 and U5 trimeric snRNPs associate with each other and undergo a stepwise 39-splicing site cleavage process. Eventually, U5 snRNP dissociates from the complex along with a lariat form of the intron. In this process, U5 snRNP accurately and dynamically swaps interacting partners with other snRNP subunits (Wahl et al., 2009).

Aging of living organisms is governed by intrinsic developmental programs, of which progression is often under the regulation of their cellular energy status. For example, calorie restriction is known to slow down aging of heterotrophic organisms from yeasts to mammals. In autotrophic plants cellular energy deprivation by perturbation of photosynthesis or sugar metabolism is also shown to induce senescence delay. However, the underlying molecular and biochemical mechanisms remain elusive. Our plant cell-based functional and biochemical assays have demonstrated that SNF1-RELATED KINASE1 (SnRK1) directly interacts, phosphorylates, and destabilizes the key transcription factor ETHYLENE INSENSITIVE3 (EIN3) in senescence-promoting hormone ethylene signaling. Combining chemical manipulation and genetic validation using extended loss-of-function mutants and gain-of-function transgenic lines, we further revealed that a SnRK1 elicitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea enables to slow down senescence-associated leaf degreening through the regulation of EIN3 in Arabidopsis. Our findings enlighten that an evolutionary conserved cellular energy sensor SnRK1 plays a role in fine-tuning of organ senescence progression to avoid sudden death during the last step of leaf growth and development.

To overcome high temperature stress, plants have developed transcriptional cascades which express a large amount of chaperone proteins called heat shock proteins (HSPs). In our recent publication, we reported that STABILIZED1, as an U5-snRNP-interacting protein, is involved in the splicing of heat shock factor (HSF) and HSP transcripts during high temperature stress. This indicates that not only transcriptional regulation, but also post-transcriptional regulation by STA1, is essential for the full activation of HSF-HSP cascades and for thermotolerance. Here, we observed that the splicing of HSP transcripts was induced independent of STA1 at room temperature after heat acclimation, indicating that STA1 acts as a high temperature-specific splicing factor for the splicing of HSP transcripts. Our findings suggest the molecular mechanism for how HSF and HSP transcripts are spliced well under high temperature stress that blocks the splicing of overall transcripts.