Populations of plants are subject to attack by pathogens and herbivores at all stages of their life cycles. Pest pressure may be devastating, as illustrated by the outbreaks of insects apd diseases in cultivated and native plants (121, 173, 178, 198, 329) and in the success of weed control by biotic instruments (6, 141, 338, 340). Epidemics or episodes of intensive herbivory may lead directly to the local extinction of a plant species, or may weaken plants and make them vulnerable to subsequent pressures. Loss of photosynthetic capacity due to disease or partial defoliation lowers resistance to abiotic stresses or to infection by other pests, and reduces competitive ability (198, 240, 270). Defense against pathogens and herbivores may be secured by many different mechanisms; it may depend on the texture and composition of the plant surface (156, 182, 196, 242), the presence of anatomical structures such as thorns or resin ducts (123, 199, 229), the absence of nutrients required by the pest (140), the presence of hormonelike substances that affect the development of insects (335), unsuitable pH or osmotic pressure (18, 199, 229), or the accumulation of secondary products (47, 92, 169, 298). These products are diverse, ranging from alkaloids to terpenes; phenolics; and steroidal, cyanogenic, and mustard oil glycosides. Some (e.g. phenolics) are ubiquitous in seed plants, others (e.g. alkaloids) occur in several thousand species and others (e.g. cyanogenic glycosides) occur in only a few hundred species (61). Secondary products may be sequestered within cells or organelles therein and released only when the cells are broken. They also may be stored in and secreted by epidermal glands which are sessile or on trichomes (19, 69, 156, 182, 187, 269). Typically, glandular exudates contain only one class of compound, but this class varies among species. The glandular exudates usually are terpens, alkaloids, res-

The Chemical Defenses of Plants to Pathogens and Herbivores

PERSPECTIVES AND SUMMARY 1006 INTRODUCTION 1007 PROLINE 1008 Proline Biosynthesis: The Glutamate Pathway 1008 Proline Biosynthesis: The Om Pathway 1015 Proline Degradation: The P5C Pathway 1018 Proline Degradation: Reguiation 1020 Proline Degradation: Other Pathways 1023 4-HYDROXYPROLlNE 1024 Natural Occurrence 1024 Metabolism General Features 1025 Catabolism of Free 4-Hyp 1034 3-HYDROXYPROLINE 1041 Metabolism 1042 3,4-DIHYDROXYPROLINE 1044

Metabolism of Proline and the Hydroxyprolines

The recent progresses in the research on proline will be described, focusing on plants and covering proline metabolism and signal transduction as well as the role of this imino acid in stress response. Furthermore, the recently described developmental role of proline in flowering and reproduction will be illustrated and discussed.

Multiple roles of proline in plant stress tolerance and development

Publisher Summary This chapter reviews the recent work on the regulatory functions of proline and pyrroline-5-carboxylate and presents a synthesis of these functions within the framework of biologic regulation. As a redox couple, proline and pyrroline-5-carboxylate provide a mechanism for the inter-compartmental and intercellular transfer of redox potential. The transfer of redox potential alters the ratio of NADP + /NADPH, thereby activating certain metabolic pathways. Although the reduction of pyrroline-5-carboxylate is the central mechanism in the transfer of redox potential, the metabolic interconversions of proline, ornithine, and glutamate with pyrroline-5-carboxylate as the obligate intermediate can also play a role. The end point of this regulation is the formation of purine ribonucleotides by both salvage and de novo mechanisms. Proline and pyrroline-5-carboxylate are metabolic signals that can be fine-tuned by humoral factors to coordinate the metabolism of amino acids and ribonucleotides. When the transfer is from cell to cell, proline and pyrroline-5-carboxylate can function as intercellular communicators.

The regulatory functions of proline and pyrroline-5-carboxylic acid.

Removal of Feedback Inhibition of Δ1-Pyrroline-5-carboxylate Synthetase, a Bifunctional Enzyme Catalyzing the First Two Steps of Proline Biosynthesis in Plants

Proline synthesis in Escherichia coli a proline-inhibitable glutamic acid kinase

Control of proline synthesis in Escherichia coli.

Glutamic acid biosynthesis in an organism lacking a Krebs tricardoxylic acid cycle

Die Beeinflussung der Prolinsynthese inEscherichia coli durch Azetidin-2-carbonsaure wird untersucht.

Annette Baich

Papers

Proline synthesis in Escherichia coli a proline-inhibitable glutamic acid kinase

Control of proline synthesis in Escherichia coli.

Glutamic acid biosynthesis in an organism lacking a Krebs tricardoxylic acid cycle

The effect of azetidine-2-carboxylic acid on the synthesis of proline in Escherichia coli.

Physiological control of nicotinic acid biosynthesis in Aerobacter aerogenes.