Do plants use root-derived proteases to promote the uptake of soil organic nitrogen?
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Do plants use root-derived proteases to promote the uptake of soil organic nitrogen? Lucy M. Greenfield
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Paul W. Hill & Eric Paterson & Elizabeth M. Baggs & Davey L. Jones
Received: 6 May 2020 / Accepted: 15 September 2020 / Published online: 23 September 2020 # The Author(s) 2020
Abstract Aims The capacity of plant roots to directly acquire organic nitrogen (N) in the form of oligopeptides and amino acids from soil is well established. However, plants have poor access to protein, the central reservoir of soil organic N. Our question is: do plants actively secrete proteases to enhance the breakdown of soil protein or are they functionally reliant on soil microorganisms to undertake this role? Methods Growing maize and wheat under sterile hydroponic conditions with and without inorganic N, we measured protease activity on the root surface (rootResponsible Editor: Ad C. Borstlap. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11104-020-04719-6) contains supplementary material, which is available to authorized users. L. M. Greenfield (*) : P. W. Hill : D. L. Jones School of Natural Sciences, Bangor University, Gwynedd LL57 2UW, UK e-mail: [email protected] E. Paterson The James Hutton Institute, Craigiebuckler, Aberdeen AB15 8QH, UK E. M. Baggs Global Academy of Agriculture and Food Security, the Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Campus, Midlothian EH25 9RG, UK D. L. Jones SoilsWest, UWA School of Agriculture and Environment, The University of Western Australia, Perth, WA 6009, Australia
bound proteases) or exogenously in the solution (free proteases). We compared root protease activities to the rhizosphere microbial community to estimate the ecological significance of root-derived proteases. Results We found little evidence for the secretion of free proteases, with almost all protease activity associated with the root surface. Root protease activity was not stimulated under N deficiency. Our findings suggest that cereal roots contribute one-fifth of rhizosphere protease activity. Conclusions Our results indicate that plant N uptake is only functionally significant when soil protein is in direct contact with root surfaces. The lack of protease upregulation under N deficiency suggests that root protease activity is unrelated to enhanced soil N capture. Keywords Aminopeptidase . Peptidase . Plant nutrition . Proteinase . Root exudation
Introduction The rhizosphere represents a zone of intense competition for nutrient resources between plant roots and soil microorganisms (Jones et al. 2009). This competition is particularly intense for low molecular weight forms of organic N such as amino acids, oligopeptides and urea which can be taken up and assimilated by both plants and microorganisms (Kuzyakov and Xu 2013; Moreau et al. 2019). Conventionally, it is thought that high molecular weight N held in soil organic matter is largely unavailable to plants and that this resource needs to be
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