Genetic analysis of mechanisms linking cell wall integrity with growth control in Arabidopsis

Cell wall materials represent the major carbohydrate component in most plants. Recent research has identified genes encoding enzymes involved in synthesising different cell wall components, but many other aspects of cell wall function and organisation remain to be investigated. We have recently shown that several mutants in genes encoding enzymes synthesising sugars destined for incorporation into cell wall polysaccharides exhibit strong sugar-dependent growth and developmental phenotypes in the dark and elevated starch levels. These phenotypes require a well characterised protein involved in regulating sugar, hormone and light responses. We also showed that changes in cell wall structure/composition are most likely responsible for the observed phenotypes. We aim to define other components of this pathway using genetic screens and establish links with known sugar- response pathways. The outcomes of this research will establish, possibly for the first time, that plant cell wall composition and levels are regulated in response to carbohydrate availability. This knowledge will provide an understanding of how resources are allocated to cell wall synthesis and may also help design crops that allocate more resources to cell wall biomass. Understanding how plant cell wall composition is controlled is a highly relevant objective at the moment because of the potential for converting cell wall material to biofuels and chemical feedstocks. A major research objective in this area is to alter cell wall composition in biomass crops such as rapidly growing willow, poplar and perennial grasses. Several studies show that attempts to alter cell wall composition lead to major, and somewhat unexpected, cellular changes such as activation of stress and defence responses. These then lead to undesirable changes in cell activity and cell wall composition. These results suggest that attempts to alter cell wall composition, for example to tailor cell wall composition for more efficient degradation, require a lot more knowledge about how the composition of the cell wall is controlled. The work described here will add to this knowledge in the model plant Arabidopsis. Knowledge of the genes involved might ultimately be used to engineer altered cell wall control in biomass crops, for example to increase levels of cell wall polysaccharides. These crops may provide sustainable and cost-effective sources for fuel production and help to ameliorate climate change by more effective use of the global carbon cycle.

We propose to identify genes in a putative signalling pathway that links changes in cell wall composition to sugar- response pathways that control growth, development and storage processes. We have characterised a gene called HSR8 in a genetic screen for sugar-responsive growth and development in Arabidopsis. HSR8 is allelic to MUR4, which catalyses arabinose formation in the Golgi compartment where it is destined for cell wall polysaccharide biosynthesis. hsr8 mutations are suppressed by prl1, a mutation affecting sugar and hormonal- responsive signalling and gene expression. This suggests that cell wall changes caused by hsr8 and two other cell wall biosynthetic mutants, mur1-1 and mur3-1, are signalled to increase sugar- dependent growth and development via PRL1. We showed that treatment of seedlings with low levels of borate reverses the growth and developmental phenotypes in these three mutants. Borate acts in the cell wall to cross link polysaccharides such as RG II and pectins. Altered cell wall polysaccharide composition weakens these cross links and high borate levels can reverse this. The sugar-dependent growth and developmental phenotypes observed in the hsr8 and other mutants are robust and quantitative and will be used in a genetic screen to identify new components in this pathway and characterise their function. We will also establish genetic interactions with known sugar signalling pathways to build up a more integrated view of how cell wall changes are sensed and signalled to the cell, for example to sugar- response pathways that regulate growth and resource allocation. In the long term we aim to establish sufficient knowledge about cell wall signalling pathways and resource allocation to help to manipulate cell wall synthesis in bioenergy crops- for example by manipulating carbohydrate incorporation into cell wall synthesis.