Abscisic acid levels in seeds of the gibberellin-deficient mutant lh-2 of pea (Pisum sativum)

The lh-2 mutation in garden pea (Pisum sativum L.) blocks an early step in the gibberellin (GA) biosynthesis pathway, the three-step oxidation of ent- kaurene to ent-kaurenoic acid. As a result, only low levels of GAs, includi ng the bioactive GA,, are found in shoots and seeds of lh-2 plants. Mutant plants are dwarf in stature, and show increased seed abortion and decreased seed weight, compared with seeds of the tall wildtype (WT) progenitor (cv. Torsdag). The aberrant seed development of lh-2 plants is associated with reduced levels of GA, and GA,, and with an accumulation of abscisic acid (A BA) in young seeds (pre-contact point). This ABA accumulation is typically 3- to 4-fold, and can be up to 6-fold, compared with control plants. To inv estigate whether the accumulation of ABA is partly responsible for causing the observed seed abortion in lh-2 plants, me constructed a double mutant b etween the lh-2 allele and wil. The wil mutation blocks ABA biosynthesis, a nd reduces ABA levels in young seeds by 10-fold. Introduction of the,wil mu tation reduces the endogenous ABA levels in young lh-2 seeds, but fails to rescue the seeds from abortion. This indicates that the effects of lh-2 on seed development are not mediated through increased ABA levels, and is cons istent with previous evidence that GAs are the controlling factor underlyin g the lh-2 seed phenotype in pea.