Changes in wall-bound polysaccharidase activities during the culture cycle of a Rubus fruticosus cell suspension

Several hydrolytic enzyme activities were detected in the wall of developing cells of Rubus truticosus in suspension culture. The corresponding substrates of the enzymes are mostly polysaccharide wall constituents, except for chitinase activity. The activities measured when the enzymes were in the free state or wall-bound showed the positive influence of the cell wall micro-environment. Changes in the activities during a cell culture cycle demonstrated that those enzymes acting on xyloglucans behaved differently from the others, and suggest that xyloglucans undergo modifications in vivo over a longer period of time during the exponential growth phase. The same activities were identified in the culture medium. Endo-1,4-β-d-glucanase activities which depolymerized car☐ymethylcellulose (CMC) and xyloglucans (XG) were assayed viscosimetrically. It was found that XG oligosaccharides exhibited an inhibitory effect on the depolymerization of xyloglucans but not on that of CMC. This suggests that true xyloglucanases are present in the culture of Rubus cells.     

参与木葡聚糖合成的糖基转移酶基因研究进展

半纤维素多糖木葡聚糖(XyG)存在于大多数植物的初生细胞壁中, 对细胞壁的结构组织和生长发育具有重要的调控作用。XyG在植物进化中存在结构的多样性。该文概述了参与XyG合成的糖基转移酶的最新研究进展, XyG合成需要多种糖基转移酶参与, 这些酶类很可能以蛋白酶复合体的形式存在并发挥作用, XyG的结构和组成的改变对植物的生长发育也产生影响。     

Developmental Expression Patterns of Arabidopsis XTH Genes Reported by Transgenes and Genevestigator

The plant cell wall is the structural basis of cellular form and thus forms a foundation on which morphogenesis builds organs and tissues. Enzymes capable of modifying major wall components are prominent candidates for regulating wall form and function. Xyloglucan endotransglucosylases/hydrolases (XTHs) are predicted to participate in xyloglucan integration and/or restructuring. XTHs are encoded by large gene families in plants; the Arabidopsis genome encodes 33 XTHs. To gain insight into the potential physiological relevance of the distinct members of this family, GUS reporter fusion genes were constructed, and plants expressing these transgenes were characterized to reveal spatial and temporal patterns of expression. In addition, Genevestigator sources were mined for comprehensive and comparative XTH expression regulation analysis. These data reveal that the Arabidopsis XTHs are likely expressed in every developmental stage from seed germination through flowering. All organs show XTH::GUS expression and most, if not all, are found to express multiple XTH::GUS genes. These data suggest that XTHs may contribute to morphogenesis at every developmental stage and in every plant organ. Different XTHs have remarkably diverse and distinct expression patterns indicating that paralogous genes have evolved differential expression regulation perhaps contributing to the maintenance of the large gene family. Extensive overlap in XTH expression patterns is evident; thus, XTHs may act combinatorially in determining wall properties of specific tissues or organs. Knowledge of gene-specific expression among family members yields evidence of where and when gene products may function and provides insights to guide rational approaches to investigate function through reverse genetics. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Function of xyloglucan endotransglucosylase/hydrolases in rice

Background and Aims

Although xyloglucans are ubiquitous in land plants, they are less abundant in Poales species than in eudicotyledons. Poales cell walls contain higher levels of β-1,3/1,4 mixed-linked glucans and arabinoxylans than xyloglucans. Despite the relatively low level of xyloglucans in Poales, the xyloglucan endotransglucosylase/hydrolase (XTH) gene family in rice (Oryza sativa) is comparable in size to that of the eudicotyledon Arabidopsis thaliana. This raises the question of whether xyloglucan is a substrate for rice XTH gene products, whose enzyme activity remains largely uncharacterized.

Methods

This study focused on OsXTH19 (which belongs to Group IIIA of the XTH family and is specifically expressed in growing tissues of rice shoots), and two other XTHs, OsXTH11 (Group I/II) and OsXTH20 (Group IIIA), for reference, and measurements were made of the enzymatic activities of three recombinant rice XTHs, i.e. OsXTH11, OsXTH20 and OsXTH19.

Key Results

All three OsXTH gene products have xyloglucan endohydrolase (XEH, EC 3·2·1·151) activity, and OsXTH11 has both XEH and xyloglucan endotransglycosylase (XET, EC 2·4·1207) activities. However, these proteins had neither hydrolase nor transglucosylase activity when glucuronoarabinoxylan or mixed-linkage glucan was used as the substrate. These results are consistent with histological observations demonstrating that pOsXTH19::GUS is expressed specifically in the vicinity of tissues where xyloglucan immunoreactivity is present. Transgenic rice lines over-expressing OsXTH19 (harbouring a Cauliflower Mosaic Virus 35S promoter::OsXTH19 cDNA construct) or with suppressed OsXTH19 expression (harbouring a pOsXTH19 RNAi construct) did not show dramatic phenotypic changes, suggesting functional redundancy and collaboration among XTH family members, as was observed in A. thaliana.

Conclusions

OsXTH20 and OsXTH19 act as hydrolases exclusively on xyloglucan, while OsXTH11 exhibits both hydrolase and XET activities exclusively on xyloglucans. Phenotypic analysis of transgenic lines with altered expression of OsXTH19 suggests that OsXTH19 and related XTH(s) play redundant roles in rice growth.