Tadesse, D.; Dai, Y.; Yang, L.; Yang, Y.; Dwivedi, N.; Kumaran, D.; Blaby-Haas, C.; Lipzen, A. M.; Santiago, K.; Barry, K. W.; Liu, C.-J.; Xie, M.

Lack of mechanistic understanding of the environmental plasticity of secondary cell wall (SCW) biosynthesis hinders the massive bioenergy production on marginal lands. Growing bioenergy crops on marginal lands is appealing to minimize competition for arable land. However, abiotic stresses, particularly iron deficiency stress, are widespread to perturb SCW biosynthesis. In poplar, a major bioenergy crop, we demonstrated that iron deprivation activates stem SCW biosynthesis and identified transcription factor PtrbHLH011 as a possible underlying regulator. PtrbHLH011 is a potent repressor of SCW, whose overexpression resulted in a reduction of stem SCW by over 65%. Our genomic and molecular studies discovered that PtrbHLH011 binds to the AAAGACA sequence and represses essential genes for SCW biosynthesis, flavonoid biosynthesis, and iron homeostasis. Wood formation and iron deprivation downregulates PtrbHLH011 to release the transcriptional repression. Our findings reveal a regulatory mechanism coordinating SCW biosynthesis in response to environmental iron availability and suggest that PtrbHLH011 manipulation may help engineer bioenergy crops with improved performance under marginal conditions.