Amylolytic activities in cereal seeds under aerobic and anaerobic conditions

Abstract

An adequate carbohydrate supply contributes to the survival of seeds under conditions of limited oxygen availability. The amount of soluble, readily fermentable carbohydrates in dry cereal seeds is usually very limited, with starch representing the main storage compound. Starch breakdown during the germination of cereal seeds is the result of the action of hydrolytic enzymes and only through the concerted action of α-amylase (EC 3.2.1.1), β-amylase (EC 3.2.1.2), debranching enzyme (EC 3.2.1.41), and α-glucosidase (EC 3.2.1.20) can starch be hydrolyzed completely. We present here data concerning the complete set of starch-degrading enzymes in three cereals, rice (Oryza sativa L.), which is tolerant to anaerobiosis, and wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.), which are unable to germinate under anoxia. Among the cereal seeds tested under anoxia, only rice is able to degrade nonboiled, soluble starch, reflecting the ability to degrade the starch granules in vivo. This is explained by the presence of the complete set of enzymes needed to degrade starch completely either as the result of de novo synthesis (α-amylase, β-amylase) or activation of preexisting, inactive forms of the enzyme (debranching enzyme, α-glucosidase). These enzymes are either absent or inactive in wheat and barley seeds kept under anaerobic conditions.