Differentiation of Tracheary Elements in Sugarcane Suspension Cells Involves Changes in Secondary Wall Deposition and Extensive Transcriptional Reprogramming.

Marcella Siqueira Simões,Staffan Persson,André Ferraz,Jorge Rencoret,Adriana Grandis,José C. del Río,Eny Iochevet Segal Floh,Sávio Siqueira Ferreira,Marcos Silveira Buckeridge,Igor Cesarino

doi:10.3389/fpls.2020.617020

Abstract

Plant lignocellulosic biomass, mostly composed of polysaccharide-rich secondary cell walls (SCWs), provides fermentable sugars that may be used to produce biofuels and biomaterials. However, the complex chemical composition and physical structure of SCWs hinder efficient processing of plant biomass. Understanding the molecular mechanisms underlying SCW deposition is, thus, essential to optimize bioenergy feedstocks. Here, we establish a xylogenic culture as a model system to study SCW deposition in sugarcane; the first of its kind in a C4 grass species. We used auxin and brassinolide to differentiate sugarcane suspension cells into tracheary elements, which showed metaxylem-like reticulate or pitted SCW patterning. The differentiation led to increased lignin levels, mainly caused by S-lignin units, and a rise in p-coumarate, leading to increased p-coumarate:ferulate ratios. RNAseq analysis revealed massive transcriptional reprogramming during differentiation, with upregulation of genes associated with cell wall biogenesis and phenylpropanoid metabolism and downregulation of genes related to cell division and primary metabolism. To better understand the differentiation process, we constructed regulatory networks of transcription factors and SCW-related genes based on co-expression analyses. Accordingly, we found multiple regulatory modules that may underpin SCW deposition in sugarcane. Our results provide important insights and resources to identify biotechnological strategies for sugarcane biomass optimization.

Highlights

Sugarcane (Saccharum spp. hybrids), as a C4 plant, is very efficient in generating high yields of biomass with minimal inputs and has the unique ability within the Poaceae family to accumulate high amounts of sucrose in its mature stem (Diniz et al, 2019)
To establish a xylogenic culture to study SCW deposition in sugarcane, we generated a suspension cell system from friable callus derived from meristematic stem tissues of greenhouse grown SP80-3280, a cultivar used in Brazilian breeding programs for which a 373k gene space of its genome was recently sequenced and assembled (Souza et al, 2019)
We determined efficient in vitro conditions to induce tracheary elements (TEs) differentiation by transferring suspension cells into various media containing different concentrations of 2,4-D and brassinolide (BL), based on a protocol previously established for lignification of suspension cells of switchgrass (Shen et al, 2013; Rao et al, 2017), a grass species phylogenetically close to sugarcane

Summary

Introduction

Sugarcane (Saccharum spp. hybrids), as a C4 plant, is very efficient in generating high yields of biomass with minimal inputs and has the unique ability within the Poaceae family to accumulate high amounts of sucrose in its mature stem (Diniz et al, 2019). The SCW-rich residue produced after sucrose extraction, is currently burned to generate power and electricity for the production of sugar and ethanol in the mills. It can be used in part as lignocellulosic feedstock, in which SCW polysaccharides are converted into monomeric sugars for fermentation (Klein et al, 2019). Due to the complex chemical composition and physical structure of SCWs, processing of plant biomass (including sugarcane bagasse) into downstream products is still considered to be relatively expensive, negatively affecting the transition from an oil-based economy toward a sustainable bio-based economy. Unraveling the molecular mechanisms underlying SCW deposition in sugarcane is essential for unlocking the economic potential of the bagasse as lignocellulosic feedstock

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Conclusion