Calcium and Nitrogen Availability Controls Root Exudation in Hydroponically Cultured Barley

Root exudation is a key component of plant-rhizosphere interactome. It is increasingly evident that root exudates influence rhizospheric microbial communities and in turn can benefit plants through improved resource allocation. However, how suboptimal nutrient availability relates to control of root exudation is poorly understood. This study explores effects of calcium and nitrogen availability on barley (Hordeum vulgare L.) root architecture and metabolism with focus on metabolite exudation. Upon depletion of NO₃ ^- from the medium (-N), both sugar and amino acid exudation dropped rapidly within 6 h, but this fast effect was absent for amino acids if also Ca²⁺ was omitted from the medium (-Ca/-N). In contrast, Ca²⁺ depletion alone (-Ca) led to a fourfold higher amino acid exudation. Further, a modulatory role for Ca²⁺ was evident, as the exudation varied in Ca²⁺-channel inhibitor concentration- and treatment time-dependent manner. Among the 51 detected exuded metabolites, N-deprivation increased the release of specific sugars, i.e. sucrose, fructose, and cellobiose (1.6 to 8-fold), with potential roles as chemoattractants. This pattern is reflected in the increased C/N ratio in -N-exudates. The results show a predominance of nitrogen availability and a modulatory role of Ca²⁺ in tight regulation of root exudation in dependence of NO₃ ^- availability and metabolic state.