Defective APETALA2 Genes Lead to Sepal Modification in Brassica Crops.

Yanfeng Zhang,Shuhua Huang,Xuefang Wang,Xiaofeng Wang,Xupeng Guo,Jianhua Tian,Jianwei Liu,Jianxin Mu

doi:10.3389/fpls.2018.00367

Abstract

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.

Highlights

Eudicot flowers are composed of four distinct floral organs, sepals, petals, stamens, and carpels, which are arranged in four concentric whorls from the outer to the inner layer of the flower
We recovered a novel pistillike flower mutant in the F3 selfing line Nc116, which was generated from an interspecific cross between B. napus and B. rapa
To examine the expression of the AtAP2-green fluorescent protein (GFP), BrAP2a-GFP, and BrAP2b-GFP genes in independent transgenic lines, 50 mg of young leaves were ground in liquid N2 and treated with 200 μL 1×SDS loading buffer without Bromophenol Blue (50 mM pH 6.8 Tris–HCl, 5% β-mercaptoethanol, 10% Glycerol, and 2% SDS), followed by further grinding

Summary

Introduction

Eudicot flowers are composed of four distinct floral organs, sepals, petals, stamens, and carpels, which are arranged in four concentric whorls from the outer to the inner layer of the flower. Genetic analysis of floral homeotic mutants in Arabidopsis thaliana and Antirrhinum majus led to the development of the classic ABC model of floral organ identity (Bowman et al, 1991; Coen and Meyerowitz, 1991). This model proposes three classes of floral homeotic genes that act alone or cooperate with each other to specify the four floral organs: class A specifies the sepals, A and B specify the petals, B and C specify the stamens, and C specifies the carpels. A C-functional gene mutation leads to a homeotic translation of stamens to petals and carpels to sepals (Meyerowitz et al, 1991; Weigel and Meyerowitz, 1994)

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Conclusion