Cysteine and methionine contribute differentially to regulate alternative oxidase in leaves of poplar (Populus deltoides x Populus euramericana 'Nanlin 895') seedlings exposed to different salinity.

The effects of different doses of NaCl on the expression profiles of genes involved in the mitochondrial electron transport chain (miETC), H2O2 and O2- levels, and antioxidant enzymes and amino acid metabolism were investigated in the leaves of poplar (Populus deltoides x Populus euramericana 'Nanlin 895'). In the miETC, complexes II and III and bypasses of the cytochrome c pathway including AOX and UCP displayed higher transcript abundance, whereas COX6b encoding cytochrome c oxidase were suppressed at 200 and 400 mM. H2O2 accumulated at 200 mM NaCl but O2- was generated at 400 mM. Accordingly, CAT was enhanced at 200 and 400 mM, while G-POD strengthened only at 400 mM. In addition, cysteine was reduced at 400 mM but did not change at 200 mM, although methionine was accumulated at 200 mM but not altered at 400 mM. Exogenous cysteine accumulated H2S and methionine increased ACC at 200 mM NaCl. At 400 mM NaCl, cysteine elevated the expression of CGS encoding cystathionine gamma-synthase and MS2 encoding methionine synthase as well as ACC and H2S levels, and methionine increased ACC content with repressed CGS and MS2. Moreover, exogenous KCN decreased cysteine levels, with an augment in H2S and up-regulation of CYS C1 encoding β-cyanoalanine synthase at all salinity conditions, whereas antimycin A (AA) and salicylhydroxamic acid (SHAM) affected neither the levels of cysteine or H2S, nor the CYS C1 expression. However, neither KCN, AA nor SHAM affected ACC content. AOX1b was induced both by exogenous cysteine and methionine as well as KCN and AA but suppressed by SHAM at 200 and 400 mM NaCl, in negative correlation with MDA content. These results suggest that poplar leaf evolved diverse strategies in amino acid metabolism of manipulating the AOX pathway to defend against different levels of salt stress.