RHODOTORULA GLUTINIS AND CRYPTOCOCCUS LAURENTII ANTAGONISTS OF PENICILLIUM EXPANSUM: A PRELIMINARY STUDY ON THEIR POSSIBLE MECHANISMS OF ACTION Abstract. The utilization of antagonist yeasts is a promising technology for the reduction of chemicals in the control of postharvest diseases caused by fungal pathogens. The acquisition of information on the mode of action of these microorganisms is an important prerequisite for the improvement of their effectiveness as well as for the assessment of criteria for the screening of new isolates. Two yeasts, Rhodotorula glutinis (LS-11) and Cryptococcus laurentii (LS-28), were previously assayed in experiments involving different pathogens and several fruits and vegetable crops. In most cases isolate LS-28 appeared to be more effective than LS-11. In this paper we compared some possible mechanisms of action of the two isolates. We examined: the interaction of yeast cells with the hyphae of Penicillium expansum, the production of antibiosis, the competition for nutrients and the extracellular beta-1,3-glucanase activity. Microscopic observations showed a clear attachment of LS-11 cells to pathogen’s hyphae. This occurred both with alive and with heat-killed cells, suggesting that the interaction of LS-11 with P. expansum may be mediateti by a thermally stable factor(s). No attachment to the fungal hyphae was observed in the case of LS-28 cells. Competition for nutrients is probably involved in the mechanisms of action of both yeasts, since the addition of exogenous nutrients in artificially wounded apples, treated either with the antagonist and the pathogen, restored the same infection percentages as the control treatments. In our experimental conditions, no antibiosis appeared to be produced by the two yeasts. In fact, in Petri dishes inoculated with isolate LS-11 or isolate LS-28 and P. expansum, no significant inhibition of pathogen's growth was observed. Analogous results were obtained in in vivo experiments since the culture filtrates of the two antagonists did not reduce the P. expansum infection on apple wounds. Both antagonists produced extracellular beta-l,3-glucanase activity, especially in the presence of purified cell walls of P. expansum as a carbon source. On this substrate, in particular, LS-28, the most effective antagonist, was able to produce higher levels of beta-1,3-glucanase activity than LS-11. The latter results parallel those previously obtained by other authors, suggesting a possible correlation between the levels of beta-1 ,3-glucanase activity and the antagonistic behavior of the yeast isolates. The possible involvement of this enzymatic activity in competition for nutrients should also be considered. However, the actual role of this enzymatic activity in in vivo experimental conditions should be assessed. Further investigations are in progress to evaluate the possible involvement of the induction of plant tissue defence processes in the mechanisms of action of the two yeasts.
Sono stati studiati i possibili meccanismi d'azione messi in atto dai lieviti Rhodotorula glutinis (LS-11) e Cryptococcus laurentii (LS-28) antagonisti di Penicillium expansum, responsabile di marciumi su ortofrutticoli in postraccolta. I meccanismi d'azione presi in considerazione sono stati: l'interazione fìsica diretta con il patogeno, l'antibiosi,la competizione per i nutrienti e l'attività 1,3-beta glucanasica. I risultati ottenuti hanno messo in evidenza che le cellule dell'isolato LS-11, contrariamente a quelle dell'isolato LS-28, aderiscono alle ife del patogeno; entrambi gli isolati non sembrano produrre sostanze antibiotiche attive contro P. expansum, mentre la competizione per i nutrienti potrebbe avere, fra i possibili meccanismi d'azione, un ruolo non secondario nell'attività antagonistica di entrambi i lieviti. L'isolato LS-28, inoltre, ha mostrato un'attività beta 1,3-glucanasica più elevata dell'isolato LS-11.
|Titolo:||Studio preliminare sui meccanismi d'azione dei lieviti Rhodotorula glutinis e Cryptococcus laurentii antagonisti di Penicillium expansum|
|Appare nelle tipologie:||1.1 Articolo in rivista|