The parthenocarpic gene Pat-k is generated by a natural mutation of SlAGL6 affecting fruit development in tomato (Solanum lycopersicum L.)

Abstract

Background: Parthenocarpy is a desired trait in tomato because it can overcome problems with fruit setting under unfavorable environmental conditions. A parthenocarpic tomato cultivar, 'MPK-1', with a parthenocarpic gene, Pat-k, exhibits stable parthenocarpy that produces few seeds. Because 'MPK-1' produces few seeds, seedlings are propagated inefficiently via cuttings. It was reported that Pat-k is located on chromosome 1. However, the gene had not been isolated and the relationship between the parthenocarpy and low seed set in 'MPK-1' remained unclear. In this study, we isolated Pat-k to clarify the relationship between parthenocarpy and low seed set in 'MPK-1'. Results: Using quantitative trait locus (QTL) analysis for parthenocarpy and seed production, we detected a major QTL for each trait on nearly the same region of the Pat-k locus on chromosome 1. To isolate Pat-k, we performed fine mapping using an F4 population following the cross between a non-parthenocarpic cultivar, 'Micro-Tom' and 'MPK-1'. The results showed that Pat-k was located in the 529 kb interval between two markers, where 60 genes exist. By using data from a whole genome re-sequencing and genome sequence analysis of 'MPK-1', we could identify that the SlAGAMOUS-LIKE 6 (SlAGL6) gene of 'MPK-1' was mutated by a retrotransposon insertion. The transcript level of SlAGL6 was significantly lower in ovaries of 'MPK-1' than a non-parthenocarpic cultivar. From these results, we could conclude that Pat-k is SlAGL6, and its down-regulation in 'MPK-1' causes parthenocarpy and low seed set. In addition, we observed abnormal micropyles only in plants homozygous for the 'MPK-1' allele at the Pat-k/SlAGL6 locus. This result suggests that Pat-k/SlAGL6 is also related to ovule formation and that the low seed set in 'MPK-1' is likely caused by abnormal ovule formation through down-regulation of Pat-k/SlAGL6. Conclusions:Pat-k is identical to SlAGL6, and its down-regulation causes parthenocarpy and low seed set in 'MPK-1'. Moreover, down-regulation of Pat-k/SlAGL6 could cause abnormal ovule formation, leading to a reduction in the number of seeds.