Research Associate - Dr Patrick Loughlin
Phone: +61 2 9351 8668
Fax: +61 2 9351 4771
A12 - Macleay
The University of Sydney
NSW 2006 Australia
My research in the Plant Molecular Biology lab is focused on the symbiotic relationship between legumes and soil bacteria of the Rhizobia spp. This relationship is of considerable economic and environmental significance and is responsible for a large proportion of nitrogen input into the biosphere. This is because rhizobia are able to convert inert atmospheric nitrogen to the bioavailable form ammonium. Symbiotic rhizobia, termed bacteroids, are housed in novel plant root organs called nodules. Rhizobia enter the nodule primordium through a membrane invagination (the infection thread) and are endocytosed into ‘infected’ cells, enclosed within the symbiosome membrane. This membrane is of plant origin and has a unique complement of proteins required for mediating nutrient exchange between the plant and bacteroid.
It is a particularly exciting time for the genetic dissection of the interaction between soybean and its rhizobial symbiont Bradyrhizobium japonicum with the recent release of the soybean genome sequence to complement the already available rhizobia genome. My research specifically involves identifying and characterising transport proteins that localise to the symbiosome membrane of the model legume soybean (Glycine max). At present only a few symbiosome membrane proteins have been characterised however biochemical evidence suggests many more proteins are localised to this membrane. This is not surprising considering the intimate nature of this symbiosis with the rhizobia being entirely dependent on its legume host. Indeed the symbiosome may be considered a facultative organelle and is suggested by some to be an evolutionary intermediate step towards the formation of an organelle like the mitochondrion or plastid. Of particular interest for myself is the provision of reduced carbon and iron to the bacteroid. Transport of reduced carbon from the plant to the bacteroid, probably in the form of dicarboxylates, is an essential component of the symbiosis as the bacteroid requires large amounts of energy (approximately 16 ATP molecules hydrolysed/N2 fixed) to fix nitrogen. Another requirement for the bacteroid is iron, which is needed both in the nitrogenase enzyme, and proteins in the electron transport chain. I am building on previous work which has identified a number of soybean genes that share homology with known dicarboxylate and iron transporters from other plant species.
I am examining their expression level in developing and mature nodules using qPCR as well as GFP fusion and immuno-localisation techniques for subcellular localisation of the proteins. We have a robust, simple method for genetic transformation of soybean roots and nodules, allowing use of a suite of vectors for GFP fusion, silencing, overexpression and promoter analysis of various soybean genes. Future work will include identification and characterisation of other symbiosome membrane transport proteins and the functional characterisation of identified candidates in heterologous systems such as yeast, Xenopus oocytes and A. thaliana.