ABI2 promotes flowering by inhibiting OST1/ABI5-dependent FLC activation in Arabidopsis.

The plant hormone abscisic acid (ABA) is an important regulator of plant growth and development and plays a crucial role in both biotic and abiotic stress responses. ABA modulates flowering time but the precise molecular mechanism remains poorly understood. Here we report ABA INSENSITIVE 2 (ABI2), as the only phosphatase from ABA-signaling core that positively regulates the transition to flowering in Arabidopsis. Loss-of-function abi2-2 mutant shows significantly delayed flowering both under long day (LD) and short day (SD) conditions. Expression of floral repressor genes such as FLOWERING LOCUS C (FLC) and CYCLING DOF FACTOR 1 (CDF1) was significantly up-regulated in abi2-2 plants while that of the flowering promoting genes FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) was down-regulated. Through genetic interactions we further found that ost1-3 and abi5-1 mutations are epistatic to abi2-2, as both of them individually rescued the late flowering phenotype of abi2-2. Interestingly, ABA INSENSITIVE 5 (ABI5) phosphorylation and protein stability were enhanced in abi2-2 plants suggesting that ABI2 dephosphorylates ABI5, thereby reducing protein stability and the capacity to induce FLC expression. Our findings therefore uncovered the unexpected role of ABI2 to promote flowering by inhibiting ABI5-mediated FLC activation in Arabidopsis.