Chemistry of Plant Hormones

Nobutaka Takahashi

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780849354700
• eBook ISBN: 9781351461177
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

The chemistry of the five principal plant hormone groups is discussed in detail in this volume. Contributing authors review history and occurrence of each hormone group, methods of isolation and detection, biosynthesis and metabolism, and structural determination. Through these analyses, the authors clarify the role of endogenous plant growth regulators in the life cycle of higher plants. The text is supplemented with over 350 figures and structures of various plant hormones.

Chapter 1

INTRODUCTION

Nobutaka Takahashi

The life cycle of higher plants is regular, though complex. Each stage, together with the shift to the next stage, is controlled by endogenous plant growth regulators. The situation is further complicated by the need for the life cycle to accomodate to environmental conditions such as light intensity, daylength, humidity, and nutritional conditions. Such responses to environmental change are believed to be due to the quantity and availability of endogenous plant hormones and other bioactive substances. In this sense the life cycle and its sensitive adjustment to outside stimuli may be due to the mediating role of such compounds, which thus fulfill a most important role.

At this stage we know of many kinds of plant growth regulators associated with a wide variety of physiological functions in higher plants. In many cases these have been isolated and chemically characterized. Among the most important of these are the so-called plant hormones (phytohormones).

The concept of plant hormones differs substantially from that of hormones in animals and insects, because the differentiation of organ tissues in plants is less extensive than in animals. Plant hormones can be broadly defined as follows: (1) they must be chemically characterized and shown to be biosynthesized in some plant organ, (2) they must be broadly distributed within the plant kingdom, (3) they must show specific biological activity in very low concentration and must be shown to play a fundamental role in regulating physiological phenomena in vivo, and (4) they are usually translocated within the plant from a biosynthesis site to an action site.

At present, five groups of plant growth regulators — auxins, gibberellins, cytokinins, abscisic acid, and ethylene — are regarded as plant hormones; however, the distinction between plant hormones and plant growth regulators other than plant hormones is not always easily seen. At this stage one new group of compounds that regulate plant growth, brassinolide and related compounds, must be considered as true plant hormones on account of their wide distribution in the plant kingdom and their unique biological activity. A less clear cut case is that of “florigen”, the hypothetical flower-inducing hormone. Logical as the hypothesis may be, the evidence to support the existence of such a hormone is incomplete and it has never been isolated. Despite this, it is logical to expect that various new plant hormones will be isolated and chemically characterized in the future; the wide variety of physiological function required to maintain the complicated life cycle of the higher plants requires that a considerable complexity of chemical control should exist.

Quite apart from these compounds that might be considered plant hormones, there are many natural products that show interesting physiological activity in the higher plants. In general these compounds are of limited distribution in the plant kingdom and show a rather restricted range of activities. They have been isolated not only from plant tissues but also as metabolites of microorganisms. They can be grouped according to their origin (plant, microorganism) and physiological activity as follows:

•Plant growth regulators of plant origin

1. Plant growth promotors (Figure 1)

Brassinolide and related compounds: brassinolide, castasterone, dolicholide, 6-deoxocastasterone, 2-deoxocastasterone, etc.

Strigol (germination stimulant from witchweed)

Phaseolic acid

FIGURE 1. Structures of plant growth regulators of plant origin.

Dihydroconiferyl alcohol (synergist of gibberellin)

2. Plant growth inhibitors (Figure 2)

Benzoic acid and related compounds

Cinnamic acid and related compounds

Phenolic compounds including flavonoids

Unsaturated lactones: scoporetin, parasorbic acid, psoraen, seselin, etc.

Growth inhibitors isolated from Podocarpus: podolactones A—E, inumakilactones A— D, ponalactone A and its glucoside, hallactones A, B, sellowins A—C, nagilactones A—G, podolide, etc.

Momilactone and related compounds: momilactones A, B, annonalide Jasmonic acid and related compounds: jasmonic acid and its methyl ester, cucurbic acid and its glucoside

Asparagusic acid and related compounds: asparagusic acid and its sulfoxide, dihydroasparagusic acid, S-acetyldihydroasparagusic acid, etc.

Poly-yne-ene compounds: matricariaester, 2-(Z)-dehydromatricariaester, methyl 2-(Z)-decene-4,6-diynoate, etc.

Batatacins: batatacins I, II, III

Rooting inhibitors from Eucalyptus: G-regulators, G1, G2, G3, grandinol

Growth inhibitors in liverwort and algae: lunularic acid and related compounds

Growth inhibitors from bulbs of Lycoris radiata: lycoricidinol and lycoricidine

Others: harrintonolide, methyl pheoborides, juglone, 3-acetyl-6-methylbenzaldehyde, lignans with germination inhibitory activity, 4,8,13-duvatriene-1,3-diol

•Plant Growth Regulators of Microbial Origin

1. Plant growth promotors (Figure 3)

Helminthosporol and related compounds from Helminthosporium sativum: helminthosporoland cis-sativendiol, etc.

FIGURE 2. Structures of plant growth inhibitors.

Sclerin and related compounds from Sclerotinia spp.: sclerin, sclerotinins A, B

Malformins from Aspergillus spp. (malformation inducing substances): malformins A₁, A₂, B₁, B₂, C

Cotylenol and related compounds from Clodosporium sp.: cotylenol, cotylenins A—F

Radiclonic acid from penicillium sp.

Synergist to gibberellins from Pestalotia crytmeraecola and other unidentified fungi: pestalotin and related compounds

FIGURE 3. Structures of plant growth promotors of microbial origin.

FIGURE 4. Structures of host-specific phytotoxins.

2. Plant growth inhibitors and phytotoxins

Host-specific toxins (Figure 4) have been isolated mainly from Helminthosporium and Alternaria spp. and have been shown to be used for host recognition by plant pathogens: AM-toxins I—III (Alternaria mali), AK-toxin (A. kikuchiana), HMT-toxins (Helminhosporium madys race T), HC-toxin (H. carbonum), toxin from A. alternata F. sp. cycopersici.

FIGURE 5. Structures of non-specific phytotoxins.

Nonspecific toxins (Figure 5) involve compounds with a wide variety of structures and cause very divergent symptoms on host plants. Some examples follow.

a. Terpenoids: diacetoxysciprenol (Fusarium equiseti), aphidicolin (Harziella entomophilla), ophiobolins (Ophiobolus and Helminthosporium spp.), Fusicoccins (Fusicoccum amygdali).

b. Other carbocyclic compounds: phyllosinol (epoxidon) and related compounds (Phyllosticta sp.).

c. Aromatic and heteroaromatic compounds: piryculol (Pyricularia oryzae), fusaric acid (Gibberella fujikuroi, Fusarium, and Nectria spp.), α-picolic acid (Pyricularia oryzae), tenuazoic acid (P. oryzae, Alternaria longipes).

d. Amino acids and peptides: rhizobitoxine and dihydro derivative (Rhizobium japonicum), coronatine (Pseudomonas coronafaciens), lycomarasmins (Aspergillus flavus and oryzae), tentoxin (Alternaria spp.), phaseotoxin (Pseudomonas phaseolicola), tabtoxin (Pseudomonas tabaci).

Clarification of the mechanism for the regulation of the life cycle in higher plants has been one of the most important areas of research in plant physiology. An ideal approach to such research should include all the following aspects: (1) isolation and characterization of endogenous compounds responsible for the physiological phenomenon; (2) exogenous application to other species to check for the appropriate phy...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Introduction
• Chapter 2 Auxins
• Chapter 3 Gibberellins
• Chapter 4 Cytokinins
• Chapter 5 Abscisic Acid
• Chapter 6 Ethylene
• Index