Plant hormones and the control of physiological processes

This review examines contemporary views of the role of plant hormones in the control of physiological processes. Past and present difficulties with nomenclature encapsulate the problems inherent in using the ‘classic’ hormone concept in plants, with their distinctive multicellular organization. Chemical control may be a more relevant notion. However, control may also reside in the responding tissue via changes in sensitivity, or as combined control, where response is dictated by both sensitivity and concentration. Criteria for demonstrating these modes of action are reviewed, as well as frameworks for deciding whether hormone transport is involved. Problems of measuring relevant hormone concentrations are discussed. Methods for measuring and comparing tissue sensitivity to hormones are outlined and relative control is introduced as a means of assessing the importance of hormonal control against a background of other influences. While animals and plants appear to have coinherited homologueous intracellular signalling systems, at the whole organism level modes of hormone action may diverge. It is postulated that the synthesis‐transport‐action mechanism of action may be just one of several possible ways that phytohormones could control physiological processes. Twelve separate roles are discussed, and it is suggested that some of these could operate simultaneously to the plant’s advantage. Contents Summary 375 I. Introduction 376 II. The history of the hormone concept in plant systems 376 III. Issues of nomenclature 380 IV. The need for sound conceptual frameworks in plant hormone research 382 V. Development of criteria for chemical control 384 VI. Identification and quantitative analysis of plant hormones 387 VII. Hormone transport in plants 389 VIII. Hormone sensitivity and its quantification 390 IX. Roles of receptors, second messengers and signal amplification in hormone sensitivity changes 393 X. Relative control as a pivotal concept 395 XI. Diversity of physiological roles for chemical influences in plants 397 XII. Concluding remarks 400 Acknowledgements 402 References 402