A phosphorylation-gated GAPDH-MAPK6-LBD1 module controls drought-responsive transcription

Drought adaptation requires rapid spatial reorganization of signaling proteins to reprogram stress-responsive transcription, yet how phosphorylation coordinates this process remains unclear. Here we aimed to define how phosphorylation-gated protein relocalization regulates a BpLBD1-centered drought pathway in Betula platyphylla (birch). Drought activated two kinase branches: BpAGC phosphorylated BpMAPK6 at Ser318 within a cryptic nuclear localization signal, promoting importin α recognition, whereas BpCKII phosphorylated BpGAPDH at Thr109, Thr270, Ser279 and Thr298, promoting importin β recognition. Dual phosphorylation enabled coordinated nuclear accumulation of BpMAPK6 and BpGAPDH. In the nucleus, phosphorylated BpGAPDH acted as a scaffold-like adaptor coupling BpMAPK6 to BpLBD1, enabling BpMAPK6-mediated phosphorylation of BpLBD1 at Ser29. This weakened BpLBD1 binding to the BpMYB123 promoter, reduced expression of the drought-sensitivity regulator BpMYB123 , and de-repressed drought-responsive genes such as BpRD29B . Disrupting BpGAPDH or BpLBD1 Ser29 phosphorylation compromised drought tolerance. Thus, a phospho-spatial GAPDH-MAPK6-LBD1 switch couples nuclear protein assembly to drought-responsive transcription.