ARR17 controls dioecy in by repressing B-class MADS-box gene expression.

The number of dioecious species for which the genetic basis of sex determination has been resolved is rapidly increasing. Nevertheless, the molecular mechanisms downstream of the sex determinants remain largely elusive. Here, by RNA-sequencing early-flowering isogenic aspen () lines differing exclusively for the sex switch gene , we show that a narrowly defined genetic network controls differential development of female and male flowers. Although encodes a type-A response regulator supposedly involved in cytokinin (CK) hormone signalling, clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9-mediated knockout only affected the expression of a strikingly small number of genes, indicating a specific role in the regulation of floral development rather than a generic function in hormone signalling. Notably, the () gene, encoding an F-box protein acting as a transcriptional cofactor with () to activate B-class MADS-box gene expression, and the B-class gene (), necessary for male floral organ development, were strongly de-repressed in the CRISPR mutants. Our data highlight a CK-independent role of the poplar response regulator ARR17 and further emphasize the minimal differences between female and male individuals. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.