The objective of the project is to investigate the transcriptional network regulating secondary wall biosynthesis in plants. Secondary walls are the major constituent of tracheary elements and fibers in wood, which is the most abundant biomass produced by plants and is widely used for pulping, paper-making, and many other commodities. To make secondary walls, genes involved in the biosynthesis of secondary wall components, including cellulose, hemicellulose and lignin, need to be coordinately turned on. Therefore, it is conceivable that there exist genetic switches responsible for turning on the secondary wall biosynthetic program. Currently, little is known about the molecular mechanisms underlying the regulation of secondary wall biosynthesis during wood formation.
The investigators have recently discovered that the secondary wall-associated NAC domain transcription factor SND1 is a key switch in activating the developmental program of secondary wall biosynthesis, and that SND1 regulates the expression of several other secondary wall-associated transcription factors. One of them, MYB46, was demonstrated to be a direct target of SND1, and act upstream of the transcription factors MYB85 and KNAT7 in the regulation of secondary wall biosynthesis. Based on these findings, the investigators hypothesize that a hierarchy of transcriptional factors is involved in the regulation of secondary wall biosynthesis, and plan to unravel the SND1-mediated transcriptional network by functional characterization of the secondary wall-associated, SND1-regulated transcription factors and dissection of their interrelationships in the network. Broader impacts. The results from this project will contribute to our understanding of the molecular mechanisms underlying wood formation, and the knowledge gained will potentially provide valuable tools for genetically improving the quality and quantity of wood, the most important raw materials for traditional forest products and potentially for biofuel production. The investigators will actively integrate the proposed research with teaching activities and training of graduate and undergraduate students, especially from underrepresented groups, and will make every effort to disseminate the results of the project to enhance the understanding of plant biology research and its benefits to the society.
The goal of this NSF-funded project was to study the molecular mechanisms underlying the transcriptional regulation of secondary cell wall biosynthesis in plants. Secondary walls are the major constituent of tracheary elements and fibers in wood, which is the most abundant biomass produced by plants. Understanding how secondary walls are produced would not only contribute to our knowledge of basic plant biology but also have economic and environmental implications because wood is one of the most environmentally cost-effective and renewable sources of bioenergy and widely used for lumber and pulping. We hypothesized that a transcriptional regulatory network comprising of the master transcriptional switch SND1 and its downstream targets is involved in the regulation of secondary wall biosynthesis, and proposed to unravel the SND1-mediated transcriptional regulatory network by functional characterization of the secondary wall-associated, SND1-regulated transcription factors and dissection of their interrelationships in the network. We have discovered that a battery of SND1-regulated transcription factors is required for normal secondary wall biosynthesis in Arabidopsis. Two of these SND1-regulated MYB transcription factors, MYB58 and MYB63, were found to specifically expressed in cells undergoing secondary wall thickening and are important regulators of lignin biosynthesis. Two other SND1-regulated MYB transcription factors, MYB46 and MYB83, were demonstrated to act as second-level master switches regulating secondary wall biosynthesis. Our study has revealed that the transcription program regulating secondary wall biosynthesis involves a multi-leveled feed-forward loop regulatory structure in which MYB46 and MYB83 together with their regulator SND1 and their direct targets, such as MYB58 and MYB63, regulate an array of downstream genes, thereby activating the secondary wall biosynthetic program. We have also extended our study from Arabidopsis into poplar and grasses and demonstrated that the transcriptional program regulating secondary wall biosynthesis is evolutionarily conserved throughout vascular plants. The findings from our study greatly contribute to our understanding of the molecular mechanism underlying the transcriptional control of biomass production, which not only opens an unprecedented avenue to further dissect the molecular mechanisms underlying wood formation but also provides molecular tools to genetically engineering the quality and quantity of wood in tree species. We have published thirteen papers reporting our findings during this NSF funding period. One research associate, three graduate students and nine undergraduate students participated in this project and received extensive trainings in molecular biology and genetics.
