Heat stress is a major environmental stress type that can limit plant growth and development. To survive sudden temperature increases, plants utilize the heat shock response, an ancient signaling pathway. Initial results had suggested a role for brassinosteroids (BRs) in this response. Brassinosteroids are growth-promoting steroid hormones whose activity is mediated by transcription factors of the BES1/BZR1 subfamily. Here, we provide evidence that BES1 can contribute to heat stress signaling. In response to heat, BES1 is activated even in the absence of BRs and directly binds to heat shock elements (HSEs), known binding sites of heat shock transcription factors (HSFs). HSFs of the HSFA1 type can interact with BES1 and facilitate its activity in HSE binding. These findings lead us to propose an extended model of the heat stress response in plants, in which the recruitment of BES1 is a means of heat stress signaling cross-talk with a central growth regulatory pathway.

UV-B led to the inactivation and fragmentation of mitochondria, which are removed by mitophagy. Mitophagy might be one of the important repair mechanisms for UV-B-induced damage in Arabidopsis.

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Highlights In Arabidopsis, autophagy is not essential for completion of the life cycle under normal temperatures. High temperature (HT) stress induces autophagy in developing anther wall cells and microspores. Autophagy deficient atg mutants become almost completely male-sterile at moderate HT. Autophagy plays a role in tapetum degeneration and pollen development during HTcaused abortion of tapetal program cell death. AbstractAutophagy is one of the cellular processes that break down cellular components during senescence, starvation, and stress. The susceptibility of plant pollen development to hightemperature (HT) stress is well known, but the involvement of autophagy in HT injury is yet to be clarified. Here, we found that following transfer to 30 °C, all autophagy-deficient (atg) mutants (atg2-1, 5-1, 7-2, and 10-1) of Arabidopsis thaliana tested displayed visibly impaired pollen development and anther dehiscence. HT-induced male sterility significantly increased in the atg mutants, but the degree of HT-induced obstacles did not change between the wild type (WT) and mutants from the seedling stage to the bolting stage. Cytological analyses showed that 30 °C promoted autophagy and autolysosome formation in both anther wall cells and microspores in developing anthers of WT, but the atg5-1 mutant did not show completion of tapetum degeneration and microspore maturation. HT upregulated hydrogen peroxide and 2 dehydroascorbate reductase 1 production in both WT and atg5-1 anthers, but the basal levels were already higher in the mutant. HT repressed expression of UNDEAD and its regulator MYB80, which are required for tapetal programmed cell death (PCD) for proper pollen development. Taken together, our results suggest that autophagy functions in tapetum degeneration and pollen development during HT-caused tapetal PCD abortion.