Differential expression of α- and β-expansin genes in the elongating leaf of Festuca pratensis

Grasses contain a number of genes encoding both alpha- and beta -expansins. These cell wall proteins are predicted to play a role in cell wall modifications, particularly during tissue elongation. We report here on the characterisation of five alpha- and three vegetative beta -expansins expressed in the leaf elongation zone (LEZ) of the forage grass, Festuca pratensis Huds. The expression of the predominant alpha -expansin (FpExp2) was localised to the vascular tissue, as was the beta -expansin FpExpB3. Expression of another beta -expansin (FpExpB2) was not localised to vascular tissue but was highly expressed in roots and initiating tillers. This is the first description of vegetative beta -expansin gene expression at the organ and tissue level and also the first evidence of differential expression between members of this gene family. In addition, an analysis of both alpha- and beta -expansin expression along the LEZ revealed no correlation with growth rate distribution, whereas we were able to identify a novel xyloglucan endotransglycosylase (FpXET1) whose expression profile closely mimicked leaf growth rate. These data suggest that alpha- and beta -expansin activities in the grass leaf are associated with tissue differentiation, that expansins involved in leaf growth may represent more minor components of the spectrum of expansin genes expressed in this tissue, and that XETs may be useful markers for the analysis of grass leaf growth.