Cell wall degradation is required for normal starch mobilisation in barley endosperm

Abstract

Starch degradation in barley endosperm provides carbon for early seedling growth, but the control of this process is poorly understood. We investigated whether endosperm cell wall degradation is an important determinant of the rate of starch degradation. We identified iminosugar inhibitors of enzymes that degrade the cell wall component arabinoxylan. The iminosugar 1,4-dideoxy-1, 4-imino-l-arabinitol (LAB) inhibits arabinoxylan arabinofuranohydrolase (AXAH) but does not inhibit the main starch-degrading enzymes α- and β-amylase and limit dextrinase. AXAH activity in the endosperm appears soon after the onset of germination and resides in dimers putatively containing two isoforms, AXAH1 and AXAH2. Upon grain imbibition, mobilisation of arabinoxylan and starch spreads across the endosperm from the aleurone towards the crease. The front of arabinoxylan degradation precedes that of starch degradation. Incubation of grains with LAB decreases the rate of loss of both arabinoxylan and starch, and retards the spread of both degradation processes across the endosperm. We propose that starch degradation in the endosperm is dependent on cell wall degradation, which permeabilises the walls and thus permits rapid diffusion of amylolytic enzymes. AXAH may be of particular importance in this respect. These results provide new insights into the mobilization of endosperm reserves to support early seedling growth.

Multiplex Fluorescent, Activity-Based Protein Profiling Identifies Active α-Glycosidases and Other Hydrolases in Plants

“…These compounds have occasionally been used to conduct research in plants; for instance, a-amylase was inhibited to study barley (Hordeum vulgare) seed germination (Frandsen et al, 2000;Stanley et al, 2011). Iminosugars also have been used to study plant cell wall metabolism and starch remobilization (Andriotis et al, 2016).…”

Soybean LEC2 Regulates Subsets of Genes Involved in Controlling the Biosynthesis and Catabolism of Seed Storage Substances and Seed Development

“…The starch metabolism and turnover in developing and mature seeds is a complex pathway ( Andriotis et al, 2016 ). During early developmental stages in Arabidopsis seeds, starch is transiently accumulated; however, very low amount remains in mature dry seeds.…”

“…Several studies reported that seed starch level behaves reciprocally with the other main storage compounds such as oil and protein ( Angeles-Núñez and Tiessen, 2010 , 2011 ). Another study reported that starch accumulation is caused by the absence of certain metabolic enzymes ( Andriotis et al, 2016 ). The assumption of metabolic competition between oil and starch biosynthesis pathways is further supported by atwri1 ( Focks and Benning, 1998 ) and atlec2 ( Angeles-Núñez and Tiessen, 2011 ) with elevated starch and reduced oil contents.…”

Molecular Mechanism of Functional Ingredients in Barley to Combat Human Chronic Diseases

“…Arabinoxylan in barley plays an important role in quality traits of malt and beer product [56]; however, arabinoxylan arabinofuranohydrolase I can be used as novel enzyme products in the beer industry [57]. The starch degradation for seedling relies on cell wall degradation, where the iminosugar 1,4dideoxy-1,4-imino-l-arabinitol inhibits dextrinase and arabinoxylan arabinofuranohydrolase but permits rapid diffusion of αand β-amylase [58]. Arabinoxylan contents in barley grains range from 4.2% to 5.4% [59] where it ranged from 0.53 mg/100 g to 0.90 mg/100 g at an average value of 0.67 mg/100 g in barley endosperm in 128 spring 2-row barley; its two QTLs include glycosyltransferases and glycoside hydrolases [55].…”