Roles of Aquaporins in Setaria viridis Stem Development and Sugar Storage.

Samantha A Mcgaughey,Robert T Furbank,Caitlin S Byrt,Hannah L Osborn,Joseph L Pegler,Lily Chen,Christopher P L Grof,Stephen D Tyerman

doi:10.3389/fpls.2016.01815

Abstract

Setaria viridis is a C4 grass used as a model for bioenergy feedstocks. The elongating internodes in developing S. viridis stems grow from an intercalary meristem at the base, and progress acropetally toward fully expanded cells that store sugar. During stem development and maturation, water flow is a driver of cell expansion and sugar delivery. As aquaporin proteins are implicated in regulating water flow, we analyzed elongating and mature internode transcriptomes to identify putative aquaporin encoding genes that had particularly high transcript levels during the distinct stages of internode cell expansion and maturation. We observed that SvPIP2;1 was highly expressed in internode regions undergoing cell expansion, and SvNIP2;2 was highly expressed in mature sugar accumulating regions. Gene co-expression analysis revealed SvNIP2;2 expression was highly correlated with the expression of five putative sugar transporters expressed in the S. viridis internode. To explore the function of the proteins encoded by SvPIP2;1 and SvNIP2;2, we expressed them in Xenopus laevis oocytes and tested their permeability to water. SvPIP2;1 and SvNIP2;2 functioned as water channels in X. laevis oocytes and their permeability was gated by pH. Our results indicate that SvPIP2;1 may function as a water channel in developing stems undergoing cell expansion and SvNIP2;2 is a candidate for retrieving water and possibly a yet to be determined solute from mature internodes. Future research will investigate whether changing the function of these proteins influences stem growth and sugar yield in S. viridis.

Highlights

The panicoid grasses sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), switchgrass (Panicum virgatum), and miscanthus (Miscanthus X giganteum) provide the majority of soluble sugars and lignocellulosic biomass used for food and biofuel production worldwide (Somerville et al, 2010; Waclawovsky et al, 2010)
The mature stems of grasses such as sugarcane can accumulate up to 1M sucrose, with up to 428 mM sucrose stored in the apoplasm (Hawker, 1985; Welbaum and Meinzer, 1990)
Published S. viridis elongating internode transcriptome data (Martin et al, 2016), and protein sequences of aquaporins identified in Arabidopsis, S. italica, barley, maize and rice were used to identify genes predicted to encode aquaporins that were highly expressed in stages of cell expansion and sugar accumulation

Summary

Introduction

The panicoid grasses sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), switchgrass (Panicum virgatum), and miscanthus (Miscanthus X giganteum) provide the majority of soluble sugars and lignocellulosic biomass used for food and biofuel production worldwide (Somerville et al, 2010; Waclawovsky et al, 2010). Grass stems have repeating units consisting of an internode positioned between two nodes that grow from intercalary meristems at the base; sugar, primarily sucrose, accumulates and is stored in mature cells at the top of the internode (Grof et al, 2013). Along this developmental gradient there is a transition from synthesis and deposition of primary cell walls through to establishment of thicker secondary cell walls. In addition to a high capacity for soluble sugar storage, carbohydrates are stored in cell walls of stem parenchyma cells (Botha and Black, 2000; Ermawar et al, 2015; Byrt et al, 2016a)

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Conclusion