Pestie. Sei. 1978, 9, 161

The Relevance of Auxin Binding to Plant Membrane Fractions”

George Murphy

Unit of Developmental Botany, Cambridge

(Munuscript received 5 April 1977)

Synopsis

The binding of plant hormones, particularly auxins, to plant membrane fractions at physiological concentrations and in a specific manner has been investigated by several groups of workers. In attempting to evaluate the relevance of such binding to plant growth it was decided to adopt as a working hypothesis the suggestion that auxin binding to plasma membrane in elongating tissue might stimulate a membrane-bound proton translocating system, inducing growth.

As a pre-requisite for this study a method was developed for the unambiguous identification of plasma-membrane fractions from plant tissue. This involved the isolation of protoplasts from the plants studied followed by labelling of the external surface of the plasma membrane with 1251.

Disruption of the protoplasts followed by sucrose density gradient centrifugation enabled the equilibrium density of the plasma-membrane to be determined. In almost all the plants studied the plasma-membrane equilibrated at a density of 1.21-1.24 g ml-l.

The binding of auxins to plant membranes has been extensively studied using extracts of maize coleoptile and leaf. Results with either 1-naphthylacetic acid or indol-3-ylacetic acid were qualita- tively similar. 1-Naphthylacetic acid binds to a proportionately greater extent than indol-3-ylacetic acid and was the auxin normally used. Binding of 1-naphthylacetic acid to maize membranes is virtually instantaneous and is rapidly reversible. Studies with other auxins, auxin transport inhibitors and compounds chemically related to I-naphthylacetic acid suggest a high degree of specificity. However, of the plants studied (maize, pea, bean, cucumber, mustard, rye, wheat, barley and the water plant Regnillidium diphyllum) only maize coleoptile and leaf tissue show a high degree of binding. Some other plants bind auxin to a much lesser extent, too small to investigate accurately the specificity of binding. Maize mesocotyl contains very few binding sites and none could be detected in maize root.

The binding of 1-naphthylacetic acid over a wide concentration range has been analysed with a computerised curve-fitting program. This indicates that only one high affinity auxin-binding site is present in coleoptile extracts but that another site or sites exists with zero affinity but infinite capacity.

Although plasma-membrane fractions from maize bind auxin with a high affinity, binding also occurs in every other membrane fraction. No consistent effect of indol-3-ylacetic acid on membrane ATPases of maize or pea could be detected in vivo or in vitro. The relevance of the binding of auxins to membrane fractions remains an open question. However, it is certain that further investiga- tions of such binding in relation to physiological responses will prove extremely valuable.

the Physicochemical and Biophysical Panel (Pesticides Group), Society of Chemical Industry. a Presented at a meeting Physical mechanisms involved in plant growth regulation on 1 March 1977, organised by

0031-613X/78/o4ocrol61 $02.00 0 1978 Society of Chemical Industry

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