SUMMARYTranspiration and gas exchange occur through stomata. Thus, the control of stomatal aperture is important for the efficiency and regulation of water use, and for the response to drought. Here, we demonstrate that SIZ1-mediated endogenous salicylic acid (SA) accumulation plays an important role in stomatal closure and drought tolerance. siz1 reduced stomatal apertures. The reduced stomatal apertures of siz1 were inhibited by the application of peroxidase inhibitors, salicylhydroxamic acid and azide, which inhibits SA-dependent reactive oxygen species (ROS) production, but not by an NADPH oxidase inhibitor, diphenyl iodonium chloride, which inhibits ABA-dependent ROS production. Furthermore, the introduction of nahG into siz1, which reduces SA accumulation, restored stomatal opening. Stomatal closure is generally induced by water deficit. The siz1 mutation caused drought tolerance, whereas nahG siz1 suppressed the tolerant phenotype. Drought stresses also induced expression of SA-responsive genes, such as PR1 and PR2. Furthermore, other SA-accumulating mutants, cpr5 and acd6, exhibited stomatal closure and drought tolerance, and nahG suppressed the phenotype of cpr5 and acd6, as did siz1 and nahG siz1. Together, these results suggest that SIZ1 negatively affects stomatal closure and drought tolerance through the accumulation of SA.

Cold shock triggers an immediate rise in the cytosolic free calcium concentration ([Ca2+]cyt) in Arabidopsis thaliana and this cold-induced elevation of [Ca2+]cyt is inhibited by lanthanum or EGTA. It is suggested that intracellular calcium mainly contributes to the cold-induced [Ca2+]cyt response by entering into the cytosol. Two calcium-permeable mechanosensitive channels, MCA1 and MCA2 (mid1-complementing activity), have been identified in Arabidopsis. Here, we demonstrate that MCA1 and MCA2 are involved in a cold-induced increase in [Ca2+]cyt. The cold-induced [Ca2+]cyt increase in mca1 and mca2 mutants was markedly lower than that in wild types. The mca1 mca2 double mutant exhibited chilling and freezing sensitivity, compared to wild-type plants. Expression of At5g61820, At3g51660, and At4g15490, which are not regulated by the CBF/DREB1s transcription factor, was down-regulated in mca1 mca2. These results suggest that MCA1 and MCA2 are involved in the cold-induced elevation of [Ca2+]cyt, cold tolerance, and CBF/DREB1-independent cold signaling.

Many abiotic and biotic stresses can reduce plant growth and development. Low temperature is one of the most harmful abiotic stresses, particularly for plants that are tropical or subtropical in origin. To elucidate the molecular mechanisms underlying the cold-stress response, components involved in the signal transduction of cold stress have been characterized. In this study, we characterized a basic helix-loop-helix (bHLH) transcription factor encoding gene, SlICE1, from tomato (Solanum lycopersicum), which shows similarity with Arabidopsis ICE1. e expression of SlICE1 was observed in younger leaves, owers, and green and red fruits. To characterize the function of SlICE1, overexpressing tomato lines were produced. SlICE1-overexpressing tomatoes exhibited chilling tolerance, and SlICE1 enhanced the expression of coldresponsive genes, such as SlCBF1 and SlDRCi7, as well as accumulation of ascorbic acid. e SlICE1 protein was degraded a er cold treatment. ese results indicate that SlICE1 enhances cold tolerance in tomatoes.

Antioxidants and antioxidant activity confer important protective e ects in plants against the e ects of free radicals, which are generated by biotic and abiotic stresses, such as cold. In another study, we identi ed SlICE1 as a basic helix-loop-helix transcription factor to improve cold tolerance. Here, we demonstrate that SlICE1 plays an important role in the accumulation of antioxidants and in the regulation of antioxidant activity in tomato Solanum lycopersicum. Overexpression of SlICE1 in tomatoes enhanced the accumulation of antioxidants, such as β-carotene, lycopene, and ascorbic acid, as well as antioxidant activity, measured as the scavenging of 2,2-diphenyl-1-picrylhydazyl (DPPH) free radicals and O 2 − radicals. Furthermore, the sugar content in SlICE1-overexpressing tomatoes red fruits was higher than that in wild-type red fruits. Metabolite pro ling analysis performed by capillary electrophoresis time-of-ight mass spectrometry (CE-TOFMS) revealed that several amino acids and amines were more highly accumulated in SlICE1-overexpressing tomato red fruits compared to those in wild-type tomatoes. ese results suggest that SlICE1 plays a role in the regulation of antioxidant activity through the accumulation of several antioxidants.