Melatonin Deficiency Confers Tolerance to Multiple Abiotic Stresses in Rice via Decreased Brassinosteroid Levels

Abstract

Melatonin has long been recognized as a positive signaling molecule and potent antioxidant in plants, which alleviates damage caused by adverse conditions such as salt, cold, and heat stress. In this study, we found a paradoxical role for melatonin in abiotic stress responses. Suppression of the serotonin N-acetyltransferase 2 (snat2) gene encoding the penultimate enzyme in melatonin biosynthesis led to simultaneous decreases in both melatonin and brassinosteroid (BR) levels, causing a semi-dwarf with erect leaf phenotype, typical of BR deficiency. Here, we further characterized snat2 rice in terms of grain morphology and abiotic stress tolerance, to determine whether snat2 rice exhibited characteristics similar to those of BR-deficient rice. As expected, the snat2 rice exhibited tolerance to multiple stress conditions including cadmium, salt, cold, and heat, as evidenced by decreased malondialdehyde (MDA) levels and increased chlorophyll levels, in contrast with SNAT2 overexpression lines, which were less tolerant to stress than wild type plants. In addition, the length and width of grain from snat2 plants were reduced relative to the wild type, which is reminiscent of BR deficiency in rice. Other melatonin-deficient mutant rice lines with suppressed BR synthesis (i.e., comt and t5h) also showed tolerance to salt and heat stress, whereas melatonin-deficient rice seedlings without decreased BR levels (i.e., tdc) failed to exhibit increased stress tolerance, suggesting that stress tolerance was increased not by melatonin deficiency alone, but by a melatonin deficiency-mediated decrease in BR.

Simultaneous Suppression of Two Distinct Serotonin N-Acetyltransferase Isogenes by RNA Interference Leads to Severe Decreases in Melatonin and Accelerated Seed Deterioration in Rice

“…Similarly, GA 3 treatments of s1, s2, and snat1+2 caused a similar 1.5-fold increase in leaf sheath lengths, although the total leaf sheath lengths were still shorter than those of the wild-types, indicating that the shorter leaf sheath lengths in the RNAi lines were not rescued by GA treatments. These data are consistent with a previous report that s2 shoot lengths are insensitive to GA 3 after pretreatments with the GA-inhibitor paclobutrazol [15].…”

“…DWARF4 is a rate-limiting step enzyme that catalyzes C22-hydroxylation from 6-oxo-campestanol into cathasterone. The failure or reduction of DWARF4 expression leads to dwarf, erect leaf, and aberrant skotomorphogenic phenotypes in plants [14,15]. These observations suggest different physiological roles for the SNAT1 and SNAT2 genes, although they are both involved in melatonin biosynthesis in rice plants.…”

“…Previous research has revealed that the s2 rice exhibits a phenotype of dwarf plants with erect leaves [14], whereas the s1 phenotype is a dwarf without erect leaves [13,15]. The erect leaves in s2 rice were attributed to a decrease in BR content [14].…”

“…The overexpression of SNAT1 conferred tolerance against abiotic stress [12], whereas its downregulation (snat1) led to a susceptibility to abiotic stress and retarded seedling growths, in conjunction with altered melatonin levels [13]. In marked contrast, downregulation of SNAT2 in rice (snat2) led to an enhanced tolerance to abiotic stress, a semidwarf phenotype with erect leaves, and a decrease in melatonin content [14,15]. The major difference between snat1 and snat2 rice was the level of brassinosteroid (BR), of which snat2 was more deficient than the wild-type, while snat1 had a BR content comparable to that of the wild-type.…”

“…The major difference between snat1 and snat2 rice was the level of brassinosteroid (BR), of which snat2 was more deficient than the wild-type, while snat1 had a BR content comparable to that of the wild-type. Both snat1 and snat2 rice had decreased melatonin levels, but they had different phenotypes and abiotic stress responses, depending on their BR levels [15,16]. These contradictory results prompted us to generate a double-suppression rice mutant (snat1+2) to observe the effects of the simultaneous downregulation of both genes.…”