The role of glycosylated amino acids of wall-bound protein in cell extension of stems of Pisum sativum L. plants

Theeffect of cell extension regulating plant hormones. GA and IA A were studied in view of differences in protein properties of the primary cell wall. Lamport's hypothesis that wall-bound protein is a polysaccharidecross-link ing agent and hence affects wallextensibility was tested. The results show that during cell extension new synthesis of wall-bound protein occurs, though the highest rate coincides with a declining rate of cell extension. Consistent with Lamport’s hypothesis, the elongating cells of dwarf variety (Rondo) plants have a higher glycosylated hydroxyproline content than the standard variety (Alaska). The application of GA to the top of the slow growing dwarf variety increased the growth rate until comparable with the standard variety, while decreasing the glycosylated part of the hydroxyproline content. Experiments performed with excised elongating stem segments, grown in a culture solution containing phosphate, demonstrated that the glycosylated hydroxyproline content of the cell walls at least doubled during an incubation time of 24 hrs. IAA strongly inhibited this increase while stimulating elongation. However, culture solutions stimulating growth in length do not always cause a reduction in glycosylated hydroxyproline synthesis. Moreover, during the period of rapid cell extension, large differences in growth rate may occur with hardly any difference in glycosylated hydroxyproline synthesis. It was concluded that an inverse linear relationship between glycosylated hydroxyproline content of primary cell walls and growth rate does not necessarily exist. This conclusion also holds true for the glycosylated wall-bound serine content. No significant change in concentration during the period of rapid cell extension could be detected and therefore the conclusion that wall-bound glycosylated aminoacids play a role as a stiffening agent of the primary cell wall is not justified.