Submitted to: Annual Plant Reviews
Publication Type: Review article
Publication Acceptance Date: 4/26/2011
Publication Date: 2/2/2012
Citation: Harpaz-Saad, S., Yoon, G.M., Mattoo, A.K., Keiber, J.J. 2012. The formation of ACC and competition between polyamines and ethylene for SAM. Annual Plant Reviews. 44:53-81. Interpretive Summary: Ethylene is a gaseous plant hormone that is a pro-senescence and pro-ripening regulator in plants. It is synthesized from an important substrate called S-adenosylmethionine (SAM). SAM is also a donor of one methyl carbon for critical reactions in living cells. It is used as a substrate also for the synthesis of regulators that are antagonistic to senescence and ripening , which are known as polyamines. How the two pathways are regulated in plants is of immense interest because this knowledge would help in tweaking the metabolic program in plants such that cells could be shifted from growth to senescence or in delaying the program of senescence or fruit ripening. This invited review summarizes our knowledge in the field to date and presents explanations for the contradictory conclusions in the literature. It is also suggested that apart from developmental and environmental regulation of SAM homeostasis, any competition for SAM in plant cells is likely of a transient, signaling nature, which switches on a homeostatic mechanism that maintains a certain threshold of intracellular SAM. This information is of wide interest to plant biologists, biotechnologists, physiologists and horticulturists.
Technical Abstract: Ethylene biosynthesis involves the conversion of S-adenosylmethionine (SAM) to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase (ACS). ACC is then converted to ethylene. The genes that encode enzymes in this pathway all belong to a family of genes. Differential transcriptional regulation of nine tomato LeACS genes occurs during fruit ripening. Based on the level of ethylene production during fruit development, two systems of ethylene regulation have been proposed. System 1 is responsible for producing the basal level of ethylene during normal vegetative growth and during non-climacteric fruit development. System 2 refers to the high rate of ethylene production observed during ripening in climacteric fruit and in certain senescent flowers. System 1 ethylene biosynthesis is subjected to negative feedback regulation that reduces ACS gene expression and activity. In the case of system 2, there is a positive feedback regulation such that ethylene increases its own biosynthesis. Elucidation of ethylene biosynthetic pathway in plants brought to light the possibility of a developmental inter-relationship with another class of plant growth regulators commonly called polyamines. Polyamines share with ethylene SAM as a common substrate and a common product, methylthioadenosine. This generated considerable interest in lieu of the opposite functions associated with them, ethylene as a pro-senescence, pro-ripening regulator while polyamines are pro-growth, anti-senescence regulators. It also raised a question about competition for SAM by the two pathways. Two transgenic approaches were used to test this question, which involved shifting the flux from SAM to polyamine accumulation in a transgenic fruit and in a transgenic poplar cell system. It was shown that the two pathways can coexist in a plant cell without limiting SAM levels. It was proposed that polyamine levels and their inter-relationship with ethylene pathway may be developmentally regulated in a tissue- and cell-specific manner. If there is a competition for SAM in plant cells, it is likely of a transient, signaling nature, which switches on a homeostatic mechanism that maintains a certain threshold of intracellular SAM.