Metabolic basis of the borate cross-linking of rhamnogalacturonan-II a plant cell wall polysaccharide

Unlike animals and microbes, plants have a clear requirement for the element boron (B), which they obtain from soluble boric acid naturally present in soil. This unique and agriculturally important feature of plant life is poorly understood biochemically, and will be explored here from several novel perspectives. Adequate boron is essential throughout the plant, but particularly in growing tissues; boron is thought to be central to the mechanism of plant cell expansion (growth). Although boron deficiency can be cured by fertilisers, excess boron in the soil is a serious and intractable agricultural problem in some places. Understanding why plants require boron, and why excess boron is toxic, will facilitate progress towards both a fundamental understanding of the mechanism of plant growth and the optimisation of crop yields. Boron is necessary for the plant to assemble normal cell walls, which are key to the mechanism of cell expansion. The principal role of the boron is to form a bridge between two molecules of a minor type of cell-wall pectin called rhamnogalacturonan-II (RGII). Although minor quantitatively, RGII evidently serves a major purpose in the life of the plant since its boron bridges are essential for plant growth. Previous work has revealed the exact atoms of RGII which are bridged by the boron atom. A second role of boron is in the proper functioning of the plant's membranes, for example controlling the uptake of other important nutrients such as potassium. Boron also seems to help the cell membrane to remain correctly attached to the cell wall. However, we don't yet know what is the particular molecule of the plant membrane that interacts with boron for these purposes. Despite our detailed knowledge of the (static) chemical structure of boron-RGII bridges in the plant cell wall, we know nothing about how the bridging process occurs: when and where in the cell, and whether catalysed by enzymes. This project will explore the (dynamic) bridging process, potentially leading to the discovery of highly novel enzyme activities: to date, there are no known enzymes that act on boron compounds. Specifically, we will work out at what stage in the 'career' of an RGII molecule, and where in the cell, it normally becomes boron-bridged in healthy plant tissues. We will also discover whether boron-RGII bridges are permanent or if the boron can later be re-cycled to different RGII molecules. Importantly, we do not yet know what the 'players' are in the boron-bridging process. We will explore the idea that the (negatively charged) RGII is 'chaperoned' by some other (positively charged) molecule at the moment of bridging; and we will test whether the boron used for making the bridge is donated by boric acid itself or by some special boron carrier such as a water-soluble sugar-related substance or a water-insoluble lipid. A boron-carrying lipid could in principle be the 'particular molecule of the plant membrane that interacts with boron', mentioned above. We will explore these possibilities, developing new methodologies where necessary. At this stage, the ideas outlined above are mainly hypotheses (for testing), not asserted as 'facts'. Some will turn out to be false starts and we will not waste time pursuing them beyond their useful lives. However, it is valuable to have such hypotheses in mind during the exploration of new scientific avenues. The impact of the knowledge generated in this project would allow us in the future to manipulate boron bridging and bridge severance in crop plants, thus potentially enhancing crop production on soils with excess or insufficient boron. This could be done either by plant breeding or by optimised use of borate-containing fertilisers.

The project will explore the following new hypotheses, mainly by a combination of in-vivo and in-vitro radiolabelling studies: 1. The borate-bridging of rhamnogalacturonan-II (RGII) in vivo normally occurs in a specific subcellular compartment and at a particular time after the RGII polysaccharide structure has been assembled. 2. The borate-bridges in RGII may undergo turnover in vivo, with B transferring to different RGIIs. 3. The true polysaccharide substrate for B-bridging (= borate acceptor substrate) may not be pure RGII but a complex in which the RGII is a component of a large pectin chain and/or chaperoned by a polycation. 4. The true borate donor substrate for B-bridging of RGII may not be free boric acid but a complex of borate with a non-pectic organic ligand -- either water-soluble in the apoplast or membrane-bound. 5. The borate-bridging of RGII may be catalysed by novel enzymes: a borate esterase acting in the 'ester synthase' direction, and/or a boryltransferase transferring borate from donor substrates to RGII. 6. The severance (turnover) of RGII-borate bridges may be catalysed by a novel borate esterase and/or a 'boryltransferase'. 7. A plant-specific, B-binding membrane component may exist, accounting for the role of B in plant membrane function. 8. Such a membrane component may form mixed-function borate bridges with RGII, establishing membrane/wall attachment sites. 9. I hypothesise that in the presence of a supra-optimal boric acid supply most RGII domains quickly bind one B atom, leaving very little B-free RGII, and thus preventing bridging reactions. 10. Methodologically, gel electrophoresis may be useful for RGII monomer/dimer fractionation and [14C]apiose may be valuable for in-vivo radiolabelling of RGII. The impact of the results relates to food/fodder/biomass production in soils containing excess or insufficient boron, and to our fundamental understanding of the mechanics of plant growth.