Changes in xyloglucan endotransglycosylase (XET) activity during hormone-induced growth in lettuce and cucumber hypocotyls and spinach cell suspension cultures

The promotion of stem elongation in dwarf pea plants by GA3 has been correlated with an increase in extractable xyloglucan endotransglycosylase (XET) activity (Potter and Fry, 1993). Doses of auxin that caused either elongation or lateral swelling in pea stems have not been found to increase XET activity (Fry et al., 1992). We therefore explored the generality of the association between GA3 action and XET activity by testing three other bioassay systems. A similar correlation was found in the hypocotyls of intact lettuce seedlings, where GA3 strongly promoted elongation and doubled the extractable XET activity per unit fresh weight. In the hypocotyls of intact cucumber seedlings, GA3 evoked a prolonged promotion of elongation for at least 96 h; this was correlated with only a small increase in XET activity per unit fresh weight of tissue. IAA promoted elongation for only 48 h, but this effect was correlated with a larger increase in XET activity per unit fresh weight than that due to GA3. In cell suspension cultures of spinach, much of the XET activity was present in solution in the culture medium. GA3 had little effect on this fraction for the first 9 d, but thereafter the hormone suppressed a sudden burst in soluble extracellular XET activity that occurred in the untreated controls. A further proportion of the XET activity was ionically wall-bound; this fraction was enhanced by GA3 in the early phase of phases. Pre-adaptation of the culture by growth for 6 years in the presence of 10(-7) M GA3 intensified the response of the cells to re-addition of GA3. We conclude that there is no simple or unique relationship between total extractable XET activity and GA3 action. Nevertheless, we have extended the evidence that XET is often subject to the effects of GA3 in systems where this hormone influences growth.