Sulphur supply and infection with Pyrenopeziza brassicae influence L-cysteine desulphydrase activity in Brassica napus L.

Different field surveys have shown that sulphur (S) fertilization can increase the resistance of agricultural crops against fungal pathogens. The mechanisms of this sulphur-induced resistance (SIR) are, however, not yet known. Volatile S compounds are thought to play an important role because H2S is toxic to fungi. A field experiment was conducted to analyse the influence of S fertilization and the activity of H2S-releasing enzymes on fungal infections. Two levels of N and S fertilizers and two varieties of oilseed rape were investigated with respect to their potential to release H2S by the enzymatic activity of L-cysteine desulphydrase (LCD) and O-acetyl-L-serine(thiol)lyase (OAS-TL). LCD releases H2S during cysteine degradation, while OAS-TL consumes H2S during cysteine synthesis and free H2S is only released in a side reaction. All plots of the field trial showed an infection with Pyrenopeziza brassicae and leaf disc samples were taken from visibly infected leaf areas and apparently uninfected areas to investigate the reaction to the infection in relation to the treatments. Different S fractions and the activities of LCD and OAS-TL were measured to evaluate the potential to release H2S in relation to S nutrition and fungal infection. S fertilization significantly increased the contents of total S, sulphate, organic S, cysteine, and glutathione in the plants, but decreased LCD activity. Infection with P. brassicae increased cysteine and glutathione contents, as well as the activity of LCD. Therefore crops were able to react to a fungal infection with a greater potential to release H2S, which is reflected by an increasing LCD activity with fungal infection.