Genome-wide identification and expression analysis of calmodulin and calmodulin-like genes in apple (Malus x domestica)

Changes in intracellular calcium (Ca2+) levels in response to developmental processes or external stimuli serve as signals in eukaryotic cells. These Ca2+ signals are likely perceived through sensor proteins that bind Ca2+ by EF-hand (a helix-loop-helix structure) motif. Calmodulins (CaMs), a group of well-characterized Ca2+ sensors, and calmodulin-like (CMLs) are implicated in a large number of diverse cellular processes, including plant development and stress responses. In this study, apple (Malus x domestica) genes encoding CaM and CML proteins that only possess EF-hand motifs with no other functional domains were analyzed. A total of 4 MdCaM and 58 MdCML genes were identified, which are spread among 16 out of the 17 apple chromosomes. Bioinformatics analyses, including protein characteristics, conserved domain, evolutionary relationships and chromosomal locations, demonstrated the conservation and divergence of MdCaMs/CMLs. In addition, expression analysis showed that MdCaMs/CMLs are expressed in more than one tissue, including shoot tips, roots, mature leaves, flowers and fruit. Furthermore, the expression of some MdCaM/CML members responded to plant hormones (abscisic acid, jasmonic acid) and salt stress, suggesting a potential role of these genes in responses to biotic and abiotic stress. Overexpression of stress-induced MdCML3 gene significantly improved the tolerance of apple calli to salinity and ABA. The identification and characterization of MdCaMs/CMLs in apple lays a foundation for future functional studies of these genes.