A 48‐bp deletion within the promoter of the <scp> <i>BnaC9</i> </scp> . <scp> <i>APT5</i> </scp> gene results in elevated seed number per silique in <i>Brassica napus</i>

Seed number per silique (SNPS) is an important determinant for seed yield in rapeseed (Brassica napus). Here, the molecular mechanism of BnaC9.APT5 (Adenine phosphoribosyltransferase 5) regulating SNPS was identified by combining gene function with biochemical analyses. Overexpression (OE) of Arabidopsis Adenine Phosphoribosyltransferase 5 (AtAPT5) and ectopic expression of BnaC9.APT5 in Arabidopsis thaliana increased SNPS by c. 9%. By contrast, the Arabidopsis knockout mutant had 18% fewer seeds per silique. OE of BnaC9.APT5 in rapeseed increased SNPS by c. 11% through upregulating cytokinin (CK) biosynthesis genes. The enzyme activity analysis showed that BnaC9.APT5 converts adenine to adenosine monophosphate. Spraying the Arabidopsis knockout mutant with 6-benzylaminopurine increased SNPS by 34%. The treatment of the Arabidopsis AtAPT5 and rapeseed BnaC9.APT5 OE lines with the CK biosynthesis inhibitor lovastatin reduced SNPS by c. 7% and 9%, respectively. Thus, BnaC9.APT5 controls SNPS via the CK metabolism in rapeseed. A 48-bp InDel in the BnaC9.APT5 promoter was significantly associated with gene expression and SNPS and has been selected during rapeseed 'double-low' quality breeding. Our data provide clear evidence of the role of the BnaC9.APT5 gene for the genetic improvement of modern rapeseed.