Detection and cellular localization of elemental sulphur in disease-resistant genotypes of Theobroma cacao

DISEASE-RESISTANT genotypes are the basis for controlling many major microbial pathogens of economic plants. Resistance is often linked with organic, antimicrobial phytoalexins, produced de novo in cells surrounding apoptotic or 'hypersensitive' cells that die rapidly after contact with incompatible pathogens(1-3). Here we report the production by resistant genotypes of Theobroma cacao of four phytoalexins in response to a xylem-invading fungal pathogen; these comprised two phenolics, a triterpenoid and, highly unusually in a higher eukaryote, elemental sulphur as cyclooctasulphur S-8. Energy dispersive X-ray microanalysis revealed a high accumulation of sulphur only in cells and structures in potential contact,vith the vascular pathogen; that is, xylem parenchyma, xylem vessel cell walls and gels occluding vessels. Our data provide a rare example of cellular localization of an antimicrobial substance and evidence for the first time for accumulation of elemental sulphur in a plant Linked with a resistance response; this discovery comes centuries after man first used elemental sulphur as a potent fungicide(4,5).