INTERACTIONS BETWEEN THE BIOCONTROL AGENT PSEUDOMONAS-FLUORESCENS CHA0 AND THIELAVIOPSIS-BASICOLA IN TOBACCO ROOTS OBSERVED BY IMMUNOFLUORESCENCE MICROSCOPY

Pseudomonas fluorescens CHAO protects plants from damage caused by sev eral soilborne fungi. In this work, immunofluorescence microscopy was used to investigate the colonization of tobacco roots by CHAO and its physical relationship with the black root rot fungus Thielaviopsis bas icola. The pseudomonad colonized the rhizoplane shortly after planting of tobacco seedlings in sterile soil microcosms, in which it had been introduced as soil inoculant. CHAO was found between and inside cells in the epidermis and the cortex, as well as in the xylem vessels, wit hin 4-7 days after planting of seedlings. The presence of CHAO delayed the colonization of the interior of tobacco roots by T. basicola comp ared with the treatment in which only the fungus had been inoculated. Likewise, the pseudomonad reduced the extent of black root rot from 82 % to 28%. However, CHAO was seldom found in contact with the mycelium of T. basicola or in its vicinity, indicating that direct colonization of the mycelium of T. basicola by CHAO was not required for protectio n of tobacco against black root rot. Overall, the results suggest that the interior of the root is a key site for implementation of the stra in's biocontrol activity against soilborne plant-pathogenic fungi.