FUNCTIONAL CHARACTERIZATION OF POLYKETIDE-DERIVED SECONDARY METABOLITES SOLANAPYRONES PRODUCED BY THE CHICKPEA BLIGHT PATHOGEN, ASCOCHYTA RABIEI: GENETICS AND CHEMICAL ECOLOGY
MetadataShow full item record
Ascochyta rabiei causes Ascochyta blight of chickpea. The pathogen is known to produce polyketide-derived secondary metabolites, solanapyrones A, B and C, of which solanapyrone A has long been considered a key virulence factor in the chickpea-A. rabiei interaction, due to its phytotoxicity to chickpea. In order to determine the role of solanapyrones during infection process, solanapyrone-minus mutants were generated from A. rabiei strains of different pathotypes by targeting sol5 and sol4 genes, which encode the last step enzyme and a pathway-specific regulator for solanapyrone biosynthesis, respectively. In addition, phytotoxicity of solanapyrones was examined with various legumes including chickpea plants with varying degree of resistance to the disease. As a result, purified solanapyrone A showed a broad spectrum of phytotoxicity, causing necrotic lesions on all tested plants. The resulting solanapyrone-minus mutants were equally virulent as their corresponding wild-type progenitors, indicating that solanapyrone A is neither a host-selective toxin nor a virulence factor of A. rabiei. Despite the dispensability in parasitic growth of the pathogen, solanapyrones are produced by all strains investigated in this and several previous studies. The universal production of solanapyrones prompted us to examine possible ecological roles other than parasitism.Synteny analyses of A. rabiei genome with genomes of related species revealed that the solanapyrone biosynthesis gene cluster was placed in a ‘genomic island’, where genes implicated in niche adaptation are often found. The expressions levels of the solanapyrone cluster genes were high during saprobic growth, but negligible during infection processes. Also, the gene expressions culminated at the formation of pycnidia, indicating that solanapyrones are produced in a growth and tissue-specific manner. To investigate ecological roles of solanapyrones, wild-type strains or solanapyrone-minus mutants were co-cultured with saprobic fungi that have been isolated from chickpea debris left in a field. Wild-type strains effectively suppressed the growth of the saprobic fungi, whereas solanapyrone-minus mutants did not. The half maximal inhibitory concentration of solanapyrone A against the saprobic fungi ranged from 50 to 80 μM. These results suggest that solanapyrone A plays an important role in competition and presumably in survival of the pathogen in nature.