Elemental sulphur as an induced antifungal substance in plant defence

Man’s oldest fungicide has probably long functioned in this role in plants, as a natural component of induced antifungal defence. Elemental sulphur (S⁰) is the only inorganic phytoalexin and the only phytoalexin produced by so many different taxa. S⁰ (detected by GC-MS as ³²S₈ ) is produced in representative spp. of Sterculaceae (cacao), Solanaceae (tomato, tobacco), Malvaceae (cotton) and Leguminosae (French bean) in response to xylem-invading fungal and bacterial pathogens. Production was more rapid and intensive in disease resistant genotypes. Gene expression for S⁰ production may be xylem-specific as S⁰ was not present in leaves of six spp. undergoing hypersensitivity to Pseudomonas syringae. Anomalously, high constitutive S⁰ levels occurred in leaves of Arabidopsis and Brassica oleracea. S⁰ was highly toxic (ED50 0.8–3 μg/ml) to many fungal pathogens representing ascomycetes, basidiomycetes and deuteromycetes, but not to an oomycete, Phytophthora, or to bacteria. Levels in tomato xylem and Arabidopsis leaves were potentially inhibitory, but in other interactions were below theoretically toxic concentrations. However, S⁰ accumulation is highly localised, suggesting the element is produced in sufficient amounts, at the right time and place to be effective. SEM-EDX revealed S in tomato and cacao xylem walls, xylem parenchyma and vascular gels, all sites appropriate to counter vascular pathogenic Verticillium dahliae. Transient increases in sulphate, glutathione and cysteine occurred in tomato xylem. The sulphate may reflect over-expression of sulphate transporters, but the thiols might be possible precursors. Analysis of differential gene expression should reveal what may be a novel biosynthetic pathway of S⁰ formation in eukaryotes.