Abiotic stresses such as drought, heat or salinity are a major cause of yield loss worldwide. Recent studies revealed that the acclimation of plants to a combination of different environmental stresses is unique and cannot be directly deduced from studying the response of plants to each of the different stresses applied individually. Here we report on the response of Arabidopsis thaliana to a combination of salt and heat stress using transcriptome analysis, physiological measurements and mutants deficient in abscisic acid, salicylic acid, jasmonic acid or ethylene signaling. Arabidopsis plants were found to be more susceptible to a combination of salt and heat stress compared to each of the different stresses applied individually. The stress combination resulted in a higher ratio of Na+/K+ in leaves and caused the enhanced expression of 699 transcripts unique to the stress combination. Interestingly, many of the transcripts that specifically accumulated in plants in response to the salt and heat stress combination were associated with the plant hormone abscisic acid. In accordance with this finding, mutants deficient in abscisic acid metabolism and signaling were found to be more susceptible to a combination of salt and heat stress than wild type plants. Our study highlights the important role abscisic acid plays in the acclimation of plants to a combination of two different abiotic stresses.

Plants are exposed to multiple abiotic stresses that simultaneously occur under natural environmental conditions. Studies deciphering acclimation of plants to stress combinations are, however, still scarce. ISOCHORISMATE SYNTHASE 1 (ICS1) is known as a crucial enzyme required for synthesis of salicylic acid and phylloquinone, one of the components of the photosystem I complex. Although the significance of ICS1 in the regulation of abiotic stress response and pathogen defense in plants has been evidenced in previous studies, the role of this enzyme in the acclimation of plants to stress combinations is still largely unknown. In this study, we demonstrated the enhanced tolerance of Arabidopsis salicylic acid induction deficient 2-1(sid2-1) mutant deficient in ICS1 to a combination of heat stress and drought. H O -dependent stomatal closure and accumulation of total soluble sugars are associated with the enhanced tolerance of sid2-1 plants to this stress combination. In addition, sid2-1 plants showed higher accumulation of reaction center proteins (D1 and D2) in photosystem II accompanied by enhanced expression of transcripts involved in repair of these reaction center proteins. Furthermore, investigation of chlorophyll fluorescence indicated that mechanisms for dissipating the excess energy might be activated in sid2-1 plants specifically under a combination of heat stress and drought. Taken together, our findings suggest that maintenance of photosynthetic apparatus as well as prevention of excess water loss might enhance the tolerance of sid2-1 plants deficient in ICS1 to a combination of heat stress and drought.