Elucidating molecular level details of the plant Borate transporter structure and function.

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All cells are surrounded by a membrane made up of fatty lipid molecules. This membrane acts as an effective barrier separating the contents of the cell from the external environment. The lipid membrane itself is impermeable to all but a limited number of molecules, however cells need to have a means of taking up key nutrients and removing waste products. The import and export of a wide range of molecules across the membrane is mediated via a system of specialized proteins called membrane transporters, which are embedded into the lipid layer. These transporter proteins bind a specific substrate or cargo on one side of the membrane, undergo a reconfiguration and then release the substrate on the other side of the membrane. The ability to transport key nutrients into and out of plant cells is fundamental to healthy plant growth and development. However, our current understanding of how plants take up minerals and precisely regulate their concentration inside cells is very limited due to a lack of detailed information on the transport proteins responsible for their uptake and distribution. One key nutrient is boron which is taken up from the soil by the roots and has essential roles in the formation and strength of the plant cell wall. Plants are tolerant of a very narrow range between deficient and toxic soil solution concentration. Boron deficiency causes reduced plant size and impaired seed production, both of which have a major impact on crop yield. Boron toxicity also results in compromised plant growth and development. Vast areas of agricultural land in China and West and Central Africa have poor fertility as a result of boron deficiency, while land in Australia, Turkey and Chile is affected by boron toxicity. BOR1 is a membrane transporter with roles in boron distribution from plant roots to other growing tissues. It is very important to understand precisely in what form and how boron is taken up into cells through BOR1. Central to increasing understanding of the operation of BOR1 is a technique called X-ray crystallography which allows us to obtain very detailed information on the arrangement of the atoms within a protein structure. Obtaining such detailed structures of membrane proteins such as transporters remains very challenging as it is necessary to remove the proteins from their membrane environment into a detergent containing solution so they can be isolated and crystallised. We have made substantial progress towards obtaining a detailed structure of BOR1 having obtained well diffracting crystals of BOR1 from both the model plant, mouse-ear cress, and the important crop, rice. We will additionally complement the structural studies with analysis of the protein in plant cells and whole plants. The techniques we will use allow us to study the protein in a native environment and also assess the functionality of a range of mutant BOR1s. Taken together with the structure, the plant based functional analysis will provide an opportunity to build a uniquely detailed picture of how BOR1 works. This should form the basis of future studies attempting to generate plants with improved tolerance to suboptimal soil boron concentrations.