BAM1/2-mediated Phosphorylation of ABCG16 Reduces its Stability and ABA Export Activity to Suppress Root Growth in Arabidopsis

The phytohormone abscisic acid (ABA) governs plant stress responses depending on dynamic regulation of its cellular distribution by ABA transporters, yet the mechanisms controlling ABA transporter activity remain poorly understood. Here, we uncover a phosphorylation-dependent regulatory mechanism that controls cellular ABA level and Arabidopsis root growth responses. We show that ABA mediates the subcellular distribution of its transporter ABCG16, which is negatively involved in ABA-induced root growth inhibition. Contrary to ABCG16, the receptor-like kinases BAM1/2 are found to be required for root responses to ABA. It is further shown that BAM1/2 physically interact with ABCG16 and phosphorylate it at threonine 45 (Thr45). Relative intensity and cell degradation assays reveal enhanced ABCG16 accumulation in the bam1;+/- bam2 mutant, pointing to that ABCG16 protein stability is at least partially dependent on BAM1/2. ABA transport and root growth assays show that whilst the non- phosphorylated variant of ABCG16 facilitates ABA efflux activity and rescues root growth responses to ABA as wild-type ABCG16, the phosphor-mimic ABCG16 impairs cytosolic ABA efflux and fails to rescue root growth under ABA treatment. Moreover, the bam1;+/-bam2 mutant exhibits constantly improved ABA efflux activity compared with wild-type Ler, further verifying that BAM1/2-mediated phosphorylation of ABCG16 dampens its transport activity. Genetic hierarchy analyses show that the double mutants g16 bam1 and g16 bam2 phenocopy the single mutant g16, with ABA hypersensitivities in root growth responses. These findings together suggest that BAM1/2-mediated phosphorylation of ABCG16 dampens its stability and ABA export activity, thus maintaining cellular ABA level for inhibiting root growth.