The Plant Hormone Concept:Concentration, Sensitivity and Transport

Abstract

The concept of control by changing concentrations is crucial to the original concept of hormones in mammals. A few years ago, a great stir was created amongst biologists working with plant hormones by the suggestion of Trewavas (56) that there is no evidence that plant hormones act via changes in the amount or concentration of the hormone, and that all change in response must be attributed to changes in the sensitivity of the tissue. The reason for this suggestion is the frequent lack of correlation between hormone concentrations measured in a tissue and the response of the tissue. In addition, in most plants, growth is proportional to the logarithm of the applied hormone concentration, such that there may be an increasing response over three orders of magnitude in concentration. However, changes in the endogenous concentration in tissues are usually far smaller than would be expected to produce the vast changes in growth or development observed; (plant hormone workers tend to regard a doubling in concentration as a large change!). If concentration changes cannot account for the differences in growth and development then something else must be responsible, and changing tissue sensitivity is the only other logical alternative. While our knowledge on factors influencing sensitivity (or responsiveness) to hormones is still elementary, documentation of differential sensitivities, elucidation of hormone binding (Chapters D4 and E3), and understanding of the mechanisms of regulation of gene expression are steadily increasing, so that the potential mechanisms for the regulation of hormone sensitivity are becoming more evident. Our best examples of changing sensitivity are to ethylene. Immature flower or fruit tissue show no response to ethylene, but mature tissue responds to ethylene with ripening or senescence (see Chapter G2). However, an important question is what we mean by the rather vague term ``sensitivity''. As Firn (11) has pointed out, a change in sensitivity simply refers to an observation that the response to a given amount of hormone has changed.