Into the Shoot Apical Meristem of Arabidopsis with bZIP AREB3: long and dry days integrated signals into a fast flowering transition

Arabidopsis thaliana is a long-day annual plant. Under 16 hours of light Arabidopsis flowers early, the vegetative phase is short, and only 12-13 leaves are produced in contrast to about 80 leaves under short days (8-hour light). Further anticipation of the reproductive phase occurs under drought conditions in long days, by about 1-2 leaves. Central to regulation of flowering time and shoot structure development is the shoot apical meristem (SAM) that switches to producing flowers instead of leaves during the floral transition. The SAM has a central stem cell niche and continuous organogenesis at its flanks enables the indeterminate growth of the shoot. When the vegetative-to-reproductive transition occurs the SAM changes shape while increasing in size. This is coordinated with the identity switch of its leaf primordia to flower ones (Bertran Garcia de Olalla et al., 2024). These changes are triggered by a long-distance signal, the “florigen”. It originates in the leaves, where day length, but also drought, are known to regulate its production. Thus, florigen and hormones at the SAM regulate the floral transition and affect the inflorescence structure (Kinoshita et al., 2020; Gao et al., 2025; Romera-Branchat et al., 2025). At the SAM, the florigen binds a bZIP transcription factor dimer trough 2 14-3-3 adaptors to regulate expression of flowering but also of drought-hormonal, and bZIP genes (Collani et al., 2019; Romera-Branchat et al., 2020; Gao et al., 2025). Other bZIPs, as AREB3, could participate in the florigen activation complex, and perhaps do so to integrate drought signalling in the SAM (Martignago et al., 2020, 2023, 2025). AREB3 is phosphorylated by drought signalling. I will show data on the generation of phospho-null mutants, on the phospho-proteome and interactome of AREB3 and their impact on the flowering transition under drought.