Transcriptional regulation of secondary cell wall formation and lignification

Secondary xylem is the chief form of lignocellulosic biomass on the planet and consists mainly of fibers, vessels and tracheids with lignified secondary cell walls (SCWs). These cells develop from xylem mother cells produced from the meristematic vascular cambium, undergoing cell expansion, SCW deposition, programmed cell death and lignification within a narrow developmental zone. Understanding the transcriptional control of this complex process has been an activate area of research for over two decades, where significant contributions in the model annual Arabidopsis thaliana (Arabidopsis) and the model tree genus Populus have provided detailed insights into a semi-hierarchical transcriptional regulatory network (TRN) underlying SCW deposition and lignification. In this chapter, I summarize the classical three-tiered SCW TRN model and elaborate on its expanded structure, discussing key transcription factors underlying lignin formation in Arabidopsis. I present a comprehensive new model of the SCW TRN of poplar, with a commentary on the conservation and divergence of the SCW TRN across plant lineages and the possible existence of lineage-specific regulators that act as “accessory” components to the core TRN. Finally, I discuss key post-translational modifications that affect the activity of the transcription factors comprising the SCW TRN and recent insights into cooperation and antagonism between transcription factors through the formation of protein complexes. We are learning that the SCW TRN has undergone substantial re-wiring in well-studied models and that plants have adopted a myriad of combinatorial and post-translational mechanisms to modulate SCW transcriptional control according to particular biological contexts.