Leaf and seed development in pea (Pisum sativum L.)
Creators: name: Yaxley, JL
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis examines the development of the compound leaf in pea (Pisum sativum L.), focusing on regional identities within the leaf. Novel interactions between the pea leaf mutants insecatus(ins), cachleata(cach), unifaliata(uni), stipules reduced(st), tendril-less(tl) and afila(aj) are utilised to investigate the role of regional identity in the determination of leaf form. Secondly, a new viviparous pea mutant, viviparaus(vip), is characterised and used to investigate the processes controlling normal seed maturation and the prevention of precocious germination. Leaf development. Plants which are homozygous for both the each and st mutations show that the each mutation transforms the stipules into organs which act as leaf tissue. The stipules of each mutants respond to other mutant genes as a leaf would, and do not respond to the mutant gene st, which alters only stipule tissue. Plants of the genotype each each Tl tl show reduced Tl expression at the base of the leaf. This provides evidence for gradients of gene expression within the pea leaf, which have been proposed in other studies. Examination of the apical meristems of each and uni mutants has revealed that the phenotypes of these mutants are visible earlier in the development of the leaf primordia than other previously characterised pea leaf mutants. Plants homozygous for the each mutation show greatly retarded development of the stipule primordia. Also, the apical domes of uni mutants are smaller than those of wild-type plants. A difference in apical dome size has not been previously reported for any pea leaf mutant. A new allele of a poorly characterised mutant, ins, has been found and ins has been shown to be a dominant mutation with poor penetrance rather than a recessive mutation. Plants homozygous for the ins mutation have a range of tendril and notched structures at the tip of some leaflets. This has been found to be due to additional growth at the tip of the leaflet, which occurs late in development. The double mutant ins tl has leaflet-like structures, rather than tendril structures, present at the affected leaflet tips. This indicates that the ins mutation alters the identity of the leaf tip. The double mutant ins uni shows that the simple leaf of the uni mutant has a region at its tip which corresponds to the tip of the leaflet in a compound pea leaf. Previous studies of pea leaf development have focused on the af and tl mutants, which affect the leaflets and tendrils. This thesis provides information about the development of the stipules at the leaf-base which has not been addressed in detail in previous models of pea leaf development. The vip mutant is the first viviparous mutant to be reported in a legume species. The seeds of the vip mutant germinate in the pod near contact point (when the liquid endosperm disappears). They stay green and will die through desiccation if they remain in the pod. The phenotype of the seed is determined by the embryo. Seeds of the vip mutant will survive if they are removed from the pod and planted before they dehydrate. However, growth of seedlings homozygous for the vip mutation is slow for the first few weeks, and there is increased seedling death, compared with wild-type seedlings. Mature vip mutant plants produce one pod per inflorescence rather than two. Two vip alleles have been isolated (vip-1 and vip-2). The mutation is inherited as a monogenic recessive, with the gene located at the bottom of linkage-group five. A paucity of vip seeds from crosses suggests that vip gametes have reduced viability, as there is no increased abortion of vip seeds. Compared with wild-type seeds, seeds of vip-1 and vip-2 mutants show some alteration in free-abscisic acid (ABA) levels during development. However, comparison with seeds of the pea ABA-deficient mutant wilty, which have very low ABA levels and are not viviparous, suggests that the altered ABA levels in vip seeds are not responsible for their precocious germination. Free-ABA levels in the shoots of the vip mutants, and their ABA synthesis in response to droughting, are the same as wild-type shoots. Seeds of the vip mutants are somewhat insensitive to ABA during their early growth. There is no evidence of overproduction of bioactive gibberellins in vip seeds. Messenger RNAs for proteins found during seed maturation, which are reduced in wild-type germinating seeds, continue to accumulate in vip seeds even though they are germinating precociously. Thus, vip mutant seeds are deficient in a single factor, which is necessary to prevent precocious germination, but does not reduce expression of maturation genes. This factor does not appear to be homologous to the Arabidopsis gene ABSCISIC ACID INSENSITIVE3 (ABJ3). The vip gene may relate to ABA sensitivity early in seed development, which would indicate that this is the critical time when ABA prevents precocious germination. Alternatively, the ABA insensitivity found may be a secondary effect, and vip seeds may be altered in a novel factor that normally prevents precocious germination. Such factors have been proposed to exist by other studies examining the control of germination.