Xyloglucan endotransglycosylase: a role after growth cessation in harvested asparagus

Abstract

Little is known about the mode of xyloglucan endotransglycosylase (XET) activity in cell walls once the turgor, which drives expansion, is reduced. Such a situation exists when growing shoots are excised from the parent plant, and is the case for many commercially valuable vegetable crops, e.g. asparagus, Asparagus officinalis L. XET activity was present in all zones of rapidly growing, immature asparagus spears, but with highest levels at the spear base where elongation growth had ceased. Activity increased in all parts of the spear for up to 72 h after harvest. Two members of the XET-related gene family in asparagus (AoXET1 and AoXET2) were isolated and mRNA corresponding to these clones accumulated at low levels, particularly in the basal zone during spear growth. Transcript levels increased in all parts of the asparagus spear after harvest, but this increase did not coincide with the increase in XET activity. The harvest-related changes to xyloglucan molecular weight were restricted to slight, segment-specific, up- or down-shifts. However, this may hide strategic alterations to linkages leading to a more rigid wall without major changes in overall molecular weight. The initial postharvest surge in XET activity could be related to harvest stresses such as water deficit, but we propose that the later induction of AoXET1 and AoXET2 is linked to the development of lignified secondary cell walls.