Endophytic bacterial communities in the periderm of potato tubers and their potential to improve resistance to soil-borne plant pathogens

To evaluate whether the location of bacterial endophyte communities contrib utes to disease resistance in potato tubers (Solanum tuberosum), the popula tion density, biodiversity and antibiotic activity of endophytic bacteria w as examined from the tuber peel (periderm plus top 3 mm of tissue) of four cultivars (Russet Burbank, Kennebec, Butte and Shepody). There were no sign ificant differences for population density of bacteria among the layers of peel examined and no cultivar x peel layer interaction. Endophytic bacteria from several layers of peel were challenged in in vitro bioassays to the s oil-borne plant pathogens Fusarium sambucinum, Fusarium avenaceum, Fusarium oxysporum and Phytophthora infestans (mating types A1 and A2). In general, antibiosis of bacterial endophytes against these pathogens was significant ly higher (P = 0.01) in isolates recovered from the outermost layer of tube r peel and decreased progressively toward the centre of the tuber. Antibios is against P. infestans was variable, with a progressive decrease in antibi otic activity from outer to inner layers of peel occurring in cvs Russet Bu rbank and Kennebec only. For antibiosis there were significant cultivar x p eel, and cultivar x pathogen interactions (P = 0.01). In all cases the inhi bitory activity of endophytic bacteria was significantly greater (P = 0.01) against the A1 than the A2 mating type of P. infestans. In four of seven c ases, where the same species of bacteria were recovered from all three peel layers, antibiosis to pathogens decreased significantly (P = 0.01)with dep th of recovery (from the periderm to inside the tuber), indicating that in certain communities of endophytic bacteria, defence against pathogens may b e related to bacterial adaptation to location within a host and may be tiss ue-type and tissue-site specific.