Metabolism and function of polyamines during osmotically induced senescence in oat leaves and protoplasts

Introduction Polyamines (PAs) are biologically ubiquitous aliphatic nitrogencontaining compounds of low molecular weight and polycationic nature. The diamine putrescine (Put) and the triamine spermidine (Spd) are probably synthesized by all organisms, while eukaryotes contain the tetraamine spermine (Spm) as well (Cohen, 1971). In plants, PAs are metabolically related to the basic amino acids arginine and ornithine and therefore also to glutamic acid, a key intermediate in nitrogen metabolism (Tiburcio, Kaur-Sawhney & Galston, 1990). The biosynthetic and degradative pathways for the formation of Put, Spd and Spm in plants are now well established; the key enzymes have been characterized, but their regulation at molecular level is still obscure (Tiburcio et al , 1990; Galston & Tiburcio, 1991). Suggested roles for PA function, and the evidence for these functions, has recently been reviewed (Tiburcio et al . 1993). These include membrane stabilization, free radical scavenging, effects on DNA, RNA and protein synthesis, effects on the activities of RNase, protease and other enzymes, the interaction with ethylene biosynthesis, and effects on second messengers. It was concluded that, in addition to interacting with plant hormones, PAs are able to modulate plant development through (a) fundamental mechanism(s) common to all living organisms (Tiburcio et al , 1993a). This chapter deals with the study of PA metabolism and function during osmotically induced senescence in Avena sativa L. (oat) leaves and protoplasts.