Abstract
Elevated CO2 stimulates plant growth and affects quantity and composition of root exudates, followed by response of its microbiome. Three scenarios representing nitrate fertilization regimes: limited (30 ppm), moderate (70 ppm) and excess nitrate (100 ppm) were compared under ambient and elevated CO2 (eCO2, 850 ppm) to elucidate their combined effects on root-surface-associated bacterial community abundance, structure and function. Wheat root-surface-associated microbiome structure and function, as well as soil and plant properties, were highly influenced by interactions between CO2 and nitrate levels. Relative abundance of total bacteria per plant increased at eCO2 under excess nitrate. Elevated CO2 significantly influenced the abundance of genes encoding enzymes, transporters and secretion systems. Proteobacteria, the largest taxonomic group in wheat roots (~ 75%), is the most influenced group by eCO2 under all nitrate levels. Rhizobiales, Burkholderiales and Pseudomonadales are responsible for most of these functional changes. A correlation was observed among the five gene-groups whose abundance was significantly changed (secretion systems, particularly type VI secretion system, biofilm formation, pyruvate, fructose and mannose metabolism). These changes in bacterial abundance and gene functions may be the result of alteration in root exudation at eCO2, leading to changes in bacteria colonization patterns and influencing their fitness and proliferation.
Highlights
The atmospheric concentration of carbon dioxide (CO2) has been increasing since the industrial revolution due to fossilfuel burning
We examined the combined effect of elevated CO2 (eCO2) and nitrate levels on the structure and function of the root-surfaceassociated bacterial community structure and functions based on metagenome extracted from the background of host-plant genomic data
Wheat was grown for 6 weeks in a greenhouse under 400 ppm CO2 or 850 ppm CO2, and three nitrate levels representing three possible wheat-growthsustaining fertilization scenarios: limited (30 ppm), moderate (70 ppm) and excess (100 ppm)
Summary
The atmospheric concentration of carbon dioxide (CO2) has been increasing since the industrial revolution due to fossilfuel burning. The studies published so far have focused on the structure of rhizosphere microbial communities and not on root-surfaceassociated communities [16,17,18], that are likely to be even more affected by eCO2 and increased nitrate supply [19]. We examined the combined effect of eCO2 and nitrate levels on the structure and function of the root-surfaceassociated (i.e., bacteria attached to root surface) bacterial community structure and functions based on metagenome extracted from the background of host-plant (wheat) genomic data. We anticipated that changes in function of the root-surface-associated bacterial community over a period of weeks might react to and indicate the conditions developing in the root system, as a result of changing levels of atmospheric CO2 and nitrate supply. By observing changes in gene functions of the rootsurface-associated microbiome, we sought to reveal possible conditions to which the roots will be exposed in the future, information that might enable accommodation of wheat growth to future environmental conditions
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