Regulation from Within: The Hormone Dilemma

Abstract

The classical theory of the hormonal regulation of plant growth and development proposes that the five classical types of plant hormones are responsible for the regulation of many different aspects of plant development. Furthermore, it is also thought that the flexibility and diversity required for such control are provided by modulation of hormonal levels and interactions between different hormones. However, the central question as to how far the controls exerted by plant hormones result from modulation of the hormone levels or from modulation of the 'sensitivity' of plant cells to the hormonal signals, remains largely unanswered. Furthermore, a growing body of evidence showing a variety of plant responses to small oligosaccharides strongly suggests that the diversity of chemical messages used by plants to react to external signals and/or to integrate their functions at the whole plant level is far greater than that corresponding to the 'classical' plant hormones. This calls for some re-evaluation of our concept of plant hormones. Ideas about the mechanisms of action of plant hormones have been strongly influenced by the concepts and the methodologies relative to animal systems. The few pieces available for this large puzzle suggest interesting similarities between plants and animals in terms of reception, transduction and amplification of the hormonal signal. However, this is mostly documented for auxins and no coherent picture is emerging from our present knowledge of the mechanisms by which hormones control plant development. The characterization of receptors for plant hormones is still in its infancy in spite of many descriptions of different proteins or membrane fractions able to bind hormones. Progress in this domain is likely to be linked to a reappraisal of some basic aspects of the methodologies, to the characterization of functional responses associated with hormone binding and to the use of more appropriate biological systems, including mutants affected in their sensitivity to hormones. The developmental regulation of plant hormone receptors, and more specifically the hypothesis that the amount of receptors could be controlled by the hormonal levels, is one of the most exciting questions for the next few years.