Colonization Of Wheat Roots Research Articles

The recA gene is crucial to mediate colonization of Bacillus cereus 905 on wheat roots.

Bacillus cereus 905, one of the plant growth-promoting rhizobacteria (PGPRs), is capable of colonizing wheat roots in a large population size. From previous studies, we learned that the sodA2-encoding manganese-containing superoxide dismutase (MnSOD2) is important for B. cereus 905 to survive in wheat rhizosphere. In this investigation, we demonstrated that deletion of the recA gene, which codes for the recombinase A, significantly reduced MnSOD2 expression at both the mRNA and the protein levels. Through comparison with the wild-type, the ∆recA showed a dramatic decrease in cell survival after exposure to 50 μM paraquat or 15 mM H2O2. Evidence indicated that the recA gene of B. cereus 905 also notably regulated nutrition utilization efficiency, biofilm formation, and swarming motility. The root colonization examination showed that the ∆recA had a 1000- to 2500-fold reduction in colonization on wheat roots, suggesting that RecA plays an indispensable role in effective colonization on wheat roots by B. cereus 905. Taken together, the recA gene positively regulates MnSOD2 production and nutrition utilization and protects B. cereus 905 cells against paraquat and H2O2. Besides, biofilm formation and swarming motility of B. cereus 905 are promoted by RecA. Finally, RecA significantly contributes to wheat root colonization of B. cereus 905. Our results showed the important role of RecA during physiological processes in B. cereus 905, especially for colonization on wheat roots. Our findings will point out a research direction to study the colonization mechanisms of B. cereus 905 in the future and provide potential effective strategy to enhance the biocontrol efficacy of PGPR strains. KEY POINTS : • RecA plays an indispensable role in root colonization of B. cereus.

Transcriptome plasticity underlying plant root colonization and insect invasion by Pseudomonas protegens

Pseudomonas protegens shows a high degree of lifestyle plasticity since it can establish both plant-beneficial and insect-pathogenic interactions. While P. protegens protects plants against soilborne pathogens, it can also invade insects when orally ingested leading to the death of susceptible pest insects. The mechanism whereby pseudomonads effectively switch between lifestyles, plant-beneficial or insecticidal, and the specific factors enabling plant or insect colonization are poorly understood. We generated a large-scale transcriptomics dataset of the model P. protegens strain CHA0 which includes data from the colonization of wheat roots, the gut of Plutella xylostella after oral uptake and the Galleria mellonella hemolymph after injection. We identified extensive plasticity in transcriptomic profiles depending on the environment and specific factors associated to different hosts or different stages of insect infection. Specifically, motor-activity and Reb toxin-related genes were highly expressed on wheat roots but showed low expression within insects, while certain antimicrobial compounds (pyoluteorin), exoenzymes (a chitinase and a polyphosphate kinase), and a transposase exhibited insect-specific expression. We further identified two-partner secretion systems as novel factors contributing to pest insect invasion. Finally, we use genus-wide comparative genomics to retrace the evolutionary origins of cross-kingdom colonization.

Different wheat cultivars exhibit variable responses to inoculation with arbuscular mycorrhizal fungi from organic and conventional farms.

The present study aimed to investigate the effects of arbuscular mycorrhizal (AM) fungal communities originating from organic and conventional agriculture on wheat growth and yield. Six different spring wheat cultivars released in different years in north and central European countries were considered. We hypothesised that AM fungal inoculum collected from organic agricultural fields would elicit a greater positive growth response than inoculum collected from conventional agricultural fields; and that older cultivars, which were developed under conditions of low fertilizer input, would exhibit overall greater growth responses to the presence of AM fungi, compared with more recent cultivars, and that AM fungal inoculum from conventional fields might have the most beneficial effect on the growth and yield of recent cultivars. The results showed that the overall effects on the growth and yield of spring wheat grown with organic and conventional AM fungal inocula did not differ greatly. However, the inoculation growth response, showing the difference of the effects of organic and conventional inocula, varied between particular wheat cultivars. Inoculation growth response of the cultivar Pikker (released in 1959) was the most positive, while that of the cultivar Arabella (released in 2012) was the most negative. The use of AM fungal inoculum from organic fields resulted in slightly taller plant individuals. Pikker showed relatively higher yield and stronger growth when the organic AM fungal inoculum was used. Arabella exhibited relatively lower yield and weaker growth when the organic inoculum was used. Whether the positive response of Pikker to Estonian organic inoculation reflects adaptation to the locally occurring AM fungal community needs to be established by further studies of the communities of AM fungi colonizing wheat roots.

Quantification of Trichoderma afroharzianum, Trichoderma harzianum and Trichoderma gamsii inoculants in soil, the wheat rhizosphere and in planta suppression of the crown rot pathogen Fusarium pseudograminearum.

Develop quantitative assays (qPCR) to determine the detection threshold limits, colonization and persistence of Trichoderma gamsii, Trichoderma afroharzianum and T. harzianum inoculants in cropping soils, the wheat rhizosphere and their in planta suppressive efficacy against the crown rot pathogen Fusarium pseudograminearum. Trichoderma qPCR primers were designed from the internal transcribed spacer region of 5.8S rDNA and from sequences of DNA fragments diagnostic for each inoculant genotype. The minimum detection thresholds of qPCR assays varied between 1×103 (log 3) and 8×104 (log 4·9) conidia (genome) equivalents per gram of soil for multi- and single-copy target sequences, respectively and were independent of soil type. There was a strong correlation (r>0·974) between culture-dependent and culture-independent (qPCR) quantification methods. In wheat bioassays, Trichoderma inoculants colonized rhizosphere soils and wheat roots at 56-112days postemergence to a depth of 20cm but were more abundant (P<0·001) at 0-10cm root depth, means ranging from 2×102 (log 2·3) to 4×105 (log 5·6) conidia equivalents per gram of rhizosphere soil or root tissue. Inoculants reduced (P<0·001) F. pseudograminearum biomass in wheat crown and root tissue by up to 5754-fold and increased (P=0·008) plant biomass, relative to the pathogen control. The qPCR assays provided sensitive and accurate assessment of wheat root and rhizosphere soil colonization of Trichoderma inoculants. Strains persisted through to grain maturity at levels shown to significantly suppress F. pseudograminearum in planta. The qPCR assays developed here were used to determine the wheat rhizosphere dynamics of T. harzianum, T. afroharzianum and T. gamsii inoculants and their suppressive efficacy against F. pseudograminearum in planta. These assays can be applied to monitor inoculant dynamics in suppressing crown rot and other wheat root diseases in the field.

LysM Proteins Regulate Fungal Development and Contribute to Hyphal Protection and Biocontrol Traits in Clonostachys rosea.

Lysin motif (LysM) modules are approximately 50 amino acids long and bind to peptidoglycan, chitin and its derivatives. Certain LysM proteins in plant pathogenic and entomopathogenic fungi are shown to scavenge chitin oligosaccharides and thereby dampen host defense reactions. Other LysM proteins can protect the fungal cell wall against hydrolytic enzymes. In this study, we investigated the biological function of LysM proteins in the mycoparasitic fungus Clonostachys rosea. The C. rosea genome contained three genes coding for LysM-containing proteins and gene expression analysis revealed that lysm1 and lysm2 were induced during mycoparasitic interaction with Fusarium graminearum and during colonization of wheat roots. Lysm1 was suppressed in germinating conidia, while lysm2 was induced during growth in chitin or peptidoglycan-containing medium. Deletion of lysm1 and lysm2 resulted in mutants with increased levels of conidiation and conidial germination, but reduced ability to control plant diseases caused by F. graminearum and Botrytis cinerea. The Δlysm2 strain showed a distinct, accelerated mycelial disintegration phenotype accompanied by reduced biomass production and hyphal protection against hydrolytic enzymes including chitinases, suggesting a role of LYSM2 in hyphal protection against chitinases. The Δlysm2 and Δlysm1Δlysm2 strains displayed reduced ability to colonize wheat roots, while only Δlysm1Δlysm2 failed to suppress expression of the wheat defense response genes PR1 and PR4. Based on our data, we propose a role of LYSM1 as a regulator of fungal development and of LYSM2 in cell wall protection against endogenous hydrolytic enzymes, while both are required to suppress plant defense responses. Our findings expand the understanding of the role of LysM proteins in fungal-fungal interactions and biocontrol.

Trichoderma harzianum- and Methyl Jasmonate-Induced Resistance to Bipolaris sorokiniana Through Enhanced Phenylpropanoid Activities in Bread Wheat (Triticum aestivum L.).

The aim of the present study was to evaluate the impact of Trichoderma harzianum UBSTH-501- and methyl jasmonate-induced systemic resistance and their integration on the spot blotch pathogen, Bipolaris sorokiniana through enhanced phenylpropanoid activities in bread wheat (Triticum aestivum L.). It was found that the application of MeJA (>100 mg L–1) inhibits the germination of B. sorokiniana spores under controlled laboratory conditions. To assess the effect of MeJA (150 mg L–1) in combination with the biocontrol agent T. harzianum UBSTH-501 in vivo, a green house experiment was conducted. For this, biocontrol agent T. harzianum UBSTH-501 was applied as seed treatment, whereas MeJA (150 mg L–1) was applied 5 days prior to pathogen inoculation. Results indicated that application of MeJA (150 mg L–1) did not affect the root colonization of wheat by T. harzianum UBSTH-501 in the rhizosphere. The combined application of T. harzianum UBSTH-501 and MeJA also enhanced indole acetic acid production in the rhizosphere (4.92 μg g–1 of soil) which in turn helps in plant growth and development. Further, the combined application found to enhance the activities of defense related enzymes viz. catalase (5.92 EU min–1 g–1 fresh wt.), ascorbate peroxidase [μmol ascorbate oxidized (mg prot)–1 min–1], phenylalanine ammonia lyase (102.25 μmol cinnamic acid h–1 mg–1 fresh wt.) and peroxidase (6.95 Unit mg–1 min–1 fresh wt.) significantly in the plants under treatment which was further confirmed by assessing the transcript level of PAL and peroxidase genes using semi-quantitative PCR approach. The results showed manifold increase in salicylic acid (SA) along with enhanced accumulation of total free phenolics, ferulic acid, caffeic acid, coumaric acid, and chlorogenic acid in the leaves of the plants treated with the biocontrol agent alone or in combination with MeJA. A significant decrease in the disease severity (17.46%) and area under disease progress curve (630.32) were also observed in the treatments with biocontrol agent and MeJA in combination as compared to B. sorokiniana alone treated plant (56.95% and 945.50, respectively). Up-regulation of phenylpropanoid cascades in response to exogenous application of MeJA and the biocontrol agent was observed. It was depicted from the results that PAL is the primary route for lignin production in wheat which reduces cell wall disruption and tissue disintegration and increases suberization and lignification of the plant cell as seen by Scanning Electron microphotographs. These results clearly indicated that exogenous application of MeJA with T. harzianum inducing JA- and/or SA-dependent defense signaling after pathogen challenge may increase the resistance to spot blotch by stimulating enzymatic activities and the accumulation of phenolic compounds in a cooperative manner. This study apparently provides the evidence of biochemical cross-talk and physiological responses in wheat following MeJA and biocontrol agent treatment during the bio-trophic infection.

A PilZ-Containing Chemotaxis Receptor Mediates Oxygen and Wheat Root Sensing in Azospirillum brasilense.

Chemotactic bacteria sense environmental changes via dedicated receptors that bind to extra- or intracellular cues and relay this signal to ultimately alter direction of movement toward beneficial cues and away from harmful environments. In complex environments, such as the rhizosphere, bacteria must be able to sense and integrate diverse cues. Azospirillum brasilense is a microaerophilic motile bacterium that promotes growth of cereals and grains. Root surface colonization is a prerequisite for the beneficial effects on plant growth but how motile A. brasilense navigates the rhizosphere is poorly studied. Previously only 2 out of 51 A. brasilense chemotaxis receptors have been characterized, AerC and Tlp1, and only Tlp1 was found to be essential for wheat root colonization. Here we describe another chemotaxis receptor, named Aer, that is homologous to the Escherichia coli Aer receptor, likely possesses an FAD cofactor and is involved in aerotaxis (taxis in an air gradient). We also found that the A. brasilense Aer contributes to sensing chemical gradients originating from wheat roots. In addition to A. brasilense Aer having a putative N-terminal FAD-binding PAS domain, it possesses a C-terminal PilZ domain that contains all the conserved residues for binding c-di-GMP. Mutants lacking the PilZ domain of Aer are altered in aerotaxis and are completely null in wheat root colonization and they also fail to sense gradients originating from wheat roots. The PilZ domain of Aer is also vital in integrating Aer signaling with signaling from other chemotaxis receptors to sense gradients from wheat root surfaces and colonizing wheat root surfaces.

Effects of cover crops on soil biota, soil fertility and weeds, and Pratylenchus suppression in experimental conditions

Summary Cover crops are used in Mediterranean agrosystems to reduce soil degradation and restore soil function. We carried out an experiment to test the effects of two cover crops (Brachypodium distachyon and Trifolium subterraneum, mulched or incorporated) and two bare soil controls (fertilised and unfertilised) on soil properties, biodiversity, and soil natural suppressiveness against Pratylenchus neglectus in pot microcosms. Cover crop performance, weed infestation, litter decomposition rates, soil properties, and the response of soil biota (nematode indicators, mesofauna feeding activity, enzymatic activity, and mycorrhizal spores) were measured. A short-term suppressiveness experiment was performed to determine P. neglectus colonisation of wheat roots after each treatment. Trifolium subterraneum incorporation significantly enhanced the fungal-mediated decomposition channel and decomposition rates, but enzymatic activities and mycorrhizal spore abundances did not respond to the experimental treatments. Cover crops were effective at controlling weeds, but did not increase soil suppressiveness against P. neglectus. Brachypodium distachyon cover cropping increased root infection.

Basidiomycetes fungi as biocontrol agents against take-all disease of wheat

Take-all caused by Gaeumannomyces graminis var. tritici (Ggt) is one of the most important root and crown diseases of wheat. In this research, four basidiomycetes isolates as fungal endophytes were isolated from healthy cereal plants and were identified as Coprinopsis urticicola M2, Rhizoctonia endophytica M9, Rhizoctonia zeae M32 and Fomes fomentarius M40 based on phenotypic characteristics and molecular phylogenetic analysis of the ITS rDNA sequences (ITS1-5.8S rDNA-ITS2). Then, their effects as potential biological control agents (BCAs) against Ggt were tested in vitro and greenhouse conditions. All isolates had inhibitory effects on the development of Ggt growth in vitro by dual-culture method with inhibition percentage ranging from 39.1% to 54.8%. Volatile antifungal effects were observed for M32 isolate with 23.7% inhibition, followed by the isolate M2 (18.7%). However, culture filtrates of M2 isolate showed maximum inhibition (29.8%) against Ggt. Only two of the four isolates (M2 and M32) had significant effects on controlling take-all in greenhouse conditions. M2 isolate produced IAA and gibberellin, which had the most effect on increasing of growth parameters of wheat plant in greenhouse conditions. M2 isolate was able to colonize the root cortex of wheat seedlings more rapidly compared to pathogen, as assessed by both cultural and microscopic methods. Therefore, its mode of action may depend on the internal root colonization of wheat by C. urticicola M2 and competition for space and nutrient with the Ggt. This is the first research which shows that the C. urticicola M2 could colonize the wheat roots and has great potential to be used as effective biocontrol agent of take-all. Additionally, it can improve growth parameters in wheat under greenhouse conditions. Therefore, the results of this research indicated that C. urticicola M2, due to its ability to colonize the root and crown internal tissues, may provide an advantage as biocontrol agents to be used in the field conditions.

Correction: Root colonization and growth promotion of soybean, wheat and Chinese cabbage by Bacillus cereus YL6.

[This corrects the article DOI: 10.1371/journal.pone.0200181.].

Root colonization and growth promotion of soybean, wheat and Chinese cabbage by Bacillus cereus YL6.

Although phosphate-solubilizing bacteria (PSBs) are used in agricultural production, comprehensive research on PSB that colonize the rhizosphere of different plants and promote plant growth is lacking. This study was conducted to examine the growth-promoting effects and colonizing capacity of strain YL6, a PSB. YL6 not only increased the biomass of soybean and wheat in pot experiments but also increased the yield and growth of Chinese cabbage under field conditions. The observed growth promotion was related to the capacity of YL6 to dissolve inorganic and organic phosphorus and to produce indole-3-acetic (IAA) and gibberellin (GA). After applying YL6 to soybean, wheat and Chinese cabbage, the rhizosphere soil available phosphorus (available P) content increased by 120.16%, 62.47% and 7.21%, respectively, and the plant total phosphorus content increased by 198.60%, 6.20% and 78.89%, respectively, compared with plants not treated with YL6. To examine plant colonization, YL6 labeled with green fluorescent protein (YL6-GFP) was inoculated into the plant rhizosphere and found to first colonize the root surface and hairs and then to penetrate into the intercellular spaces and vessels. Collectively, these results demonstrate that YL6 promotes the growth of three different crops and colonizes them in a similar manner. The findings therefore provide a solid foundation for probing the mechanisms by which PSB affect plant growth.

The phosphotransferase system gene ptsH plays an important role in MnSOD production, biofilm formation, swarming motility, and root colonization in Bacillus cereus 905

The rhizosphere bacterium Bacillus cereus 905 is capable of promoting plant growth through effective colonization on plant roots. The sodA2-encoding manganese-containing superoxide dismutase (MnSOD2) is important for survival of B. cereus 905 in the wheat rhizosphere. However, the genes involved in regulating sodA2 expression and the mechanisms of rhizosphere colonization of B. cereus 905 are not well elucidated. In this study, we found that the deletion of the ptsH gene, which encodes the histidine-phosphorylatable protein (HPr), a component of the phosphotransferase system (PTS), causes a decrease of about 60% in the MnSOD2 expression. Evidences indicate that the ptsH dramatically influences resistance to oxidative stress, glucose uptake, as well as biofilm formation and swarming motility of B. cereus 905. Root colonization assay demonstrated that ΔptsH is defective in colonizing wheat roots, while complementation of the sodA2 gene could partially restore the ability in utilization of arabinose, a non-PTS sugar, and root colonization caused by the loss of the ptsH gene. In toto, based on the current findings, we propose that PtsH contributes to root colonization of B. cereus 905 through multiple indistinct mechanisms, involving PTS and uptake of PTS-sugars, up-regulation of MnSOD2 production, and promotion of biofilm formation and swarming motility.

Isolation and characterization of antagonistic bacteria with the potential for biocontrol of soil-borne wheat diseases.

The aim of this study was to identify efficient plant-beneficial rhizobacterium that has the potential to be developed as biocontrol agent for the control of wheat soil-borne diseases. Rhizosphere soil samples were collected from a wheat field located in Taian City. Numerous bacteria were isolated and screened for antagonistic activity against soil-borne plant pathogenic fungi by performing dual-culture assays. Among them, XH-9 was selected for its highly antagonistic activity and others growth-promoting characteristics. Subsequently, the strain was identified as Bacillus amyloliquefaciens subsp. plantarum based on phylogenetic analysis of 16S rDNA sequence. Pot experiment indicated that XH-9 has good capacities for wheat, corn, and chili root colonization and considerably increased the biometric parameters of wheat seedlings. Quantitative real-time polymerase chain reaction experiments showed that the amount of Fusarium oxysporum associated with the XH-9 after treatment significantly decreased compared with control group. Bacillus amyloliquefaciens subsp. plantarum XH-9 has the potential as biocontrol agent when applied in local arable land to prevent damage caused by F. oxysporum and other phytopathogens. The development of biocontrol strategies for reducing the damage caused by plant pathogens is fully in accord with the current principles of sustainability.

Trichothecene production is detrimental to early root colonization by Fusarium culmorum and F. graminearum in fusarium crown and root rot of wheat

Fusarium culmorum (Fc) and F. graminearum (Fg) belong to the predominant causal agents of fusarium crown and root rot (FCR) in wheat. While many studies have been done to investigate crown rot, including stem base infection, root colonization and mycotoxin production associated with root rot is not well understood. In this study the impact of mycotoxins on the colonization of wheat roots and stem bases was analysed by using Fc and Fg isolates that varied in both quantity and types of trichothecenes they produce. Seedling inoculations in growth chambers with a high deoxynivalenol (DON)‐ and 3‐acetyldeoxynivalenol (3ADON)‐producing isolate led to more severe symptoms and 20‐times greater colonization of the stem base, as measured by Fc DNA accumulation, than isolates that produced less DON/3ADON. In contrast to stem base colonization, in vitro inoculations of roots with a Tri5 deletion mutant deficient in Fg trichothecene production led to three‐times higher colonization than the wildtype. Furthermore, an Fc isolate that produced low levels of zearalenone resulted in twice the level of colonization of a high DON/3ADON‐producing isolate included in the study. When root inoculation with a low DON/3ADON‐producing Fc isolate was supplemented with exogenous DON, DON production decreased by more than half per unit weight of Fc DNA, and root colonization doubled compared to the untreated control. Therefore, in contrast to its potential role as an aggressiveness factor in stem base infection, trichothecene production by Fc and Fg is detrimental to the early stages of wheat root colonization in FCR.

Versatile use of Azospirillum brasilense strains tagged with egfp and mCherry genes for the visualization of biofilms associated with wheat roots

This study reports the introduction of egfp or mCherry markers to the Sp245, Sp7, and M40 wild-type strains of Azospirillum brasilense and the hhkB (encoding for a putative hybrid histidine kinase) minus mutant an isogenic strain of A. brasilense Sp245 to monitor colonization of wheat (Triticum aestivum). Two plasmids were constructed: (1) the pJMS-2 suicide plasmid derived from pSUP202 and harboring the mCherry gene expressed under the constitutive kanamycin resistance promoter to create a cis tag and (2) the broad-range plasmid pMP2449-5 that carries the mCherry gene under the lac promoter, which is derived from the plasmid pMP2444; to create the in trans tag. The stability of the plasmids encoding egfp and mCherry were confirmed in vitro for seven days of bacterial growth, and then, the A. brasilense strains harboring the plasmids were studied under nonselective conditions for adherence to seeds and, at seven or 14 days post-inoculation, for wheat root colonization. The utility of the labeled strains was proven by observation, using fluorescence microscopy and confocal laser scanning microscopy (CLSM) in wheat plants inoculated with the labeled strains and compared with the CFU g–1 for seed and wheat root. The method was suitable for observation of the in situ formation of mini-colonies, enabled visualization of bacterial colonization sites on large root fragments, and showed adherence to germinated seeds and root colonization of all strains by cell counts and direct microscopic examination. Thus, we are able to quantify the structures of the biofilms formed by each strain.

Genetic markers associated to arbuscular mycorrhizal colonization in durum wheat

In this work we investigated the variability and the genetic basis of susceptibility to arbuscular mycorrhizal (AM) colonization of wheat roots. The mycorrhizal status of wild, domesticated and cultivated tetraploid wheat accessions, inoculated with the AM species Funneliformis mosseae, was evaluated. In addition, to detect genetic markers in linkage with chromosome regions involved in AM root colonization, a genome wide association analysis was carried out on 108 durum wheat varieties and two AM fungal species (F. mosseae and Rhizoglomus irregulare). Our findings showed that a century of breeding on durum wheat and the introgression of Reduced height (Rht) genes associated with increased grain yields did not select against AM symbiosis in durum wheat. Seven putative Quantitative Trait Loci (QTLs) linked with durum wheat mycorrhizal susceptibility in both experiments, located on chromosomes 1A, 2B, 5A, 6A, 7A and 7B, were detected. The individual QTL effects (r2) ranged from 7 to 16%, suggesting a genetic basis for this trait. Marker functional analysis identified predicted proteins with potential roles in host-parasite interactions, degradation of cellular proteins, homeostasis regulation, plant growth and disease/defence. The results of this work emphasize the potential for further enhancement of root colonization exploiting the genetic variability present in wheat.

The Catabolite Repressor/Activator Cra Is a Bridge Connecting Carbon Metabolism and Host Colonization in the Plant Drought Resistance-Promoting Bacterium Pantoea alhagi LTYR-11Z.

Efficient root colonization is a prerequisite for application of plant growth-promoting (PGP) bacteria in improving health and yield of agricultural crops. We have recently identified an endophytic bacterium, Pantoea alhagi LTYR-11Z, with multiple PGP properties that effectively colonizes the root system of wheat and improves its growth and drought tolerance. To identify novel regulatory genes required for wheat colonization, we screened an LTYR-11Z transposon (Tn) insertion library and found cra to be a colonization-related gene. By using transcriptome (RNA-seq) analysis, we found that transcriptional levels of an eps operon, the ydiV gene encoding an anti-FlhD4C2 factor, and the yedQ gene encoding an enzyme for synthesis of cyclic dimeric GMP (c-di-GMP) were significantly downregulated in the Δcra mutant. Further studies demonstrated that Cra directly binds to the promoters of the eps operon, ydiV, and yedQ and activates their expression, thus inhibiting motility and promoting exopolysaccharide (EPS) production and biofilm formation. Consistent with previous findings that Cra plays a role in transcriptional regulation in response to carbon source availability, the activating effects of Cra were much more pronounced when LTYR-11Z was grown within a gluconeogenic environment than when it was grown within a glycolytic environment. We further demonstrate that the ability of LTYR-11Z to colonize wheat roots is modulated by the availability of carbon sources. Altogether, these results uncover a novel strategy utilized by LTYR-11Z to achieve host colonization in response to carbon nutrition in the environment, in which Cra bridges a connection between carbon metabolism and colonization capacity of LTYR-11Z.IMPORTANCE Rapid and appropriate response to environmental signals is crucial for bacteria to adapt to competitive environments and to establish interactions with their hosts. Efficient colonization and persistence within the host are controlled by various regulatory factors that respond to specific environmental cues. The most common is nutrient availability. In this work, we unraveled the pivotal role of Cra in regulation of colonization ability of Pantoea alhagi LTYR-11Z in response to carbon source availability. Moreover, we identified three novel members of the Cra regulon involved in EPS synthesis, regulation of flagellar biosynthesis, and synthesis of c-di-GMP and propose a working model to explain the Cra-mediated regulatory mechanism that links carbon metabolism to host colonization. This study elucidates the regulatory role of Cra in bacterial attachment and colonization of plants, which raises the possibility of extending our studies to other bacteria associated with plant and human health.

Intensified dryland crop rotations support greater grain production with fewer inputs

Rising use and costs of agri-chemical inputs to support agricultural production have placed an economic burden on farmers while contributing to environmental and human health issues. Ecologically based nutrient and weed management – the use of ecological processes to replace external chemical inputs – may represent a strategy to support crop growth while achieving positive environmental and economic outcomes. In dryland agroecosystems around the world, farmers are increasingly transitioning toward no-till and intensified cropping systems, in which unvegetated fallow periods are replaced with crops. This study seeks to determine if cropping system intensification represents an ecologically based strategy for managing nutrients and weeds relative to traditional crop-fallow systems, and to understand the implications for crop production and profitability. We quantified total and potentially mineralizable nitrogen (N), arbuscular mycorrhizal fungal (AMF) colonization of wheat roots and implications for plant phosphorus (P) uptake, 6 years of crop yields, fertilizer and herbicide use, and net operating income across dryland, no-till cropping systems in the semi-arid High Plains, USA. Three levels of cropping system intensity were represented ranging from wheat-fallow (unvegetated fallow every other year) to continuous cropping (no fallow years). After accounting for variability due to environment and site characteristics, total and potentially mineralizable N were 12% and 30% greater in continuous rotations relative to wheat-fallow, respectively. Mid-intensity (fallow every 2 or 3 years) and continuous rotations had roughly 2 and 3 times more AMF colonization than wheat-fallow, respectively, and AMF colonization was positively correlated with wheat plant P concentration. Farmers practicing continuous cropping applied 22 and 34 kg ha−1 less N fertilizer per crop compared to wheat-fallow and mid-intensity, respectively, despite similar and 60% greater annualized crop production than mid-intensity and wheat-fallow rotations, respectively. Additionally, farmers who practiced continuous cropping used less than half the total herbicide used by wheat-fallow farmers. Net operating incomes of continuous and mid-intensity rotations were an estimated 47 USD ha−1 yr−1 (80%) and 42 USD ha−1 yr−1 (70%) more than wheat-fallow, respectively. These results suggest that cropping system intensification, and especially continuous cropping, represents an opportunity to achieve more grain production while managing weeds and nutrients with fewer agri-chemical inputs, leading to greater profitability and improved environmental outcomes in no-till agroecosystems.

Assessment of bacterial inoculant formulated with Paraburkholderia tropica to enhance wheat productivity.

Paraburkholderia tropica is an endophytic nitrogen-fixing bacterium isolated from the rhizosphere, rhizoplane, and internal tissues of sugarcane and corn plants in different geographical regions. Other plant-growth-promoting abilities, such as phosphate solubilization and antifungal activity, have also been reported for this bacterium. With an aim at investigating the potential use of P. tropica as an inoculant for improving the performance of wheat crop, in this work we evaluated an experimental inoculant formulated with P. tropica MTo-293 with respect to root colonization, the practical aspects of its application, and the effects under field conditions when applied to wheat seeds. Bacterial colonization was monitored by culture dependent techniques and the wheat yield determined by quantifying the total grain production in two different seasons. Rhizoplane and endophytic colonization in wheat roots was achieved efficiently (on average, 8 and 4 log colony-forming units/g fresh weight, respectively) even at relatively low concentrations of viable bacteria in the inoculum under controlled conditions. P. tropica was compatible with a widely used fungicide, maintained viability for 48h once applied to seeds, and was also able to colonize wheat roots efficiently. Furthermore, we were able to formulate an inoculant that maintained bacterial viability for relatively long time periods. Preliminary field assays were realized, and even though the average yields values for the inoculated treatments remained above the uninoculated ones, no significant effects of inoculation were detected with or without fertilization. The correct physiologic behavior of P. tropica suggests the necessity to continue with field experiments under different conditions.

The root endophytes Trametes versicolor and Piriformospora indica increase grain yield and P content in wheat

Soil phosphorus (P) deficiency occurs in many developing and transition countries. One method of resolving soil P deficiency is a strong application of mineral and organic fertilizers in order to saturate the P binding capacity of soil. Another promising method is the implementation of crop-endophyte symbioses in combination with the application of smaller amount of P fertilizer. This study comparatively examined the effect of the fungal endophytes Trametes versicolor and Piriformospora indica in P-deprived and P-rich conditions on P uptake and yield in wheat (Triticum aestivum L., cv. Bobwhite). Three-day-old wheat seedlings were dip-inoculated with mycelia of (a) T. versicolor WC16GW axenically isolated from Galium album, a dicotyledonous plant obtained from grassland in Linden near Giessen, Germany, and (b) axenic cultures of P. indica DSM 11827 freshly re-isolated from surface-sterilized barley roots. Seedlings were subsequently grown in 6 l Mitscherlich pots (eight seedlings per pot) in soil containing mono-calcium phosphate [CP, Ca (H2PO4)2] with 100 mg P kg−1 soil and control (CO) with 6.3 mg CAL-P kg−1 soil P in an open-air pot experiment station for three months. Colonization of wheat roots by T. versicolor and P. indica increased plant biomass, yield and P content. T. versicolor-colonized plants exhibited a significant increase in grain yield of 37% (CO treatment) and 8.5% (CP treatment), as well as straw yield of 27% (CO treatment) as compared to non-colonized plants. P. indica-colonized plants showed a significant increase in grain yield of 10% under high P (CP treatment) and straw yield of 22% (CO treatment). Moreover, P. indica improved grain P content by 30% (CO treatment), 16% (CP treatment) and straw P content by 33% (CO treatment), while T. versicolor increased grain P content by 16% (CP treatment) and straw by 35% (CP treatment). Both T. versicolor and P. indica improved wheat P uptake in both P-deprived and P-rich conditions. T. versicolor supported a high grain yield under the CO and CP treatments, suggesting this fungus has a promising potential for P management in cereal crops.