FL7 is an ancient ABA-independent inhibitor of PP2C-As regulating plant stress responses.

Clade A protein phosphatase 2Cs (PP2C-As) play crucial roles in plant stress responses. Although the ABA receptors PYLs inhibit PP2C-As in an ABA-dependent manner, other modulators of these phosphatases remain largely unknown. Here, we identify the FORKED-LIKE 7 (FL7) protein as a broad PP2C-A interactor that effectively suppresses PP2C activity through an ABA-independent, noncompetitive mechanism. By inhibiting PP2C-A activity, FL7 positively regulates osmotic tolerance and plant immunity in an ABA-independent manner. The N-terminal auxin canalisation (AC) domain of FL7 is required for its PP2C-As inhibitory activity. Further evolutionary analyses reveal that FL7 homologues containing an AC domain belong to an ancient family that emerged in a common ancestor of Klebsormidiophyceae algae and land plants. Genetic analyses indicate that algal FL7 homologues have a conserved function as PP2C-A inhibitors. Our study reveals an ABA-independent layer of PP2C-A modulation that regulates biotic and abiotic stress responses, which is likely conserved across a billion years of streptophyte evolution and predated the PYL-ABA regulation established in the common ancestor of land plants.