ZmAGA1 Hydrolyzes RFOs Late during the Lag Phase of Seed Germination, Shifting Sugar Metabolism toward Seed Germination Over Seed Aging Tolerance

Raffinose family oligosaccharides (RFOs) are accumulated during the late stage of seed development and hydrolyzed during seed germination. The process of raffinose hydrolysis during seed germination and how this process affects seed vigor remains unknown. We report here that maize alkaline α-galactosidase 1 (ZmAGA1) protein is translationally induced and is capable of hydrolyzing RFOs as well as a precursor, galactinol, during seed germination. Constitutively overexpressing ZmAGA1 in Arabidopsis decreased both RFOs and galactinol contents of mature, desiccated, and 30 hours after imbibition (HAI) seeds, yet enhanced the seed germination percentage under either salt or somewhat osmotic-stress conditions at earlier times during the time course. However, ZmAGA1 overexpression also decreased the seed aging tolerance of mature, desiccated seeds as compared with wild type (WT) or those overexpressing GFP. Compared to that of WT control seeds, the atsip2 (mutant of Arabidopsis AtSIP2 (seed imbibition protein 2, encoding alkaline α-galactosidase)) seeds have similar RFOs and galactinol contents in mature, desiccated seeds but significantly increased the amount of these metabolites at 30 HAI. This retention of RFOs and galactinol in atsip2 results in seeds that exhibit lowered seed germination percentage under either salt or osmotic stress conditions, and yet, increased seed aging tolerance relative to WT. Similarly, when maize seeds were imbibed in the presence of a specific α-galactosidase inhibitor (1-deoxygalactonojirimycin) as compared to those imbibed in water, greater amounts of raffinose and galactinol were detected; the seeds exhibited decreased seed germination percentages but increased seed aging tolerance. Taken together, these data suggest that both maize seed germination and seed aging tolerance can be simultaneously regulated through careful temporal manipulation of ZmAGA1 expression.