Transcriptional Regulation of the Lignin Biosynthetic Pathway Revisited: New Players and Insights

The discovery that AC elements coordinated the regulation of genes belonging to the entire lignin biosynthetic pathway was the first breakthrough in understanding how lignin biosynthesis is regulated. Since then, tremendous progress has been made in the identification and characterization of many transcription factors (TFs) that regulate the genes of the phenylpropanoid branch pathway leading to lignin. A major breakthrough consisted in the discovery of a hierarchical transcriptional network regulating the biosynthesis of lignified secondary walls (SWs) in Arabidopsis. The NAC TFs (VND/NST/SND) work as the first layer of master switches activating the whole SW biosynthetic network through the regulation of a cascade of downstream TFs. Among these, MYB46/83 act as a second layer of master switches. Recent findings, however, reveal that the regulation of SW formation is far more complex than initially thought, involving both positive and negative regulators, dual function regulators, feedback loops, combinatorial complexes and cross talk between pathways. Finally, because of the great potential that lignocellulosic biomass represents for the production of bioenergy, there is a great interest in further elucidating the molecular mechanisms underlying the regulation of lignified SW and subsequently applying this knowledge to improve their saccharification potential for the generation of biofuels