Differential expression of abscisic acid metabolism and signalling genes induced by seed-covering structures or hypoxia in barley (Hordeum vulgare L.) grains

Dormant barley grains cannot germinate at 30 C and this inability to germinate is imposed mostly by the glumellae which have been suggested to limit oxygen supply to the embryo. Hypoxia imposed either artificially or by the glumellae to embryos from dormant grains, increases embryo sensitivity to abscisic acid (ABA) and promotes the accumulation of ABA during the first hours after imbibition. Expression of candidate genes involved in ABA synthesis (HvNCED), catabolism (HvABA8OH1) and signalling (HvABI5, HvVP1 and HvPKABA) was analysed in embryos isolated from dormant whole or de-hulled grains incubated in air or in hypoxia (5% oxygen). The presence of the glumellae enhanced the expression of genes involved in ABA metabolism and signalling with respect to that observed in de-hulled grains incubated in air. These results suggest that at least part of the observed physiological responses to the presence of the glumellae are regulated at the level of gene expression. However, hypoxia imposed on dormant de-hulled grains did not mimic the presence of the glumellae in terms of expression of candidate genes. Hypoxia mimics the presence of the glumellae in terms of dormancy maintenance and ABA accumulation and sensitivity, but its effects appear to operate through different mechanisms.