Is organic P solubilisation predicable by the genetic or chemical potential of a soil?

Phosphorus (P) is an essential element for all life. Mined rock phosphate is the principle P source for chemical fertilisers – but these non-renewable stocks are dwindling rapidly, threating future agricultural production at a time when global food demands will peak1. Phytate-P in soil accounts for an important proportion of the total in many soils, sometimes, as with UK soils, it is dominant2. The accumulation of phytate-P in soil is an indicator of low P availability to the plants. Two mechanisms explain why phytate is unable to be utilized by plant roots i) low activity of phytase in soil and ii) the strong binding of phytate to the soil-solid phase. The importance of these ‘bottlenecks’ of phytase-P bioavailability vary between soil types and across a continuum of environmental gradients (pH, temperature, DOC concentration) encompassing the landscape, field and the sub-mm reaction zone of the rhizosphere/root-soil interface.

Isolating these opposing pathways of P-release is challenging. Current methodologies for quantifying these shifting soil processes are far from ideal, tending to address either only the biotic or chemical component. While the importance of spatial trends, especially at the fine-scale remains unknown. A further, overlying factor is the recent observation that root exudates/LMWOA, mechanistically have a critical concentration threshold for mobilising soil P for plant use3, whilst having an unknown positive/negative impact on either microbial or chemical-Phytate-P release.

However, there is scope to refine/combine method approaches to provide tailored in information about how a soil can be managed to enhance release of existing soil-P reserves. Here, we will combine both novel molecular tools alongside state-of-the-art, dynamic chemical sampling (DGT)4, 5, 6 to assess P mobilisation fluxes in range of soil types across Northern Ireland and assess the impact of plant root exudates on the pathways of P-release.

Bacterial root symbionts such as Paenibacillus polymyxa can release P from phytate thereby improving its bioavailability to promote plant growth. We have recently identified a genetic mutant (ccpA) of Paenibacillus polymyxa showing a near 50% reduction in its ability to solubilise P from phytate (see Fig 1). CcpA is a major regulator of gene expression suggesting the observed reduction in P solubilisation from phytate is due to abnormal levels of gene products controlled by CcpA. We are currently performing Next Generation RNA sequencing to compare the gene expression profiles between ccpA mutant and wildtype Paenibacillus polymyxa. This will enable us to identify specific genes that could be manipulated via genetic engineering for maximal solubilisation of P from phytate. We have also created Paenibacillus polymyxa strains that report the level of phytase expression via the generation of fluorescent signals. By combing the DGT gel system with the genetically engineered and fluorescent reporter strains of Paenibacillus polymyxa the student will study in real time the dynamics of the architecture of phytase expression by this plant growth promoting bacterium within the rhizosphere and its effect on localised chemical fluxes thus enabling the isolation of chemical vs biological P mobilisation around features such as plant root hairs or tip apices.