Myc-like transcriptional factors in wheat: structural and functional organization of the subfamily I members

Abstract

Background: Myc-like regulatory factors carrying the basic helix-loop-helix (bHLH) domain belong to a large superfamily of transcriptional factors (TFs) present in all eukaryotic kingdoms. In plants, the representatives of this superfamily regulate diverse biological processes including growth and development as well as response to various stresses. As members of the regulatory MBW complexes, they participate in biosynthesis of flavonoids. In wheat, only one member (TaMyc1) of the Myc-like TFs family has been studied, while structural and functional organization of further members remained uncharacterized. From two Myc-subfamilies described recently in the genomes of Triticeae tribe species, we investigated thoroughly the members of the subfamily I which includes the TaMyc1 gene.

Results: Comparison of the promoter regions of the Myc subfamily I members in wheat suggested their division into two groups (likely homoeologous sets): TaMyc-1 (TaMyc-A1/TaMyc1, TaMyc-B1, TaMyc-D1) and TaMyc-2 (TaMyc-A2 and TaMyc-D2). It was demonstrated that the TaMyc-D1 copy has lost its functionality due to the frame shift mutation. The study of functional features of the other four copies suggested some of them to be involved in the biosynthesis of anthocyanins. In particular, TaMyc-B1 is assumed to be a co-regulator of the gene TaC1-A1 (encoding R2R3-Myb factor) in the MBW regulatory complex activating anthocyanin synthesis in wheat coleoptile. The mRNA levels of the TaMyc-A1, TaMyc-B1, TaMyc-A2 and TaMyc-D2 genes increased significantly in wheat seedlings exposed to osmotic stress. Salinity stress induced expression of TaMyc-B1 and TaMyc-A2, while TaMyc-A1 was repressed.

Conclusions: The features of the structural and functional organization of the members of subfamily I of Myc-like TFs in wheat were determined. Myc-like co-regulator (TaMyc-B1) of anthocyanin synthesis in wheat coleoptile was described for the first time. The Myc-encoding genes presumably involved in response to drought and salinity were determined in wheat. The results obtained are important for further manipulations with Myc genes, aimed on increasing wheat adaptability.

Keywords: Anthocyanin biosynthesis; Flavonoid biosynthesis; Gene duplication; Myc; Osmotic stress; Salinity stress; Stress response; Transcription factor; Triticum; Wheat; bHLH.

Fig. 1

Genetic similarity of Myc-like genes (bHLH motif). Phylogenic tree was constructed in MEGA 7.0 with Neighbor-Joining method with 1000 bootstrap replicates. Green colour – homoeologous group 2 chromosomes genes (subfamily I). Pink colour – homoeologous group 4 chromosomes genes (subfamily II)

Fig. 2

The multiple alignment of the Myc subfamily I proteins. Analysed protein motif is Myc-type, basic helix-loop-helix (bHLH) domain (IPR011598). Multiple sequence alignment was performed using MultAlin program. Red - high consensus, blue - low consensus, black - neutral

Fig. 3

Diverse promoter structure of the Myc subfamily I genes in wheat. Promoter analysis was performed using New PLACE database. Each square represents a corresponding colour motif

Fig. 4

The expression of the Myc subfamily I genes in the coleoptile of wheat genotypes having different coloration (the 5th day after germination). The data are presented as mean value ± standard error. *differences are statistically significant between coloured genotypes and S29 at p ≤ 0.005 (U-test)

Fig. 5

The expression of the Myc subfamily I genes in the coleoptile during wheat seedling development (from the 2nd to the 5th day). Selected genotype: sister lines CS (uncolored) and CS(H7A) (dark-colored) and the unrelated genotype S29 (light-coloured). The data are presented as mean value ± standard error. *differences are statistically significant between coloured genotypes and S29 at p ≤ 0.05 (U-test)

Fig. 6

Methylation patterns of the TaMyc-A1 promoter regions in the pericarp of isogenic lines differing in anthocyanin pigmentation of this tissue. ATG – the triplet code for the first amino acid methionine, e – exon, TSS – transcription start site

Fig. 7

a The expression of the Myc-like genes in wheat coleoptiles under salinity and drought stress (the 4th day after germination). b The relative level of anthocyanin content under the same conditions. The data are presented as mean value ± standard error. *differences are statistically significant between stressed and control S29 at p ≤ 0.05 (U-test)