Gibberellins and seed development in Pisum

The lhi and ls mutations have been used to investigate the role of the gibberellins (GAs) in seed development of the garden pea (pjsum salivum L.). These mutations were originally identified by their effects on internode elongation. Plants homozygous for lhi or ls possess reduced levels of endogenous GAl in developing shoots, resulting in a dwarf phenotype compared with wild-type plants. The Lh locus has been shown to be linked tO the Le locus at a distance of ca. ScM. In conjunction with wild-type plants and other GA-deficient mutants, lhi, ls and le5839 plants have been used to demonstrate a log-linear relationship between endogenous GAl levels and internode elongation, further supporting a role for GA1 as the major native GA controlling internode elongation is this species. However, the lh' mutation differs from other GA-deficient mutations, such as lh, since the response of lh' plants to paclobutrazol (an inhibitor ofGA-biosynthesis) is dramatically increased. The lh' and ls mutations also reduce endogenous GA levels in developing seeds. This has allowed the site of action of the ls mutation to be identified. Incubation of cell-free enzyme systems from developing wild-type and ls seeds suggests that ls plants possess reduced ent-kaurene synthetase A activity. The lh' mutation also reduces endogenous GA1 and GA3 levels in young seeds (a few days after fertilization), while ls seeds possess similar GA1 and GA3 levels at this stage compared with wild-type seeds. Comparison of GA levels in lh1lh1, lsls, Lsls and wild-type seeds suggests that GA-biosynthcsis may vary within different tissues (embryo, endosperm and testa) of developing seeds. Seeds homozygous for lhi are more likely to abort during development, and weigh less at harvest, compared with wild-type seeds and seeds homozygous for Is. Altering the source/sink relations of developing lhi plants, and 14C-photoassimilate studies, both suggest that lhi seeds possess reduced sink strength compared with wild-type seds. Fertilizing lhi plants with wild-type pollen produces seeds with nonnal GA levels and restores normal seed development. Culturing of lh' embryos with GAl in vitro also increases embryo size. These results have been used to suggest that GAl and GA3 play an important role early in pea seed development. By contrast, the high GA levels found in maturing wild-type seeds do not have a physiological role in seed development.