High ethylene level impedes amino acid biosynthesis in rice grains

Ethylene (ETH) is a gaseous plant growth regulator that affects grain yield and quality in rice (Oryza sativa L.). Little is known about whether and how ETH regulates amino acid contents in milled rice. In this study, three categories of rice cultivars differing in protein content were field grown and the relationship of ethylene evolution rate (EER) and its synthetic precursor (1-aminocylopropane-1-carboxylic acid, ACC) in filling grains with amino acid biosynthesis were investigated. The results showed that rice cultivars with higher levels of ETH and ACC exhibited lower amino acid contents than the others. The EER and ACC content were highly significantly and negatively correlated with the activities of glutamate synthetase (GOGAT), aspartate transaminase (AST) and alanine transaminase (ALT) involved in amino acid biosynthesis, and the contents of essential amino acids (EAAs), non-essential amino acids (NEAAs), and total amino acids (TAAs), serine (Ser), alanine (Ala), lysine (Lys), threonine (Thr) and leucine (Leu) in milled rice. The application of ethephon (an ethylene-releasing agent) or ACC to rice panicles markedly reduced the activities of the above three enzymes and contents of amino acids, especially Ser, Ala, Lys, Thr and Leu, whereas the use of aminoethoxyvinylglycine (AVG, an inhibitor of ACC synthase) had the opposite effect. These results suggest that high ETH level negatively affects amino acid biosynthesis mainly by inhibiting the activities of GOGAT, AST, and ALT during the grain filling in rice.