Lignocellulosic biomass was used for thousands of years as animal feed and is now considered a great sugar source for biofuels production. It is composed mostly of secondary cell walls built with polysaccharide polymers that are embedded in lignin to reinforce the cell wall structure and maintain its integrity. Lignin is the primary material responsible for biomass recalcitrance to enzymatic hydrolysis. During plant development, deep reductions of lignin cause growth defects and often correlate with the loss of vessel integrity that adversely affects water and nutrient transport in plants. The work presented here describes a new approach to decrease lignin content while preventing vessel collapse and introduces a new strategy to boost transcription factor expression in native tissues. We used synthetic biology tools in Arabidopsis to rewire the secondary cell network by changing promoter-coding sequence associations. The result was a reduction in lignin and an increase in polysaccharide depositions in fibre cells. The promoter of a key lignin gene, C4H, was replaced by the vessel-specific promoter of transcription factor VND6. This rewired lignin biosynthesis specifically for vessel formation while disconnecting C4H expression from the fibre regulatory network. Secondly, the promoter of the IRX8 gene, secondary cell wall glycosyltransferase, was used to express a new copy of the fibre transcription factor NST1, and as the IRX8 promoter is induced by NST1, this also created an artificial positive feedback loop (APFL). The combination of strategies—lignin rewiring with APFL insertion—enhances polysaccharide deposition in stems without over-lignifying them, resulting in higher sugar yields after enzymatic hydrolysis.

Glycosyltransferases of the Cellulose Synthase Like D (CSLD) subfamily have been reported to be involved in tip growth and stem development in Arabidopsis. The csld2 and csld3 mutants are root hair defective and the csld5 mutant has reduced stem growth. In this study, we produced double and triple knockout mutants of CSLD2, CSLD3, and CSLD5. Unlike the single mutants and the csld2/csld3 double mutant, the csld2/csld5, csld3/csld5, and csld2/ csld3/csld5 mutants were dwarfed and showed severely reduced viability. This demonstrates that the cooperative activities of CSLD2, CSLD3, and CSLD5 are required for normal Arabidopsis development, and that they are involved in important processes besides the specialized role in tip growth. The mutant phenotypes indicate that CSLD2 and CSLD3 have overlapping functions with CSLD5 in early plant development, whereas the CSLD2 and CSLD3 proteins are non-redundant. To determine the biochemical function of CSLD proteins, we used transient expression in tobacco leaves. Microsomes containing heterologously expressed CSLD5 transferred mannose from GDP-mannose onto endogenous acceptors. The same activity was detected when CSLD2 and CSLD3 were co-expressed but not when they were expressed separately. With monosaccharides as exogenous acceptors, microsomal preparations from CSLD5-expressing plants mediated the transfer of mannose from GDP-mannose onto mannose. These results were supported by immunodetection studies that showed reduced levels of a mannan epitope in the cell walls of stem interfascicular fibers and xylem vessels of the csld2/csld3/csld5 mutant.

Stem cells and progenitors in many lineages undergo self- renewing divisions, but the extracellular and intracellular proteins that regulate this process are largely unknown. Glucocorticoids stimulate red cell formation by promoting self-renewal of early erythroid burst forming unit-erythrocyte (BFU-E) progenitors1-4. Here we show that the RNA binding protein Zfp36l2 is a transcriptional target of the glucocorticoid receptor (GR) in BFU-Es and is required for BFU-E self-renewal. Zfp36l2 is normally downregulated during erythroid differentiation from the BFU-E stage but its expression is maintained by all tested GR agonists that stimulate BFU-E self-renewal, and the GR binds to several potential enhancer regions of Zfp36l2. Knockdown of Zfp36l2 in cultured BFU-E cells did not affect the rate of cell division but disrupted glucocorticoid-induced BFU-E self-renewal, and knockdown of Zfp36l2 in transplanted erythroid progenitors prevented expansion of erythroid lineage progenitors normally seen following induction of anemia by phenylhydrazine treatment. Zfp36l2 preferentially binds to mRNAs that are induced or maintained at high expression levels during terminal erythroid differentiation and negatively regulates their expression levels. Thus Zfp36l2 functions as part of molecular switch promoting BFU-E self-renewal and thus a subsequent increase in the total numbers of CFU-E progenitors and erythroid cells that are generated.

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