Study of Phenylpropanoid Gene Regulation by MYB Transcription factors in Maize, Sorghum and Rice

ZmMYB31 and ZmMYB42 are R2R3 MYB regulatory factors implicated in the regulation of phenylpropanoid genes in maize (Fornale, Sonbol et al. 2006, Fornale, Shi et al. 2010). Here we tested the hypothesis that the regulatory functions of MYB31 and MYB42 are conserved in other monocots specifically sorghum and rice. The expression profile of syntelogs of MYB31 and MYB42 and of four phenylpropanoid genes, caffeic acid O-methyltransferase (Comt), 4-coumarate-CoA ligase (4CL), ferulate-5-hydroxylase ( F5H), and caffeoyl shikimate esterase ( Cse), was determined and found to vary along the developmental gradient of young seedling leaves. There was a general trend towards reduced expression of these phenylpropanoid genes in mature leaf tip tissues as compared to the basal leaf tissues. Specific antisera were developed and used to demonstrate, by chromatin immunoprecipitation (ChIP), that Comt is also a target of MYB31 and MYB42 in the mature leaf tissues, not only of maize but also in sorghum and rice. In addition, 4CL2, F5H, and Cse were discovered to be regulatory targets of MYB31 and/ or MYB42 in seedling leaves of all three species but their binding profile differed between species. A survey of promoter occupancy revealed 9 out of 54 instances of common tissue specific regulation by MYB31 or MYB42 across 2 or more species and two (CSE and MYB31) that occur in all three species. In two instances enriched promoter occupancy by these regulators paralleled reduced expression of the target genes (Comt and 4CL2). The Cse gene is targeted by MYB42 in all three species and may represent a conserved regulatory module that functions early in the phenylpropanoid pathway. Similarly the MYB31 regulatory gene was a common target of MYB42 in the leaf tip of all three species and evidence is provided of cross regulation and MYB42 autoregulation. In summary, it was found that apart from a few instances of conserved regulatory patterns, the functions of MYB31 and MYB42 syntelogs appear to have undergone rapid change following divergence of these grass species. Elucidating these different regulatory roles may help guide attempts to alter the flux of intermediates towards lignin production in grasses that may be used as biofuels.