Endoxyloglucan transferase is a novel class of glycosyltransferase recently purified from Vigna angularis (Nishitani, K,, and Tominaga, R. (1992) J. Biol. Chem. 267, 21058-21064). This enzyme is the first transferase identified that catalyzes molecular grafting between polysaccharide cross-links in the cell wall matrix and participates in reconstruction of the network structure in the cell wall. Based on the NH2-terminal amino acid sequence information of the purified transferase, we have here cloned and sequenced cDNAs derived from five different plant species, V. angularis, Triticum aestivum, Arabidopsis thaliana, Lycopersicon esculentum, and Glycine max. In the five plant species, the amino acid sequence of the mature proteins is conserved in the range of 71-90% throughout their length. The consensus sequence for N-linked glycosylation, and four cysteine residues are all conserved in the five species. Thus, the endoxyloglucan transferase protein is ubiquitous among higher plants. The highly conserved DNA sequence will serve as a promising tool for exploring the molecular process by which cell wall construction, and hence cell growth, is regulated.

Xyloglucan, the primary hemicellulosic cell wall polysaccharide in dicotyledons, undergoes substantial modification during auxin-stimulated cell expansion. To identify candidates for mediating xyloglucan turnover, the expression and auxin regulation of tomato Cel7 and LeEXT, genes encoding an endo-1,4-beta-glucanase (EGase) and a xyloglucan endotransglycosylase (XET), respectively, were examined. LeEXT mRNA was present primarily in elongating regions of the hypocotyl and was induced to higher levels by hormone treatments that elicited elongation of hypocotyl segments. Cel7 mRNA abundance was very low in both elongating and mature regions of the hypocotyl but was induced to accumulate to high levels in both hypocotyl regions by auxin application. Analysis of the time dependence of expression of Cel7 and LeEXT during auxin treatment suggested that induction of these genes is not required for rapid growth responses but may participate in the cell wall changes involved in sustained cell elongation. Localization of Cel7 and LeEXT mRNA by in situ hybridization revealed that both genes are expressed in outer cell layers of the hypocotyl. In untreated etiolated seedlings, LeEXT mRNA was detected in epidermal cells of the elongating region, a tissue considered to play a key role in auxin-induced elongation. After auxin treatment, Cel7 and LeEXT mRNA showed an overlapping spatial distribution in the epidermis and outer cortical cell layers. We conclude that LeEXT and Cel7 exhibit both unique and overlapping patterns of expression and have the potential to act cooperatively in mediating cell wall disassembly associated with expansive growth.

The polysaccharide xyloglucan is thought to play an important structural role in the primary cell wall of dicotyledons. Accordingly, there is considerable interest in understanding the biochemical basis and regulation of xyloglucan metabolism, and research over the last 16 years has identified a large family of cell wall proteins that specifically catalyze xyloglucan endohydrolysis and/or endotransglucosylation. However, a confusing and contradictory series of nomenclatures has emerged in the literature, of which xyloglucan endotransglycosylases (XETs) and endoxyloglucan transferases (EXGTs) are just two examples, to describe members of essentially the same class of genes/proteins. The completion of the first plant genome sequencing projects has revealed the full extent of this gene family and so this is an opportune time to resolve the many discrepancies in the database that include different names being assigned to the same gene. Following consultation with members of the scientific community involved in plant cell wall research, we propose a new unifying nomenclature that conveys an accurate description of the spectrum of biochemical activities that cumulative research has shown are catalyzed by these enzymes. Thus, a member of this class of genes/proteins will be referred to as a xyloglucan endotransglucosylase/hydrolase (XTH). The two known activities of XTH proteins are referred to enzymologically as xyloglucan endotransglucosylase (XET, which is hereby re-defined) activity and xyloglucan endohydrolase (XEH) activity. This review provides a summary of the biochemical and functional diversity of XTHs, including an overview of the structure and organization of the Arabidopsis XTH gene family, and highlights the potentially important roles that XTHs appear to play in numerous examples of plant growth and development.

Depolymerization of cell wall xyloglucan has been proposed to be involved in tomato fruit softening, along with the xyloglucan modifying enzymes. Xyloglucan endotransglucosylase/hydrolases (XTHs: EC 2.4.1.207 and/or EC 3.2.1.151) have been proposed to have a dual role integrating newly secreted xyloglucan chains into an existing wall-bound xyloglucan, or restructuring the existing cell wall material by catalyzing transglucosylation between previously wall-bound xyloglucan molecules. Here, 10 tomato (Solanum lycopersicum) SlXTHs were studied and grouped into three phylogenetic groups to determine which members of each family were expressed during fruit growth and fruit ripening, and the ways in which the expression of different SlXTHs contributed to the total XET and XEH activities. Our results showed that all of the SlXTHs studied were expressed during fruit growth and ripening, and that the expression of all the SlXTHs in Group 1 was clearly related to fruit growth, as were SlXTH12 in Group 2 and SlXTH6 in Group 3-B. Only the expression of SlXTH5 and SlXTH8 from Group 3-A was clearly associated with fruit ripening, although all 10 of the different SlXTHs were expressed at the red ripe stage. Both total XET and XEH activities were higher during fruit growth, and decreased during fruit ripening. Ethylene production during tomato fruit growth was low and experienced a significant increase during fruit ripening, which was not correlated either with SlXTH expression or with XET and XEH activities. We suggest that the role of XTH during fruit development could be related to the maintenance of the structural integrity of the cell wall, and the decrease in XTHs expression, and the subsequent decrease in activity during ripening may contribute to fruit softening, with this process being regulated through different XTH genes.

Xyloglucan endotransglucosylase/hydrolase (XTHs: EC 2.4.1.207 and/or EC 3.2.1.151), a xyloglucan modifying enzyme, has been proposed to have a role during tomato and apple fruit ripening by loosening the cell wall. Since the ripening of climacteric fruits is controlled by endogenous ethylene biosynthesis, we wanted to study whether XET activity was ethylene-regulated, and if so, which specific genes encoding ripening-regulated XTH genes were indeed ethylene-regulated. XET specific activity in tomato and apple fruits was significantly increased by the ethylene treatment, as compared with the control fruits, suggesting an increase in the XTH gene expression induced by ethylene. The 25 SlXTH protein sequences of tomato and the 11 sequences MdXTH of apple were phylogenetically analyzed and grouped into three major clades. The SlXTHs genes with highest expression during ripening were SlXTH5 and SlXTH8 from Group III-B, and in apple MdXTH2, from Group II, and MdXTH10, and MdXTH11 from Group III-B. Ethylene was involved in the regulation of the expression of different SlXTH and MdXTH genes during ripening. In tomato fruit fifteen different SlXTH genes showed an increase in expression after ethylene treatment, and the SlXTHs that were ripening associated were also ethylene dependent, and belong to Group III-B (SlXTH5 and SlXTH8). In apple fruit, three MdXTH showed an increase in expression after the ethylene treatment and the only MdXTH that was ripening associated and ethylene dependent was MdXTH10 from Group III-B. The results indicate that XTH may play an important role in fruit ripening and a possible relationship between XTHs from Group III-B and fruit ripening, and ethylene regulation is suggested.