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Chapter 17

Biosynthesis of Hormones
and Elicitor Molecules

CHAPTER OUTLINE
Introduction
17.1 Gibberellins
17.2 Abscisic acid
17.3 Cytokinins
17.4 Indole-3-acetic acid
17.5 Ethylene
17.6 Brassinosteroids
17.7 Polyamines
17.8 Jasmonic acid
17.9 Salicylic acid
17.10 Prospects

Alan Crozier
Yuji Kamiya
Gerard Bishop
Takao Yokota

 

 

Plant hormones are signal molecules, present in trace quantities. Changes in hormone concentration and tissue sensitivity mediate a whole range of developmental processes in plants, many of which involve interactions with environmental factors. This chapter is concerned with the biosynthetic, catabolic, and conjugation pathways that together control the homeostasis of plant hormone pools. Attention will be focused on the first five plant hormones discovered— gibberellins, abscisic acid, cytokinins, indole-3-acetic acid, and ethylene— as well as on compounds shown more recently to have a regulatory role in plant development, namely, brassinosteroids, polyamines, jasmonic acid, and salicylic acid (Fig. 17.1). Each of these compounds has its own particular properties, so the pathways regulating their production and degradation are quite diverse and have been elucidated by synergistic use of many disciplines, including chemistry, biochemistry, plant physiology, genetics and, more recently, molecular genetics. The most powerful tool has been the use of mutant plants that are unable to catalyze an enzymatic step leading to the formation or degradation of a bioactive hormone. Such mutants have been invaluable in hormone analysis studies and in phenotypic rescue experiments using intermediates in the biosynthetic pathway. They also provide the essential material for cloning the genes that encode the biosynthetic enzymes. In addition, inhibitors of plant hormone biosynthesis have been used to dissect biosynthetic reactions for which relevant mutants have not yet been identified. More recently, transgenic technology has provided insights into pathways by altering the expression of genes involved in the biosynthesis or catabolism of hormones. Furthermore, plants expressing transgenic constructs in situations in which the promoters of the biosynthetic genes are linked to reporter genes have been used to visualize the specific tissues where bioactive hormones are produced. In summary, each hormone biosynthetic pathway has been elucidated by using similar techniques but with biases toward certain methodologies according to the nature of the biosynthetic pathway and the availability of mutants.


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