The role of amino acid metabolism in signaling and metabolic adaptation to stress-induced energy deficiency in plants

The adaptation of plant metabolism to stress-induced energy deficiency involves profound changes in amino acid metabolism. Anabolic reactions are suppressed, whereas respiratory pathways that use amino acids as alternative substrates are activated. This review highlights recent progress in unraveling the stress-induced amino acid oxidation pathways, their regulation, and the role of amino acids as signaling molecules. We present an updated map of the degradation pathways for lysine and the branched-chain amino acids. The regulation of amino acid metabolism during energy deprivation, including the coordinated induction of several catabolic pathways, is mediated by the balance between TOR and SnRK signaling. Recent findings indicate that some amino acids might act as nutrient signals in TOR activation and thus promote a shift from catabolic to anabolic pathways. The metabolism of the sulfur-containing amino acid cysteine is highly interconnected with TOR and SnRK signaling. Mechanistic details have recently been elucidated for cysteine signaling during the abscisic acid-dependent drought response. Local cysteine synthesis triggers abscisic acid production and, in addition, cysteine degradation produces the gaseous messenger hydrogen sulfide, which promotes stomatal closure via protein persulfidation. Amino acid signaling in plants is still an emerging topic with potential for fundamental discoveries.