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 through dynamic control of its cellular distribution by ABA transporters, yet the mechanisms controlling ABA transporter activity remain poorly understood. Here, we identify a phosphorylation-dependent regulatory mechanism that modulates cellular ABA levels and root growth responses in Arabidopsis. We show that ABA alters the subcellular distribution of its transporter ATP-binding cassette G16 (ABCG16), which functions as a negative regulator of ABA-induced root growth inhibition. In contrast to ABCG16, the receptor-like kinases BARELY ANY MERISTEM 1 and 2 (BAM1/2) are essential for proper root responses to ABA. BAM1/2 physically interact with ABCG16 and phosphorylate it at threonine 45. Relative intensity analyses and cell-free degradation assays reveal enhanced ABCG16 accumulation in the bam1;+/-bam2 mutant, indicating that ABCG16 protein stability is at least partially dependent on BAM1/2. ABA transport and root growth assays further show that the non-phosphorylated ABCG16 variant promotes ABA efflux and restores ABA-induced root growth inhibition similar to the wild-type protein, whereas the phospho-mimic ABCG16 variant impairs cytosolic ABA efflux and fails to restore root growth under ABA treatment. Consistently, the bam1;+/-bam2 mutant shows constitutively elevated ABA efflux activity compared with wild-type landsberg erecta (Ler), supporting the notion that BAM1/2-mediated phosphorylation dampens ABCG16 transport activity. The abcg16 bam1 and abcg16 bam2 double mutants phenocopy the abcg16 single mutant, showing ABA hypersensitivity in root growth. Together, these findings demonstrate that BAM1/2-mediated phosphorylation of ABCG16 reduces its stability and ABA export activity, thereby maintaining cellular ABA levels required for root growth inhibition.