PLANT CELL WALL · Functional analysis of the sugar based signalling process coordinating plant cell wall, primary metabolism and photosynthesis

Functional analysis of the sugar based signalling process coordinating plant cell wall, primary metabolism and photosynthesis

Dwindling supplies of fossil fuels and growing energy demands make biomass more and more important for bioenergy production. Biomass consists mostly of plant cell walls, which are complex and highly dynamic structures, changing composition and structure during growth/development and in response to biotic and abiotic stresses. Cellulose is the most abundant biopolymer in the world, the main load-bearing element in plant cell walls, represents both a major sink for carbon fixed during photosynthesis in plant cells and a possible source for glucose to generate biofuels. The aim of this project was to understand the role of hexoses in the response of the plant cell to wall damage and to characterise the regulatory mechanism that coordinates cellulose biosynthesis with photosynthetic activity and carbohydrate metabolism. In this study, the pre-emergence herbicide isoxaben was used to cause cell wall damage through cellulose biosynthesis inhibition (CBI) in Arabidopsis seedlings grown in liquid culture. Here we show that cellulose biosynthesis inhibition does not cause changes in cytosolic glucose or sucrose contents, possibly due to a redistribution of sugars into the vacuole or because extracellular sugar levels are affected. The necessary tools/transgenic lines to pursue the alternative possibilities have been generated. Our preliminary data suggested two cell wall invertase genes (ATCWIN1 and six) could function as initial detectors for extracellular sugars during the response to CBI. Analysis of knock out mutants demonstrated that ATCWINV6 is required for the CBI-induced lignin deposition, whereas ATCWINV1 is not. Transgenic lines for a detailed functional analysis of ATCWINV6 have been generated and are now available. In addition, we demonstrate that CBI leads to transcriptional shutdown of genes involved in photosynthesis, the Calvin cycle and starch degradation. We show that CBI causes redirection of metabolic flux from the cell wall towards starch biosynthesis and reduction of Rubisco activity. The observed effects can be suppressed in a concentration dependent manner by providing osmotic support using PEG. In parallel, hyper-osmotic stress treatments (using KCl or NaCl) have opposite effects on starch and sucrose levels compared to CBI (a hypo-osmotic stress). In mid1 complementing activity (mca1, putative stretch activated Ca2+ channel), Arabidopsis histidine kinase (ahk4/cre1, osmosensor) and respiratory burst oxidase homolog DF (rbohDF, NADPH oxidase) seedlings the CBI-induced effects on starch metabolism and gene expression are not suppressed by osmotic support. Our findings reveal a novel regulatory mechanism coordinating cellulose biosynthesis with primary metabolism and photosynthetic activity. We demonstrate that the osmotic state of the cell affects carbohydrate metabolism and identify MCA1, AHK4/CRE1 and NADPHoxidase-derived ROS as components of a signaling mechanism translating osmotic signals into metabolic changes. These results are subject of a manuscript currently in preparation for PLoS Biology. A detailed final report including figures is attached as 'Final_report.pdf'

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