Transcriptomic analysis of Picea abies tissue culture reveals the impact of culture conditions and the presence of glucuronoxylan on extracellular lignin production

Tissue cultures are an important study model for woody plant tissue and can be used to study lignin biosynthesis. The greatest disadvantage of protocols based on extraction of lignin from wood biomass is the almost inevitable alteration of the native structure of lignin. Using a Norway spruce tissue culture with the ability to secrete monolignols into a liquid culture medium, fundamental aspects of lignin have been studied in the past, such as its structure, the enzyme activity related to its polymerization, and its interactions with a secondary cell wall hemicellulose. In this study, parameters that can induce monolignol production and secretion in the tissue culture are investigated via gene expression analysis. The impact of the composition of the solid growth medium, which was in some cases supplemented with xylan, was studied in depth through transcriptomic investigation. We find that the state (i.e. liquid or solid) and the xylan content of the medium can impact gene expression, although microscopic analysis suggests that cellular morphology is consistent. Extracellular lignin was collected from a formulation of liquid medium with the same composition as that used for cellular growth, which was previously presumed to be "non-inducing" of lignin biosynthesis. Chemical analysis of this lignin was performed using nuclear magnetic resonance spectroscopy and size exclusion chromatography, which revealed changes in its structure compared to the polymer produced in the previously developed "inducing" liquid medium. These experiments show that there is still much we do not understand about an oft-used tissue culture system, but show the way to a deeper understanding of the genetic control of lignin biosynthesis.