Four genes encoding MYB28, a major transcriptional regulator of the aliphatic glucosinolate pathway, are differentially expressed in the allopolyploid Brassica juncea

Abstract

Glucosinolates are Capparales-specific secondary metabolites that have immense potential in human health and agriculture. Unlike Arabidopsis thaliana, our knowledge about glucosinolate regulators in the Brassica crops is sparse. In the current study, four MYB28 homologues were identified (BjuMYB28-1,-2,-3,-4) from the polyploid Brassica juncea, and the effects of allopolyploidization on the divergence of gene sequence, structure, function, and expression were assessed. The deduced protein sequences of the four BjuMYB28 genes showed 76.1–83.1% identity with the Arabidopsis MYB28. Phylogenetic analysis revealed that the four BjuMYB28 proteins have evolved via the hybridization and duplication processes forming the B. juncea genome (AABB) from B. rapa (AA) and B. nigra (BB), while retaining high levels of sequence conservation. Mutant complementation and over-expression studies in A. thaliana showed that all four BjuMYB28 genes encode functional MYB28 proteins and resulted in similar aliphatic glucosinolate composition and content. Detailed expression analysis using qRT-PCR assays and promoter-GUS lines revealed that the BjuMYB28 genes have both tissue- and cell-specific expression partitioning in B. juncea. The two B-genome origin BjuMYB28 genes had more abundant transcripts during the early stages of plant development than the A-genome origin genes. However, with the onset of the reproductive phase, expression levels of all four BjuMYB28 increased significantly, which may be necessary for producing and maintaining high amounts of aliphatic glucosinolates during the later stages of plant development. Taken together, our results suggest that the four MYB28 genes are differentially expressed and regulated in B. juncea to play discrete though overlapping roles in controlling aliphatic glucosinolate biosynthesis.

Characterization of RsMYB28 and RsMYB29 transcription factor genes in radish (Raphanus sativus L.)

“…The subcellular localization of MYB28 in A. thaliana and B. juncea was found to be in the nucleus (Gigolashvili et al, 2007b;Augustine et al, 2013a). Our GFP transient expression indicated that RsMYB28 and RsMYB29 were both located in the nuclei in onion epidermal cells and also possessed transcriptional activity.…”

“…In this study, the genomic DNA and cDNA of RsMYB28 and RsMYB29 were isolated. Like the BjuMYB28 gene of other R2R3-MYB members (Augustine et al, 2013a), both genes were found to be conserved with three separated exons and two introns (Figure 1). The amino acid sequence alignment showed that both RsMYB28 and RsMYB29 contained two highly conserved domains, and showed high homology with AtMYB28 and AtMYB29, respectively.…”

“…Recently, many studies have revealed that the GSL biosynthesis is a tripartite pathway consisting of three stages: side-chain elongation, development of the core structure, and secondary modification of the amino acid side chain (Chen et al, 2011;Augustine et al, 2013a). The most prominent GSL biosynthetic precursor amino acids include methionine, tryptophan, and phenylalanine, which result in aliphatic, indolic, and aromatic GSLs, respectively (Kos et al, 2012).…”

“…In recent years, a great number of functional genes involved in the GSL biosynthetic pathway have been successfully identified in several plant species including Arabidopsis thaliana (Sønderby et al, 2010), Brassica juncea (Augustine et al, 2013a), and B. rapa (Kim et al, 2013). The R2R3-MYB proteins are plant specific and constitute one of the largest transcription factor families in plants (Wilkins et al, 2009).…”

“…In Arabidopsis, the AtMYB28, AtMYB29, and AtMYB76 genes positively regulate aliphatic GSL biosynthesis (Gigolashvili et al, 2008;Sønderby et al, 2010;Li et al, 2013), whereas the AtMYB34, AtMYB51, and AtMYB122 genes have been shown to regulate indolic GSL biosynthesis (Gigolashvili et al, 2007a). In B. juncea, four homologous genes encoding MYB28 were found to participate in regulating the aliphatic GSL biosynthesis (Augustine et al, 2013a). Moreover, both the MYB28 and MYB29 genes showed high expression levels in flowers, seeds, and stems of Chinese cabbage (Kim et al, 2013).…”