Overexpression of a wheat alpha-amylase type 2 impact on starch metabolism and abscisic acid sensitivity during grain germination

Despite being of vital importance for seed establishment and grain quality, starch degradation remains poorly understood in organs such as cereal or legume seeds. In cereals, starch degradation requires the synergetic action of different isoforms of alpha-amylases. Ubiquitous overexpression of TaAmy2 resulted in a 2.0-437.6-fold increase of total alpha-amylase activity in developing leaf and harvested grains. These increases led to dramatic alterations of starch visco-properties and augmentation of soluble carbohydrate levels (mainly sucrose and alpha-gluco-oligosaccharide) in grain. Interestingly, the overexpression of TaAMY2 led to an absence of dormancy in ripened grain due to abscisic acid (ABA) insensitivity. Using an allosteric alpha-amylase inhibitor (acarbose), we demonstrated that ABA insensitivity was due to the increased soluble carbohydrate generated by the alpha-amylase excess. Independent from the TaAMY2 overexpression, inhibition of alpha-amylase during germination led to the accumulation of soluble alpha-gluco-oligosaccharides without affecting the first stage of germination. These findings support the hypotheses that (i) endosperm sugar may overcome ABA signalling and promote sprouting, and (ii) alpha-amylase may not be required for the initial stage of grain germination, an observation that questions the function of the amylolytic enzyme in the starch degradation process during germination.