Hormone Metabolism and Signaling in Plants

Jiayang Li, Chuanyou Li, Steven M Smith

• Publisher: Academic Press
• Year: 2017
• Print ISBN: 9780128115626
• eBook ISBN: 9780128115633
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

Plant Hormones: Biosynthesis and Mechanisms of Action is based on research funded by the Chinese government's National Natural Science Foundation of China (NSFC). This book brings a fresh understanding of hormone biology, particularly molecular mechanisms driving plant hormone actions. With growing understanding of hormone biology comes new outlooks on how mankind values and utilizes the built-in potential of plants for improvement of crops in an environmentally friendly and sustainable manner.This book is a comprehensive description of all major plant hormones: how they are synthesized and catabolized; how they are perceived by plant cells; how they trigger signal transduction; how they regulate gene expression; how they regulate plant growth, development and defense responses; and how we measure plant hormones.This is an exciting time for researchers interested in plant hormones. Plants rely on a diverse set of small molecule hormones to regulate every aspect of their biological processes including development, growth, and adaptation. Since the discovery of the first plant hormone auxin, hormones have always been the frontiers of plant biology.Although the physiological functions of most plant hormones have been studied for decades, the last 15 to 20 years have seen a dramatic progress in our understanding of the molecular mechanisms of hormone actions. The publication of the whole genome sequences of the model systems of Arabidopsis and rice, together with the advent of multidisciplinary approaches has opened the door to successful experimentation on plant hormone actions.- Offers a comprehensive description of all major plant hormones including the recently discovered strigolactones and several peptide hormones- Contains a chapter describing how plant hormones regulate stem cells- Offers a fresh understanding of hormone biology, particularly molecular mechanisms driving plant hormone actions- Discusses the built-in potential of plants for improvement of crops in an environmentally friendly and sustainable manner

1

Hormone function in plants

Steven M. Smith^1,2, Chuanyou Li¹, and Jiayang Li^{1 1}Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing, China ²University of Tasmania, Hobart, Australia

Summary

There are many endogenous signaling and regulatory molecules which can influence the growth, development and physiology of plants. Hormones are produced specifically for signaling. They are often transported from sites of synthesis to distant sites of action and they operate at very low concentrations. In contrast, some other chemicals may provide signals, but it may not be their main function or activity, such as primary metabolites, reactive oxygen species (ROS) and inorganic ions, and they often act locally within individual cells. The hormone family includes auxins, cytokinins (CK), gibberellins (GA), abscisic acid (ABA), ethylene (ETH), brassinosteroids (BR), strigolactones (SL), salicylic acid (SA), jasmonates (JA), and peptides. They are synthesized from common metabolic precursors, but use specialized pathways, and their production is very strictly controlled, both spatially and temporally. All hormones influence multiple aspects of plant function, and they influence the synthesis and actions of each other. The interactions between hormones, environmental signals, and developmental programs are so complex that the description and modeling of the whole system is very challenging. Some hormone receptors are membrane anchored (CK, ETH, BR, and peptides) while others are soluble (auxin, GA, ABA, SA, jasmonic acid (JA), and SL). Co-receptor complexes are formed during perception of hormones including auxin (with IAA/AUX transcriptional repressor proteins), JA (with JAZ transcriptional repressor proteins), and ABA (with phosphoprotein phosphatase PPC2). Hormone perception can lead to signal transduction through protein phosphorylation cascades (e.g., ABA, CK, BR, and peptides). Other hormone-receptor complexes trigger interaction with F-box proteins and ubiquitination enzymes that target proteins such as transcriptional repressors for degradation by the 26S proteasome (e.g., auxin, GA, SA, JA, and SL). Such signaling changes protein activities and gene transcription, with consequent changes to plant development and physiology. The effects of hormones are so profound that through breeding and agrochemical approaches in the 20th century, they gave us high-yielding, nutritious and resilient crops. In the 21st century we look to plant hormones to help meet the increasing demand for food production under ever-more challenging environmental conditions.

Keywords

Cell differentiation; Growth and development; Hormone biosynthesis; Hormone receptors; Plant genetics; Plant hormones; Signal transduction; Transcription; Transport proteins

1.1. The nature of hormones

1.1.1. Hormones and signals

Since the discovery of the auxin indole-3-acetic acid in the 1930s, many endogenous signaling and regulatory molecules have been discovered, and more are yet to be discovered. There is great diversity among this array of signaling molecules, which includes small organic compounds, gases and volatiles, inorganic ions, oligosaccharides, peptides, and RNAs. Some of these substances are produced and act within individual cells, others pass between cells, others are transported between tissues and organs to regulate processes at the whole-plant level and yet others may be released into the environment to influence neighboring plants and other organisms.

These observations raise the important question of how we define plant hormones. It is not easy to define them, and there is no absolute definition. The hormone concept in plants was borrowed from that of animals. A classical and simple definition of a hormone in animals is an organic chemical produced in one organ and transported at very low concentrations to other sites in the animal to regulate specific processes in target tissues. Examples include adrenaline, thyroxine, growth hormone (somatotropin) and the steroidal sex hormones, testosterone and estrogen. There are several aspects of such a definition that do not fit so well for plants. Firstly it is more difficult to identify discrete source organs and specific target tissues. Many such chemical signals are produced throughout the plant and can act locally, as well as distally. Some signaling substances have multiple functions, such as sugars which serve as a source of carbon and energy in addition to signaling, or calcium ions which serve signaling, enzymatic, and structural roles. Some signals such as reactive oxygen species (ROS) and nitric oxide (NO) are probably produced in all cells as an inevitable consequence of conducting metabolic activities in an oxygen-containing environment. Sugars and some other metabolic signals operate at high (mM) concentrations while others such as calcium ions operate at low (μM) concentrations, while hormones typically function at very low levels (nM to pM).

For the present discussion and for the content of this book (Li et al., 2017a), we focus on nine hormones which are all small endogenous organic signaling chemicals, plus a 10th group comprising small peptides involved in the control of plant development (Fig. 1.1). Each of these hormones is discussed individually in more detail in subsequent chapters in this book. They are auxin (Qu et al., 2017), cytokinins (CK) (Feng et al., 2017), gibberellic acid (GA) and gibberellins (Gao et al., 2017), abscisic acid (ABA) (Li et al., 2017b), ethylene (ETH) (Hao et al., 2017), jasmonic acid (JA) and jasmonates (Zhai et al., 2017), salicylic acid (SA) (Jin et al., 2017), brassinosteroids (BR) (Wang et al., 2017a), strigolactones (SL) (Wang et al., 2017b), and peptides (Song et al., 2017). The structures of these hormones (Fig. 1.1) show that they are small organic chemicals, ranging in size from the 2-carbon ETH (C₂H₄) to BR with approximately 25 carbons.

The classical growth hormones are sometimes considered to be auxin, CK, GA, ABA, and ETH, because they have been known and understood for the longest period of time and they have profound effects on plant physiology and development (Davies, 2010). In the past, ABA was considered to be primarily a growth inhibitor and a stress hormone because it acts during exposure of plants to abiotic stress such as water deficit, but it is now known to be also required for many growth and developmental processes (Li et al., 2017b). Similarly JA and SA are often considered to be stress or defense hormones because they act when plants are challenged by pests, pathogens, and environmental stresses. However, it is clear that both JA and SA are also involved in other aspects of plant function (Zhai et al., 2017; Jin et al., 2017). Although BR and SL have been known for several decades, their essential roles in plant growth and development have become recognized now, with SL being identified as a multifunctional plant hormone only between 2000 and 2010 (Wang et al., 2017a,b). Peptide hormones were first recognized for their role in defense against pests but have since been found to participate in symbiotic interactions and plant development (Song et al., 2017). It is now clear that the concept of separate “growth” hormones and “stress” hormones does not hold up, because they all have multiple and diverse roles.

Figure 1.1 Structures of plant hormones. The hormones are: the auxin indole-3-acetic acid (IAA); the cytokinin (CK) trans-zeatin; Gibberellin A₄ which is an oxidized form of gibberellic acid (GA) also denoted as GA₃; Abscisic acid (ABA); Ethylene (ETH); Jasmonyl-isoleucine (JA-Ile) which is a conjugate of jasmonic acid (JA); Salicylic acid (SA); Brassinolide (BL), the first brassinosteroid (BR) to be studied; 5-Deoxystrigol (5DS), a precursor of strigol, both active strigolactones (SL); Systemin, a peptide first identified in ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• About the editors
• Foreword
• 1. Hormone function in plants
• 2. Auxins
• 3. Cytokinins
• 4. Gibberellins
• 5. Abscisic acid
• 6. Ethylene
• 7. Jasmonates
• 8. Salicylic acid
• 9. Brassinosteroids
• 10. Strigolactones
• 11. Peptide hormones
• 12. Plant hormones and stem cells
• 13. Phytohormonal quantification based on biological principles
• 14. Quantitative analysis of plant hormones based on LC-MS/MS
• Author Index
• Subject Index