Specification of floral meristem identity at the Arabidopsis shoot apex

After floral induction the Arabidopsis shoot apical meristem becomes an indeterminate inflorescence meristem whose lateral primordia must be assigned floral, rather than shoot, fate. Floral integrators supply the input: the florigen FT, a PEBP-family protein made in leaves, reaches the apex and acts as a transcriptional coactivator of the bZIP factor FD to activate APETALA1 (AP1) and the MADS-box integrators SOC1 and FRUITFULL (FUL); SOC1 (as a heterodimer with AGL24) binds the LEAFY (LFY) promoter and activates it, and FUL promotes the reproductive transition partially redundantly with SOC1. LFY, a plant-specific DNA-binding transcription factor, directly induces AP1 in the floral anlagen, and AP1 in turn up-regulates LFY, so the two lock in floral fate through positive feedback. The SQUAMOSA-clade MADS factors AP1, CAULIFLOWER (CAL) and FUL act partially redundantly in this step (ap1 cal ful triple mutants make no flowers); AP1 also acts as a master repressor in the young flower, directly silencing the flowering-time/inflorescence genes SOC1, AGL24 and SVP so that the shoot program does not continue in floral meristems. The F-box protein UNUSUAL FLORAL ORGANS (UFO) binds LFY and DNA to form a LFY-UFO transcriptional complex that redirects LFY to region-specific targets in the young flower. TERMINAL FLOWER1 (TFL1), a PEBP paralog of FT, is expressed in the centre of the inflorescence meristem and acts as an FD-dependent corepressor that keeps LFY and AP1 out of the shoot apex, maintaining indeterminate inflorescence identity; AP1 and LFY reciprocally repress TFL1 in floral primordia. The A-class factor APETALA2 contributes to floral meristem establishment alongside AP1. Floral organ identity (the ABCE program activated later by LFY, UFO and AP1 together with SEP proteins), the leaf-side photoperiod pathway that produces FT, and the age, vernalization and gibberellin pathways converging on the integrators are separate modules.

Concrete Arabidopsis instance. Every annoton is grounded on the Swiss-Prot accession of a reviewed gene in genes/ARATH and its function term is taken from that review's core_functions. GO has no process term for floral meristem identity specification as distinct from determinacy; GO:0010228 (vegetative to reproductive phase transition of meristem) is used as the module concept, with GO:0010582 (floral meristem determinacy) for the floral-fate step, and the AP1 review carries the proposed new term. FUL is modelled twice because it has two separable roles in this module: an integrator role at the shoot apex (with SOC1) and a redundant floral-fate role with AP1/CAL in primordia; ap1 cal ful plants never flower. AGL24 and SVP (SOC1 partners and AP1 repression targets), the 14-3-3 proteins of the florigen activation complex, the SPL/miR156 age-pathway inputs and BOP1/BOP2 are referenced in prose only. FD is included as the DNA-binding partner of FT; FT production in leaves is covered by the photoperiodic_flowering_co_ft module. The ABCE floral organ identity program (AP3/PI/AG/SEP) downstream of LFY, UFO and AP1 is covered by floral_organ_identity_abce. No production GO-CAM in gocams/index.tsv contains these Arabidopsis gene products, so no gocam_associations are given. PANTHER family ids are not asserted; all participants are concrete gene products.