Phosphorus cycling in the soil-microbe-plant continuum of agri-ecosystems

The element phosphorus (P) is an essential nutrient required by crops to ensure good growth and yields. Crops get their P from the soil via their roots in the form of phosphate (a phosphorus atom surrounded by four oxygen atoms, Pi). The availability of Pi for the crop in the soil depends on the soil type, its pH, the growth of bacteria and fungi in the soil and the amount of Pi the crop takes up. Unfortunately, P is very reactive and can get locked away in the soil either with other elements or in organic compounds, making it hard for the crop to acquire sufficient Pi. To overcome this, farmers add Pi fertilisers to the crop. However, Pi fertilisers are made from rock phosphate, a non-renewable resource, the availability of which is set to decline, and the price increase, over the coming decades. Excessive use of Pi fertilisers is also a problem as the Pi can be washed into local rivers and lakes and contributes to the process of eutrophication.

Since plants evolved over millions of years without Pi fertilisers, they are well equipped with adaptations to help improve the availability of Pi near their roots. Many of these adaptations have not been selected for directly when breeding crop varieties or they are not optimised for rapidly growing, high yielding crops. These adaptations included making more roots, releasing acids from their roots to free Pi bound to the soil, releasing enzymes from their roots to release Pi trapped in organic compounds and recruiting soil bacteria and fungi to help acquire Pi.

To help reduce our need for Pi fertilisers we will study these plant adaptations and the bacteria that grow near the roots of oilseed rape. We will begin by identifying the bacteria that live near the roots of these crop plants using next generation sequencing technology. This allows us to sequence the genomes of most of the bacteria living in the soil near the roots and identify them. We will also investigate the enzymes and proteins made by the bacteria and the root. These approaches will tell us about bacterial activity in the soil near the root and which processes they are contributing towards. Since the P can be in different forms in the soil, such as bound to the soil or trapped in organic compounds, we will use 31P-NMR spectroscopy to investigate what forms the P is in and how they change.

The growth of bacteria around the roots of the crop is largely controlled by sugars and other products released by the roots; the content and concentrations of these are genetically determined. We will reduce the expression of some of the genes that determine the release of these compounds and study the effects on the types of bacteria present near the roots and the processes they affect in relation to P availability.

Finally, the P requirement of the crop changes during the growing season, declining towards harvest. We will study how the root and the bacteria growing near to it change overtime and regulate the availability of P to the crop.

These studies will provide valuable information on how a crop controls the bacteria growing near its root, how the bacteria help the crop acquire P and how these processes change during the growing season. This information will help develop agricultural systems that use existing P in the soil more efficiently and optimise the amount of Pi fertiliser required to grow a successful crop. It will also provide targets for breeding crops that are more efficient at acquiring Pi from the soil, either by themselves, or with help from some soil bacteria.

To provide a holistic understanding of phosphorus (P) cycling in the rhizosphere of a non-mycorrhizal crop, we will use metagenomics, transcriptomics, metaproteomics and 31P-NMR analyses on the rhizospheres of soil grown Brassica rapa and Brassica napus plants. This will deliver novel insights into the structure and function of rhizosphere microbial communities and their role in P cycling for a crop plant (Objective 1). We will utilise TILLING resources available for B. rapa to manipulate the expression of genes regulating the amount or content of root exudates and quantify the impact on rhizosphere P pools and microbial community structure and function. This will provide knowledge on the role of root exudates in making P directly and indirectly available through their contribution to microbial growth (Objective 2). Since the demand for P by the crop changes during development, we will evaluate changes in rhizosphere P pools and microbial community structure and function during the development of B. rapa under controlled environment conditions and B. napus (oilseed rape) under field conditions, with and without P fertiliser. This will deliver temporal information on crop P demand and how this impacts on root exudates and P cycling. The addition of P fertilisers and their effects on labile and non-labile P pools, specifically organic P pools, will also be evaluated (Objectives 3 & 4).

Knowledge gaps on P cycling in the rhizosphere of a major crop and the roles of the crop and microbial community will be addressed. This will deliver information on the microbes functionally responsible for P cycling, the enzymes involved and the role of the root in delivering carbon for these functions. Novel opportunities for breeding crops that are more efficient at acquiring P, either directly or indirectly through the rhizosphere microbial community will be possible, together with potential biotechnological applications for microbes and enzymes identified by this research.

The proposal will deliver novel insights into 1) the structure and function of rhizosphere microbial communities and their roles in rhizosphere phosphorus (P) cycling of a crop plant, 2) the role of root exudates in making P available directly and indirectly through their contribution to microbial community growth and 3) temporal information on crop P demand and how this impacts on root exudates and rhizosphere P cycling. The results of the project will highlight the predominant bacterial genes actively expressed in the rhizosphere and provide data on proteins to be isolated through metagenomic clone library screening approaches. We hope to enhance collaboration with industry with the new approaches to understanding microbial activity in soil and further develop techniques for analysis of the soil bacterial metagenome and metaproteome. The latter will improve understanding of soil enzyme activity and impacts of plant growth on bacterial activity below ground. We will also improve understanding of how soil conditions impact on microbial diversity both at the structural and functional levels. This project will provide vital information to farmers and government agencies, such as Defra, on the potential effects of soil conditions on plant health. The relevant individuals from Defra and HGCA (cereals and oilseeds division of the Agriculture and Horticulture Development Board (AHDB)) already have strong contacts with Warwick School of Life Sciences so this represents an opportunity to extend these contacts. In addition to the academic beneficiaries to these advancements in our knowledge, the following could benefit from this research:

PDRAs: PDRAs recruited to the project will benefit from formal training supplied through University centres for continuing professional development, which includes management and leadership training, and through the development of skills in metagenomics, metaproteomics, plant genetics and rhizosphere biology. These will generate future research scientists focused on rhizosphere processes with the skills required to develop and lead their own research programs.

Industry: Potential immediate impacts will result from the identification of novel microbes and/or enzymes from the rhizosphere and improved knowledge on the role of a microbial inoculant in plant nutrition. These may be of interest to biotechnology companies for use in bioprocessing or for the development of biofertilisers. Research on the genes involved in root exudate production could be of interest to breeding companies for the development of new crops with enhanced abilities to acquire P or encourage the growth of beneficial bacteria in their rhizosphere. Knowledge of these rhizosphere processes could lead to the development of improved cultural practices for crop production or reduced inputs of P fertilisers, benefitting growers. Savings in input costs could be passed on to consumers. Capacity building for young researchers in the exploitation of metagenomics for discovery of novel microbial enzymes and metabolic processes which will improve UK based commercial exploration and exploitation of the uncultured microbial diversity in soil.

Public: Ultimately, potential impact will be felt by the wider society in the longer term. To feed the world's burgeoning population, agricultural production must double in the next three decades within unpredictable environmental constraints. Better understanding of how crops interact with the soil they grow in will facilitate improvements in crop varieties and growing practices to improve yields and increase the food supplied from a given area. This will contribute to agricultural sustainability and greater food security.

Increasing the efficiency with which crops acquire P could reduce inputs of P fertilisers, which will help sustain this non-renewable resource and benefit society through greater food security and lower production costs.