Functional characterisation of Nicotiana tabacum xyloglucan endotransglycosylase (NtXET-1): generation of transgenic tobacco plants and changes in cell wall xyloglucan

To study the function of xyloglucan endotransglycosylase (XET) in vivo we i solated, a tomato (Lycopersicon esculentum Mill.) XET cDNA (GenBank AA82498 6) from the homologous tobacco (Nicotiana tabacum L.) clone named NtXET-1 ( Accession no. D86730). The expression pattern revealed highest levels of Nt XET-1 mRNA in organs highly enriched in vascular tissue. The levels of NtXE T-1 mRNA decreased in midribs with increasing age of leaves. Increasing lea f age was correlated with an increase in the average molecular weight (MW) of xyloglucan (XG) and a decrease in the relative growth rates of leaves. T ransgenic tobacco plants with reduced levels of XET activity were created t o further study the biochemical consequences of reduced levels of NtXET-1 e xpression. In two independent lines, total XET activity could be reduced by 56% and 37%, respectively, in midribs of tobacco plants transformed with a n antisense construct. The decreased activity led to an increase in the ave rage MW of XG by at least 20%. These two lines of evidence argue for NtXET- 1 being involved in the incorporation of small XG molecules into the cell w all by transglycosylation. Reducing the incorporation of small XG molecules will result in a shift towards a higher average MW. The observed reduction in NtXET-1 expression and increase in the MW of XG in older leaves might b e associated with strengthening of cell walls by reduced turnover and hydro lysis of XG.