Metabolic Interactions between Brachypodium and Pseudomonas fluorescens under Controlled Iron-Limited Conditions

Abstract

Iron (Fe) availability has well-known effects on plant and microbial metabolism, but its effects on interspecies interactions are poorly understood. The purpose of this study was to investigate metabolite exchange between the grass Brachypodium distachyon strain Bd21 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 plants grown semihydroponically with and without SBW25 inoculation and Fe amendment. Liquid chromatography-mass spectrometry analysis of the hydroponic solution revealed an increase in the abundance of the 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 that in sterile roots, but external phytosiderophore abundances decreased. P. fluorescens siderophores were not detected in treatments without Fe. Rather, expression of SBW25 genes encoding a porin, a 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. IMPORTANCE Rhizosphere bacteria influence the growth of their host plant by consuming and producing metabolites, nutrients, and antibiotic compounds within the root system that affect plant metabolism. Under Fe-limited growth conditions, different plant and microbial species have distinct Fe acquisition strategies, often involving the secretion of strong Fe-binding chelators that scavenge Fe and facilitate uptake. Here, we studied interactions between P. fluorescens SBW25, a plant-colonizing bacterium that produces siderophores with antifungal properties, and B. distachyon, a genetic model for cereal grain and biofuel grasses. Under controlled growth conditions, bacterial siderophore production was inhibited in the root system of Fe-deficient plants, bacterial inoculation altered transcription of genes involved in defense and stress response in the roots of B. distachyon, and SBW25 degraded phytosiderophores secreted by the host plant. These findings provide mechanistic insight into interactions that may play a role in rhizosphere dynamics and plant health in soils with low Fe solubility.

Changes in the Cell Wall Proteome of Leaves in Response to High Temperature Stress in Brachypodium distachyon

“…Interestingly, proteins with this domain were previously observed to be overaccumulated in response to heat stress (after 6 h and 12 h) in rice [70]. The gene encoding this protein with a Barwin domain was also found to be upregulated in B. distachyon roots inoculated with Pseudomonas fluorescens SBW25 [71]. Taken together, this suggests the importance of Bradi4g14920 in the response to heat stress as well as in root defence mechanisms in response to bacterial colonisation.…”

Shared and host-specific transcriptomic response of Pseudomonas donghuensis P482 to the exudates of tomato (Dicot) and maize (Monocot) shed light on the host-adaptive traits in the promiscuous root colonizing bacteria

“…P. donghuensis strains also produce 7-hydroxytropolone (7-HT) – a compound with iron-scavenging and antimicrobial properties (21, 106, 107). Some microorganisms can use exogenous siderophores, therefore shifting the energetic cost of their production to other organisms (108). This phenomenon is sometimes referred to as ‘sierophore piracy’ (109).…”

“…Boiteau et al, (108) have found that the production of phytosiderophores was increased in B. distachyon following inoculation with P. fluorescens SBW25 compared to the amount in sterile roots. Moreover, in response to Fe deficiency, the P. fluorescens consumed the plant-derived siderophores present in the exudates rather than producing its own (108). It therefore seems that rhizosphere-associated pseudomonas generally do not suffer from iron starvation when supplied with root exudates.…”

Root-Associated Bacteria Are Biocontrol Agents for Multiple Plant Pests

“…In this context, being able to kill these organisms would provide nutrients for the biocontrol organisms, thus augmenting the carbon-rich metabolites in the root exudates. A comparison of the root exudate composition shows that colonization of the roots by beneficial microbes alters the composition of amino acids [ 69 , 70 , 71 ]. Some amino acids presumably supply essential N for microbial growth, which is limited in the soil pore water.…”

“…Some amino acids presumably supply essential N for microbial growth, which is limited in the soil pore water. For instance, Boiteau et al [ 70 ] found that the concentration of two N-containing amino acids in root exudates from Brachypodium decreases after colonization by Pseudomonas fluorescens . Microbes that are rhizosphere colonists have higher numbers of amino acid and sugar transporters than non-colonists [ 69 ].…”