Xyloglucan endotransglycosylase/hydrolase genes in cotton and their role in fiber elongation

Plant cell wall extensibility is mediated, in part, by xyloglucan endotransglycosylases/hydrolases (XTH) that are able to cleave and reattach xyloglucan polymers that make up the hemicelluloses matrix of type I cell walls. In Arabidopsis and other plants, XTHs are encoded by relatively large gene families that are regulated in specific spatial and temporal patterns. In silico screening of a cotton expressed sequence tag (EST) database identified 23 sequences with close sequence similarity to Arabidopsis XTH coding sequences. Analysis of full-length cotton cDNAs derived from these ESTs allow for the identification of three distinct GhXTH cDNAs (denoted GhXTH1, GhXTH2 and GhXTH3) based primarily on their 3' untranslated sequences. The three GhXTH genes were expressed differently with GhXTH1 predominantly expressed in elongating cotton fibers. The function of GhXTH1 in mediating cotton fiber elongation was analyzed in transgenic cotton plants that express a transgene consisting of the GhXTH1 coding sequence under transcriptional control of the CaMV 35S promoter. Plants that over-expressed GhXTH1 had increased XTH activity and produced mature cotton fibers that were between 15 and 20% longer than wild-type cotton plants under both greenhouse and field growth conditions. Segregation analysis showed that the 35S::GhXTH1 transgene acts as a dominant fiber length allele in transgenic cotton. These results confirm that GhXTH1 is the predominant XTH in elongating fibers and its expression limits cotton fiber elongation.