Nitrogen is quantitatively the most important nutrient that plants acquire from the soil. It is well established that plant roots take up nitrogen compounds of low molecular mass, including ammonium, nitrate, and amino acids. However, in the soil of natural ecosystems, nitrogen occurs predominantly as proteins. This complex organic form of nitrogen is considered to be not directly available to plants. We examined the long-held view that plants depend on specialized symbioses with fungi (mycorrhizas) to access soil protein and studied the woody heathland plant Hakea actites and the herbaceous model plant Arabidopsis thaliana, which do not form mycorrhizas. We show that both species can use protein as a nitrogen source for growth without assistance from other organ- isms. We identified two mechanisms by which roots access protein. Roots exude proteolytic enzymes that digest protein at the root surface and possibly in the apoplast of the root cortex. Intact protein also was taken up into root cells most likely via endocytosis. These findings change our view of the spectrum of nitrogen sources that plants can access and challenge the current paradigm that plants rely on microbes and soil fauna for the breakdown of organic matter.
This article provides evidence that plants can assimilate protein without assistance from soil organisms. Whether the ability to use protein as a nitrogen source is limited to nonmycorrhizal plant species or is more widespread in the plant kingdom has to be established. In the experimental systems used here, protein did not support plant growth to the same extent as inorganic nitrogen sources. The slower growth of plants supplied with protein alone suggests that there may be metabolic bottlenecks associated with protein catabolism in the absence of inorganic nitrogen sources.