Apoplastic peroxidases enable an additional sulphite detoxification strategy and act as first line of defence upon exposure to sulphur containing gas

SO2 poses severe stress on plants due to sulphite formation in the apoplastic space leading to cell death and necrosis. Therefore, sulphite detoxification is of utmost importance in plants and sulphite concentration is strictly controlled. This study used laboratory fumigation as well as naturally occurring SO2 from fumaroles to provide new insights into sulphite detoxification. To elucidate, if guaiacol peroxidases act as first line of defence, Arabidopsis thaliana wild-type, sulphite oxidase knockout, as well as peroxidase 34 knockout lines were fumigated with 600 nL L−1 SO2 and analysed for gene expression levels, enzymatic activities and phytohormone contents. Arabidopsis sulphite oxidase knockout mutants displayed strong up-regulation of the genes peroxidase33 and peroxidase34 after fumigation, followed by increased guaiacol activity. Apparently, loss of the key enzyme sulphite oxidase is compensated through increased guaiacol peroxidase activity. Recombinant expression of purified Prx34-twin-strepII in Nicotiana benthamiana confirmed sulphite oxidising capacity of guaiacol peroxidases on the molecular level. Using plant material collected on the Aolian Islands with naturally occurring SO2/ H2S containing volcanic gases, the important role of guaiacol peroxidases as first line of defence for sulphite detoxification was highlighted under field conditions.