XYLOGLUCAN ENDOTRANSGLYCOSYLASE AND PLANT-GROWTH

Xyloglucan endotransglycosylase (XET) catalyses the breaking and re-joining of xyloglucan molecules. It may be involved in plant growth, functioning either to anchor newly synthesized xyloglucan polymers into the cell wall or to reversibly loosen the cellulose-xyloglucan network permitting turgor-driven cell expansion. Consistent with a role in growth, levels of XET activity have been shown to correlate with elongation in, for example, maize roots (Pritchard et al., 1993). Our group has recently isolated a cDNA clone from nasturtium (de Silva et al., 1993) encoding a seed enzyme with XET activity (Edwards et al., 1986; Fanutti et al., 1993). The biological function of XET is being investigated via in vivo manipulation. We have used the nasturtium sequence to isolate several cDNA clones from tomato whose encoded polypeptide sequences exhibit approximately 40% homology with nasturtium XET. Heterologous expression in E. coli has been used to confirm XET activity. An isoform of XET has been purified from tomato fruit and a polyclonal antibody raised. The antibody and gene-specific nucleic acid probes are being used to monitor developmental changes in XET protein and mRNA levels, respectively. Preliminary results suggest that tomato XET is encoded by a family of genes which show different developmental patterns of expression. Agrobacterium transformation of tomato has been carried out using the nasturtium cDNA (sense orientation) and a fruit-expressed tomato cDNA (antisense orientation) under the control of a constitutive promoter. The phenotypic consequences of altering levels of XET are being investigated.