Hydrogen sulfide and ethylene regulate sulfur-mediated stomatal and photosynthetic responses and heat stress acclimation in rice

The gaseous signaling molecules, ethylene (ET) and hydrogen sulfide (H2S) are well known for their ability to mitigate abiotic stress, but how they interact with mineral nutrients under heat stress is unclear. We have studied the involvement of ET and H2S in adaptation of heat stress on the availability of sulfur (S) levels in rice (Oryza sativa L.). Heat stress (40 degrees C) negatively impacted growth and photosynthetic -sulfur use efficiency (p -SUE), with accumulation of reactive oxygen species (ROS) in six rice cultivars, namely PS 2511, Birupa, Nidhi, PB 1509, PB 1728, and Panvel. Supplementation of S at 2.0 mM SO42- in the form of MgSO4, improved growth and photosynthetic attributes more than 1.0 mM SO42- under control (28 degrees C), and mitigated heat stress effects more prominently in PS 2511 (heat -tolerant) than in PB 1509 (heat -sensitive) cultivar. The higher heat stress mitigation potential of 2.0 mM SO42- in heat -tolerant cultivar was correlated with higher S -assimilation, activity of antioxidant enzymes, stomatal (stomatal conductance) and non-stomatal limitations, activity of carbonic anhydrase and Rubisco, and mesophyll conductance. The use of norbornadiene (NBD) and hypotaurine (HT), ET and H2S inhibitors, respectively, resulted in the lowest values for photosynthetic efficiency, stomatal and non-stomatal factors, implying the mediation of ET and H2S in heat stress acclimation. The connectivity of ET and H2S with S -assimilation through a common metabolite cysteine (Cys) improved heat stress adaptation in which H2S acted downstream to ET-mediated responses. Thus, the better adaptability of rice plants to heat stress may be obtained through modulation of ET and H2S via S.