Research Student Profile: Victoria Clarke

Project Title: Unraveling the mechanisms of solute transport across the soybean symbiosome membrane: An interface between plant and bacteria

Supervisors

Dr. Penny Smith, Prof. Robyn Overall and Prof. David Day

Project Overview

Soybeans are able to form a symbiotic association with soil bacteria, Bradyrhizobium japonicum, whereby atmospheric nitrogen is fixed by the bacteria and made available to the plant in exchange for organic acids and other nutrients. This symbiotic relationship occurs within specialised root structures termed nodules. Free-living Bradyrhizobium bacteria invade the cortical cells of soybean roots and become engulfed within the plant cell, surrounded by a membrane of plant origin known as the symbiosome membrane. It is this membrane that regulates the movement of solutes from plant to bacteroid (the symbiotic form of the rhizobium) and vice versa. Previous work has identified several transporters on this membrane, but many are still to be classified. These include transporters for the main flux of nutrients in and out of the bacteriod: ammonium (bacteroid to plant), and malate (plant to bacteroid). My project aims to identify and characterise additional transporters on the symbiosome membrane. I will be undertaking the first comprehensive proteomic analysis of the membrane, and also specifically investigating amino acid transport across the membrane.

The Symbiosome

Investigating the entire protein content of the symbiosome membrane has previously been attempted, but was hindered by the lack of comprehensive genome data. This problem has now been overcome by the complete sequencing of the soybean genome in 2008. Soybean is the first legume to have its genome sequence completed and hence is an excellent model for studying bacteroid-plant interactions. Isolation of the symbiosome membrane is also a well established technique in soybean and yields a high-purity membrane fraction. This aspect of my project aims to create a comprehensive database of integral symbiosome membrane proteins, of which promising transporter candidates will be further investigated.

There is evidence to suggest that the majority of nitrogen produced by bacteroids is exported and assimilated into the plant, making the bacteroids auxotrophs for nitrogen. Recent work in pea suggests the plant host rescues the bacteroids by providing the bacteroids nitrogen as branched chain amino acids, and it is expected that there will be amino acid transporters on the symbiosome membrane to facilitate this. Using previously characterized amino acid transporters from Arabidopsis thaliana, I have identified a large number of soybean gene candidates, and these will be characterised as part of my project. As well as classifying symbiosome membrane localised transporters, it is hoped that transporters on plasma and vacuolar membranes in various tissues will be identified.

Predicted Structure of a soybean amino acid transporter

Predicted Structure of a soybean amino acid transporter

Background

I obtained a Bachelor of Biotechnology degree at the University of Tasmania in 2007. In 2008 I completed an Honours year (first class) in plant genetics, also at the University of Tasmania and I received the Professor Newton Barber Honours scholarship. My Honours thesis investigated DELLA proteins (integrators of plant hormone signals) and their varied roles within the plant. My project focused on what role DELLA proteins may be playing in the integration of the auxin and gibberellin, through effects on the biosynthesis of these two phytohormones. I relocated to Sydney in 2009 to undertake post-graduate research at the University of Sydney.