By subjecting various plant tissues to conditions which influencegrowth, such as treatment with the growth hormone, 3-indoleacetic acid(IAA), changes in certain cytoplasmic constituents were observed whichparallel simultaneous changes in the mechanical properties of the cellwall.
Treatment of maize roots for one hour in the presence of concentrationsof IAA that normally promote shoot growth and, therefore, inhibitroot growth followed by measurement of the deformability of therapidly elongating region under artificially imposed load, demonstrateda concentration-dependent, IAA-induced plasticity component, not observablein untreated roots. The response is transient; half-maximal plasticityis induced by ca. 5 x 10^(-7) M IAA, one hour after treatment (aconcentration comparable to that of Avena sections). Increased deformabilityis paralleled by increased growth of root sections, also transient,with maximum IAA-induced increase in growth rate coinciding with themaximum for increased wall plasticization. The initial response of maizeroots to IAA, therefore, resembles that of Avena coleoptiles both qualitativelyand quantitatively. The end result, however, is to effectivelyshorten the period over which the root can elongate.
Associated with increased plasticity and growth rate of maize rootsis rapid formation (or maintenance) of a protein-bound carbohydratefraction. Disruption of the complex by such agents as extremes of pH ororganic solvents results in increased turbidity of aqueous solutions andacquisition of solutility properties characteristic of lipides. Resultsof preliminary characterization studies also suggest that material is oflipoprotein origin.
Evidence for a similar fraction, increased in amount by treatment ofthe tissue with IAA, has been extended to include Avena coleoptiles, peaepicotyls 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 the2- to 4-fold increase in amount of protein-bound PGL is a decrease in theheat coagulability of cytoplasmic proteins. Similarily, a portion of anapparent IAA-induced increase in acid phosphatase activity may beattributable to increased stabilization as well. An electron microscopicsurvey of IAA effects on the fine structure of subcellular organellesrevealed no major structural changes, however, the number of vesiclesassociated with the central Golgi structure of both maize roots and Avenasections may be increased by IAA treatment.
A study of cell wall pectic constituents has revealed that althoughversene-soluble pectin does represent a solubility class distinct fromhot water-soluble pectin, the conditions for extraction do not correspondto those of the classical residual pectin fraction. The existence ofpectin in the cold buffer-soluble, 70% ethanol-insoluble fraction ofAvena coleoptiles (cold water-soluble pectin) seems doubtful, however.
