The PeaGIGASGene Is aFLOWERING LOCUS THomolog Necessary for Graft-Transmissible Specification of Flowering but Not for Responsiveness to Photoperiod  

Abstract

Garden pea (Pisum sativum) was prominent in early studies investigating the genetic control of flowering and the role of mobile flowering signals. In view of recent evidence that genes in the FLOWERING LOCUS T (FT) family play an important role in generating mobile flowering signals, we isolated the FT gene family in pea and examined the regulation and function of its members. Comparison with Medicago truncatula and soybean (Glycine max) provides evidence of three ancient subclades (FTa, FTb, and FTc) likely to be common to most crop and model legumes. Pea FT genes show distinctly different expression patterns with respect to developmental timing, tissue specificity, and response to photoperiod and differ in their activity in transgenic Arabidopsis thaliana, suggesting they may have different functions. We show that the pea FTa1 gene corresponds to the GIGAS locus, which is essential for flowering under long-day conditions and promotes flowering under short-day conditions but is not required for photoperiod responsiveness. Grafting, expression, and double mutant analyses show that GIGAS/FTa1 regulates a mobile flowering stimulus but also provide clear evidence for a second mobile flowering stimulus that is correlated with expression of FTb2 in leaf tissue. These results suggest that induction of flowering by photoperiod in pea results from interactions among several members of a diversified FT family.

VEGETATIVE1 is essential for development of the compound inflorescence in pea

“…S4a). Consistent with that, a time-course expression analysis showed that VEG1 upregulation occurs during the floral transition, after FTb2, a leaf marker of floral induction 16 , and before the floral meristem identity gene PIM (Supplementary Fig. S4b).…”

“…In wild-type plants, the floral transition is associated with transcriptional induction of DETERMINATE (DET) and PROLIFERATING INFLORESCENCE MERISTEM (PIM) [14][15][16][17] (Fig. 3a), which, like their Arabidopsis orthologues TERMINAL FLOWER 1 (TFL1) 18 and APETALA 1 (AP1) 19 , control the identity of the inflorescence and the floral meristems 14 expressed in wild-type plants, expression was not observed in veg1 (Fig.…”

“…This indicates that DET and PIM repress VEG1 expression and restrict it to the I 2 , and suggests that the ectopic expression of VEG1 in det and pim mutants causes the conversion of I 1 and floral meristems, respectively, into I 2 meristems. Severe mutations in the VEGETATIVE2 (VEG2) or GIGAS genes cause a non-flowering phenotype similar to the veg1 mutant (gigas only under long-day photoperiods), with no formation of secondary inflorescences 16,31 . As in veg1, the plants of both mutants show outgrowth of vegetative lateral branches at nodes occupied in wild-type plants by I 2 s (ref.…”

“…As in veg1, the plants of both mutants show outgrowth of vegetative lateral branches at nodes occupied in wild-type plants by I 2 s (ref. 16) (Sussmilch et al, unpublished). This similarity suggests that these genes participate in the same genetic network as VEG1 and raises the question of what is their relative position in that network.…”

“…This indicates that VEG1 acts downstream of VEG2 and GIGAS and suggests that they participate in the activation of VEG1 expression. This seems particularly likely for GIGAS 16 , in view of the fact that its Arabidopsis homologue FLOWERING LOCUS T (FT) is a direct activator of the floral identity genes AP1 and FUL [32][33][34][35] , which are MADS-box genes from the same lineage as VEG1 (Supplementary Fig. S3).…”