Use of Solanum stenotomum for introduction of resistance to bacterial wilt in somatic hybrids of potato

The wild tuber-bearing relative Solanum stenotomum was used as source of resistance to bacterial wilt caused by Ralstonia solanacearum. In order to transfer resistance, somatic hybrids between a dihaploid clone of potato (S. tuberosum) cv. BF15 and S. stenotomum were produced by electrofusion of mesophyll protoplasts. A total of thirty hybrid plants were regenerated. When transferred to the greenhouse, they exhibited a strong vigour and showed morphological intermediate traits, including leaf form, flowers and tuber characteristics. DNA analysis using flow cytometry revealed that 25 were tetraploids (4×; 48 chromosomes), three hexaploids (6×; 72) and two aneuploids (< 4×; 48). Their hybrid nature was confirmed by examining isoenzyme patterns for esterases, and analysis of DNA simple sequence repeat (SSR) markers. Analysis of chloroplast (ct) DNA microsatellites of fourteen somatic hybrids revealed that six hybrids possessed ctDNA of S. stenotomum, and eight contained S. tuberosum ct-type. Six tetraploid hybrid clones were evaluated for resistance to bacterial wilt by using race 1 and 3 strains of R. solanacearum, originating from Reunion Island. Inoculations were performed by an in vitro root dipping method. The cultivated potato was susceptible to both bacterial strains and died within a few days. Interestingly, all somatic hybrids tested showed a resistance level as high as that of the wild species.