PNNL

Abstract

Iron (Fe) availability has well known effects on plant and microbial metabolism, but effects on interspecies interactions are poorly understood. The purpose of this study was to investigate metabolite exchange between the grass, Brachypodium distachyon strain Bd21 (B. distachyon), and the soil bacterium, Pseudomonas fluorescens SBW25::gfp/lux (SBW25), during Fe limitation under axenic conditions. We compared the transcriptional profiles and root exudate metabolites of B. distachyon when grown semi-hydroponically with and without SBW25 inoculation and Fe amendment. Liquid chromatography mass spectrometry analysis of the hydroponic solution revealed an increase in the abundance of phytosiderophores, mugineic acid and deoxymugineic acid, under Fe-limited conditions compared to Fe-replete conditions, indicating greater secretion by roots presumably to facilitate Fe uptake. In SBW25 inoculated roots, expression of genes encoding phytosiderophore biosynthesis and uptake proteins increased compared to sterile roots, but external phytosiderophore abundances decreased. P. fluorescens siderophores were not detected in treatments without Fe. Rather, expression of SBW25 genes encoding an outer membrane porin, a transmembrane transporter, and a monooxygenase was significantly upregulated in response to Fe deprivation. Collectively, these results suggest that SBW25 consumed root exuded phytosiderophores in response to Fe deficiency, and we propose target genes that may be involved. SBW25 also altered the expression of root genes encoding defense related enzymes and regulators, including thionin and cyanogenic glycoside production, chitinase and peroxidase activity, and transcription factors. Our findings provide insights into the molecular bases for the stress response and metabolite exchange of interacting plants and bacteria under Fe-deficient conditions.