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Many vegetable and oilseed crops belong to  Brassica  species. The seed production of these crops is hampered often by abnormal floral organs, especially under the conditions of abiotic conditions. However, the molecular reasons for these abnormal floral organs remains poorly understood. Here, we report a novel pistil-like flower mutant of  B. rapa . In the flower of this mutant, the four sepals are modified to one merged carpel that look like a ring in the sepal positions, enveloping some abnormal stamens and a pistil, and resulting in poor seed production. This novel mutant is named  sepal-carpel modification  ( scm ). DNA sequencing showed that the  BrAP2a  gene, the ortholog of  Arabidopsis APETALA2  ( AP2 ) that specifies sepal identity, losses the function of in  scm  mutant due to a 119-bp repeated sequence insertion that resulted in an early transcription termination.  BrAP2b , the paralog of  BrAP2a  featured two single-nucleotide substitutions that cause a single amino acid substitution in the highly conserved acidic serine-rich transcriptional activation domain. Each of the two  BrAP2  genes rescues the sepal defective phenotype of the  ap2-5  mutant of  Arabidopsis . Furthermore, the knockout mutation of the corresponding  BnAP2  genes of oilseed rape ( B. napus ) by CRISPR/Cas9-mediated genome editing system resulted in  scm -like phenotype. These results suggest that  BrAP2  gene plays a key role in sepal modification. Our finding provides an insight into molecular mechanism underlying morphological modification of floral organs and is useful for genetic manipulation of flower modification and improvement of seed production of  Brassica  crops.

Defective APETALA2 Genes Lead to Sepal Modification in Brassica Crops