Elucidation of the Molecular Identity and Characterization of the Tomato Fruit Weight Gene FW3.2

Fruit weight is an important agronomic trait that has been selected during the domestication of many crops. In tomato, fruit weight is controlled by several quantitative trait genes including FW2.2, FASCIATED (FAS) and LOCULE NUMBER (LC). Fw3.2 is a major fruit weight QTL contributing up to 19% of the variance in certain populations. We fine mapped the locus to a 52.3 kb region encompassing eight candidate genes.  At the time of anthesis, ovary size was the same between large and small fruited NILs.  However, two weeks after anthesis fruit size started to differ, resulting in larger fruit size and weight for the NILs carrying the cultivated allele of fw3.2. Yield per plant is not significantly different between NILs. Smaller fruit in the NILs carrying the wild allele of fw3.2 was offset by more branches while the number of flowers per inflorescence was the same compared to NILs carrying the cultivated allele. Fruit from the NILs carrying the wild allele ripened approximately one week earlier than fruit from NILs carrying the large fruit allele. Northern blot analyses showed that two candidate genes were differentially expressed between large and small fruited NILs. Knock down lines were generated using RNA interference (RNAi) approach for one of the two candidate genes, ORF6. Silencing of ORF6 was shown to correlate with reduced fruit weight. A few silenced lines carried small fruits with a sufficient number of viable seeds. For those, the seed count per fruit weight showed that smaller fruit weight is not due to poor seed set. Reduced expression of ORF6 also resulted in smaller vegetative and reproductive organs and reduced plant stature. The findings suggest that ORF6 is the most likely gene to underlie the fruit weight QTL fw3.2. Its role in plant development will be discussed.