TTG1 complex MYBs, MYB5 and TT2, control outer seed coat differentiation

A suite of epidermal characters in Arabidopsis is under the transcriptional control of a combinatorial complex containing WD repeat, bHLH and MYB proteins. Many genetic, molecular and biochemical means have been employed to identify and characterize a complete minimal set of complex members required for the trichome initiation, root hair spacing, anthocyanin production and seed coat tannin production pathways. In addition, the WD and bHLH proteins required for outer seed coat differentiation have been identified. However, until now the MYB complex member(s) required for this last WD-bHLH-MYB complex-dependent character have remained elusive. Here we identify two MYBs, AtMYB5 and TT2, as partially redundant in regulating this outer seed coat developmental process with MYB5 having the major role. MYB5 and TT2 are shown to be expressed in this outer seed coat domain. We also show that MYB5 has weak pleiotropic control over trichome development and tannin production and is also expressed in the appropriate places for these functions. TT8 and the downstream GL2 and TTG2 regulators of seed coat development are found to be downregulated in the MYB mutants. Although the TTG1-dependent R2R3 MYBs are considered to be highly pathway specific, identification of MYBs responsible for outer seed coat development allowed for the elucidation of previously undetected novel developmental pleiotropy among these elements.