Oliver Baars

Many critical processes depend on metalloenzymes, and scarcity of the trace metals required for these enzymes may limit their activity, thus causing potential bottlenecks biogeochemical cycles. A recent revision to the microbial tree of life has revealed widespread and abundant soil bacteria that produce lanthanum-dependent methanol dehydrogenase, an enzyme potentially important in their metabolism and the cycling of carbon in soil. This exciting discovery further expands the periodic table of life and raises many questions about the biogeochemistry of lanthanum and other rare earth elements (REYs). Our research project’s central assertion is that microbes—specifically those utilizing REY-dependent methanol dehydrogenase—will require a specific REY uptake strategy, akin to other biologically necessary trace metals. We propose to utilize cutting-edge coordination, soil, and analytical chemistry approaches to identify and characterize ligands that promote solubilization and binding of REYs from soils. The successful completion of the project will result in a transformative new paradigm for the transport of REYs in biological systems, and may provide significant advance in other related fields.

Our Vision is to provide a science-based platform for new agricultural practices enabling plant producers to manage their production ecosystems in a resource-efficient way with limited environmental footprint based on an in-depth understanding of key ecological functions in the soilplant interphase (rhizosphere). Our Motivation is to address the major research gaps in deciphering the complexity of microbemicrobe and microbe-plant interactions in the rhizosphere, and thereby provide new conceptual understanding on how these interactions influence plant performance. This motivation is timely due to recent developments in methodology and will enable us to provide the knowledge-base for unlocking the potential of the soil rhizobiota (microbes living on in the rhizosphere) as the key to development of sustainable and resilient plant production systems. Our Focus is to identify and quantify main determinants of microbial interactions and networks in the rhizosphere leading toward a resilient ecological unit, and thus reveal the importance and potential of microbial interactions and functions in the rhizosphere. The proposed research will take advantage of a multi-faceted, integrative and cross-disciplinary approach, which is fundamental for 1) achieving a deep understanding of the chemical and biological factors that control microbe-microbe and plantmicrobe interactions and functions under natural soil conditions, 2) establishing improved predictive models for microbial interactions in soil and 3) exploiting the microbial potential in plant-soil production systems for the benefit of plant growth and resilience. INTERACT will decode these important, yet often transient, microbial interactions in the complex soil matrix, in relation to soil biogeochemical status, water stress as well as pathogen attack, and the impact of these interactions on plant performance. We will challenge the currently accepted view among scientists that plants are the primary drivers for rhizobiome assembly. Hence, we will determine whether in fact soil microbes, largely through chemical communication and signaling, play a greater role in rhizobiome development and function than has been previously appreciated. INTERACT will provide critical insight into the rhizosphere ecology, as a basis for actively influencing the assembly of effective rhizosphere communities to support plant health and productivity, either through biotechnological or agronomic approaches.

Solubilization and cellular acquisition of inorganic species in the rhizosphere mediated by biogeochemical processes is the nutritional underpinning of terrestrial ecosystems and most human food stores. Graminaceous plants and soil microbes exude siderophores, chelating agents with a high affinity for ferric iron, which are critical to the acquisition of essential nutrients. The traditional paradigm for siderophore activity in soil involves: (1) release of siderophore from an organism; (2) solubilization of iron from a bio-inaccessible source; and (3) cellular uptake of iron via a membrane bound receptor. However, for a siderophore to function properly, it must travel from the cell or root to an iron source and then back again to the organism for uptake. Because soils are complicated, dynamic systems, a myriad of antagonistic reactive soil components may disrupt this nutrient uptake process. Although many studies have focused on quantifying the ability of siderophores to solubilize metals from minerals and elucidating the details of cellular uptake systems, the roles of common soil components in regulating the fate and transport of siderophores and siderophore-metal complexes have largely been ignored. Our objectives are to: (1) identify which soil components (viz., mineral surfaces, soil organic matter, extracellular soil enzymes, and reactive oxygen species) have the most significant antagonistic effects on siderophore-mediated micronutrient uptake; (2) elucidate major mechanisms of reactions of siderophores, and their complexes, with major soil components; and (3) verify these interactions in soils. We will approach these issues with a targeted set of laboratory based biochemical and geochemical experiments, complemented by spectroscopic approaches, which will determine the rates and mechanisms of reactions between reactive soil components and both plant and microbial siderophores. By using distinct siderophores that are representative of those produced by plants, soil bacteria, and fungi, we will improve our understanding of nutrient dynamics in the rhizosphere and the micronutrient-mediated ecological relationships between bacteria, fungi, and plants.

Overview. A fundamental question about the origin of life revolves around how nitrogen became bioavailable via biological nitrogen fixation to support the proliferation of life. N2-fixation requires nitrogenases, which requires metal cofactors for catalytic activity. Nitrogen isotopes of sedimentary rocks and phylogenetic analysis strongly suggest that a nitrogenase using molybdenum as a catalytic cofactor was the predominant class of enzymes responsible for N2 fixation in the mid-late Archaean ocean, long before the Great Oxidation Event. However, the emergence of Mo-based nitrogenase Nif, before the Great Oxidation Event, raises an apparent paradox: during early earth reducing conditions, Mo was present as particle reactive species or locked in mineral structures and was thus likely not readily bioavailable and potentially limited nitrogen fixation. The objective of this project is to resolve this paradox by testing the following hypotheses: (A) under limiting concentrations of dissolved Mo, N2-fixing bacteria have developed strategies to extract Mo directly from minerals and rocks to make it bioavailable for use in Mo-based nitrogenase; and (B) the utilization efficiency of mineral-associated Mo depends on microbial “sensing” of the mineral surface. These hypotheses will be tested in three experiments using incubations with (1) two N2-fixing bacteria, one aerobic, and one anaerobic, to assess extraction of Mo via secreted Mo binding metabolites from Mo-bearing rocks that are common on early Earth; (2) two anaerobic cultures, one Fe oxidizer and one methanogen to study other possible biochemical mechanisms of Mo release; and (3) a coculture of one methanogen with one anaerobic N2 fixer to determine the importance of microbial interaction (e.g., sharing of Mo and fixed nitrogen) on Mo bioavailability. Analyses will be done to measure Mo release by various microbial metabolites (measurements done at NCSU using LC-MS and LC-ICP-MS). N2 fixation rates will be determined by the ARA assay, 15N labelling experiments, and nano-SIMS imaging. RT-qPCR will be performed to correlate expression levels of specific functional genes with N2 fixation rate. XRD, SEM, and TEM will be used to characterize mineralogical changes. Biosignatures from microbial weathering of Mo-bearing rocks will be determined by TOF-SIMS and XPS. The results of this project will provide an alternative answer to the Mo paradox with important implications for the co-evolution of geosphere and biosphere.

Borlaug Fellowship Program for Mexico fellow to spend 12 weeks at NC State conducting research on applying UV and/or visible lights as abiotic stress in tomato plants to increase the bioactive compound synthesis (carotenoids, flavonoids and phenolic acids) with antioxidant capacity.

We propose a quantum spin technology to image biochemical pathways in the rhizosphere with unprecedented chemical detail and sensitivity. Specifically, the proposed technology transfers the quantum entangled nuclear spin order of hydrogen gas, to metabolites, including nitrate, amino acids, nitrogen and pyruvate, to enable molecular imaging of their metabolic transformations without any penetration depth limitations such that molecular turnover and metabolism can be observed directly in soil.

Using microbial metabolic services is gaining significant interest in both academia and industry as a cost-effective approach to maintain and increase crop production sustainably. What drives high rhizosphere microbial activity is organic carbon from root exudates generated by the plant’s photosynthetic activity. Root exudates also serve as chemical signals and biocontrol molecules for colonization by rhizosphere microbes. Recent studies with Arabidopsis show that roots actively secrete secondary metabolites under nutrient-limitation stress. What is lacking are links to understand how stress-induced exudates affect assembly and activity of rhizobiomes and studies beyond Arabidopsis to progress microbiome approaches with agriculturally important crops. Based on our preliminary data that shows tomato bacterial wilt resistance related to stress-induced root exudation, our overall goal is to develop new microbiome-based applications for improved and sustainable crop production. We will use root exudates for isolating microbial inoculants as biofertilizers or biocontrols or use our insight to screen for plants with optimized root exudation profiles.