Soilborne pathogen resistance and agronomic performance of tobacco DMR6 mutant lines and hybrids

Induction of deleterious mutations in pathogen susceptibility genes may provide a route for increasing levels of disease resistance in cultivated tobacco (Nicotiana tabacum L.). In other species, mutations in DMR6 genes have been demonstrated to have the effect of increasing salicylic acid levels with corresponding activation of systemic acquired resistance. In this research, gene editing was used to create deleterious mutant alleles of DMR6 genes at two loci in the tobacco genome. In anticipation of potential adverse effects such as reduced plant productivity caused by mutations in members of this gene family, different genetic combinations of mutant alleles were established in five recipient genotypes, including those with genetic potential for increased leaf number. Relative to wild-type control lines, double homozygous DMR6 mutants exhibited significantly improved and impressive resistance to black shank and bacterial wilt caused by economically important soilborne pathogens Phytophthora nicotianae and Ralstonia solanacearum, respectively. This mutant combination was associated with late-season physiological leaf spotting prior to harvesting, however, resulting in significantly reduced cured leaf yields. Improved resistance of single homozygous mutants was inconsistent across recipient genotypes. Double heterozygous mutant genotypes exhibited greater levels of soilborne pathogen resistance but did not exhibit the leaf spotting characteristic. Although slightly lower yielding than wild-type genotypes, this genetic combination may offer a satisfactory tradeoff for tobacco breeders when considering both improved pathogen resistance and yielding ability. Strategic temporal or spatial modification of DMR6 expression might be pursued to achieve maximal pathogen resistance with minimal corresponding adverse effects in this species.