The availability of the genome sequence of the bacterial plant pathogen Xylella fastidiosa, the causal agent of citrus variegated chlorosis, is accelerating important investigations concerning its pathogenicity. Plant vessel occlusion is critical for symptom development. The objective of the present study was to search for information that would help to explain the adhesion of X. fastidiosa cells to the xylem. Scanning electron microscopy revealed that adhesion may occur without the fastidium gum, an exopolysaccharide produced by X. fastidiosa, and X-ray microanalysis demonstrated the presence of elemental sulfur both in cells grown in vitro and in cells found inside plant vessels, indicating that the sulfur signal is generated by the pathogen surface. Calcium and magnesium peaks were detected in association with sulfur in occluded vessels. We propose an explanation for the adhesion and aggregation process. Thiol groups, maintained by the enzyme peptide methionine sulfoxide reductase, could be active on the surface of the bacteria and appear to promote cell-cell aggregation by forming disulfide bonds with thiol groups on the surface of adjacent cells. The enzyme methionine sulfoxide reductase has been shown to be an auxiliary component in the adhesiveness of some human pathogens. The negative charge conferred by the ionized thiol group could of itself constitute a mechanism of adhesion by allowing the formation of divalent cation bridges between the negatively charged bacteria and predominantly negatively charged xylem walls.

A levedura Saccharomyces cerevisiae estimula o acúmulo de fitoalexinas e tem potencial para ser utilizada como agente de controle alternativo no tratamento de doenças fúngicas em sorgo. São descritos aqui os procedimentos iniciais para a purificação de elicitores de fitoalexinas em sorgo, os quais são extraídos das células da levedura S. cerevisiae por autoclavagem, indicando serem termoestáveis. Após precipitacão com etanol, em concentrações finais de 50 e 80%, as moléculas elicitoras permanecem em solução. O acúmulo de fitoalexinas nos mesocótilos é mais elevado quanto maiores os teores de proteínas das amostras elicitoras.

The yeast Saccharomyces cerevisiae stimulates phytoalexin accumulation and is a potential agent for biological control of fungal diseases in sorghum. The present investigation establishes the initial steps to purify elicitor molecules of phytoalexins in sorghum from S. cerevisiae. These molecules are extracted using heat and remain in solution after ethanol precipitation. They are active even after autoclaving, thus showing to be thermostable. A correlation between phytoalexin accumulation in mesocotyls and increasing amounts of protein on elicitor samples was observed.

O trabalho foi desenvolvido com o objetivo de se verificar a influência de Saccharomyces cerevisiae na forma de produto comercial, fermento biológico fresco para panificação, na atividade e no padrão eletroforético da peroxidase em folhas de milho e sorgo. As preparações de Saccharomyces utilizadas foram representadas por suspensões de células e seus respectivos filtrados, autoclavados ou não. A análise dos extratos obtidos dessas gramíneas mostrou que suspensões de células e os filtrados dessas suspensões, tratadas termicamente ou não, foram efetivos em ocasionar alterações na atividade e padrão eletroforético da peroxidase em milho e sorgo. Além de se mostrarem termoestáveis, os extratos obtidos de células intactas ou homogeneizadas de S. cerevisiae apresentaram baixa atividade de peroxidase, indicando que as alterações na atividade e perfil eletroforético da enzima nas plantas eram decorrentes do próprio tecido vegetal. Finalmente, os resultados sugerem que as alterações ocorridas com a peroxidase nas gramíneas estudadas, em resposta ao tratamento com S. cerevisiae, refletem o "reconhecimento" das células ou metabólitos da levedura por parte das plantas, acarretando uma alteração no metabolismo normal das mesmas.

The aim of this work was to test the effect of Saccharomyces cerevisiae, in a commercial baker's yeast form, on the activity and changes in the electrophoretic pattern of peroxidase in maize and sorghum leaves. The yeast treatment included spray of cell suspensions and their filtrates, autoclaved or not, which were able to cause changes in the peroxidase activity and electrophoretic pattern in maize and sorghum. Besides being thermostable, the extracts from intact or homogenized S. cerevisiae cells exhibited low peroxidase activity, indicating that the changes in the enzyme, activity and pattern, were due only to the plant tissue. Finally, the results suggest that the changes in peroxidase in maize and sorghum leaves in response to the treatment with S. cerevisiae, indicate a possible recognition of the yeast cells or their metabolites by these plants.