Compound Inflorescence (S) represses fruit growth and seed development in tomato

In most fruit-bearing plants, the growth and ripening of the fruit are highly coordinated with its seeds. In tomato (Solanum lycopersicum), fruit growth is regulated by a number of transcription factors. The WUSCHEL-RELATED HOMEOBOX (WOX) transcription factor COMPOUND INFLORESCENCE (S)/SlWOX9 is a key regulator of inflorescence branching and embryogenesis. In this study, we show that S activity is indispensable for fruit and seed development. Ectopic S expression led to a decrease in seed weight and markedly suppressed endoreduplication-associated fruit growth. Conversely, the s-classic allele promoted fruit growth in the small-fruited S. pimpinellifolium accession LA1781 though the promotion was offset by fruit overload and vegetative reversion of flowers in large-fruited cultivars. Through transcriptome profiling and DNA affinity purification sequencing (DAP-seq), we discovered that S targeted a set of genes involved in seed development. Moreover, we demonstrate that S regulates fruit and seed development mainly through direct transcriptional repression of the Mei2-like gene SlTEL1 (Solanum lycopersicum TERMINAL EAR-LIKE 1). These findings uncover a new aspect of transcription regulation of fruit growth and seed development by WOX transcription factors in tomato.