Tomato (Solanum lycopersicum) Homologs of TRIPTYCHON (SlTRY) and GLABRA3 (SlGL3) are involved in anthocyanin accumulation

Abstract

In Arabidopsis thaliana the CPC-like MYB transcription factors [CAPRICE (CPC), TRIPTYCHON (TRY), ENHANCER OF TRY AND CPC 1, 2, 3/CPC-LIKE MYB 3 (ETC1, ETC2, ETC3/CPL3), TRICHOMELESS 1, 2/CPC-LIKE MYB 4 (TCL1, TCL2/CPL4)] and the bHLH transcription factors [GLABRA3 (GL3) and ENHANCER OF GLABRA 3 (EGL3)] are central regulators of trichome initiation and root-hair differentiation. By transforming the tomato orthologous genes SlTRY (TRY) and SlGL3 (GL3) into Arabidopsis, we demonstrated that SlTRY inhibited trichome initiation and enhanced root-hair differentiation. These results suggest that tomato and Arabidopsis partially use similar transcription factors for epidermal cell differentiation, and that a CPC-like R3 MYB may be a key common regulator of plant trichome and root-hair development. CPC and GL3 are also known to be involved in anthocyanin biosynthesis. Here, we show that anthocyanin accumulation was repressed in the CPC::SlTRY and GL3::SlGL3 transgenic plants, suggesting that SlTRY and SlGL3 can influence anthocyanin pigment synthesis.

The differentiation of plant epidermal cells into trichomes and root hairs has proven to be a very useful paradigm for investigating early events in plant cell development. In Arabidopsis thaliana, the CAPRICE (CPC) gene, a member of the R3-type MYB transcription factor family, is an important regulatory factor for trichome initiation and root hair formation.¹ Genes closely related to CPC in the Arabidopsis genome include: TRYPTICHON, TRY; ENHANCER OF TRY AND CPC1, ETC1; ENHANCER OF TRY AND CPC2, ETC2; ENHANCER OF TRY AND CPC3/CPC-LIKE MYB3, ETC3/CPL3; TRICHOMELESS1, TCL1 and TRICHOMELESS2/CPC-LIKE MYB4, TCL2/CPL4.^2-10 The TRY protein functions primarily during trichome initiation.^2,11 Another type of transcription factor in Arabidopsis, the bHLH transcription factor GLABRA3 (GL3), also controls trichome initiation and root hair formation.¹² Previously, we reported that the CPC, TRY, ETC1, ETC2 and CPL3 proteins interact with GL3 in yeast and might function together as a transcription factor complex.⁷

Recently, we identified tomato SlTRY and SlGL3 genes as orthologs of the Arabidopsis TRY and GL3 genes, respectively.¹³ The CPCp::SlTRY transgenic plants produced more root hairs and had no trichomes, a phenotype analogous to that of the 35S::CPC transgenic plants.¹³ In contrast, GL3p::SlGL3 transgenic plants closely resembled wild-type. These results suggest that Arabidopsis and tomato have orthologous transcription factors for epidermal cell differentiation including trichome initiation and root-hair differentiation. Furthermore, SlTRY-like R3 MYB appears to function as an important common transcription factor for both trichome initiation and root-hair differentiation.¹³

In addition to epidermal cell differentiation, Arabidopsis CPC and GL3 genes are known to regulate anthocyanin biosynthesis.^14,15 To verify that tomato SlTRY and SlGL3 also play roles in anthocyanin accumulation, we measured the anthocyanin content of SlTRY and SlGL3 transgenic plants. Total plant pigments were extracted from leaves, and anthocyanin levels were measured according to previously reported protocols.^16,17 Compared with the wild-type Col-0, the CPC::SlTRY transgenic plants had significantly reduced levels of anthocyanin accumulation (Fig. 1A). Compared with the cpc-2 mutant, cpc-2 transgenic plants transformed with CPC::SlTRY also accumulated significantly less anthocyanin (Fig. 1A). Anthocyanin accumulation was somewhat reduced in the GL3::SlGL3 transgenic plants compared with wild type Col-0 (Fig. 1B). In comparison with gl3-7454 mutants, gl3-7454 transgenic plants transformed with GL3::SlGL3 also had significantly lower levels of anthocyanin accumulation (Fig. 1B). Therefore, the tomato homologs to the TRY and GL3 genes, SlTRY and SlGL3, also function as repressors of anthocyanin accumulation.

Figure 1. Anthocyanin accumulation is inhibited by SlTRY and SlGL3. (A) Anthocyanin content in wild-type Col-0, CPC::SlTRY, cpc-2 and CPC::SlTRY in cpc-2. Error bars indicate the standard deviations. Bars marked with asterisks indicate a significant difference between the wild-type Col-0 and the CPC::SlTRY transgenic line, or the cpc-2 mutant and the CPC::SlTRY in the cpc-2 transgenic line by Student’s t-test (p < 0.050). (B) Anthocyanin content in wild-type Col-0, GL3::SlGL3, gl3–7454 and GL3::SlGL3 in gl3-7454. Error bars indicate the standard deviations. Bars marked with asterisks indicate a significant difference between the wild-type Col-0 and the GL3::SlGL3 transgenic line, or the gl3-7454 mutant and the GL3::SlGL3 in gl3-7454 transgenic line by Student’s t-test (p < 0.050).

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.