Plant cell expansion is regulated by O-glycoproteins of the AGP type

The major mode of morphogenesis in plants is oriented cell expansion. The fibrous polymers of the plant cell wall have to be properly deposited and undergo specific remodelling processes to enable cell expansion. The machinery of cell wall performance and integrity control is thought to play a crucial role in the coordination of these intricate processes. There is evidence that O-glycoproteins of the arabinogalactan-protein (AGP) type are required for the process of plant cell elongation. The root cell elongation defect caused by mutations in the Arabidopsis thaliana FASCICLIN LIKE AGP 4 gene (FLA4) is a genetic paradigm for this role and was used as a background for a phenotypic suppressor screen. The identified loci define genetic intersections between the role of FLA4 in cell elongation, in the control of functional cellulose biosynthesis, in abiotic stress and potentially in ethylene independent ACC signalling. Here it is proposed to investigate the molecular identities of several suppressor loci of fla4 (slof) that are specifically involved in the requirement for FLA4 to obtain normal cell length. In a collaboration with the lab of Jose Manuel Estevez, University of Buenos Aires who have isolated genetic suppressors of the role of AGPs in hypocotyl elongation we plan to integrate our two mutant collections identified in independent but functionally related screens and to investigate the cellular properties of the newly identified gene products. This joint effort will expand the knowledge on the regulation of plant cell expansion and thereby widen the understanding of growth processes in crop species.

The growth of plant cells is a complex process that integrates developmental programs with environmental stress to establish the growing cell wall - a mechanically strong and dynamic network of carbohydrates and glycosylated proteins. While the role of cell wall polysaccharides such as cellulose, hemicellulose and pectin is increasingly well understood there are huge gaps of knowledge on the function of glycoproteins. Here we focused on a particular cell wall protein called Fasciclin like Arabinogalactan Protein 4 (FLA4). Mutant plants lacking a normal FLA4 gene grow abnormally and are less tolerant to salt and produce seeds that are less sticky than from normal plants. The FLA4 protein has different parts such as two Fasciclin domains that can be found in many other organisms including humans, and is thought to be decorated with a lipid anchor and diverse types of sugar molecules. We wanted to find out how FLA4 functions in living plants and how its different parts partake in its function. We succeeded to restore normal growth to fla4 mutants by expressing a normally FLA4 gene that is fused to a gene from jelly fish coding for yellow fluorescent protein (YFP). Thus we could microscopically observe YFP-FLA4 in great detail. To study its different parts and modifications we also constructed mutated forms of YFP-FLA4. Moreover combined the fla4 mutant with numerous other mutants defective in the control of growth and development to better understand the role of FLA4 in this process. We found that FLA4 might perform a complex role in the cell wall sending signals to the cell in order to control development using parts of the machinery that responds to the growth regulators brassinosteroid and abscisic acid. However, FLA4 might also to make physical connections to the adherent part of cell wall polysaccharides called pectin in the seed coat. For the first time a member of the large family of Arabinogalactan Proteins (AGPs) was thoroughly analyzed to understand the biological meaning of its extremely complex structure. Future applications of this basic work could potentially contribute to breed better growing or stress tolerant improved agricultural crops.