Summary
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.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.0028-646X.2001.00281.x

Plant hormones and the control of physiological processes

1 Xylem and Phloem Differentiation in Perspective.- 1.1 Introduction.- 1.2 Procambium Initiation in Embryos.- 1.3 General Mechanisms of Programming.- 1.4 Quiescent Center.- 1.5 Quantal Mitosis and Differentiation.- 1.6 Multiple Gene Copies and Xylogenesis.- 1.7 Procambium to Cambium.- 1.8 Maintenance and Extension of the Vascular System.- 2 Hormonal Aspects of Vascular Differentiation.- 2.1 Some Characteristics of Plant Hormones.- 2.2 Auxins.- 2.3 Cytokinins.- 2.4 Gibberellins.- 2.5 Ethylene.- 2.6 Polyamines.- 2.7 Inhibitors.- 2.8 Summary.- 3 Vascular Differentiation Within the Plant.- 3.1 Major Problems.- 3.1.1 In Vivo Versus In Vitro Systems.- 3.1.2 The Difficulty of Observing the Phloem.- 3.1.3 Primary Phloem Structure and the Zero-Day Control.- 3.2 Control of Vascular Differentiation by Auxin Flow.- 3.3 Role of Cytokinin in Vascular Differentiation.- 3.4 Relation Between Phloem and Xylem Differentiation.- 3.5 Graniferous Tracheary Elements.- 3.6 Control of Conduit Size and Density.- 3.7 Sites of Xylem Initiation and Rates of Maturation in Relation to Conduction.- 3.8 Role of Gibberellin and the Control of Fiber Differentiation...- 3.9 Transfer Cells.- 3.10 Vascular Adaptation.- 4 Evidence from Wound Responses and Tissue Cultures.- 4.1 Rationale for Using Wounds and Tissue Cultures to Study Vascular Differentiation.- 4.2 Coleus Stem Wounds and Cultured Internodal Explants.- 4.3 Root and Shoot Wounds of Pisum.- 4.4 Vascular Differentiation in Other Wound Systems.- 4.5 Induced Xylogenesis in Lactuca Pith Parenchyma.- 4.6 Xylogenesis in Explants from Storage Organs.- 4.7 Differentiation in Suspension Cultures Including the Zinnia System.- 4.8 Cell Wall Metabolism During Xylem Differentiation.- 4.9 Some Final Thoughts on Tissue Culture Systems.- 5 Physical Factors, Hormones, and Differentiation.- 5.1 Identification of the Physical Factors Influencing Vascular Differentiation.- 5.2 Temperature.- 5.3 Water.- 5.4 Light.- 5.5 Gases.- 5.6 Mechanical Stress.- 5.7 Acidity.- Epilogue.- References.- Appendix: Abbreviations.

Vascular differentiation and plant growth regulators

The review will focus on recent advances in our understanding of the action of auxin on plant cell elongation Particular emphasis will be placed on the acid growth theory of auxin action and a comparison of auxin action on shoot vs root tissues Theories of the action of auxin will be evaluated in view of recent information on auxin dose/response relationships, auxin uptake kinetics, the time‐dependent adaptation of tissues to auxin, and the kinetics of plant cell responses to the hormone Considerations of research on auxin binding and auxin polar transport will be included only as they apply to the central question of the action of the hormone on cell expansion

The action of auxin on plant cell elongation.

Conjugates of abscisic acid, brassinosteroids, ethylene, gibberellins, and jasmonates

Plant hormones are an unusual group of secondary plant constituents playing a regulatory role in plant growth and development The regulating properties appear in course of the biosynthetic pathways and are followed by deactivation via catabolic processes All these metabolic steps are in principle irreversible, except for some processes such as the formation of ester, glucoside and amide conjugates, where the free parent compound can be liberated by enzymatic hydrolysis For each class of the plant hormones so-called ‘bound’ hormones have been found In the early literature this term was applied to hormones bound to other low-molecular-weight substances or associated with macromolecules or cell structures irrespective of whether structural elucidation had been achieved

Plant hormone conjugation

Throughout this chapter the terms hormone and growth regulator will be used interchangeably. The designation “hormone” was originally restricted by definition to compounds that are produced naturally and exert their action in tissues or cellular localities removed from the point of synthesis. Both terms are, however, widely used in the current literature on plant growth regulation and development to cover both natural and synthetic compounds.

M. A. Venis

Papers

Molecular and subcellular aspects of hormone action.