Do plants use root-derived proteases to promote the uptake of soil organic nitrogen?

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 (root-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.

Fig. 1

Extracellular root leucine aminopeptidase activity (μmol AMC mg⁻¹ root h⁻¹ of maize and wheat under inorganic N and zero N treatments measured using the in situ assay. Different letters represent significant difference between N treatments for each plant (p < 0.05). Values represent mean ± SEM (n = 4)

Fig. 2

¹⁴C-labelled protein respired, root and shoot uptake rate (μg ¹⁴C plant⁻¹ day⁻¹) of maize and wheat under inorganic N and zero N treatments. Different letters represent significant difference between N treatments for each plant (p < 0.05). Values represent mean ± SEM (n = 4)

Fig. 3

Comparison of leucine aminopeptidase activity in the rhizosphere and extracellular root (μmol AMC cm⁻¹ root h⁻¹) of maize and wheat. Different letters represent significant difference between N treatments for each plant (p < 0.05). Values represent mean ± SEM (n = 4)

Fig. 4

Schematic diagram for the mechanisms of root protease activity in order to obtain N for nutrition