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

Abstract

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, including the bioactive GA1, 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 wild-type (WT) progenitor (cv. Torsdag). The aberrant seed development of lh-2 plants is associated with reduced levels of GA1 and GA3, and with an accumulation of abscisic acid (ABA) 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 investigate whether the accumulation of ABA is partly responsible for causing the observed seed abortion in lh-2 plants, we constructed a double mutant between the lh-2 allele and wil. The wil mutation blocks ABA biosynthesis, and reduces ABA levels in young seeds by 10-fold. Introduction of the wil mutation 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 consistent with previous evidence that GAs are the controlling factor underlying the lh-2 seed phenotype in pea.