Polyamine Interactions with Plant Hormones: Crosstalk at Several Levels

Polyamines play important roles in diverse plant growth and development processes, including seed germination, tissue lignification, organogenesis, flowering, pollination, embryogenesis, fruit development, ripening, abscission, senescence, and stress responses. In all these processes, synergistic and antagonistic interactions among various plant hormones have been shown. Although significant progress has been made in understanding the regulation of biosynthesis and signal transduction mechanisms for most of the plant hormones, inroads into the molecular mechanisms underlying polyamine (PA) action have just begun. Little is known or understood about the signal transduction pathways regulating a myriad of PA effects. We, therefore, embarked on evaluating and collating the information on changes in the transcriptome based on metabolic engineering of the PA pathway as well as mutants of PA biosynthesis. Also taken into consideration are the studies using exogenous application of PAs. Our analysis has revealed complex and differential relationships among the three main PAs—putrescine, spermidine and spermine—in regard to regulation of plant hormone biosynthesis and signaling. Putrescine was positively linked to the expression of genes regulating ABA biosynthesis but downregulated those of ethylene, jasmonates, and gibberellin biosynthesis, and the action of spermidine was found to be exactly opposite. Spermine, on the other hand, enhanced genes for ethylene and jasmonates biosynthesis but downregulated those for gibberellins and abscisic acid biosynthesis. In regard to hormone signaling pathways, spermidine positively regulated salicylic acid signaling genes, and those of auxin and cytokinins signaling genes were linked to spermine action. Putrescine was neutral to positive in regulating jasmonates signaling. PAs seem to be neutral in regard to brassinosteroids biosynthesis or signaling pathways.