Molecular mechanisms of microbe-enhanced plant performance under limited nitrogen

Modern agriculture relies on the application of nitrogen fertilizer synthesized via the Haber-Bosch process, resulting inhigh energy costs, CO2 emissions, and often environmental pollution. Some countries now restrict fertilizer application.Additional strategies are required to increase crop performance under limited fertilizer input. Plant growth promotingrhizobacteria (PGPR) have long been researched as solutions to a more sustainable agriculture. PGPR includePseudomonas spp. naturally abundant in soils around the world and prevalent in the root microbiome of many species,including grass crops such as cereals. To date, relatively little research has gone into using Pseudomonas spp. as a sourceof N supply to roots of grasses. Our project asks if P. koreensis is a potential PGPR for sustainable nitrogen use inexperiments with the grass Brachypodium distachyon as model for cereals. Here we present mechanisms underlying P.koreensis benefits, obtained in a gnotobiotic growth system with varying N levels using whole-plant invasive and noninvasive phenotyping, N and C contents, 15NH4-15NO3 isotope labelling and proteomics. Under limited N conditions,plants with P. koreensis had greater root (31%) and shoot (69%) biomass, greater leaf area and a root phenotypedifferent to uninoculated plants. P. koreensis stimulated the expression of proteins related to nitrogen uptake andresulted in increased N:C ratios of the roots. 15N natural abundance signatures revealed that B. distachyon inoculatedwith P. koreensis under N limitation had an additional 14N source to that of uninoculated B. distachyon. Plantsincolulated with P. koreensis and supplemented with sufficient N had a distinct proteomic profile that differed from lowN plants and an unchanged ∂15N signature compared to plants with sufficient N that were not inoculated. We show thatP. koreensis functions as a new potential PGPR for grasses and likely does so by fixing atmospheric nitrogen andstimulating plant nitrogen uptake. Nitrogen supply is an abiotic factor that plays an important role in these plantmicrobe interactions, suggesting that future in-field benefits have to be investigated at reduced nitrogen fertilization.Future studies will explore the gene expression and lipidomic profiles around the experimentally confirmed time pointof growth promotion, to further elucidate the mechanisms underlying P. koreensis benefits to plant N acquisition