GNRF Represses Floral Transition and Secondary Wall Synthesis in Brachypodium distachyon

Several NAC transcription factors have been shown to play crucial roles in secondary cell wall biosynthesis and overall plant growth and development. GRASS NAC REPRESSOR OF FLOWERING (GNRF) was selected for functional characterization as it is highly expressed in stems, mirroring an expression pattern similar to that of characterized secondary cell wall regulators such as Arabidopsis thaliana NST1, SND1 and SND2. Moreover it is co-regulated with cell wall biosynthesis genes. Over expression mutants (GNRF-OE) were generated by constitutively over expressing GNRF full-length coding region under the maize ubiquitin promoter. A homozygous mutant allele (gnrf-1) harboring a non-synonymous point mutation was isolated from a TILLING mutant collection. Surprisingly, over-expression mutants failed to flower and could not transition into the reproductive stage. Conversely the gnrf-1 mutants flowered significantly earlier than control plants. Transcription profiling using a microarray revealed a fifty-fold reduction in two flowering time pathway genes BdFUL1 and BdFUL2, consistent with the persistent vegetative growth phenotype. Three genes involved in cellulose biosynthesis, BdCESA4/7/8, two genes associated with lignin biosynthesis, BdCAD1 and BdCOMT4 and a gene with a predicted role in xylan biosynthesis BdGT47-1 were analyzed for changes in transcript abundance. In most cases, these genes were significantly down-regulated in GNRF-OE and up-regulated in gnrf-1 stems. Chemical and histological analysis of stems revealed reduced lignin content compared to the controls where as no significant change was observed between gnrf-1 and the controls. Taken together, these data suggest that GNRF functions as a pleiotropic repressor regulating cell walls and flowering in Brachypodium distachyon.