Ethylene, ACC, and the Plant Growth-Promoting Enzyme ACC Deaminase

SIMPLE SUMMARY: The molecule 1-aminocyclopropane-1-carboxylate is the immediate precursor of the plant hormone ethylene in most seed plant species. Both 1-aminocyclopropane-1-carboxylate and ethylene can affect plant growth and development in a variety of ways. In addition, the bacterial enzyme 1-aminocyclopropane-1-carboxylate deaminase can cleave 1-aminocyclopropane-1-carboxylate and prevent both 1-aminocyclopropane-1-carboxylate and ethylene from affecting plant gene expression and subsequent behavior. In this review, the roles of 1-aminocyclopropane-1-carboxylate, ethylene and 1-aminocyclopropane-1-carboxylate deaminase in the development of plants and their regulation are explored. Since D-amino acids stimulate ethylene synthesis in plants, a section of the review is dedicated to this topic. It is suggested that 1-aminocyclopropane-1-carboxylate was synthesized in plants prior to the plant synthesis of ethylene, giving 1-aminocyclopropane-1-carboxylate some control of plant gene expression in response to environmental signals. ABSTRACT: Here, a brief summary of the biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) and ethylene in plants, as well as overviews of how ACC and ethylene act as signaling molecules in plants, is presented. Next, how the bacterial enzyme ACC deaminase cleaves plant-produced ACC and thereby decreases or prevents the ethylene or ACC modulation of plant gene expression is considered. A detailed model of ACC deaminase functioning, including the role of indoleacetic acid (IAA), is presented. Given that ACC is a signaling molecule under some circumstances, this suggests that ACC, which appears to have evolved prior to ethylene, may have been a major signaling molecule in primitive plants prior to the evolution of ethylene and ethylene signaling. Due to their involvement in stimulating ethylene production, the role of D-amino acids in plants is then considered. The enzyme D-cysteine desulfhydrase, which is structurally very similar to ACC deaminase, is briefly discussed and the possibility that ACC deaminase arose as a variant of D-cysteine desulfhydrase is suggested.