Unlocking a Mystery: Characterizing the First Appearance of Clavibacter nebraskensis in Mexican Cornfields.

The Goss's wilt and leaf blight is a disease of maize (Zea mays) caused by Clavibacter nebraskensis, which was widespread in the last several years throughout the Midwest in the United States, south in Texas, and north to Canada. The bacterium is included within the high-risk list of quarantine pathogens by many plant protection organizations and countries including Mexico. Severe blight symptoms on maize plants were found in different provinces from Coahuila and Tlaxcala, Mexico, in 2012 and 2021, respectively. Twenty bacterial isolates with morphology similar to C. nebraskensis were obtained from the diseased maize leaves. The isolates were confirmed by phenotypic tests and 16S rRNA and gyrB sequencing. Two strains were tested for pathogenicity tests on seven hybrid sweet corn cultivars available in Mexico, and the most sensitive cultivar was tested for all the strains to fulfill Koch's postulates. The phylogenetic reconstruction based on two single loci reveals a remarkable clustering of Mexican strains to American strains reported approximately 50 years ago. The presence of this pathogen represents a risk and a significant challenge for plant protection strategies in Mexico and maize diversity.

Extracellular ligninases production and lignin degradation by Paenibacillus polymyxa.

Bacteria represent an attractive source for the isolation and identification of potentially useful microorganisms for lignin depolymerization, a process required for the use of agricultural waste. In this work, ten autochthonous bacteria isolated from straw, cow manure, and composts were characterized for potential use in the biodelignification of the waste. A comparison of the ability to degrade lignin and the efficiency of ligninolytic enzymes was performed in bacteria grown in media with lignin as a sole carbon source (LLM, 3.5g/L lignin-alkali) and in complex media supplemented with All-Ban fiber (FLM, 1.5g/L). Bacterial isolates showed different abilities to degrade lignin, they decreased the lignin concentration from 7.6 to 18.6% in LLM and from 11.1 to 44.8% in FLM. They also presented the activity of manganese peroxidase, lignin peroxidases, and laccases with different specific activities. However, strain 26 identified as Paenibacillus polymyxa by sequencing the 16S rRNA showed the highest activity of lignin peroxidase and the ability to degrade efficiently lignocellulose. In addition, P. polymyxa showed the highest potential (desirability ≥ 0.795) related to the best combination of properties to depolymerize lignin from biomass. The results suggest that P. polymyxa has a coordinated lignin degradation system constituted of lignin peroxidase, manganese peroxidase, and laccase enzymes.

Mycorrhizal status and host genotype interact to shape plant nutrition in field grown maize (Zea mays ssp. mays).

Arbuscular mycorrhizal fungi (AMF) establish symbioses with the major cereal crops, providing plants with increased access to nutrients while enhancing their tolerance to toxic heavy metals. However, not all plant varieties benefit equally from this association. In this study, we used quantitative trait loci (QTL) mapping to evaluate the combined effect of host genotypic variation (G) and AMF across 141 genotypes on the concentration of 20 mineral elements in the leaves and grain of field grown maize (Zea mays spp. mays). Our mapping design included selective incorporation of a castor AMF-incompatibility mutation, allowing estimation of AMF, QTL and QTLxAMF effects by comparison of mycorrhizal and non-mycorrhizal plants. Overall, AMF compatibility was associated with higher concentrations of boron (B), copper (Cu), molybdenum (Mo), phosphorus (P), selenium (Se) and zinc (Zn) and lower concentrations of arsenic (As), iron (Fe), magnesium (Mg), manganese (Mn), potassium (K) and strontium (Sr). In addition to effects on individual elements, pairwise correlation matrices for element concentration differed between mycorrhizal and non-mycorrhizal plants. We mapped 22 element QTLs, including 18 associated with QTLxAMF effects that indicate plant genotype-specific differences in the impact of AMF on the host ionome. Although there is considerable interest in AMF as biofertilizers, it remains challenging to estimate the impact of AMF in the field. Our design illustrates an effective approach for field evaluation of AMF effects. Furthermore, we demonstrate the capacity of the ionome to reveal host genotype-specific variation in the impact of AMF on plant nutrition.

Soil Structure and Ectomycorrhizal Root Colonization of Pecan Orchards in Northern Mexico.

Pecan trees form a symbiotic relationship with ectomycorrhizal fungi (ECM), which actively provide nutrition to the roots and protect them from phytopathogens. Although these trees originated in the southern United States and northern Mexico, information on their root colonization by ECM is insufficient in terms of a representative number of samples, both in these regions and worldwide. Therefore, the objectives of this study were to determine the percentage of ectomycorrhizal colonization (ECM) of pecan trees of different ages in conventional and organic agronomic orchards and to identify ectomycorrhizal sporocarps, both morphologically and molecularly. The rhizospheric soil properties and the ECM percentages were analyzed for 14 Western variety pecan tree orchards between 3 and 48 years of age and grouped according to the agronomic management method. DNA extraction, internal transcribed spacer amplification, and sequencing were conducted on the fungal macroforms. The ECM colonization percentage fluctuated between 31.44 and 59.89%. Soils with low phosphorus content showed higher ECM colonization. The ECM concentrations were relatively homogeneous in relation to the ages of the trees, and organic matter content did not affect the percentage of ECM colonization. The highest ECM percentages occurred with the sandy clay crumb texture soil, with an average of 55% ECM, followed by sandy clay loam soils with 49.5%. The Pisolithus arenarius and Pisolithus tinctorius fungi were molecularly identified from sporocarps associated with pecan trees. This is the first study that reports Pisolithus arenarius as being associated with this tree.

Effect of Bacillus subtilis on antioxidant enzyme activities in tomato grafting.

Grafting generally means stress to a plant and this triggers antioxidant defense systems. An imbalance in reactive oxygen species may negatively affect the grafting success. Several research projects have studied the association with plant growth-promoting rhizobacteria (PGPR) and it has been documented that they enhance nutrient acquisition, regulate hormone levels, and influence the antioxidant response in crops. However, little is known about the strategy of inoculating grafted herbaceous plants with PGPR and its effect on the antioxidant response. The effects of inoculating a strain of Bacillus subtilis on the antioxidant metabolism of grafted tomato were evaluated. In this study, two different rootstocks were used for tomato (Solanum lycopersicum L. var. Rio Grande (RG)): [S. lycopersicum L. var. cerasiforme (Ch)] and eggplant [(Solanum melanogena L. (Ber)] to establish a compatible graft (RGCh) and a semicompatible graft (RGBer). Enzyme activities involved in the antioxidant defense system: superoxide dismutase (SOD), catalase (CAT), phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO), peroxidase (POD), and total phenols were measured during 4 weeks after grafting. The results show that for RGCh, during the first two weeks after grafting, the tendency was a decrease of the enzyme activity for SOD, CAT, PAL when inoculated with B. subtilis; while in the semicompatible graft RGBer, PPO and PAL decreased their activity after inoculation. For both combinations, the quantity of total phenols varied depending on the day. In both graft combinations, applications of B. subtilis resulted in 86 and 80% callusing compared with the uninoculated control where the percentages were 74 and 70% for RGCh and RGBer, respectively. The highest significant graft success (95%) was recorded 28 days after grafting for inoculated RGBer. These findings imply that B. subtilis induced antioxidant mechanisms in grafted plants and suggest that inoculation with this growth-promoting bacterium can represent a biotechnological approach to improve success in tomato grafting.

The genetic architecture of host response reveals the importance of arbuscular mycorrhizae to maize cultivation.

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in cultivated soils, forming symbiotic relationships with the roots of major crop species. Studies in controlled conditions have demonstrated the potential of AMF to enhance the growth of host plants. However, it is difficult to estimate the actual benefit in the field, not least because of the lack of suitable AMF-free controls. Here we implement a novel strategy using the selective incorporation of AMF-resistance into a genetic mapping population to evaluate maize response to AMF. We found AMF to account for about one-third of the grain production in a medium input field, as well as to affect the relative performance of different plant genotypes. Characterization of the genetic architecture of the host response indicated a trade-off between mycorrhizal dependence and benefit. We identified several QTL linked to host benefit, supporting the feasibility of breeding crops to maximize profit from symbiosis with AMF.

Proteomic analysis and interactions network in leaves of mycorrhizal and nonmycorrhizal sorghum plants under water deficit.

For understanding the water deficit stress mechanism in sorghum, we conducted a physiological and proteomic analysis in the leaves of Sorghum bicolor L. Moench (a drought tolerant crop model) of non-colonized and colonized plants with a consortium of arbuscular mycorrhizal fungi. Physiological results indicate that mycorrhizal fungi association enhances growth and photosynthesis in plants, under normal and water deficit conditions. 2D-electrophoresis profiles revealed 51 differentially accumulated proteins in response to water deficit, of which HPLC/MS successfully identified 49. Bioinformatics analysis of protein-protein interactions revealed the participation of different metabolic pathways in nonmycorrhizal compared to mycorrhizal sorghum plants under water deficit. In noninoculated plants, the altered proteins are related to protein synthesis and folding (50S ribosomal protein L1, 30S ribosomal protein S10, Nascent polypeptide-associated complex subunit alpha), coupled with multiple signal transduction pathways, guanine nucleotide-binding beta subunit (Rack1) and peptidyl-prolyl-cis-trans isomerase (ROC4). In contrast, in mycorrhizal plants, proteins related to energy metabolism (ATP synthase-24kDa, ATP synthase ß), carbon metabolism (malate dehydrogenase, triosephosphate isomerase, sucrose-phosphatase), oxidative phosphorylation (mitochondrial-processing peptidase) and sulfur metabolism (thiosulfate/3-mercaptopyruvate sulfurtransferase) were found. Our results provide a set of proteins of different metabolic pathways involved in water deficit produced by sorghum plants alone or associated with a consortium of arbuscular mycorrhizal fungi isolated from the tropical rain forest Los Tuxtlas Veracruz, México.

Inoculation with the mycorrhizal fungus Rhizophagus irregularis modulates the relationship between root growth and nutrient content in maize (Zea mays ssp. mays L.).

Plant root systems play a fundamental role in nutrient and water acquisition. In resource-limited soils, modification of root system architecture is an important strategy to optimize plant performance. Most terrestrial plants also form symbiotic associations with arbuscular mycorrhizal fungi to maximize nutrient uptake. In addition to direct delivery of nutrients, arbuscular mycorrhizal fungi benefit the plant host by promoting root growth. Here, we aimed to quantify the impact of arbuscular mycorrhizal symbiosis on root growth and nutrient uptake in maize. Inoculated plants showed an increase in both biomass and the total content of twenty quantified elements. In addition, image analysis showed mycorrhizal plants to have denser, more branched root systems. For most of the quantified elements, the increase in content in mycorrhizal plants was proportional to root and overall plant growth. However, the increase in boron, calcium, magnesium, phosphorus, sulfur, and strontium was greater than predicted by root system size alone, indicating fungal delivery to be supplementing root uptake.

The impact of domestication and crop improvement on arbuscular mycorrhizal symbiosis in cereals: insights from genetics and genomics.

Contents Summary 1135 I. Introduction 1135 II. Recruitment of plant metabolites and hormones as signals in AM symbiosis 1136 III. Phytohormones are regulators of AM symbiosis and targets of plant breeding 1137 IV. Variation in host response to AM symbiosis 1137 V. Outlook 1137 Acknowledgements 1139 References 1139 SUMMARY: Cereals (rice, maize, wheat, sorghum and the millets) provide over 50% of the world's caloric intake, a value that rises to > 80% in developing countries. Since domestication, cereals have been under artificial selection, largely directed towards higher yield. Throughout this process, cereals have maintained their capacity to interact with arbuscular mycorrhizal (AM) fungi, beneficial symbionts that associate with the roots of most terrestrial plants. It has been hypothesized that the shift from the wild to cultivation, and above all the last c. 50 years of intensive breeding for high-input farming systems, has reduced the capacity of the major cereal crops to gain full benefit from AM interactions. Recent studies have shed further light on the molecular basis of establishment and functioning of AM symbiosis in cereals, providing insight into where the breeding process might have had an impact. Classic phytohormones, targets of artificial selection during the generation of Green Revolution semi-dwarf varieties, have emerged as important regulators of AM symbiosis. Although there is still much to be learnt about the mechanistic basis of variation in symbiotic outcome, these advances are providing an insight into the role of arbuscular mycorrhiza in agronomic systems.

Co-ordinated Changes in the Accumulation of Metal Ions in Maize (Zea mays ssp. mays L.) in Response to Inoculation with the Arbuscular Mycorrhizal Fungus Funneliformis mosseae.

Arbuscular mycorrhizal symbiosis is an ancient interaction between plants and fungi of the phylum Glomeromycota. In exchange for photosynthetically fixed carbon, the fungus provides the plant host with greater access to soil nutrients via an extensive network of root-external hyphae. Here, to determine the impact of the symbiosis on the host ionome, the concentration of 19 elements was determined in the roots and leaves of a panel of 30 maize varieties, grown under phosphorus-limiting conditions, with or without inoculation with the fungus Funneliformis mosseae. Although the most recognized benefit of the symbiosis to the host plant is greater access to soil phosphorus, the concentration of a number of other elements responded significantly to inoculation across the panel as a whole. In addition, variety-specific effects indicated the importance of plant genotype to the response. Clusters of elements were identified that varied in a co-ordinated manner across genotypes, and that were maintained between non-inoculated and inoculated plants.

Extracellular endoglucanase activity from Paenibacillus polymyxa BEb-40: production, optimization and enzymatic characterization.

Endoglucanase activity produced by Paenibacillus polymyxa BEb-40 was studied. In submerged culture with minimal medium supplemented with carboxymethylcellulose (CMC), this microorganism produced up to 0.37 U/mL endoglucanase activity with high specific activity (14.3 U/mgtotal protein). Detection of endoglucanase activity through zymography revealed at least 14 isoenzymes with molecular weights between 38 and 220 kDa. This high variety of secreted endoglucanases has not been described previously in Paenibacillus genus. The optimum conditions, determined by response surface methodology, were 48 °C and pH 3.4, which allowed an increase of 33.7 % in the relative endoglucanase activity obtained with respect to the standard conditions. Nevertheless, high levels of hydrolysis of at least 70 % of the maximum activity could be obtained at wide ranges of pH (2-9) and temperature (40-60 °C). Under optimal conditions, high levels of CMC hydrolysis were reached, of about 40 %, after only 12 h of reaction with substrate/total protein ratios between 19 and 76. Kinetic analysis revealed that endoglucanase activity followed a mixed inhibition model (K m = 8.4 mM, K ic = 0.03 mM, K iu = 0.35 mM, V max = 33.3 U/mgtotal protein). These results allow to consider P. polymyxa BEb-40 as a promising microorganism for the production of endoglucanases, with possibilities of application in the breakdown of lignocellulosic biomass. The high specific activity at wide ranges of pH and temperature can allow its use in a wide variety of processes, under both acidic and alkaline conditions, as well as in mesophilic and thermophilic temperatures, further reducing the amount of enzymes used.

Expression of phenazine biosynthetic genes during the arbuscular mycorrhizal symbiosis of Glomus intraradices

To explore the molecular mechanisms that prevail during the establishment of the arbuscular mycorrhiza symbiosis involving the genus Glomus, we transcriptionally analysed spores of Glomus intraradices BE3 during early hyphal growth. Among 458 transcripts initially identified as being expressed at presymbiotic stages, 20% of sequences had homology to previously characterized eukaryotic genes, 30% were homologous to fungal coding sequences, and 9% showed homology to previously characterized bacterial genes. Among them, GintPbr1a encodes a homolog to Phenazine Biosynthesis Regulator (Pbr) of Burkholderia cenocepacia, an pleiotropic regulatory protein that activates phenazine production through transcriptional activation of the protein D isochorismatase biosynthetic enzyme phzD (Ramos et al., 2010). Whereas GintPbr1a is expressed during the presymbiotic phase, the G. intraradices BE3 homolog of phzD (BGintphzD) is transcriptionally active at the time of the establishment of the arbuscular mycorrhizal symbiosis. DNA from isolated bacterial cultures found in spores of G. intraradices BE3 confirmed that both BGintPbr1a and BGintphzD are present in the genome of its potential endosymbionts. Taken together, our results indicate that spores of G. intraradices BE3 express bacterial phenazine biosynthetic genes at the onset of the fungal-plant symbiotic interaction.

Expression of phenazine biosynthetic genes during the arbuscular mycorrhizal symbiosis of Glomus intraradices.

To explore the molecular mechanisms that prevail during the establishment of the arbuscular mycorrhiza symbiosis involving the genus Glomus, we transcriptionally analysed spores of Glomus intraradices BE3 during early hyphal growth. Among 458 transcripts initially identified as being expressed at presymbiotic stages, 20% of sequences had homology to previously characterized eukaryotic genes, 30% were homologous to fungal coding sequences, and 9% showed homology to previously characterized bacterial genes. Among them, GintPbr1a encodes a homolog to Phenazine Biosynthesis Regulator (Pbr) of Burkholderia cenocepacia, an pleiotropic regulatory protein that activates phenazine production through transcriptional activation of the protein D isochorismatase biosynthetic enzyme phzD (Ramos et al., 2010). Whereas GintPbr1a is expressed during the presymbiotic phase, the G. intraradices BE3 homolog of phzD (BGintphzD) is transcriptionally active at the time of the establishment of the arbuscular mycorrhizal symbiosis. DNA from isolated bacterial cultures found in spores of G. intraradices BE3 confirmed that both BGintPbr1a and BGintphzD are present in the genome of its potential endosymbionts. Taken together, our results indicate that spores of G. intraradices BE3 express bacterial phenazine biosynthetic genes at the onset of the fungal-plant symbiotic interaction.

Arbuscular mycorrhizal fungi enhance fruit growth and quality of chile ancho (Capsicum annuum L. cv San Luis) plants exposed to drought.

The effect of arbuscular mycorrhizal fungi (AMF) and drought on fruit quality was evaluated in chile ancho (Capsicum annuum L. cv San Luis). AMF treatments were (1) Glomus fasciculatum (AMFG), (2) a fungal species consortium from the forest "Los Tuxtla" in Mexico (AMFT), (3) a fungal species consortium from the Sonorian desert in Mexico (AMFD), and (4) a noninoculated control (NAMF). Plants were exposed to a 26-day drought cycle. Fruit quality was determined by measuring size (length, width, and pedicel length), color, chlorophyll, and carotenoid concentration. Under nondrought conditions, AMFG produced fruits that were 13% wider and 15% longer than the NAMF treatment. Under nondrought conditions, fruit fresh weight was 25% greater in the AMFG treatment compared to the NAMF. Under drought, fruits in the AMFT and AMFD treatments showed fresh weights similar to those in the NAMF treatment not subjected to drought. Fruits of the AMFG treatment subjected to drought showed the same color intensity and chlorophyll content as those of the nondroughted NAMF treatment and carotenoid content increased 1.4 times compared to that in the NAMF not exposed to drought. It is interesting to note that fruits in the AMFD treatment subjected to drought and the NAMF treatment not exposed to drought reached the same size. AMFD treatment increased the concentration of carotenes (1.4 times) under nondrought conditions and the concentration of xanthophylls (1.5 times) under drought when compared to the nondroughted NAMF treatment.