Auxin-induced elongation of pea stem segments and the role of calcium

Cunninghame, Morag Elizabeth (1983) Auxin-induced elongation of pea stem segments and the role of calcium. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Some ultrastructural aspects of auxin-induced secretion of matrix cell wall components in elongating pea stem segments have been investigated. A quantitative analysis of micrographs confirmed an initial rapid increase in dictyosome material followed by a decrease due to an increased utilization of the organelle. The increase was detected within 15 minutes of auxinpresentation, reaching a maximum around 30 minutes. The possibility that auxin may stimulate the secretion of matrix cell wall components, and hence elongation, through changes in Ca2+ levels was examined. Auxin-induced elongation of pea stem segments was measured in the presence of chelators of divalent cations (chlorotetracycline (CTC), ethyleneglycolbis-(B-amino ethyl ether) N,N1-tetraacetic acid (ECTA), ethylene diamine tetraacetic acid (EDTA)), compounds which disrupt Cat+ gradients (LaC13, verapamil, the ionophore A-23187) and inhibitors of secretory processes (colchicine, cytochalasin B (CB), monensin). Pronounced inhibitory effects on elongation were induced by all of the compounds except colchicine, and became apparent within 1-2 hours. The effects appeared to increase with increasing concentration apart from LaC13i where a maximum effect on auxininduced elongation was reached at 2 mM. The timing correlates with the maximum auxin stimulated secretion of cell wall material. The effects on elongation were also considered in relation to the ultrastructural effects of the compounds on the tissue. The most pronounced effects were induced by monensin, ECTA and LaC13, which all affected the morphology of the dictyosomes, the plasma membrane and, to some extent, themitochondria. CTC, A-23187 and verapamil showed no deleterious effects on the structure of the organelles, but there was a pronounced absence of secretory vesicles in the presence of A-23187. Two methods were used to identify sites of Ca 2+ localization in the epidermal cells, with a view to linking changes in Ca2+ distribution with auxin-induced elongation; visualization of Ca2+ binding sites and Ca2+ precipitation with antimonate. The results however were inconsistent, and it was concluded that these methods were not sufficiently sensitive to detect the small changes that might occur.The results suggest that the secretion of wall materials was affected by changes in cellular Ca2+ levels induced by the disruption of secretory mechanisms. This conclusion was supported by a reduction in the incorporation of radioactive precursors into the wall after a 5 hour incubation in the presence of some inhibitors (monensin, LaC13, CTC,A-23187). It is concluded that the secretory process in pea stem epidermal cells is dependent on the level and/or distribution ofintracellular Ca2+.

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