Candidate Resistance Gene Discovery: Resistance Gene Analog Characterisation and Differential Gene Expression Analysis in Musa- Mycosphaerella Host-Pathogen Interactions

Many banana cultivars (Musa spp.)are sterile triploids or diploids, evolving only via somatic mutation. As a consequence, this crop can lack resistance to pests and diseases. Numerous disease resistance genes (R-genes)have been characterised in plants, conferring resistance to bacteria, viruses, fungi and nematodes. Identification and cloning of R-genes in Musa will provide new opportunities for genetic improvement. Our group has identified over 50 distinct NBS-LRR-type resistance gene analogs (RGAs)in the resistant wild diploid Musa acuminata ssp. burmannicoides 'Calcutta 4'. Characterisation is ongoing in M. acuminata cultivars contrasting in resistance to Mycosphaerella leaf spot diseases, focusing on both the NBS-LRR R-gene family and cytoplasmic receptor-like kinases (RLKs)with extracellular LRRs. NBS-LRR class RGA probes applied to 'Calcutta 4', 'Grande Naine' (AAA, Cavendish subgroup), and M. balbisiana 'Pisang Klutuk Wulung' BAC libraries have revealed many putative resistance loci. Sequence data for such selected clones will provide insight into organisation and evolution of this R-gene class in Musa. Candidate gene discovery is also ongoing via analysis of differential gene expression from infected leaf cDNA during Musa-Mycosphaerella interactions. Candidate R-genes will be applicable for banana genetic improvement via both plant transformation and conventional breeding using marker-assisted selection.