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

• Published in: The Plant journal : for cell and molecular biology Vol. 108; no. 2; pp. 378 - 393
• Main Authors: Zhang, Qin, Pritchard, Jenifer, Mieog, Jos, Byrne, Keren, Colgrave, Michelle L., Wang, Ji‐Rui, Ral, Jean‐Philippe F.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01-10-2021; John Wiley and Sons Inc
• Subjects: Abscisic acid; Abscisic Acid - pharmacology; Acarbose; Allosteric properties; alpha-Amylases - genetics; alpha-Amylases - metabolism; amylase; Amylases; Carbohydrates; Cereals; Degradation; Dormancy; Endosperm; Gene Expression Regulation, Plant; Germination; Germination - physiology; Grain; Isoforms; Legumes; Metabolism; Oligosaccharides; Organs; Original; Plant Dormancy - drug effects; Plant Dormancy - physiology; Plant Proteins - genetics; Plant Proteins - metabolism; Plants, Genetically Modified; Seeds; Seeds - genetics; Seeds - growth & development; Seeds - metabolism; Starch; Starch - chemistry; Starch - genetics; Starch - metabolism; Sucrose; Sugars - metabolism; Triticum - genetics; Triticum - metabolism; wheat; germination; dormancy; degradation; starch; wheat; amylase; sucrose
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/tpj.15444

Summary
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 α‐amylases. Ubiquitous overexpression of TaAmy2 resulted in a 2.0–437.6‐fold increase of total α‐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 α‐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 α‐amylase inhibitor (acarbose), we demonstrated that ABA insensitivity was due to the increased soluble carbohydrate generated by the α‐amylase excess. Independent from the TaAMY2 overexpression, inhibition of α‐amylase during germination led to the accumulation of soluble α‐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) α‐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.
Significance Statement
This study described the application of plant biotechnology to provide evidence as to whether elevated levels of α‐amylase in the mature grain can affect germination positively or negatively. These findings support the hypotheses that (i) endosperm sugar may overcome abscisic acid signalling and promote sprouting, and (ii) α‐amylase may not be required for the initial stage of grain germination.