From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by Aminotransferases During Biosynthesis of a Major Plant Growth Hormone

Ethylene biosynthesis originates from three amino acids: aspartate, cysteine, and methionine. In the aspartate-derived amino acid pathway, the ethylene pathway requires no less than seven aminotransferases that connect the metabolisms of nitrogen (N), sulfur (S), and carbon (C). Aminotransferases are fundamental enzymes in plants involved in N, S, and C shuttling through their implication in amino acid biosynthesis and catabolism. The role of these enzymes in the biosynthesis of hormones such as ethylene and auxins (IAA and PAA) is frequently overlooked. The functioning of aminotransferases is dependent on an essential cofactor: pyridoxal-5′-phosphate (PLP). This phosphorylated form of vitamin B6 is synthesized from glutamine, the first product of N assimilation produced by the GS/GOGAT cycle after reduction of nitrate and the glyceraldehyde 3-phosphate (G3P) and ribose 5-phosphate (5RP) provided by the glycolytic and pentose phosphate pathways, respectively. Here we review the recent progress in characterization of the aspartate-derived metabolic pathway with a particular focus on methionine biosynthesis and its salvage pathway (Yang cycle) related to ethylene and polyamine biosynthesis. Emphasis is placed on the key role of aminotransferases in regulating these pathways and their relation with aromatic amino acid biosynthesis and catabolism. Indeed, the promiscuity of certain aminotransferases extends their catalytic function and gives them a key role in the metabolism of ethylene, IAA, and aromatic amino acids. In this respect, recent studies have identified specific aminotransferases as being the main targets involved in the root morphogenetic program in response to environmental cues, nutrient availability, and energy status. Thus, genetically engineered plants for some aminotransferases, such as ACC synthase and tryptophan aminotransferase, demonstrate a great potential to produce crop species with enhanced exploratory root growth and a better nitrogen use efficiency. How the network of aminotransferases is involved in nitrogen-sensing systems such as plant glutamate receptors, TOR, and GCN2 kinases is now becoming a fundamental issue. The use of specific and nonspecific inhibitors of the catalytic activity of certain aminotransferases should help future pharmacological and genetic approaches to unravel their role in N, S, and C sensory systems.