Regulation of tomato fruit growth by epidermal cell wall enzymes

Water relations of tomato fruit and the epidermal and pericarp activities of the putative cell wall loosening and tightening enzymes Xyloglucan endotransglycosylase (XET) and peroxidase were investigated, to determine whether tomato fruit growth is principally regulated in the epidermis or pericarp. Analysis of the fruit water relations and observation of the pattern of expansion of tomato fruit slices in vitro, has shown that the pericarp exerts tissue pressure on the epidermis in tomato fruit, suggesting that the rate of growth of tomato fruit is determined by the physical properties of the epidermal cell walls. The epidermal activities of XET and peroxidase were assayed throughout fruit development. Temporal changes in these enzyme activities were found to correspond well with putative cell wall loosening and stiffening during fruit development. XET activity was found to be proportional to the relative expansion rate of the fruit until growth ceased, and a peroxidase activity weakly bound to the epidermal cell wall appeared shortly before cessation of fruit expansion. No equivalent peroxidase activity was detected in pericarp tissue of any age. It is therefore plausible that the expansion of tomato fruit is regulated by the combined action of these enzyme activities in the fruit epidermis

In vitro synthesis and properties of pectin/Acetobacter xylinus cellulose composites

Pectin and cellulose are major components of most primary cell walls, yet little is known about the way in which they interact either during assembly or in subsequent functional performance of the wall. As a mimic of cell wall assembly, we studied the formation of molecular composites formed by deposition of cellulose from Acetobacter xylinus into pectin/calcium systems, and the molecular, architectural and mechanical properties of the composites obtained. The formation of interpenetrating cellulose/pectin composite networks (as envisaged in current models for primary cell walls) required a pre-existing, but not too strong, pectin network. For pectin either in solution or strongly networked, phase separation from cellulose occurred, providing two physical models for the formation of middle lamellae. Composite networks showed no evidence of direct molecular interaction between the components, but pectin networks became more aggregated following deposition of cellulose into them. The shear strength under small deformation conditions for cellulose/pectin composites was very similar to that of cellulose alone. In contrast, under uniaxial tension, extensibility was greatly increased and stiffness decreased. These major changes were due to the effect of pectin on cellulose network architecture at deposition, as they were maintained upon removal of the pectin component. These results show that the presence and physical state of pectin at the time of cellulose deposition in muro may be a significant determinant of subsequent extensibility without compromising strength