Genome-Wide Analysis of MYB Transcription Factor Gene Superfamily Reveals BjPHL2a Involved in Modulating the Expression of BjCHI1 in Brassica juncea.

Brassica juncea is an economically important vegetable and oilseed crop. The MYB transcription factor superfamily is one of the largest transcription factor families in plants, and plays crucial roles in regulating the expression of key genes involved in a variety of physiological processes. However, a systematic analysis of the MYB transcription factor genes in Brassica juncea (BjMYB) has not been performed. In this study, a total of 502 BjMYB superfamily transcription factor genes were identified, including 23 1R-MYBs, 388 R2R3-MYBs, 16 3R-MYBs, 4 4R-MYBs, 7 atypical MYBs, and 64 MYB-CCs, which is approximately 2.4-fold larger than that of AtMYBs. Phylogenetic relationship analysis revealed that the MYB-CC subfamily consists of 64 BjMYB-CC genes. The expression pattern of members of PHL2 subclade homologous genes in Brassica juncea (BjPHL2) after Botrytis cinerea infection were determined, and BjPHL2a was isolated from a yeast one-hybrid screen with the promoter of BjCHI1 as bait. BjPHL2a was found to localize mainly in the nucleus of plant cells. An EMSA assay confirmed that BjPHL2a binds to the Wbl-4 element of BjCHI1. Transiently expressed BjPHL2a activates expression of the GUS reporter system driven by a BjCHI1 mini-promoter in tobacco (Nicotiana benthamiana) leaves. Taken together, our data provide a comprehensive evaluation of BjMYBs and show that BjPHL2a, one of the members of BjMYB-CCs, functions as a transcription activator by interacting with the Wbl-4 element in the promoter of BjCHI1 for targeted gene-inducible expression.

BjMYB1, a transcription factor implicated in plant defence through activating BjCHI1 chitinase expression by binding to a W-box-like element.

We previously identified the W-box-like-4 (Wbl-4) element (GTAGTGACTCAT), one of six Wbl elements in the BjC-P promoter of the unusual chitinase gene BjCHI1 from Brassica juncea, as the core element responsive to fungal infection. Here, we report the isolation and characterization of the cognate transcription factor interacting with the Wbl-4 element. Using Wbl-4 as a target, we performed yeast one-hybrid screening of a B. juncea cDNA library and isolated an R2R3-MYB transcription factor designated as BjMYB1. BjMYB1 was localized in the nucleus of plant cells. EMSA assays confirmed that BjMYB1 binds to the Wbl-4 element. Transiently expressed BjMYB1 up-regulated the activity of the BjC-P promoter through its binding to the Wbl-4 element in tobacco (Nicotiana benthamiana) leaves. In B. juncea, BjMYB1 displayed a similar induced expression pattern as that of BjCHI1 upon infection by the fungus Botrytis cinerea Moreover, heterogeneous overexpression of BjMYB1 significantly elevated the resistance of transgenic Arabidopsis thaliana to the fungus B. cinerea These results suggest that BjMYB1 is potentially involved in host defence against fungal attack through activating the expression of BjCHI1 by binding to the Wbl-4 element in the BjC-P promoter. This finding demonstrates a novel DNA target of plant MYB transcription factors.

Identification of fungus-responsive cis-acting element in the promoter of Brassica juncea chitinase gene, BjCHI1.

Chitinases are a group of pathogenesis-related proteins. The Brassica juncea chitinase gene BjCHI1 is highly inducible by pathogenic fungal infection, suggesting that the promoter of BjCHI1 might contain specific cis-acting element responsive to fungal attack. To identify the fungus-responsive element in BjCHI1 promoter (BjC-P), a series of binary plant transformation vectors were constructed by fusing the BjC-P or its deletion-derivatives to ß-glucuronidase (GUS) reporter gene. Expression of the GUS reporter gene was systematically assayed by a transient gene expression system in Nicotiana benthamiana leaves treated with fungal elicitor Hexa-N-Acetyl-Chitohexaose, as well as in transgenic Arabidopsis plants inoculated with fungus Botrytis cinerea. The histochemical and quantitative GUS assays showed that the W-box-like element (GTAGTGACTCAT) in the region (-668 to -657) was necessary for the fungus-response, although there were another five W-box-like elements in BjC-P. In addition, gain-of-function analysis demonstrated that the fragment (-409 to -337) coupled to the W-box-like element was needed for full magnitude of the fungal induction. These results revealed the existence of a novel regulation mechanism of W-box-like element involved in plant pathogenic resistance, and will benefit the potential application of BjC-P in engineering crops.