ABA signaling and GA metabolism in sorghum lines with contrasting dormancy

RODRIGUEZ M.V., MENDIONDO G.M. Y BENECH-ARNOLD R.L.

Salamanca, España

Workshop; 2nd Workshop on Molecular Aspects of Seed Dormancy and Germination; 2007

ISSS

As physiological and genetic evidence 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 model system for studying this interaction in a cereal crop. The aims of this work were: 1) to identify genes involved in ABA signaling and genes involved in GA metabolism which are regulated differentially between lines, and correlate with active GA levels and ABA sensitivity in the imbibed grain; 2) to evaluate a possible negative interaction between ABA signalling and GA metabolism during imbibition, and 3) to identify some component of GA metabolism affected by ABA action at the transcriptional level. 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 (ABI7500 system, SYBR green; Actin as internal control) in embryo samples obtained from grains after different incubation periods. Parallel samples were taken for GA measurements by GC-MS. Also, isolated embryos were incubated in either water or 50uM ABA, and samples taken for gene expression and GA quantification. 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 as compared to RedandB2, 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. From these results, we propose that dormancy blocks the increase in GA levels (otherwise observed in germinating non-dormant grains) and this is at least partly due to a stronger ABA signaling in dormant grains; our results show that gene 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, and thus constitute one point of interaction between ABA signaling and the promotion of GA inactivation.