Domestication affected the basal and induced disease resistance in common bean (Phaseolus vulgaris)

Crop plants exhibit reduced levels of disease resistance, but little is known about the specific resistance mechanisms that are affected by breeding for increased yields. We investigated basal and chemically induced resistance of two wild accessions and four cultivars (including one landrace and three 'modern', yield-improved cultivars that have been produced by hybridisation and pedigree breeding) of common bean (Phaseolus vulgaris) under greenhouse and field conditions. After treatment with benzothiadiazole, a widely used inducer of systemic acquired resistance, plants were challenged with one of two bacterial pathogens (Pseudomonas syringae pv. syringae and Enterobacter sp. strain FCB1). Basal resistance to Pseudomonas in the wild accessions was significantly higher than in the cultivars. Moreover, benzothiadiazole-treatment elevated resistance to the same pathogen in a wild accession and the landrace, but not in the yield-improved cultivars. Similarly, benzothiadiazole-induced resistance to Enterobacter FCB1 was detected in both wild accessions and the landrace, whereas the same treatment enhanced susceptibility to Enterobacter FCB1 in two of the yield-improved cultivars. Basal resistance to Pseudomonas was highly (but negatively) correlated to induced resistance over all accessions, and basal and inducible resistance to Enterobacter FCB1 were negatively correlated for the cultivars, but not when considering all six accessions. Benzothiadiazole-treatment increased growth rates under pathogen pressure of the wild accessions but not the cultivars. Apparently, the yield-improved cultivars investigated here have lost a considerable part of the basal and induced broad-spectrum disease resistance that characterises their wild relatives and to some degree also the landrace. Two of the yield-improved cultivars even became highly susceptible to infection by an Enterobacter strain that has not yet been described as a pathogen of bean and that is likely to represent a common environmental or phyllosphere bacterium. Future studies should disentangle the effects of domestication on the various layers of plant resistance to pathogens and consider the potential of wild accessions and landraces for future breeding programmes.