H₂O₂ homeostasis in wild-type and ethylene-insensitive Never ripe tomato in response to salicylic acid treatment in normal photoperiod and in prolonged darkness

Ethylene proved to be an important modulator of salicylic acid (SA) signalling pathway. Since SA may regulate both the production and scavenging of hydrogen peroxide (H₂O₂), which show light-dependency, the aim of this study was to compare H₂O₂ metabolism in the leaves of SA-treated wild-type (WT) tomato (Solanum lycopersicum L. cv. Ailsa Craig) and in ethylene receptor Never-ripe (Nr) mutants grown in normal photoperiod or in prolonged darkness. H₂O₂ accumulation was higher in the WT than in the mutants in normal photoperiod after 1 mM SA treatment, while Nr leaves contained more H₂O₂ after light deprivation. The expression of certain superoxide dismutase (SOD) genes and activity of the enzyme followed the same tendency as H₂O₂, which was scavenged by different enzymes in the two genotypes. Catalase (CAT, EC 1.11.1.6) activity was inhibited by SA in WT, while the mutants maintained enhanced enzyme activity in the dark. Thus, in WT, CAT inhibition was the major component of the H₂O₂ accumulation elicited by 1 mM SA in a normal photoperiod, since the expression and/or activity of ascorbate (APX, EC 1.11.1.11) and guaiacol peroxidases (POD, EC 1.11.1.7) were induced in the leaves. The absence of APX and POD activation in mutant plants suggests that the regulation of these enzymes by SA needs functional ethylene signalling. While the block of ethylene perception in Nr mutants was overwritten in the transcription and activity of certain SOD and CAT isoenzymes during prolonged darkness, the low APX and POD activities led to H₂O₂ accumulation in these tissues.