Ethylene Biosynthesis in A. thaliana
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Ethylene Biosynthesis in A. thaliana

Ethylene (C2H4) is a Plant hormone involved in a large number of developmental processes including Ripening of Fruit, Abscission, Senescence and Responses to Wounding. Ethylene Biosynthesis is best understood among the plant hormone pathways (Ref.1). In higher plants, Ethylene is produced from L-Methionine. Carbons C3 and C4 of Methionine is activated by ATP to form Adomet (S-Adenosylmethionine) through the Catalytic activity of SAM (S- Adenosylmethionine Synthetase).  The Methionine can be recycled again involving several steps. Adomet formed from Methionine is fragmented to give MTA (5`-Methylthioadenosine) and ACC (1-Aminocyclopropane-1-Carboxylic Acid). MTA is hydrolyzed to MTR (5-Methylthioribose) and Adenine, and phosphorylation of MTR by MTR kinase yields MTR-1P (5-Methylthioribose-1-Phosphate). Loss of the phosphate group of MTR-1P, concurrent with the rearrangement of the Ribose Carbon atoms, leads to the synthesis of KMB (2-Keto-4-Methylthiobutyrate). In the final step of the sequence, KMB is converted to Methionine via transamination. The overall result of this cycle is that the Ribose moiety of ATP (Carbons 2`, 3`, 4`, and 5`) furnishes the 4-Carbon moiety of Methionine, and the CH3S group of Methionine is conserved for continued regeneration of Methionine. Recycling of MTA to Methionine allows for high Ethylene production rates without a required increase in the concentration of intracellular Methionine (Ref.2).

Ethylene formation from Adomet involves two specific steps. The first step produces the non-protein Amino Acid ACC. It is catalyzed by ACC Synthase (1-Aminocyclopropane-1-Carboxylate Synthase) with Pyridoxal Phosphate acting as a Co-factor. Formation of ACC is the rate-limiting step in Ethylene biosynthesis. ACC Synthase is encoded by a medium-size multigene family. In Arabidopsis, there are at least six genes that encode ACC Synthases. Two of them, ACS1 (1-Aminocyclopropane-1-Carboxylate Synthase-Like Protein-1) and ACS3 (1-Aminocyclopropane-1-Carboxylate Synthase-Like Protein-3), appear to be pseudogenes, whereas the other four are differentially regulated by a variety of biotic and abiotic factors. ACS2 (1-Aminocyclopropane-1-Carboxylate Synthase-Like Protein-2) mRNA is up regulated in young leaves and flowers and during lateral root formation. In contrast, ACS6 (1-Aminocyclopropane-1-Carboxylate Synthase-Like Protein-6) expression is induced by ozone treatment. ACS5 (1-Aminocyclopropane-1-Carboxylate Synthase-Like Protein-5) and ACS4 (1-Aminocyclopropane-1-Carboxylate Synthase-Like Protein-4) are regulated by other hormones; ACS5 is the main determinant of the increase in Ethylene production resulting from low concentrations of Cytokinin. Similarly, ACS4 is induced by Auxin, and several Auxin-Responsive Elements have been found in the promoter region of this gene. The regulation of the ACS4 and ACS5 genes takes place at different levels: activation of ACS5 by Cytokinins is mainly posttranscriptional, whereas the Auxin induction of ACS4 occurs at the level of transcription. ACC, once produced, can then follow one of two fates: it can be inactivated by conjugation to form Malonyl- or Glutamyl-ACC, or can be oxidized to produce the bioactive hormone Ethylene.  Production of Ethylene from ACC is catalyzed by ACO (ACC Oxidase). This reactions is Oxygen-dependent. At Anaerobic conditions Ethylene formation is completely suppressed. Fe2+ is a co-factor and Ascorbate a cosubstrate; CO2 was shown to activate ACC oxidase. ACC oxidases are encoded by small gene families in plants. Under normal conditions ACO is in excess and the amount of substrate, ACC, is rate-limiting. However, expression of the ACO genes, like that of the ACS genes, is affected by several different factors (Ref.3 and Ref.4).

Ethylene synthesis is also rapidly evoked in response to a variety of biotic and abiotic stresses as well as hormones. Auxin has been shown to stimulate Ethylene formation in various plant tissues. An asymmetric distribution of Auxin results in elevated Ethylene synthesis at the site of higher Auxin concentration. High levels of Ethylene in turn inhibit growth on that side resulting in curvature of the hypocotyl. In maturing fruits, Ethylene synthesis is autocatalytically enhanced, i.e., Ethylene induces its own biosynthesis. The self-enhancing synthesis and diffusion of the gaseous hormone throughout the fruit accelerate ripening and contribute to a synchronized ripening process. ABA (Abscisic Acid) and Brassinosteroids also regulate Ethylene synthesis, although their modes of action are less clear. In Arabidopsis, ABA does not influence Ethylene biosynthesis under normal circumstances, but has a negative effect in mutant plants that overproduce Ethylene. Similarly, in other plant species, increased production of Ethylene in response to the hormone IAA (Indole-3-Acetic Acid) or drought is attenuated by ABA. In these cases ABA decreases Ethylene production by decreasing the activity of ACO and by increasing ACC inactivation by conjugation (Ref.5). The mechanism of Brassinosteroid-mediated enhancement of Ethylene production is unknown. Brassinosteroids may affect the rate of transcription of an ACS gene. Hormones, such as Auxin and Cytokinin, can influence Ethylene responses by affecting the activity of the ACS and, therefore, the rate of Ethylene biosynthesis as a whole. Although the ACO is usually in excess, in some tissues and under certain developmental conditions the levels of this enzyme clearly affect the amount of Ethylene production. Thus, the regulation of ACO by several hormones and developmental factors also contributes to the overall rate of Ethylene biosynthesis. Differential regulation of multiple ACS and ACO genes by interacting hormones in a tissue- and developmental stage-dependent manner is an example of the complexity of the hormonal communication at the level of Ethylene biosynthesis. In O3 (Ozone)-exposed plants, Ethylene synthesis is a result of the specific activation of ACC Synthase and ACC Oxidase genes and is required for O3 damage. O3 degrades into various ROS (Reactive Oxygen Species) in the Apoplast, O3 does not cross the plasma membrane. These ROS include O2- (Superoxide Anion) and H2O2 (Hydrogen Peroxide). Ethylene synthesis and functional signaling are required for either making cells competent for O2- synthesis, or potentiates it in some other way. Specific Ethylene inhibitors such as Silver and high concentration of CO2 can inhibit its production and Action. Regulation of Ethylene biosynthesis by different hormonal and developmental signals provides an excellent model for the study of hormone interactions at the biosynthetic level (Ref.6).