Expression Characterization of ABCDE Class MADS-Box Genes in Brassica rapa with Different Pistil Types

Abstract

MADS-box is a vital transcription factor family that functions in plant growth and development. Apart from APETALA2, all genes in the ABCDE model that explain the molecular mechanism of floral organ development belong to the MADS-box family. Carpel and ovule numbers in plants are essential agronomic traits that determine seed yield, and multilocular siliques have great potential for the development of high-yield varieties of Brassica. In this study, ABCDE genes in the MADS-box family from Brassica rapa were identified and characterized. Their tissue-specific expression patterns in floral organs and their differential expression in different pistil types of B. rapa were revealed by qRT-PCR. A total of 26 ABCDE genes were found to belong to the MADS-box family. Our proposed ABCDE model of B. rapa is consistent with that of Arabidopsis thaliana, indicating that ABCDE genes are functionally conserved. These results of qRT-PCR showed that the expression levels of class C and D genes were significantly different between the wild-type (wt) and tetracarpel (tetrac) mutant of B. rapa. Interestingly, the expression of the homologs of class E genes was imbalanced. Therefore, it is speculated that class C, D, and E genes are involved in developing the carpel and ovule of B. rapa. Our findings reveal the potential for the selection of candidate genes to improve yield traits in Brassica crops.

Keywords: ABCDE model; Brassica rapa; Chinese cabbage; MASD-box genes; flower development; multilocular silique; tissue specificity.

Figure 1

Floral organs of two pistil types of Brassica rapa, wt and tetrac. (a) Phenotypic observation of wt and tetrac flower organs by using a stereomicroscope, the bars of open flowers and pistils = 2 mm, and the bars of nectaries = 0.5 mm; (b) Statistical data of pistil (stigma and stylus) and ovule numbers of wt and tetrac; the data shown are mean ± SD of at least three biological replicates. Significant differences between wt and tetrac determined by an independent Student’s t-test are indicated with *** (p < 0.001).

Figure 2

Chromosome distribution of 26 ABCDE class MADS-box genes in B. rapa. The different colors of the triangles indicate different types of MADS-box genes. The direction of the top corner of the triangle indicates the encoding direction of the gene: the downward direction is forward encoding, and the upward direction is reverse encoding.

Figure 3

Phylogenetic relationships, gene structure, conserved motif compositions and structural domains of B. rapa and Arabidopsis ABCDE class MADS proteins. The maximum likelihood tree was constructed with the aligned protein sequences of B. rapa and Arabidopsis ABCDE class MADS-box genes. Eight motifs were identified and displayed in different colors. The combined match p-value is defined as the probability that a random sequence (with the same length and conforming to the background) would have position p-values such that the product is smaller or equal to the value calculated for the sequence under test.

Figure 4

Prediction of cis-acting elements of the ABCDE class MADS-box gene family in B. rapa. The upstream 2 kb sequences of ABCDE class MADS-box genes in B. rapa were analyzed through PlantCARE.

Figure 5

Proposed B. rapa flower development model based on the expression patterns and referring to the ancestral functions of homeotic MADS-box genes. Blocks in different colors with distinct heights represent different expression levels.

Figure 6

Tissue-specific expression analysis of ABCDE class MADS-box genes in the pistils (a) and ovules (b) of wt and tetrac based on qRT-PCR. Data shown are mean ± SD of at least three biological replicates. Significant differences between wt and tetrac determined by an independent sample t-test are indicated with * (p < 0.05), ** (p < 0.01), and *** (p < 0.001).