Nitrogen affects cluster root formation and expression of putative peptide transporters

Non-mycorrhizal Hakea actites (Proteaceae) grows in heathland where organic nitrogen (ON) dominates the soil nitrogen (N) pool. Hakea actites uses ON for growth, but the role of cluster roots in ON acquisition is unknown. The aim of the present study was to ascertain how N form and concentration affect cluster root formation and expression of peptide transporters. Hydroponically grown plants produced most biomass with low molecular weight ON>inorganic N>high molecular weight ON, while cluster roots were formed in the order no-N>ON>inorganic N. Intact dipeptide was transported into roots and metabolized, suggesting a role for the peptide transporter (PTR) for uptake and transport of peptides. HaPTR4, a member of subgroup II of the NRT1/PTR transporter family, which contains most characterized di- and tripeptide transporters in plants, facilitated transport of di- and tripeptides when expressed in yeast. No transport activity was demonstrated for HaPTR5 and HaPTR12, most similar to less well characterized transporters in subgroup III. The results provide further evidence that subgroup II of the NRT1/PTR family contains functional di- and tripeptide transporters. Green fluorescent protein fusion proteins of HaPTR4 and HaPTR12 localized to tonoplast, and plasma- and endomembranes, respectively, while HaPTR5 localized to vesicles of unknown identity. Grown in heathland or hydroponic culture with limiting N supply or starved of nutrients, HaPTR genes had the highest expression in cluster roots and non-cluster roots, and leaf expression increased upon re-supply of ON. It is concluded that formation of cluster roots and expression of PTR are regulated in response to N suppl

Effects of externally supplied protein on root morphology and biomass allocation in Arabidopsis

Growth, morphogenesis and function of roots are influenced by the concentration and form of nutrients present in soils, including low molecular mass inorganic N(IN, ammonium, nitrate) and organic N (ON, e.g.amino acids). Proteins, ON of high molecular mass, are prevalent in soils but their possible effects on roots have received little attention. Here, we investigated how externally supplied protein of a size typical of soluble soil proteins influences root development of axenically grown Arabidopsis. Addition of low to intermediate concentrations of protein (bovine serum albumen, BSA) to IN-replete growth medium increased root dry weight, root length and thickness, and root hair length. Supply of higher BSA concentrations inhibited root development. These effects were independent of total N concentrations in the growth medium. The possible involvement of phytohormones was investigated using Arabidopsis with defective auxin (tir1-1 and axr2-1) and ethylene (ein2-1) responses. That no phenotype was observed suggests a signalling pathway is operating independent of auxin and ethylene responses. This study expands the knowledge on N form-explicit responses to demonstrate that ON of high molecular mass elicits specific responses

Turning the Table: Plants Consume Microbes as a Source of Nutrients

Interactions between plants and microbes in soil, the final frontier of ecology, determine the availability of nutrients to plants and thereby primary production of terrestrial ecosystems. Nutrient cycling in soils is considered a battle between autotrophs and heterotrophs in which the latter usually outcompete the former, although recent studies have questioned the unconditional reign of microbes on nutrient cycles and the plants' dependence on microbes for breakdown of organic matter. Here we present evidence indicative of a more active role of plants in nutrient cycling than currently considered. Using fluorescent-labeled non-pathogenic and non-symbiotic strains of a bacterium and a fungus (Escherichia coli and Saccharomyces cerevisiae, respectively), we demonstrate that microbes enter root cells and are subsequently digested to release nitrogen that is used in shoots. Extensive modifications of root cell walls, as substantiated by cell wall outgrowth and induction of genes encoding cell wall synthesizing, loosening and degrading enzymes, may facilitate the uptake of microbes into root cells. Our study provides further evidence that the autotrophy of plants has a heterotrophic constituent which could explain the presence of root-inhabiting microbes of unknown ecological function. Our discovery has implications for soil ecology and applications including future sustainable agriculture with efficient nutrient cycles

Uptake of non-pathogenic E. coli by Arabidopsis induces downregulation of heat shock proteins

We recently demonstrated that non-pathogenic and non-symbiotic microbes E. coli and yeast are taken up by roots and used as a source of nutrients by the plant. Although this process appears to be beneficial for the plant, the nutritional gain of microbe incorporation has to exceed the energy expense of microbe uptake and digestion, and the question remains whether the presence of microbes triggers pathogen- and other stress-induced responses. Here, we present evidence that digesting microbes is accompanied by strong downregulation of genes linked to stress response in Arabidopsis. Genome-wide transcription analysis shows that uptake of E. coli by Arabidopsis roots is accompanied by a pronounced downregulation of heat shock proteins. Plants upregulate heat shock proteins in response to environmental stresses including temperature, salt, light and disease agents including microbial pathogens. The pronounced downregulation of heat shock proteins in the presence of E. coli indicates that uptake and subsequent digestion of microbes does not induce stress. Additionally it suggests that resources devoted to stress resistance in control plants may be re-allocated to the process of microbe uptake and digestion. This observation adds evidences to the notion that uptake of microbes is an active, purposeful and intentional behavior of the plant

The effect of protein supplied in the growth medium on plant pathogen resistance

Externally supplied protein (bovine serum albumin, BSA) affects root development of Arabidopsis, increasing root biomass, root hair length, and root thickness. While these changes in root morphology may enhance access to soil microenvironments rich in organic matter, we show here that the presence of protein in the growth medium increases the plant’s resilience to the root pathogen Cylindrocladium sp

Yeast as a biofertilizer alters plant growth and morphology

It has been previously demonstrated that dicotyledonous plants perform rhizophagy, a process in which live microbial cells are engulfed by root cells and digested to acquire the nutrients from the microbes. Here we tested the hypothesis that rhizophagy is a mechanism of nutrient acquisition that is not restricted to dicotyledonous plants. We report that the monocotyledonous species sugarcane (Saccharum officinarum x spontaneum), grown in controlled axenic conditions, incorporated yeast cells into root cells. This suggests that rhizophagy is an evolutionarily conserved trait that predates the divergence of dicot and monocot species. To explore the potential relevance and practical application of rhizophagy, we investigated brewers' yeast (Saccharomyces cerevisiae), a waste product of the brewing industry, for its role as biofertilizer. The addition of live or dead yeast to fertilized soil substantially increased the nitrogen (N) and phosphorus (P) content of roots and shoots of tomato (Solanum lycopersicum) and young sugarcane plants. Yeast addition to soil also increased the root-to-shoot ratio in both species and induced species-specific morphological changes that included increased tillering in sugarcane and greater shoot biomass in tomato plants. These findings support the notion that brewers' yeast is a cost-effective biofertilizer that improves not only plant nutrition but also plant vigor during the early growth phase. It remains to be established which yeast-derived substances trigger the observed plant growth effects, and how rhizophagy contributes to plant nutrient acquisition

The role of a FAD cofactor in the regulation of acetohydroxyacid synthase by redox signaling molecules

Acetohydroxyacid synthase (AHAS) catalyzes the first step of branched-chain amino acid (BCAA) biosynthesis, a pathway essential to the lifecycle of plants and microorganisms. This enzyme is of high interest because its inhibition is at the base of the exceptional potency of herbicides and potentially a target for the discovery of new antimicrobial drugs. The enzyme has conserved attributes from its predicted ancestor, pyruvate oxidase, such as a ubiquinone-binding site and the requirement for FAD as cofactor. Here, we show that these requirements are linked to the regulation of AHAS, in relationship to its anabolic function. Using various soluble quinone derivatives (e.g. ubiquinones), we reveal a new path of down-regulation of AHAS activity involving inhibition by oxidized redox-signaling molecules. The inhibition process relies on two factors specific to AHAS: (i) the requirement of a reduced FAD cofactor for the enzyme to be active and (ii) a characteristic slow rate of FAD reduction by the pyruvate oxidase side reaction of the enzyme. The mechanism of inhibition involves the oxidation of the FAD cofactor, leading to a time-dependent inhibition of AHAS correlated with the slow process of FAD re-reduction. The existence and conservation of such a complex mechanism suggests that the redox level of the environment regulates the BCAA biosynthesis pathway. This mode of regulation appears to be the foundation of the inhibitory activity of many of the commercial herbicides that target AHAS