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Botrytis cinerea grows symptomlessly in host plants: how?
C. J. EMMANUEL (1), J. A. van Kan (2), M. W. Shaw (3). (1) Department of Botany, University of Jaffna, Jaffna, Sri Lanka; (2) Laboratory of Phytopathology, Wageningen University, Wageningen, Netherlands; (3) School of Agriculture, Policy and Development, University of Reading, Reading, United Kingdom

<i>Botrytis cinerea</i> also causes symptomless infection: the fungus has been isolated from the apparently healthy plants.  We wished to understand the nature of this symptomless infection. <i>A. thaliana</i> and lettuce were inoculated with spore dust at rosette growth stage and four leaves stage respectively.  Plants were sampled at different time interval and sections plated out on selective medium. The amount of fungal DNA and the relative mRNA concentrations of selected virulence genes were compared between symptomless and necrotic samples.  Symptomless samples had 100 to 1000 times less <i>B. cinerea </i>DNA than necrotic samples. In both host species, the mRNA concentration of signalling genes <i>Bcg1</i>, <i>Bmp1</i> and calcineurin, and the pathogenicity genes <i>Bcsod1</i>and <i>Bcpg1 </i>were similar or slightly greater in symptomless samples than in necrotic samples. The mRNA of signalling gene <i>Ba</i>c and pathogenicity genes <i>Bcbot1</i> and <i>Bcnep1</i>, were not detected or detected in lower abundance in symptomless<i> A. thaliana</i>. In lettuce, the leaves developing distant from the site of inoculation showed similar results to <i>A. thaliana</i>, but in healthy leaves close to the site of inoculation mRNA concentrations of <i>Bac</i> and <i>Bcnep1</i> were similar to necrotic samples. Thus, in both host species, the fungus grew along with the plant and moved to newly growing plant parts without producing symptoms; during this growth some pathogenicity genes were less expressed than in necrotic infection.

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