Genetic control of inflorescence development in pea

Angiosperm species exhibit incredible diversity in inflorescence architecture. Legumes comprise the third largest family of flowering plants, second only to the grasses in terms of agricultural importance. Several important crop legumes, including pea, lentil, common bean and chickpea, share a characteristic inflorescence form, the compound raceme, which has one extra level of inflorescence branching, the secondary inflorescence (I2), relative to the simple raceme of Arabidopsis. Historically, pea has been a popular model legume for studies of flowering, often through the characterisation of flowering and inflorescence mutants. In this study, pea genes with an apparent or putative role in inflorescence development were investigated, in order to improve understanding of the genetic control of inflorescence development in pea. Unlike Arabidopsis, where no single gene mutant has a non-flowering phenotype, mutations at any of three pea loci can prevent flowering: GIGAS/FTa1, VEG1/FULc and VEG2. In this study, the roles of VEG2 during inflorescence development were investigated using two mutant alleles: the non-flowering veg2-1 mutant, and the late-flowering veg2-2 mutant. The results indicate that VEG2 is important for the correct timing of the inflorescence transition, initial specification and maintenance of I2 identity, and specification of floral meristems, under both LD and SD conditions. Preliminary mapping results indicated a pea homolog of FD as a candidate for the VEG2 locus. In this study, the legume FD gene family was characterised and the VEG2 locus was shown to correspond to FDa. In the veg2-1 mutant, the entire coding sequence was found to be deleted but putative flanking genes were unaffected. The veg2-2 mutant was shown to contain a single nucleotide polymorphism (SNP), affecting a highly conserved amino acid within the DNA-binding, basic region of the bZIP domain. The mechanisms of FDa action were further investigated through analysis of expression patterns of FDa and protein interactions with the pea FT and TFL1 homologs. FDa was found to be expressed in the wild-type apex throughout development. FDa protein was found to be capable of interacting with all five pea FT homologs, and DET (TFL1a), but not LF (TFL1c). Flowering genes regulated (either directly or indirectly) by FDa were identified based on misregulation of expression in the veg2 mutants. These included pea homologs of FT, TFL1 and LFY, in addition to a range of MADS-box genes. The late5 mutant is a previously undescribed EMS-induced mutant that exhibits phenotypic similarity to veg2-2. To determine the role of LATE5 during pea inflorescence development, the late5 phenotype was characterised. The genetic interactions between LATE5, DET and LF were investigated through the phenotypes of double and triple mutants. The molecular roles of LATE5 were also investigated by examining the effects of the late5 mutation on expression of flowering genes. The map position of LATE5 was refined to a region of less than 3.2cM towards the base of pea linkage group I, corresponding to a syntenic region of 0.6Mb containing 95 annotated genes in Medicago. The legume family of SVP-like genes, which have important roles in flowering time, inflorescence branching and floral meristem identity in other species, was characterised and two new SVP-like genes (SVPb and SVPc) were isolated from pea. Investigation of expression patterns of pea SVPa, SVPb and SVPc genes, revealed developmental regulation of SVPc in wild-type pea, and misregulation of SVPc in the veg2-2 mutant indicating regulation of SVPc (directly or indirectly) by FDa/VEG2. Overall, the findings of this study make a significant contribution to knowledge of the genetic control of inflorescence development in pea.