CONICET | Buscador de Institutos y Recursos Humanos

Pre-harvest sprouting is predominantly a feature of cereals. These crops have been selected heavily against dormancy. Sprouting-susceptible genotypes are those in which this selection has gone too far and, consequently, grain dormancy is terminated well before harvest. As physiological and genetic evidence obtained with model organisms have shown, seed dormancy establishment during early development and its expression in the imbibed grain both depend on the counteracting effects of ABA and gibberellins (GA). We used two grain sorghum inbred lines with contrasting sprouting behavior (IS9530, sprouting resistant, and RedlandB2, sprouting susceptible) as a system for studying this interaction in a cereal crop. Orthologous sequences in sorghum for GA metabolism and ABA signaling genes were obtained by searching PlantGDB databases. Expression of these candidate genes was analyzed by RT- QPCR. Parallel samples were taken for GA measurements by GC-MS. Results show that seeds of the dormant line (IS9530) accumulate less active GA (1 and 3) upon imbibition, and that this correlates with a high expression of a gene encoding a GA-2-oxidase (GA-inactivating enzymes). Expression of this gene was not only higher in grains of the more dormant line, but was also strongly induced by exogenous ABA in isolated embryos of both genotypes. This may account for the lower content of active GA levels observed in isolated embryos incubated in ABA. We propose that dormancy blocks the increase in GA levels and this is at least partly due to a stronger ABA signaling in dormant grains; our results show that expression of transcription factors involved in ABA signaling such as SbABI5 is higher in dormant grains. The presence of several ABRE elements in the promoter region of SbGA-2-oxidase suggest that SbABI5 (or some other bzip) might activate transcription of this gene, thus constituting one point of interaction between ABA signaling and GA inactivation.