The domestication of tomato (Solanum lycopersicum) has led to a massive increase in fruit size but at the cost of decreased sugar levels, a negative correlation that is probably the consequence of a loss of high-sugar alleles. In a study published in Nature, Zhang et al. engineer a sweet tomato that maintains its high yield and weight by mutating a gene that encodes a protein that regulates sugar accumulation.
Zhang et al. performed a genome-wide association study of both wild and cultivated tomato species to search for genomic regions that are associated with fruit sweetness. This analysis identified six genes, of which only one showed lower levels of expression in sweeter tomatoes. Functional characterization of this gene, known as SlCDPK27, as well as its paralogue SlCDPK26 showed that both of them encode kinases that are increasingly expressed during fruit ripening, at which stage they reduce sugar content by phosphorylating the sucrose synthase SlSUS3 (which promotes degradation of this enzyme). The authors showed that CRISPR–Cas9-mediated double knockout of SlCDPK27 and SlCDPK26 increases the sugar content of a commonly cultivated tomato variety by up to 30% without affecting fruit size.
