Xyloglucan Endotransglycosylase Activity, Microfibril Orientation and the Profiles of Cell Wall Properties Along Growing Regions of Maize Roots

A series of physical and chemical analyses were made on the expanding zone of maize seedling roots grown in hydroponics. Comparison of longitudinal profiles of local relative elemental growth rate and turgor pressure indicated that cell walls become looser in the apical 5 mm and then tighten 5–10 mm from the root tip. Immersion of roots in 200 mol m−3 mannitol (an osmotic stress of 0·48 MPa) rapidly and evenly reduced turgor pressure along the whole growing region. Growth was reduced to a greater extent in the region 5–10 mm from the root tip than in the apical region. This indicated rapid wall-loosening in the root tip, but not in the more basal regions. Following 24 h immersion in 400 mol m−3 mannitol (an osmotic stress of 0·96 MPa) turgor had recovered to pre-stressed values. Under this stress treatment, growth was reduced in the region 4–10 mm from the root tip, despite the recovery of turgor, indicating a tightening of the wall. In the root apex, local relative elemental growth rate was unchanged in comparison to control tissue, showing that wall properties here were similar to the control values. Cellulose microfibrils on the inner face of cortical cell walls became increasingly more parallel to the root axis along the growth profile of both unstressed and stressed roots. Orientation did not correlate with the wall loosening in the apical region of unstressed roots, or with the tightening in the region 5–10 mm from the root tip following 24 h of osmotic stress. Longitudinal profiles of the possible wall-loosening enzyme xyloglucan endotransglycosylase (XET) had good correspondence with an increase in wall loosening during development. In the zone of wall tightening following osmotic stress, XET activity was decreased per unit dry weight (compared with the unstressed control), but not per unit fresh weight.