Polyamines in plants: biosynthesis from arginine, and metabolic, physiological and stress-response roles.

Biogenic amines affect a myriad of growth and developmental processes in all organisms, including plants. Therefore, there is continued interest in understanding the biosynthesis and functional roles of polyamines in regulating plant metabolism, physiology and development. The role of polyamines in plant responses to environmental stresses has also elicited considerable interest in recent years. Four major biogenic amines and their derivatives commonly found in plants are putrescine (Put), spermidine (Spd), spermine (Spm) and thermospermine (T-Spm). The biosynthesis of these polyamines derives from arginine (Arg) and/or ornithine (Orn). This chapter's focus is on the Arg pathway. Genes encoding enzymes in the various steps of polyamine biosynthesis from Arg have been identified and characterized from a number of plants. The enzymes catalysing different steps in Arg pathway are: Arg decarboxylase, agmatine iminohydrolase/agmatine deiminase, N-carbamoylputrescine amidase/N-carbamoyl Put amido hydrolase, S-adenosylmethionine decarboxylase, Spd synthase, Spm synthase and T-Spm synthase. Many of these have been purified and studied using heterologous expression systems. Advanced genetics tools and mutants have been used, including production of loss-of-function and gain-of-function plants. Thus far, the studies have provided unequivocal evidence of polyamine role in diverse aspects of plant life. Characterizations of isogenic mutants varying in their endogenous levels of different polyamines have revealed that polyamines are central to important signalling and metabolic pathways, and affect grain filling, senescence, nodulation, root development and fruit ripening, as well as having a role in the abiotic stress responses of plants.