Xanthomonas vasicola pv. musacearum down-regulates selected defense genes during its interaction with both resistant and susceptible banana

Expression of specific R, NPR1 and PR genes was investigated to ascertain their role in the resistance of Musa balbisiana to Xanthomonas vasicola pv. musacearum (Xvm). Following inoculation, wilt symptoms developed in test plants though their incidence and severity climaxed merely at 30% and 20% respectively before recovery. Xvm was limited to the inoculated and the immediate follower leaves and its population in a 50 mg tissue dropped from 3.06 × 107 cfu at 18 days to 0 cfu by 150 days after inoculation (DAI). In susceptible banana, it instead increased from 3.41 × 108 to 1.08 × 1011 cfu by 21 DAI culminating in plant death. Expression of MbNBS, MdNPR1 and PR3 sharply declined during the first 6 h, recovering in the later stages though this did not stop later wilt symptoms developed in resistant and susceptible plants. We concluded that neither hypersensitive response, nor Systemic Acquired Resistance nor Induced systemic resistance is the resistance mechanism for M. balbisiana against Xvm as it suppresses key genes in these pathways.