Abstract

Harder kernels in barley are thought to be a factor affecting the modification of the endosperm during malting by restricting water and enzyme movement within the endosperm. The objective of this study was to investigate the relationship between kernel hardness, water uptake and the endosperm composition in barley. A range of barley samples from 2003 and 2004 crops were analyzed for kernel hardness by the Single Kernel Characterization System, water uptake during steeping and chemical composition of the endosperm including (1→3; 1→4)-β-glucan, arabinoxylan and total protein. Both (1→3; 1→4)-β-glucan and arabinoxylan content of the endosperm were correlated significantly with kernel hardness in barley samples from both 2003 (r=0.873 and 0.601, respectively,p<0.01) and 2004 seasons (r=0.764 and 0.501, respectively,p<0.01). Hardness of the kernel was highly correlated with its water uptake in both 2003 and 2004 samples (r=−0.853 and −0.752, respectively,p<0.01).β-Glucan content of the endosperm was also correlated significantly with the kernel water uptake for both years (2003:r=−0.752,p<0.01; 2004:r=−0.551,p<0.01). Arabinoxylan content of the endosperm was correlated significantly with the kernel water uptake for the 2003 barley but not for 2004 barley (2003:r=−0.523,p<0.01; 2004:r=−0.151,p>0.01). Protein content of the endosperm was not correlated with the kernel hardness in either year. These results demonstrate that endosperm cell wall components may have significant impact on kernel hardness as well as water uptake of barley.

Arabinoxylan(AX) is a major polymer of cell wall in wheat grain，with the physicochemical properties, such as high molecular weight, high viscosity and oxidative gelation et al. AX has significant effects on the quality of wheat and its products, and is effective in lowering blood lipids, losing weight and immunocompetence et al. Variations in the amount and structure of AX have been demonstrated among wheat cultivars. This paper summarizes recent research in wheat AX in order to further understand and utilization of the functional polysaccharide.

阿拉伯木聚糖（AX）是小麦细胞壁的主要结构多糖，具有高分子量、高粘度、氧化胶凝等物化特性。AX对于小麦及其面制品的加工品质具有重要影响，同时它还具有显著的降血脂、减肥、免疫调节等生理活性。小麦品种间阿拉伯木聚糖存在一定的遗传变异规律。本文回顾了近年来国内外在小麦阿拉伯木聚糖研究方面取得的成果，将为更好地利用这一功能性多糖提供思路。

Abstract

(1→4)-β-Xylan endohydrolases (EC 3.2.1.8) are key enzymes in the metabolism of heteroxylans, which are particularly abundant polymers in the cell wall matrix of cereals. They hydrolyse the internal(1→4)-β-xylosidic linkages of the xylan backbone of arabinoxylan, and together with other endo- and exohydrolases contribute to the degradation and re-modelling of plant cell walls. Their catalytic mechanism and substrate binding properties are largely deduced from (1→4)-β-xylan endohydrolases of fungal and saprophytic bacterial origin, since relatively few of plant origin have been studied. Recent advances in our understanding of the structure-function properties of a barley (1→4)-β-xylan endohydrolase has prompted us to re-examine the sequence data of four related cereal enzymes and to assemble a comprehensive comparison of their primary structure and predicted functional domains. The common structural motifs in these cereal enzymes are further compared with those of 11 related (1→4)-β-xylan endohydrolases, whose sequence was deduced from theArabidopsis genome. Our structural analyses predict a role for several conserved residues and domains in either the carbohydrate-binding, catalytic activity, or sub-cellular localisation of these plant enzymes, whose future investigation could enhance our understanding of cell wall metabolism.

Abstract

The effects of genotype, harvest year and their interaction on the levels of arabinoxylans (AX), endoxylanases and endoxylanase inhibitors in wheat were studied using 14 varieties grown in three successive growing periods with diverse climatological conditions. Relations with more commonly evaluated wheat characteristics such as yield, thousand kernel weight, specific weight, protein level, Hagberg falling number (HFN) andα-amylase activity level were examined. Water extractable arabinoxylan (WE-AX) levels in wheat varied much more than total arabinoxylan (TOT-AX) levels. This variability was mainly genetically determined, but harvest year also had an important effect. Total endoxylanase activity levels varied more than a factor of 20 between the different wheat samples. Endogenous endoxylanases typically accounted for only 10–15% of this activity, while wheat-associated microbial endoxylanases accounted for the remaining 85–90%. However, when preharvest sprouting occurred, the contribution of endogenous endoxylanases could sometimes amount to over 40% of this total activity. Endogenous endoxylanase activity levels were mainly determined by the interaction of genotype and harvest year, while wheat-associated microbial endoxylanase activity levels were predominantly determined by genotype alone. Endogenous and microbial endoxylanase activity levels were strongly correlated, suggesting that wheat varieties which are susceptible to preharvest sprouting are often also susceptible to microbial contamination. The TAXI and XIP-type endoxylanase inhibitor levels varied by a factor of 8 and 1.8, respectively. They were mainly determined by genotype and were rather similar in the different growing periods.

Abstract

Fungicide treatment had a significant impact on endoxylanase activity and XIP levels, but did not affect arabinoxylan (AX) and TAXI levels. The different response of TAXI and XIP type inhibitors to fungicide treatment is interesting. N-fertilisation did not affect AX levels, but significantly increased TAXI and XIP type inhibitor levels. Wheat-associated microbial endoxylanase activity levels were also affected by nitrogen supply, but levels of the endogenous enzyme did not change, except when sprouting occurred. The weather conditions before harvest had no influence on total AX (TOT-AX) and inhibitor levels, but had a large impact on both microbial and endogenous endoxylanase activity and water extractable AX (WE-AX) levels. Under most conditions, endoxylanase activity levels were related to those of α-amylases, liquefaction numbers (LN) and specific weights. WE-AX levels were often weakly but significantly correlated with endoxylanase activity levels, indicating that it is possible that part of the WE-AX in wheat originates from AX degradation by endoxylanases in the field. These results clearly indicate that agronomic circumstances significantly affect the levels of AX, endoxylanases and their inhibitors in wheat, and consequently could affect wheat quality.

Abstract

The levels of xylanolytic activities ((1→4)-β-endoxylanases, β-d-xylosidases and α-l-arabinofuranosidases) and the levels of water extractable arabinoxylans (WEAX) in hot water extracts of malts were compared during malting of four barley cultivars of differing quality. Endoxylanase activity increased from day 1 after steeping showing a slight lag compared with the development of (1→3,1→4)-β-glucanase activity. The levels of endoxylanase activity were different between the cultivars. For example, at day 4, which would be a suitable endpoint for malting of good quality cultivars, total endoxylanase activities of Static and Chariot malts were twice that of Hart and Optic. This variation in level of endoxylanase between cultivars did not correlate with the content of water extractable arabinoxylans in hot water extracts of the malts. However, those cultivars that accumulated most water extractable arabinoxylan had a different set of endoxylanase isoforms. This suggests that expression of specific endoxylanase isoforms may influence the extent of arabinoxylan solubilisation and/or degradation.

The β-xylosidases and α-arabinosidases increased from low levels after steep to a plateau of activity at day 6 or 7 of germination. Total xylosidase was always higher than the arabinosidase activity. There were differences between cultivars but these were small compared with the variation in endoxylanase activity. (1→4)-β-d-Endomannanase activity was detected in all four cultivars at 2 days germination then rose to a maximum around 5 days.