Doctoral Student North Carolina State University Charlotte, North Carolina
Body of Abstract: Understanding the processes involved in lignin-modification-induced dwarfism (LMID) can help design bioenergy crops with improved energy extractability without sacrificing biomass accumulation. The ref8 Arabidopsis mutant produces a defective version of the lignin biosynthesis enzyme C3’H. These mutants produce less lignin with modified lignin monomer ratios, and experience severe dwarfism and infertility. A suppressor screen identified mutations that cause growth rescue in ref8, and the genes that these mutations correspond to were called GROWTH INHIBITION RELIEVED (GIR) genes. Double mutant ref8 gir2 plants experience larger growth than ref8 plants without restoring C3’H function. Grafting the double mutant rootstock to ref8 shoots was able to improve shoot growth despite the main root growth being unchanged between ref8 and ref8 gir2. The causal mutation for gir2 was mapped to the transcriptional and epigenetic regulator VAL2. Both GIR2 and closely related VAL1 have been described as having similar protein structure and redundant functions. However, while gir2 partially rescues ref8,val1 does not. This difference appears to arise from expression differences, as VAL1 gains the ability to complement gir2 in ref8 when expressed by GIR2’s promoter. While GIR2p:GIR2-GFP and GIR2p:VAL1-GFP are expressed in root protoxylem, VAL1p:VAL1-GFP is not. The localization of GIR2 in tissue that ultimately becomes highly lignified suggests that in lignin deficient environments GIR2 triggers growth restricting processes. To explore potential growth limiting targets of GIR2 in ref8 we observed the distribution of H3K27me3 in the roots of ref8 and ref8 gir2 seedlings. Through GO analyses we found that genes that lost H3K27me3 in ref8 gir2 were enriched for involvement in regulating developmental progression. From this study we conclude that VAL2 contributes to ref8 dwarfism, and likely does so via the epigenetic suppression of developmental processes.