Salt stress response in apple

Abstract

Soil salinity triggers excessive reactive oxygen species (ROS) accumulation and oxidative damage in apple (Malus domestica), severely reducing fruit yield and quality. While melatonin confers salt tolerance by enhancing ROS scavenging, hydrogen sulfide (H2S), a vital gasotransmitter in plants, also plays a crucial role in stress responses, including salt stress. However, how H2S regulates melatonin-mediated salt tolerance remains largely unknown. Here we show that MdDES1, an L-cysteine desulfhydrase, positively regulates salt tolerance by increasing endogenous H₂S production. Biochemical and mutagenesis assays reveal that H2S directly modifies MdASMT10-the rate-limiting enzyme in melatonin biosynthesis-at Cys 12, Cys 55 and Cys158 via persulfidation, thereby improving its enzymatic activity and protein stability. Transgenic analysis confirms that persulfidation-deficient MdASMT10 no longer enhances salt tolerance. And exogenous NaHS most effectively improved salt tolerance in MdASMT10-overexpressing apple lines, while its effects on persulfidation-deficient MdASMT10 and wild type (WT) plants were comparable and weaker than those on MdASMT10-overexpressing lines. H2S thus promotes melatonin biosynthesis by persulfidating MdASMT10, which in turn strengthens ROS scavenging and alleviates salt stress. Collectively, our study elucidates the MdDES1-H2S-MdASMT10 regulatory cascade in apple salt tolerance, providing a promising molecular target for breeding salt-tolerant apple cultivars.