The mechanism of resistance to Xanthomonas vasicola pv musacearum explored in Musa balbisiana

Abstract

Banana Xanthomonas wilt caused by Xanthomonas vascicola pv musacearum (Xvm) devastates banana in East and Central Africa causing total yield loss in diseased plants. Previous germplasm screening experiments has identified M. balbisiana, a wild banana type, as an important Xvm resistance source but with exact resistance mechanism still to be defined. This PhD study was therefore aimed to evaluate the possible mechanism. For the study, clean M. balbisiana plantlets were required that were obtained through a newly developed tissue culture process. In the absence of a suitable banana culture medium, the MS proliferation medium was, in a first step, modified by increasing concentrations of the antioxidants ascorbic acid and thiamine-HCl to 20 mg/l and 0.18 mg/l, respectively. In addition, culturing the explants in total darkness for the first 9 weeks after initiation improved plantlet proliferation. Modifications applied enabled production of up to 40 M. balbisiana plantlets per explant. In a further step, multiplication and migration of Xvm in M. balbisiana plantlets following injection was investigated and compared to susceptible banana EAHB cv Nakinyika . Treatment of the youngest open leaf with a bacterial suspension delayed symptom development in M. balbisiana and symptoms only appeared 10 weeks after inoculation. In addition, wilt incidence and severity were only 33% and 20%, respectively, with only the symptomatic leaves, inoculated and the immediate follower leaves, eventually dying. In addition, a general decline in the population of the bacterium in the plant occurred, though the bacterium initially migrated to the leaves that are immediate followers to the inoculated leaf. In a further molecular dissection of the active defense pathway, a sharp decline in transcription of several genes involved in pathogen resistance was found. After inoculation, transcription of MbNBS (an R gene), MdNPR1 (NPR1 gene) and PR3 (a typical PR gene) was below basal transcription in both resistant M balbisiana and susceptible banana EAHB cv Nakinyika indicating that Xvm can limit the banana?s ability to timely detect and set up effective defenses in both resistant and susceptible banana. Gene transcription recovered, however, above basal transcription in later infection stages in resistant M balbisiana, although this did not prevent wilt symptoms from eventually developing. Investigation into M balbisiana resistance also identified heat stable pre-existing compounds soluble in methanol that can inhibit Xvm growth in-vitro. Overall, this PhD study has identified resistance gene expression and formation of secondary compounds as potential contributors to M balbisiana resistance to Xvm.