Developing soybean seed germination: low ABA and high EXP1 gene expression promote embryonic axis growth whereas the seed coat delays radicle protrusion

Abstract Seed germination implies an expansion process restarting the growth of the embryonic axis (Ax) and which is completed by radicle emergence through the seed covering layers. In developing soybean seeds, abscisic acid in Ax (ABAa) mainly inhibits Ax growth. Additionally, the expression of the EXP1 gene at the elongation zone (EZ) was found to be involved in the promotion of mature soybean Ax growth, which increased during water incubation and which was repressed by exogenous ABA. This work aimed to evaluate (1) the ABAa and EXP1 levels at the EZ and (2) the role of the seed coat (SC) in developing soybean seed germination. Whole seeds (Se), embryos (Em) and Ax at 25–45 d after anthesis (DAA) germinated in vitro, and germination performance increased with DAA. ABAa decreased in planta from 25 DAA until its critical non-inhibitory threshold (ABAc) at around physiological maturity (45 DAA). At earlier ages, the ABAc was reached during the in vitro incubation. Concomitantly, EXP1 transcripts accumulated with age into the pool of long-lived mRNAs and were up-regulated during incubation. Additionally, isolated Ax germinated faster, took up more water and increased its water potential more rapidly during incubation than Ax in Se. Also, a lower osmotic gradient was required to germinate at 45 DAA, when ABAa was no longer inhibitory. Simultaneously, the pressure to protrude SC through the micropylar area increased from 25 to 45 DAA. These results support the role of ABAa and EXP1 in controlling Ax growth and the SC in delaying radicle protrusion.