Lignins, (Oligo)lignans, and

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Lewis This particular project will establish as the first objective whether an entire class of dirigent proteins exists in Eucommia, Linum, Schizandra and Larrea species, and which individually differ in the distinctive 8-8' coupling modes stipulated. It is anticipated that each selected plant species harbors a distinct dirigent protein which differs in either preferred substrate utilized and/or stereoselectivity engendered during radical-radical coupling. Using either cDNA library screening and/or RT-PCR approaches, the dirigent protein homologues from each species will be obtained, with specific coupling modes encoded determined through heterologous expression of functionally competent recombinant dirigent proteins. Additionally, this phase of the study will include identifying the tissues and developmental stages involved in dirigent protein gene expression/protein localization in these particular species, at tissue and (sub)cellular levels. In the second objective of this study, we will seek to demonstrate that lignification in the plant cell wall is stipulated through the participation of arrays of dirigent protein sites (or an equivalent thereof) which control both linkage type and monomer(s) involved. Particular attention will be given to demonstrating formation of the so-called 8-O-4' linkage in lignins obtained in vitro, since this linkage predominates in lignins in vivo. The lignin biopolymers and (oligomeric) lignans account for nearly 30% of organic carbon circulating in the biosphere, being major constituents of vascular plants. The lignins are structural cell wall constituents without which plants would be unable to either stand vertically and/or form their woody tissues; the (oligomeric) lignans, while derived from the same precursors as lignins, are employed in plant defense roles (e.g. as antioxidants, bactericides, antiviral and antifeedant compounds, etc.). The current project builds on our recent discovery of dirigent proteins which, in turn, gives incisive insight into at least one of the principal mechanisms whereby living organisms (plants) can control the outcome of radical-radical coupling. Until this discovery, such conversions had been considered as solely in the realm of non-specific oxidative conversions, such as those catalyzed by O2/laccases and peroxidases/H2O2.