Abstract
Flowering time is a key agronomic trait, directly influencing crop yield and quality. Many flowering-time genes have been identified and characterized in the model plant Arabidopsis thaliana; however, these genes remain uncharacterized in many agronomically important Brassica crops. In this study, we identified 1064, 510, and 524 putative orthologs of A. thaliana flowering-time genes from Brassica napus, Brassica rapa, and Brassica oleracea, respectively, and found that genes involved in the aging and ambient temperature pathways were fewer than those in other flowering pathways. Flowering-time genes were distributed mostly on chromosome C03 in B. napus and B. oleracea, and on chromosome A09 in B. rapa. Calculation of non-synonymous (Ka)/synonymous substitution (Ks) ratios suggested that flowering-time genes in vernalization pathways experienced higher selection pressure than those in other pathways. Expression analysis showed that most vernalization-pathway genes were expressed in flowering organs. Approximately 40% of these genes were highly expressed in the anther, whereas flowering-time integrator genes were expressed in a highly organ-specific manner. Evolutionary selection pressures were negatively correlated with the breadth and expression levels of vernalization-pathway genes. These findings provide an integrated framework of flowering-time genes in these three Brassica crops and provide a foundation for deciphering the relationship between gene expression patterns and their evolutionary selection pressures in Brassica napus.
Highlights
The phase transition from vegetative to reproductive growth is a crucial step in the plant’s life cycle, especially in relation to plant adaptation and crop productivity [1,2,3,4]
No functional domain existed in the remaining 19 members (Table S2). For those domain-containing genes, the results derived from HMMER and Basic Local Alignment Search Tool Protein (BLASTP) searches were intersected and identified a total of 985, 475, and 482 domain-containing flowering-time genes in B. napus, B. rapa, and B. oleracea, respectively (Tables S3–S5)
A comparison of chromosome locations of flowering-time genes among B. napus, B. rapa, and B. oleracea showed that a total of 511 genes were localized on the A subgenome in B. napus, which is almost equal to the number of its orthologs in B. rapa (510 genes were identified in this study)
Summary
The phase transition from vegetative to reproductive growth is a crucial step in the plant’s life cycle, especially in relation to plant adaptation and crop productivity [1,2,3,4]. Switching the shoot apical meristem (SAM) from vegetative to reproductive growth is controlled by both environmental and endogenous signals, and this process is affected mainly by photoperiodic promotion, autonomous enhancement, flowering inhibition, vernalization (the acceleration of flowering by low temperatures and the suppression of specific genes), and gibberellin induction [10]. These factors act on the inflorescence/floral meristem in combination, resulting in the conversion of the apical meristem to the inflorescence/floral meristem. Studies of putative orthologs and of the relationship between selective pressures and expression patterns provided further information on the control of flowering time in Brassica species
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