A novel strategy to produce sweeter tomato fruits with high sugar contents by fruit‐specific expression of a single bZIP transcription factor gene

Summary Enhancement of sugar content and sweetness is desirable in some vegetables and in almost all fruits; however, biotechnological methods to increase sugar content are limited. Here, a completely novel methodological approach is presented that produces sweeter tomato fruits but does not have any negative effects on plant growth. Sucrose‐induced repression of translation ( SIRT ), which is mediated by upstream open reading frames ( uORF s), was initially reported in Arabidopsis Atb ZIP 11 , a class S basic region leucine zipper ( bZIP ) transcription factor gene. Here, two Atb ZIP 11 orthologous genes, Slb ZIP 1 and Slb ZIP 2 , were identified in tomato ( Solanum lycopersicum ) . Slb ZIP 1 and Slb ZIP 2 contained four and three uORF s, respectively, in the cDNA 5′‐leader regions. The second uORF s from the 5′ cDNA end were conserved and involved in SIRT . Tomato plants were transformed with binary vectors in which only the main open reading frames ( ORF s) of Slb ZIP 1 and Slb ZIP 2 , without the SIRT ‐responsive uORF s, were placed under the control of the fruit‐specific E8 promoter. Growth and morphology of the resulting transgenic tomato plants were comparable to those of wild‐type plants. Transgenic fruits were approximately 1.5‐fold higher in sugar content (sucrose/glucose/fructose) than nontransgenic tomato fruits. In addition, the levels of several amino acids, such as asparagine and glutamine, were higher in transgenic fruits than in wild‐type fruits. This was expected because Slb ZIP transactivates the asparagine synthase and proline dehydrogenase genes. This ‘sweetening’ technology is broadly applicable to other plants that utilize sucrose as a major translocation sugar.