Induction of phytoalexins biosynthesis in orange fruit by the biocontrol yeast Rhodotorula glutinis

The biocontrol yeast Rhodotorula glutinis, isolate 21A, obtained from tomato fruit was used to control Penicillium digitatum, P. italicum and Botrytis cinerea on artificially wounded citrus fruit. Orange and satsuma mandarin fruit were treated with the biocontrol yeast, inoculated with the pathogens and stored for 7 days at 23 degrees C. On orange fruit the antagonist compared to the control reduced decay by 92.2, 88.4 and 96.2% for P. digitatum, P. italicum and B. cinerea, respectively. On satsuma mandarin fruit the same pathogens were inhibited by 96.2, 91.2 and 90.0%, respectively. Scanning electron microscope observations, focusing on the antagonist-pathogen interactions, revealed a fast colonization of the growing fungal mycelia by the yeast cells, but no sign of lytic activity on hyphae was observed. Moreover, the fruit accumulated the phytoalexins scoparone and scopoletin into artificial wounds previously treated by the yeast and either inoculated or uninoculated with the pathogen. The concentration of scoparone, which showed higher accumulation in fruit tissues, varied significantly in relation to the time lag between the application of the antagonist and the inoculation with the pathogen. In particular, the concentration of scoparone 4 days after application of the sole yeast was 69.0 microg x g(-1) fresh weight (FW), 6.3 times higher than in the uninoculated wounded tissues (11.0 microg x g(-1) FW). The phytoalexin accumulation was low (13.0 microg x g(-1)FW) applying the yeast jointly with P. digitatum into wounds, while it increased consistently (74.0 microg x g(-1)FW) when the antagonist was applied 24 h before the pathogen.