Biochemistry of Cell Extension

Citation

Morré, D. James F. (1963) Biochemistry of Cell Extension. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/G6WY-AS68. https://resolver.caltech.edu/CaltechTHESIS:11022012-144309062

Abstract

By subjecting various plant tissues to conditions which influence growth, such as treatment with the growth hormone, 3-indoleacetic acid (IAA), changes in certain cytoplasmic constituents were observed which parallel simultaneous changes in the mechanical properties of the cell wall.

Treatment of maize roots for one hour in the presence of concentrations of IAA that normally promote shoot growth and, therefore, inhibit root growth followed by measurement of the deformability of the rapidly elongating region under artificially imposed load, demonstrated a concentration-dependent, IAA-induced plasticity component, not observable in untreated roots. The response is transient; half-maximal plasticity is induced by ca. 5 x 10^(-7) M IAA, one hour after treatment (a concentration comparable to that of Avena sections). Increased deformability is paralleled by increased growth of root sections, also transient, with maximum IAA-induced increase in growth rate coinciding with the maximum for increased wall plasticization. The initial response of maize roots to IAA, therefore, resembles that of Avena coleoptiles both qualitatively and quantitatively. The end result, however, is to effectively shorten the period over which the root can elongate.

Associated with increased plasticity and growth rate of maize roots is rapid formation (or maintenance) of a protein-bound carbohydrate fraction. Disruption of the complex by such agents as extremes of pH or organic solvents results in increased turbidity of aqueous solutions and acquisition of solutility properties characteristic of lipides. Results of preliminary characterization studies also suggest that material is of lipoprotein origin.

Evidence for a similar fraction, increased in amount by treatment of the tissue with IAA, has been extended to include Avena coleoptiles, pea epicotyls and pea embryo axes. In addition to lipide-soluble components, these fractions contain approximately equimolar quantities of carbohydrate (including hexose) and of esterified phosphate. The material, therefore, has been designated as a protein-bound glycolipide (PGL).

A cytoplasmic origin of PGL is suggested. Also associated with the 2- to 4-fold increase in amount of protein-bound PGL is a decrease in the heat coagulability of cytoplasmic proteins. Similarily, a portion of an apparent IAA-induced increase in acid phosphatase activity may be attributable to increased stabilization as well. An electron microscopic survey of IAA effects on the fine structure of subcellular organelles revealed no major structural changes, however, the number of vesicles associated with the central Golgi structure of both maize roots and Avena sections may be increased by IAA treatment.

A study of cell wall pectic constituents has revealed that although versene-soluble pectin does represent a solubility class distinct from hot water-soluble pectin, the conditions for extraction do not correspond to those of the classical residual pectin fraction. The existence of pectin in the cold buffer-soluble, 70% ethanol-insoluble fraction of Avena coleoptiles (cold water-soluble pectin) seems doubtful, however.

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