An Update on the Role of NCED and CYP707A ABA Metabolism Genes in Seed Dormancy Induction and the Response to After-Ripening and Nitrate

The molecular mechanisms underlying the regulation of seed dormancy and sensu stricto germination once dormancy is removed are still largely unknown. The hormone abscisic acid (ABA), through a complex crosstalk with other plant hormones, transcription factors and cellular signals belonging to the development program, directly controls the induction and maintenance of primary seed dormancy. A working hypothesis is that loss of primary dormancy and induction of germination requires that the severe control exercised by ABA during seed maturation is down-modulated and is replaced by different signaling pathways. Therefore, it is reasonable to speculate that both ABA metabolism and/or a decrease in ABA sensitivity is operative. The ABA-metabolic NCED and CYP707A gene families are considered key for several developmental plant processes that are ABA-controlled, including seed development and sensu stricto germination. Hence, we propose that ABA metabolism and sensitivity are involved in the mechanisms of action of after-ripening (AR), a process which overcomes the seed primary dormancy status. This review summarizes the current knowledge in the last decade on the controlling role of the NCED and CYP707A gene families as well as changes in ABA sensitivity during the seed life cycle. The expression of ABA-metabolic genes and seed sensitivity to ABA during AR and in the presence of nitrate are also considered.