Higher polyamines restore and enhance metabolic memory in ripening fruit

Polyamines are ubiquitous, biogenic amines that have been implicated in diverse cellular functions in most living organisms. Ever since spermine phosphate crystals were isolated over three centuries ago, scientists have kept busy in unraveling the mystery behind biological roles of spermine and other known polyamines, viz., putrescine and spermidine. Although the pathway of polyamine biosynthesis has been elucidated, the molecular basis of their in vivo function is far from being understood. Molecular biology tools have provided a promising avenue in this direction, with success achieved in altering endogenous polyamines in plants by over-expression and knock-out of the genes responsible for polyamine biosynthesis. Such transgenic material has become a good genetic resource to learn about the biological effects of polyamines and their interaction with other signaling molecules. Interestingly, engineered accumulation of higher polyamines, spermidine and spermine, in tomato in a fruit-specific manner restored metabolic activity even at late developmental stages of fruit ripening, reviving cellular programs underlying N:C signaling, energy and glucose metabolism. Along with these, a wide array of genes regulating transcription, translation, signal transduction, chaperone activity, stress proteins, amino acid biosynthesis, ethylene biosynthesis and action, polyamine biosynthesis, isoprenoid pathway and flavonoid biosynthesis was activated. Based on various reports and our results, we suggest that polyamines act as ‘surrogate messengers’ and nudge other signaling molecules, such as plant hormones and NO, to activate a vast genetic network to regulate growth, development and senescence.