Regulation and diversity of polyamine biosynthesis in plants

In higher plants, the main route for putrescine synthesis may be from arginine. Higher accumulation of putrescine under biotic and abiotic stress conditions is attributed at least in part to stress-inducible expression of arginine decarboxylase genes. Regulation of spermidine synthesis involves translational control of the S -adenosylmethionine decarboxylase mRNA, which is responsive to cellular polyamine levels. Genes encoding S -adenosylmethionine decarboxylase and spermine synthase are also responsive to environmental stimuli or show preferential expression in certain organs. Stress-inducible expression of these genes involves plant hormone signaling of abscisic acid (ABA) or methyl jasmonate (MeJA). Thermospermine synthase is widely distributed in the plant kingdom but has a unique role in the repression control of xylem differentiation in higher plants. Expression of the gene for thermospermine synthase is under negative feedback control by thermospermine. Putrescine, spermidine, and cadaverine are also essential precursors for alkaloid biosynthesis in some plant species. Polyamines occur in various aspects of plant growth as a conjugated form with cinnamic acids and proteins. Genes responsible for these conjugations have been increasingly identified.