The pea GIGAS gene is a FLOWERING LOCUS T homolog necessary for graft-transmissible specification of flowering but not for responsiveness to photoperiod.

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.

DIE NEUTRALIS and LATE BLOOMER 1 contribute to regulation of the pea circadian clock.

The DIE NEUTRALIS (DNE) locus in garden pea (Pisum sativum) was previously shown to inhibit flowering under noninductive short-day conditions and to affect a graft-transmissible flowering signal. In this study, we establish that DNE has a role in diurnal and/or circadian regulation of several clock genes, including the pea GIGANTEA (GI) ortholog LATE BLOOMER 1 (LATE1) and orthologs of the Arabidopsis thaliana genes LATE ELONGATED HYPOCOTYL and TIMING OF CHLOROPHYLL A/B BINDING PROTEIN EXPRESSION 1. We also confirm that LATE1 participates in the clock and provide evidence that DNE is the ortholog of Arabidopsis EARLY FLOWERING4 (ELF4). Circadian rhythms of clock gene expression in wild-type plants under constant light were weaker in pea than in Arabidopsis, and a number of differences were also seen in the effects of both DNE/ELF4 and LATE1/GI on clock gene expression. Grafting studies suggest that DNE controls flowering at least in part through a LATE1-dependent mobile stimulus, and dne mutants show elevated expression of a FLOWERING LOCUS T homolog under short-day conditions. However, the early flowering of the dne mutant is not associated with altered expression of a previously described CONSTANS-like gene. Collectively, our results characterize the clock system and reveal its importance for photoperiod responsiveness in a model legume.

Update on the genetic control of flowering in garden pea.

The garden pea has been a model for the genetics of flowering for several decades and numerous flowering loci have been identified, but until recently little was known about the molecular nature of these loci. This paper presents an update on recent work on the molecular genetics of flowering in pea, outlining progress in gene and mutant isolation, expression analyses, grafting and other physiological studies, and candidate gene assessment. Work so far has led to the identification of the LATE1 and DNE loci as orthologues of Arabidopsis GIGANTEA and ELF4, respectively, and candidate genes for several other loci are being evaluated. Expression analysis of an expanded FT-like gene family suggests a more complex role for this group of genes. These results provide the first insight into the circadian clock, photoperiod response mechanism, and mobile signals in pea, and identify both conserved and divergent features in comparison with Arabidopsis.

Pea LATE BLOOMER1 is a GIGANTEA ortholog with roles in photoperiodic flowering, deetiolation, and transcriptional regulation of circadian clock gene homologs.

Genes controlling the transition to flowering have been studied in several species, including Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), but have not yet received much attention in legumes. Here, we describe a new allelic series of late-flowering, photoperiod-insensitive mutants in the pea (Pisum sativum) LATE BLOOMER1 (LATE1) gene and show that LATE1 is an ortholog of Arabidopsis GIGANTEA. Mutants display defects in phytochrome B-dependent deetiolation under red light and in the diurnal regulation of pea homologs of several Arabidopsis circadian clock genes, including TIMING OF CAB1, EARLY FLOWERING4, and CIRCADIAN CLOCK ASSOCIATED1/LATE ELONGATED HYPOCOTYL. LATE1 itself shows strongly rhythmic expression with a small but distinct acute peak following dark-to-light transfer. Mutations in LATE1 prevent the induction of a FLOWERING LOCUS T (FT) homolog FTL in long days but cause only minor alteration to the rhythmic expression pattern of the only known group Ia CONSTANS homolog COLa. The late-flowering phenotype of late1 mutants can be completely rescued by grafting to the wild type, but this rescue is not associated with a significant increase in FTL transcript level in shoot apices. Genetic interactions of late1 with the photoperiod-insensitive, early-flowering sterile nodes (sn) mutant and impairment of the LATE1 diurnal expression rhythm in sn plants suggest that SN may also affect the circadian clock. These results show that several functions of Arabidopsis GIGANTEA are conserved in its pea ortholog and demonstrate that genetic pathways for photoperiodic flowering are likely to be conserved between these two species. They also suggest that in addition to its role in the floral transition, LATE1 also acts throughout reproductive development.