Inoculation of Herbaspirillum Seropedicae Increases Biomass in Maize Roots in the Early Stages of Plant Development

Herbaspirillum seropedicae is a plant growth-promoting bacteria isolated from diverse plant species. In this work, the main objective was to investigate the efficiency of H. seropedicae strain SmR1 in colonizing and increasing maize growth in the early stages of development under greenhouse conditions. Inoculation with H. seropedicae resulted in 10.51 and 19.43% in mean of increase of root biomass concerning non-inoculated controls, mainly in the initial stages of plant development, at 21 days after emergence (DAE). Quantification of H. seropedicae in roots and leaves was performed by quantitative PCR.. H. seropedicae was detected only in maize inoculated roots by qPCR, and a slight decrease in DNA copy number g−1 of fresh root weight was observed from 7 to 21 DAE, suggesting that there was initial effective colonization on maize plants. H. seropedicae strain SmR1 efficiently increased maize root biomass exhibiting its potencial to be used as inoculant in agricultures systems.

Plant Growth Promotion and Biocontrol Properties of Aneurinibacillus migulanus Isolated from Maize Roots (Zea mays)

Aims: To isolate Plant Growth Promoting Bacillus strain from maize roots, to evaluate its biocontrol potentials and to characterize the isolate using16S rRNA sequencing. Place and Duration of Study: Department of Applied Microbiology and Brewing, Nnamdi Azikiwe University, Awka, between February 2019 and March 2020. Methodology: The isolation of Plant Growth Promoting Rhizobacteria (PGPR) from maize roots was done using Pikovskaya (PVK) agar. Quantitative determination of phosphate was carried out using PVK broth. Evaluations of other plant growth promoting properties were carried out such as IAA, etc. Fusarium and Enterobacter plant pathogens were isolated from diseased maize plants. The in vitro antagonism effects of the PGPR isolates against the pathogens were analyzed using the dual culture plate technique. The pot experiment was carried out in a completely randomized design. Plant characteristics such as plant height, shoot and root weight, chlorophyll content, as well as disease assessment were recorded accordingly. The organisms were identified using phenotypic and molecular methods. Results: Seven PGPR bacteria were isolated from maize (Zea mays) roots using PVK agar. Aneurinibacillus migulanus gave the highest solubilization index of 4.21 while isolate IS48 gave the lowest solubilization index of 1.47. A. migulanus produced IAA, ammonia and cellulase enzyme but no hydrogen cyanide. The organism showed antagonism activity against the two tested phytopathogens. In the pot experiment, A. migulanus treated plants showed a statistically insignificant difference in maize plant height at P=0.05 but gave significant increases in shoot and root wet weights. The organism offered 83.33% and 71.43% protection against Enterobacter and Fusarium pathogens respectively in the pot experiment. Conclusion: A. migulanus solubilized phosphate in addition to other plant growth promoting properties. It showed biocontrol potentials both in vitro and in vivo and thus can be used as substitute for synthetic agrochemicals.

TopoRoot: a method for computing hierarchy and fine-grained traits of maize roots from 3D imaging

Background 3D imaging, such as X-ray CT and MRI, has been widely deployed to study plant root structures. Many computational tools exist to extract coarse-grained features from 3D root images, such as total volume, root number and total root length. However, methods that can accurately and efficiently compute fine-grained root traits, such as root number and geometry at each hierarchy level, are still lacking. These traits would allow biologists to gain deeper insights into the root system architecture. Results We present TopoRoot, a high-throughput computational method that computes fine-grained architectural traits from 3D images of maize root crowns or root systems. These traits include the number, length, thickness, angle, tortuosity, and number of children for the roots at each level of the hierarchy. TopoRoot combines state-of-the-art algorithms in computer graphics, such as topological simplification and geometric skeletonization, with customized heuristics for robustly obtaining the branching structure and hierarchical information. TopoRoot is validated on both CT scans of excavated field-grown root crowns and simulated images of root systems, and in both cases, it was shown to improve the accuracy of traits over existing methods. TopoRoot runs within a few minutes on a desktop workstation for images at the resolution range of 400^3, with minimal need for human intervention in the form of setting three intensity thresholds per image. Conclusions TopoRoot improves the state-of-the-art methods in obtaining more accurate and comprehensive fine-grained traits of maize roots from 3D imaging. The automation and efficiency make TopoRoot suitable for batch processing on large numbers of root images. Our method is thus useful for phenomic studies aimed at finding the genetic basis behind root system architecture and the subsequent development of more productive crops.

SCARECROW is deployed in distinct developmental contexts during rice and maize leaf development

The flexible deployment of developmental regulators is an increasingly appreciated aspect of plant development and evolution. The GRAS transcription factor SCARECROW (SCR) regulates the development of the endodermis in Arabidopsis and maize roots, but during leaf development it regulates the development of distinct cell-types; bundle-sheath in Arabidopsis and mesophyll in maize. In rice, SCR is implicated in stomatal patterning, but it is unknown whether this function is additional to a role in inner leaf patterning. Here, we demonstrate that two duplicated SCR genes function redundantly in rice. Contrary to previous reports, we show that these genes are necessary for stomatal development, with stomata virtually absent from leaves that are initiated after germination of mutants. The stomatal regulator OsMUTE is down-regulated in Osscr1;Osscr2 mutants indicating that OsSCR acts early in stomatal development. Notably, Osscr1;Osscr2 mutants do not exhibit the inner leaf patterning perturbations seen in Zmscr1;Zmscr1h mutants and Zmscr1;Zmscr1h mutants do not exhibit major perturbations in stomatal patterning. Taken together, these results indicate that SCR was deployed in different developmental contexts after the divergence of rice and maize around 50 million years ago.

Metabolic analysis of a bacterial synthetic community from maize roots provides new mechanistic insights into microbiome stability

Stability is a desirable property for agricultural microbiomes, but there is a poor understanding of the mechanisms that mediate microbial community stability. Recently, a representative bacterial synthetic community from maize roots has been proposed as a model system to study microbiome stability (Niu 2017, PNAS, 114:E2450). This SynCom assembles stably when all seven members are present, but community diversity collapses without the keystone E. cloacae strain. The aim of this study was to assess the role of metabolites for the stability of this SynCom, by defining the metabolic niches occupied by each strain, as well as their cross-feeding phenotypes and B-vitamin dependencies. We show that the individual member strains occupy complementary metabolic niches, measured by the depletion of distinct metabolites in exometabolomic experiments, as well as contrasting growth phenotypes on diverse carbon substrates. Minimal medium experiments show that the established seven-member community comprises a mixture of prototrophic and auxotrophic strains. Correspondingly, experimental cross-feeding phenotypes showed that spent media harvested from the prototrophic strains can sustain growth of two auxotrophs. We suggest that the metabolic mechanisms exhibited by this SynCom could serve as design principles to inform the rational assembly of stable plant-associated microbial communities.

The Pellicle–Another Strategy of the Root Apex Protection against Mechanical Stress?

In grasses, the apical part of the root is covered by a two-layered deposit of extracellular material, the pellicle, which together with the outer periclinal wall of protodermal cells forms the three-layered epidermal surface. In this study, the effect of mechanical stress on the pellicle was examined. An experiment was performed, in which maize roots were grown in narrow diameter plastic tubes with conical endings for 24 h. Two groups of experimental roots were included in the analysis: stressed (S) roots, whose tips did not grow out of the tubes, and recovering (R) roots, whose apices grew out of the tube. Control (C) roots grew freely between the layers of moist filter paper. Scanning electron microscopy and confocal microscopy analysis revealed microdamage in all the layers of the epidermal surface of S roots, however, protodermal cells in the meristematic zone remained viable. The outermost pellicle layer was twice as thick as in C roots. In R roots, large areas of dead cells were observed between the meristematic zone and the transition zone. The pellicle was defective with a discontinuous and irregular outermost layer. In the meristematic zone the pellicle was undamaged and the protodermal cells were intact. The results lead to the conclusion that the pellicle may prevent damage to protodermal cells, thus protecting the root apical meristem from the negative effects of mechano-stress.

Insights Into Manganese Solubilizing Bacillus spp. for Improving Plant Growth and Manganese Uptake in Maize

Manganese (Mn) is an essential micronutrient for plant growth that is involved in the structure of photosynthetic proteins and enzymes. Mn deficiency is widespread mainly in dry, calcareous, and sandy soil, which leads to a significant decrease in crop yield. Mn-reducing bacteria promote the solubilization of Mn minerals, thus increasing Mn availability in soil. The present study aimed to assess the Mn solubilizing ability and plant growth-promoting potential of Bacillus spp. strains for maize plants with insoluble Mn compounds. Several Mn-solubilizing bacterial (MSB) strains were isolated from the maize rhizosphere using nutrient agar media amended with 50 mM MnO2. These strains were screened based on qualitative and quantitative solubilization of Mn, phosphorus, potassium, and zinc and production of ammonia. The majority of MSB strains were positive for catalase, protease, amylase, and oxidase activity, while more than 60% of tested strains were positive for lipase activity, and the production of indole-3-acetic acid and siderophores. Forty-five percent of the tested strains also showed solubilization of potassium. All the MSB strains were evaluated for their ability to promote plant growth and Mn uptake in the presence of MnO2 under axenic sand culture conditions. The results revealed that inoculation with MSB strains under sand culture significantly improved the growth of maize seedlings except for strains ASH7, ASH10, and ASH12. Comparatively, strains ASH6, ASH11, ASH19, ASH20, and ASH22 demonstrated a better increase in plant growth, fresh and dry biomass, and Mn uptake in roots and shoots than the other strains tested. All of these strains were identified as Bacillus spp. through 16S rRNA partial gene sequencing. Maize inoculation with these selected identified MSB strains also resulted in an increase in maize growth and nutrient uptake in maize roots and shoots under soil culture conditions in the presence of native soil Mn. The current study highlights the importance of MSB strain inoculation which could be a potential bioinoculants to promote plant growth under Mn deficiency.