ISOLATION AND CHARACTERIZATION OF AN ENDO- PHYTE FROM ACTINIDIA SP. SHOWING A STRONG ANTAGONISTIC ACTIVITY AGAINST PSEUDOMONAS SYRINGAE PV. ACTINIDIAE

The bacterial canker of kiwifruit is the most destructive disease of cultivated Actinidia spp. The causal agent is the Gram negative bacterium Pseudomonas syringae pv. actinidiae (Psa). The pathogen grows inside the trunk, vines and leaves of its host plants, causing wilting and formation of cankers, with subsequent death of the plant. During the past three years, samples have been collected from various parts of asymptomatic Actinidia spp. within infected kiwi orchards and many endophytic bacteria have been isolated. Among them, a few isolates were identified and proved to be able to strongly inhibit Psa namely an isolate of Pantoea agglomerans, two pseudomonads belonging to the fluorescens/putida group, and one novel Pseudomonas sp., which proved to be highly effective in inhibiting, in vitro, several important phytopathogenic bacteria. This last isolate was chosen for further studies. Concentrated supernatant of its liquid culture in LB medium could inhibit Psa, indicating that the active compound produced by the antagonist is excreted from the cell into the environment. The nature of that biotoxin is not proteic, and its size is smaller than 3 kDa, as a result of its partial purification. Moreover, a bank of mutants deficient in their antagonistic activity has been constructed by triparental mating, in order to search and identify genes involved in antagonism against Psa. Fifty six prospective mutants have been already obtained: twenty two of them were subject to Southern blotting to verify a single transposon insertion. Gene identification is under way.