Plant Growth and Development

Hormones and Environment

Lalit M. Srivastava

• Publisher: Academic Press
• Year: 2002
• eBook ISBN: 9780080514031
• Topic: Technology & Engineering
• Subtopic: Botany
• Index: Technology & Engineering

This book provides current information on synthesis of plant hormones, how their concentrations are regulated, and how they modulate various plant processes. It details how plants sense and tolerate such factors as drought, salinity, and cold temperature, factors that limit plant productivity on earth. It also explains how plants sense two other environmental signals, light and gravity, and modify their developmental patterns in response to those signals. This book takes the reader from basic concepts to the most up-to-date thinking on these topics.* Provides clear synthesis and review of hormonal and environmental regulation of plant growth and development* Contains more than 600 illustrations supplementary information on techniques and/or related topics of interest* Single-authored text provides uniformity of presentation and integration of the subject matter* References listed alphabetically in each section

SECTION III

HORMONAL REGULATION OF DEVELOPMENTAL AND PHYSIOLOGICAL PROCESSES

INTRODUCTION TO: HORMONAL REGULATION OF DEVELOPMENTAL AND PHYSIOLOGICAL PROCESSES

… many—perhaps all—growth phenomena are the result of interaction and balance between several hormones, …

(Kenneth Thimann, 1977, p. 203)

All plant hormones have multiple roles in plant growth or defense, i.e., they are pleiotropic in their effects. To this basic theme, a layer of complexity is added by the fact that two or more hormones bring about the same, or similar, responses. For example, cell division is mediated by cytokinins (CKs), auxins, or gibberellins (GAs). Cell enlargement involves action by auxin, GA, brassinosteroids (BRs), or ethylene. Abscisic acid (ABA), ethylene, and jasmonates (JAs) are involved in the ability of a plant to cope with biotic or abiotic stresses. This redundancy is a hallmark of plant development, although it is not clear whether it is real or only apparent, i.e., two hormones regulate closely related but different aspects of the same process.

Plant hormones also regulate some activities that are specific to each hormone. For instance, patterning in embryo development and polar phenomena such as apical dominance, vascular differentiation, and tropic growth under the influence of light or gravity are principally regulated by the endogenous auxin, indoleacetic acid or IAA; mobilization of seed food reserves in cereal grains is specific to GAs; seed dormancy is induced by ABA; fruit ripening is associated with ethylene; and JAs are uniquely involved in the deposition of vegetative storage proteins.

The chapters in Section III of this book reflect the apparent redundancy and uniqueness of hormone action. They also show that even relatively simple processes, such as apical dominance or production of lateral roots, involve an interaction of two or more hormones and their relative levels, which are affected by environmental and/or developmental cues. Such interaction is often antagonistic, as, for example, IAA and CK interaction in lateral root formation, although it is not clear whether such antagonism is used for regulation of the process in nature. Synergistic interaction also occurs, e.g., for ethylene and JA in induction of some genes in plant defense against pathogens. Finally, several hormones may act in concert, one after another, to regulate a sequence of developmental events. For example, fruit set may be regulated by IAA, fruit growth by GA, fruit ripening by ethylene, and seed maturation and dormancy by ABA. Because of these interactions among homones and between hormones and environmental factors, the extent of which we have only recently begun to appreciate, an understanding of plant hormonal response is a complex and difficult fabric to unentangle.

In the past, plant hormones have been credited with a bewildering array of responses—there is hardly any plant process that has not been attributed to one or another hormone. Such conclusions were often drawn from treatment of whole plant or isolated organs/tissues with exogenous hormones, often at unnaturally high concentrations. These treatments disturb the natural homeostasis and generally do not provide a true indication of the role of a hormone. That role is deciphered better by the use of mutants that are deficient in or are insensitive to a specific hormone or by a defined biochemical response to a hormone. For this reason, not all responses attributed to hormones are covered in these chapters; rather the chapters are selective in that they deal with only the better understood and, hopefully, major responses and processes.

The uptake and transport of hormones in the plant body are covered first in Chapter 13; Chapter 14 deals with IAA and CK interaction and plant responses related to the polar transport of IAA (some others are covered in Section V of this book). Hormonal regulation of cell division and cell enlargement, both important for plant growth, are covered in Chapter 15. Chapter 16 deals with the mechanisms plants use to cope with the main types of abiotic stresses and the role of ABA in induction of those mechanisms. Not surprisingly, hormones that are involved in growth-related processes play a minimal role in stress-related responses, and vice versa. Fruit and seed development and seed germination, covered in Chapters 17, 18, and 19, are growth-related processes in which CKs, IAA, and GAs play important although still little understood roles, whereas fruit ripening and seed maturation and dormancy are culminating phases of growth, akin to senescence, and are regulated by ethylene, ABA, and possibly JAs. Chapter 20 covers several plant processes, accumulation and role of vegetative storage porteins, tuber formation in potato, senescence and abscission of plant parts and organs, and the roles of various hormones in these processes. Finally, Appendix 3 describes the major food reserves and their accumulation in seeds.

Reference

Thimann, K.V. Hormone Action in the Whole Life of Plants. The University of Massachusetts Press; 1997.

CHAPTER 13

Uptake and Transport of Hormones

1 INTRODUCTION

2 CHARACTERISTICS OF UPTAKE AND ACCUMULATION

2.1 Uptake by Diffusion

2.2 Carrier-Mediated Uptake

3 TRANSLOCATION OF HORMONES

3.1 Translocation by Diffusion

3.2 Transport in Xylem or Phloem

4 POLAR TRANSPORT OF AUXIN

4.1 Demonstration of Polar Transport of Indoeacetic Acid

4.2 Directionality of Transport in Shoots and Roots

4.3 Characteristics of Polar Transport

4.4 Polar Transport Is Mediated via Specific Efflux Carriers

4.5 Polar Transport of Different Auxins Is Differentially Regulated by Auxin Influx and Efflux Carriers

4.6 Polar Transport Is Inhibited by Chemicals

4.7 Cloning of the Gene for an Auxin efflux Carrier

4.8 Efflux Carriers Can Be Randomized and Rearranged

5 CHAPTER SUMMARY

REFERENCES

1. INTRODUCTION

Hormones are taken up from the external medium by plant tissues. They are also translocated short or long distances in the plant from their sites of synthesis and/or application. This chapter will first consider the characteristics of hormone uptake and accumulation, then their long-distance translocation in vascular tissues, and finally the polar transport of auxins, which underlies many developmental phenomena in plants as well as their tropic responses to environmental signals.

2. CHARACTERISTICS OF UPTAKE AND ACCUMULATION

Plant hormones are small molecules, which can easily diffuse across cell boundaries in response to their concentration gradient. However, such movement is governed by a complex interaction of several factors, and how uptake occurs in a specific case in vivo is still not fully understood. This section outlines some general concepts on hormone uptake by diffusion and via specific carriers, followed by a discussion on the auxin influx carrier.

Auxins, gibberellins (GAs), abscisic acid (ABA), and jasmonic acid (JA) are weak acids, which in solution dissociate into anions and protons (Fig. 13-1). The pK_a values of IAA, ABA, and JA measured at 20°C are about 4.7-4.8; the pK_a of GA₁ is lower, about 3.85. Accordingly, at neutral pH in a free solution, these hormones exist predominantly in a dissociated form, but at pH closer to the pK_a or below it, the proportion of the undissociated form increases. The situation in planta, however, is not known. Cytokinins (CKs) and brassinosteroids (BRs) occur predominantly as undissociated molecules. Ethylene is nonpolar and gaseous and freely permeable, although its precursor, 1-a...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• List of Reviewers
• SECTION I: SOME SPECIAL ASPECTS OF PLANT GROWTH AND DEVELOPMENT
• SECTION II: STRUCTURE AND METABOLISM OF PLANT HORMONES
• SECTION III: HORMONAL REGULATION OF DEVELOPMENTAL AND PHYSIOLOGICAL PROCESSES
• SECTION IV: MOLECULAR BASIS OF HORMONE ACTION
• SECTION V: ENVIRONMENTAL REGULATION OF PLANT GROWTH
• Index
• Errata for Plant Growth and Development by Lalit M. Srivastava Corrected text is in boldface