Exploiting root exudation of organic acids and phytases to enhance plant utilisation of soil phosphorus

Phosphorus (P) is a non-renewable resource, essential for crop production. Uncertainties over mineral P supplies coupled with concerns for food security and environmental impact of P on waters all necessitate an improvement in agronomic P efficiencies, based on sound knowledge of the range of P forms in soils. Our research is different to previous approaches to recover P from soils in that it focuses on the organic P (Po) components and uses a novel combination of root exudates from different plants to solubilise organic P and make it bioavailable as inorganic orthophosphate.

The two fundamental problems associated with the use of soil organic P by plants are that much of it is strongly attached to soil particles and therefore inaccessible to plants, and secondly, even when not firmly attached to soil particles, the forms in which it exists are not readily available for plant uptake. Some plants possess traits that can help access organic P in soils; firstly some plants can produce organic acids from their roots, which can release the P attached to soil particles, and secondly some can release phosphatases (e.g. phytase), which can hydrolyse the organic P into forms which plants can take-up. However, crop plants generally do not possess both these traits and so combinations of plants are required, each carrying out a different role. Systems that rely on clover undersown into cereal crops to provide nitrogen (N) to the current crop, act as an overwinter green manure, and to provide N to following crops are fairly common in organic farming enterprises. We have clover lines which can produce phytase required to mineralize organic P, while some strains of barley have been shown to release organic acids, making the organic P available for mineralisation. We will investigate a clover/barley bi-cropping system as an exemplar sustainable alternative to intensive applications of P and N fertilizers, thus potentially making the arable system more efficient both economically and environmentally. Thus we shall test the overarching hypothesis that: Cropping systems with roots exuding both organic acid anions and phytase can facilitate more sustainable agricultural production by accessing soil organic P forms.

Specifically, we propose to investigate and understand the role of organic acids and phosphatases in plant mixtures in accessing the organic P from the inositol phosphate pool. We shall then explore what happens to these organic P forms in the soils and the rhizosphere and examine their potential (or otherwise) for leaching from the soil to surface waters where they may cause eutrophication. A range of experiments will be carried out to identify potentially suitable strains of barley and clover for such a system, followed by experiments incorporating different combinations of these strains, grown in soil, to assess what combinations can most efficiently access different forms of organic P, while minimizing leaching losses. By increasing the amount of P utilized from the P stored in soils we can reduce the reliance on inorganic fertilisers, increasing agricultural sustainability and improving our ability to deliver food security in coming decades.

Organic phosphorus is a common constituent of the P in many soils and comprises mostly of esters of fully oxidised P where P is attached to C through an O atom. Other compounds such as phosphonates and organic polyphosphates occur in lesser amounts. Our appreciation of the extent to which crop plants can utilise organic P pools in soils is limited. The overarching aim of the project is to test the hypothesis that: Cropping systems with roots exuding both organic acid anions and phytase can facilitate more sustainable agricultural production by accessing soil organic P forms.

We will investigate and attempt to understand the role of organic acid anions and phosphatases in exemplar plant mixtures in accessing organic P in soils. We shall then explore what happens to these compounds in the soils and the rhizosphere. A range of experiments will be carried out to identify potentially suitable strains of barley and clover for such a system, followed by experiments incorporating different combinations of these strains, grown in soil, to assess what combinations can most efficiently access different forms of organic P, while minimizing the losses of P via leachate. Experiments and techniques employed will include screening of plant populations for abilities to exude organic acids and phosphatases using anion exchange resins (AERs) and DGT (diffusive gradients in thin films), HPLC, enzyme hydrolysis and 31P nuclear magnetic resonance (NMR) analysis, for identification of organic P compounds and groups in soil extracts and soil water.

The subject of this proposal is highly relevant to current policy in the UK, with innovation and efficiency of nutrient use being highlighted as one of the key factors in developing both national and global sustainable food security. This was the subject of a recent Foresight Workshop on 'Stimulating Innovation and Efficiency in Fertiliser Production and Use' (20th September 2012), convened by the UK Government Office for Science and attended by two of the proposing group. At the workshop the Government Chief Scientific Advisor, Sir John Beddington, highlighted the urgent need to address food production issues of which sustainable nutrient use is a vital component, but stated that governments have been slow to realise this urgency and to acknowledge the indisputable role fertiliser use will play. One of the conclusions from the workshop, attended by leading UK and international academic researchers and fertiliser industry specialists, was that improved phosphorus acquisition traits in crops and cropping systems will play an important role in achieving these goals.

This proposed project will explore technologies to improve crop productivity and nutrient use efficiency by enhancing the availability to plants of organic P in soils (which is relatively unavailable, but plentiful), whilst also minimising any potential negative environmental consequences. It therefore has implications to help agricultural producers and scientists, fertiliser suppliers, crop breeders, land and catchment managers through to policy makers. Knowledge of organic P species in soils is fundamental to the development of new plant varieties able to hydrolyse the organic P resources which may occur in different soil types and therefore this project is of particular interest to plant breeders. Additionally, it will assist with development of cropping techniques such as bi-cropping for maximizing the benefits of different plant traits.

Ultimately this work could potentially inform policy through providing information on how to maximize crop yields while minimizing fertiliser inputs. Results could influence the way in which cropping systems are considered in the future both nationally and internationally, providing some fundamental science supporting their development, based on more than just yield/productivity, but also on the specific soil/plant processes involved. In the long term this work could contribute to the nation's wealth by providing guidance on more nutrient efficient cropping techniques and indicating which traits in plants should be developed, and how they may work in tandem to maximize productivity while minimizing fertiliser usage. It may also mean that the nation becomes less reliant on the increasingly scarce global mineral P resources, which are not only becoming more expensive, but also in a time of political instability, could potentially become inaccessible. Internationally, the approaches developed here could mean increases in crop yields in under-developed regions where fertilisers are unavailable. The basic principle behind the science proposed here is relatively simple to convey, and in combination with its strategic importance to BBSRC with regard to Food Security, makes it an ideal topic to showcase to the public. This will be done at all the participating Institutes' Open days/School Science Week events, meaning it would not only have benefits to the current scientific community, but may also stimulate a new generation of scientists into this field of research. This project addresses the BBSRC strategic research priority areas of crop science (food security) and global security.