Genome-wide identification and expression analysis of the ASMT gene family reveals their role in abiotic stress tolerance in apple

Apple (Malus domestica Borkh.), an economically important fruit crop, is widely cultivated worldwide. However, apple trees often suffer from environmental stressors which affect the quality and yield of apple fruit. Acetylserotonin methyltransferase (ASMT) is a key enzyme in melatonin biosynthesis and is known to play critical roles in melatonin production and stress responses of plants. In this study, a total of 37 ASMT genes in apple were identified and unequally distributed on 10 apple chromosomes, with fragment duplication detected in four pairs of genes. The ASMT genes were classified into three groups based on their sequence compositions and phylogenetic relationship. Sequence analysis showed that apple ASMT genes were generally conserved in each group, with minor variations in gene structure and motif distribution. Transcriptome analysis revealed that most apple ASMT genes were nearly unchanged or downregulated under heat, cold, drought, and NaCl stress, whereas MdASMT6, MdASMT11, MdASMT14 and MdASMT19 were highly induced by these stressors. Subsequent qRTPCR of four apple ASMT genes showed that MdASMT11 and MdASMT14 were extremely up-regulated under heat, cold, drought, and NaCl stress. They were expressed in all tissues, especially in the roots. Subcellular localization showed that MdASMT11 mainly distributed on cell membranes and MdASMT14 mainly anchored to the nucleus and cell membrane, indicating that MdASMT11 and MdASMT14 are potential genes regulating abiotic stress resistance in apple. These results facilitate further investigation to the functional characterization of MdASMT genes in the synthesis of melatonin and the response to abiotic stress and in apple.