ALA reverses ABA-induced stomatal closure by modulating PP2AC and SnRK2.6 activity in apple leaves

5-Aminolevulinic acid (ALA), known as a new natural plant growth regulator, can reverse abscisic acid (ABA)-induced stomatal closure. The protein phosphatase 2A (PP2A) played an important role in regulation of stomatal movement by ALA and ABA; however, the underlying molecular mechanisms remain unclear. Here, we report that ALA promotes MdPP2A activity and gene expression in the leaf epidermis of apple (Malus × domestica Borkh.), and expression of the catalytic subunit MdPP2AC was most significantly correlated with stomatal aperture. Western blotting showed that ALA enhanced MdPP2AC protein abundance and phosphorylation. Y2H (yeast two hybrid), FLC (firefly luciferase complementation imaging) and BiFC (Bimolecular fluorescence complementation) assays showed that MdPP2AC interacted with several other MdPP2A subunits as well as MdSnRK2.6 (Sucrose non-fermenting 1-related protein kinase 2.6), and the latter interaction was further verified by pull-down and MST (microscale thermophoresis) assays. ALA downregulated ABA-induced MdSnRK2.6 gene expression, kinase activity, and protein phosphorylation. In transiently transgenic apple leaves, OE-MdPP2AC promoted stomatal aperture by reducing Ca(2+) and H(2)O(2) levels but increasing flavonol levels in guard cells. Conversely, OE-MdSnRK2.6 induced stomatal closure by increasing Ca(2+) and H(2)O(2) but reducing flavonols. Partial silencing of these genes had opposite effects on Ca(2+), H(2)O(2), flavonols, and stomatal movement. Application of exogenous ALA stimulated PP2A activity, which promoted SnRK2.6 dephosphorylation and lower kinase activity in wild-type and transgenic apple leaves. We therefore propose that PP2AC, which dephosphorylates SnRK2.6 and represses its enzyme activity, mediates ALA signaling to inhibit ABA-induced stomatal closure in apple leaves.