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
Figure 1: Characteristics the sepal carpeloid mutants of Brassica. (A–C): inflorescence; (D,E1,E2,F1–F3): flower; (G,H): floral organs; (I–K): SEM photos of flowers; (L1,L2,M1,M2,N1,N2): SEM photos of floral organs; (A,D,G,I,L1,L2): wild type Brassica (WT). (L1,L2) Are sepal (left) and stamen (right); (B,E1,E2,H,J,M1,M2): the typical sepal carpeloid mutant (scm). M1 is a sepal carpeloid organ, and M2 is an incomplete stamen to carpel organ with one ovule; (C,F1–F3,K,N1,N2): the incomplete sepal carpeloid mutant (scm2). Arrows indicate sepal carpel-like transition in F2 and petal stamen-like transition in F3. N1 is an incomplete petal to stamen organ, and N2 is a stamen of scm2. Bars in E1 and E2 are 5 mm; bars in I–K,L1,M1 are 1 mm; bars in L2,M2,N1,N2 are 500 μm; and bars in the others are 10 mm.