MdBAM17, a novel member of the β-amylase gene family, positively regulates starch degradation in ALA-induced stomatal opening in apple (Malus × domestica)

5-Aminolevulinic acid (ALA) is a novel plant growth regulator that has shown outstanding capability to promote stomatal opening. Starch degradation, catalyzed by β-amylase (EC3.2.1.2, BAM), plays an important role in stomatal opening. However, whether the starch breakdown is involved in ALA-regulating stomatal movement is unclear. In the current study, we found that exogenous ALA effectively stimulated the starch breakdown in guard cells, increased β-amylase activity and promoted stomatal opening in leaves of apple ( × ). Based on genome-wide identification, we identified a total of 119 members of gene family in ten commonly Rosaceae crops. Analyses of gene structure, motif identification, and gene pair collinearity revealed relative conservation among members within the same group or subgroup. Among these genes, and other 12 genes were identified as the orthologous genes of , which is responsible for starch degradation to modulate the stomatal movement in . RT-qPCR analysis revealed a positive correlation between the expressions of and stomatal aperture, as well as β-amylase activity, whereas a negative correlation was observed with the starch content. Subcellular localization analysis confirmed that MdBAM17 is a chloroplast protein, consistent with the AtBAM1. was mainly expressed in guard cells and responsive to exogenous ALA. Overexpressing increased β-amylase activity and promoted starch breakdown, leading to stomatal opening, which was further strengthened by ALA. RNA-interfering decreased β-amylase activity, resulting in starch accumulation, and impairing the stomatal opening by ALA. However, modulation of expression did not affect the levels of flavonols and HO in guard cells, suggesting that -promoted starch degradation may function at downstream of ROS signaling in the ALA-regulated stomatal opening. Our findings provide new insights into the mechanisms of ALA-regulated stomatal movement.