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 GA(3) has been correlated with an increase in extractable xyloglucan endotransglycos ylase (XET) activity (Potter and Fry, 1993). Doses of auxin that cause d either elongation or lateral swelling in pea stems have not been fou nd to increase XET activity (Fry et al., 1992). We therefore explored the generality of the association between GA(3) action and XET activit y by testing three other bioassay systems. A similar correlation was f ound in the hypocotyls of intact lettuce seedlings, where GA(3) strong ly promoted elongation and doubled the extractable XET activity per un it fresh weight. In the hypocotyls of intact cucumber seedlings, GA(3) evoked a prolonged promotion of elongation for at least 96 h; this wa s correlated with only a small increase in XET activity per unit fresh weight of tissue. IAA promoted elongation for only 48 h, but this eff ect was correlated with a larger increase in XET activity per unit fre sh weight than that due to GA(3). In cell suspension cultures of spina ch, much of the XET activity was present in solution in the culture me dium. GA(3) had little effect on this fraction for the first 9 d, but thereafter the hormone suppressed a sudden burst in soluble extracellu lar XET activity that occurred in the untreated controls. A further pr oportion of the XET activity was ionically wall-bound; this fraction w as enhanced by GA(3) in the early phase of the growth cycle of the cul ture and inhibited in later phases. Pre-adaptation of the culture by g rowth for 6 years in the presence of 10(-7) M GA(3) intensified the re sponse of the cells to re-addition of GA(3). We conclude that there is no simple or unique relationship between total extractable XET activi ty and GA(3) action. Nevertheless, we have extended the evidence that XET is often subject to the effects of GA(3) in systems where this hor mone influences growth.