Xyloglucan endotransglycosylase activity in pine hypocotyls. Intracellular localization and relationship with endogenous growth

Since xyloglucan depolymerization has been proposed as one of the biochemical bases for cell wall-loosening in gymnosperms, we characterized xyloglucan endotransglycosylase (XET) activity during pine hypocotyl growth to establish a possible relationship. XET activity was measured as the incorporation of [H-3]XXXGol into partially purified pine hypocotyl xyloglucan. XET specific and total activity was determined in the subapical and basal segments of pine hypocotyls at two different stages of growth in different subcellular fractions. XET activity was found in the apoplastic fluid, the symplastic fluid, and in the fraction of proteins ionically and covalently bound to the cell walls with different distribution profiles. The results showed a relationship between XET activity and hypocotyl growth in all the fractions, suggesting an important role for XET during growth. Consequently, the suggested growth-promoting effect of XET in angiosperms can also be extended to gymnosperms. Also, the results demonstrate that XET bound to the cell wall is able to act on endogenous wall-bound xyloglucan as well as soluble polymeric xyloglucan, using them as substrates for the endotransglycosylation reaction.