Differential effects of nitric oxide on peroxidase and H2O2 production by the xylem of Zinnia elegans

Abstract

IWF, intercellular washing fluid pCMB, p‐chloromercuribenzoic acid SNAP, S‐nitroso‐N‐acetyl‐penicillamine SNP, sodium nitroprusside TMB, 3,3’,5,5’‐ tetramethylbenzidine Sodium nitroprusside (SNP) and S‐nitroso‐N‐acetyl‐penicillamine (SNAP) are two nitric oxide (NO)‐releasing compounds that, when used at 5·0 mol m–3 concentrations, are capable of releasing NO in the aqueous phase at a rate of 35 ± 4 and 47 ± 5 μmol m–3 s–1, respectively. For this reason, the effect of SNP and SNAP on coniferyl alcohol peroxidase and on H2O2 production by the lignifying xylem of Zinnia elegans (L.) has been studied in order to ascertain whether NO, which is a synchronizing chemical messenger in animals and an air pollutant, has any effect on these plant‐specific metabolic aspects. The results showed that both SNP and SNAP provoke an inhibition in the mol m–3 concentration range of the coniferyl alcohol peroxidase activity of a basic peroxidase isoenzyme present in the intercellular washing fluid of Z. elegans. The effect of these NO‐releasing compounds on peroxidase was confirmed through histochemical studies, which showed that xylem peroxidase was totally inhibited by treatment with these NO donors at 5·0 mol m–3, and by NO at a concentration change rate of 55 ± 5 and 110 ± 9 μmol m–3 s–1. However, SNP, at 5·0 mol m–3, does not have any effect on H2O2 production by the xylem of Z. elegans. The fact that SNP and SNAP are two structurally dissimilar compounds which only share the common ability to release NO in aqueous buffer, and that similar results were obtained when using NO itself, suggest that NO could be considered as an inhibitor of coniferyl alcohol peroxidase which does not affect H2O2 production in the xylem of Z. elegans.