Callose Synthases: What do they do in the Arabidopsis root?

University Of Washington, Seattle WA

Investigators

Abstract

Callose, a b-1,3-glucan, is an important polysaccharide in higher plants. it is produced in response to wounding, and is part of the plant's defenses against pathogens. But callose is also a constituent of intact plants, where its role is far from clear. It may be essential for the formation of cross walls during plant cell division, and it may be required for pollen tube growth. But beyond this, little is known. It is known that callose can cause the closure of plasmodesmata, the protoplasmic bridges that interconnect plant cells. This will isolate cells from each other. It is known that there are groups of cells in plants that are isolated from other groups of cells; we call such a group of cells a "symplasmic domain". We have proposed that a major role of callose is to generate and maintain these symplasmic domains. Synthesis of callose is mediated by a large multisubunit complex, of which callose synthase (CalS) is the catalytic subunit. The "model" plant Arabidopsis thaliana contains 17 putative CalS genes. We have shown that 8 of these are expressed in intact Arabidopsis roots. The goal of this research is to determine the role of each of these CalS genes, and to determine whether one or more of these CalS proteins is involved in the production of callose that closes plasmodesmata. We will use two approaches to determine this. First, we are preparing a series of single-amino acid substitution mutants in each of these genes, using the TILLING procedure. These should be weak mutants that will affect the phenotype of the plants without being lethal. We have T-DNA insertion mutants for 5 of these genes, and as expected, some of these strong mutants are lethal. The ones that are not lethal will be used, along with the TILLING mutants, to study the role of each of these genes in the root. The second approach will be to generate promoter::Green Fluorescent Protein fusions for each of the 8 CalS genes, and then introduced them into Arabidopsis plants. This will allow us to determine in which cells each of these genes is expressed, and to study the conditions that activate each gene. Together, these studies should show us where and when each of the callose synthase genes is active, and should demonstrate the effects on the root when any particular callose synthase gene is reduced in activity. It should provide a starting place for future work on the role of callose in all parts of Arabidopsis, and in other plant species.