Plant-Microbe Symbiosis

The role of MtCEP1, a member of the CEP (C-terminally encoded peptide) signaling peptide family, was examined in Medicago truncatula root development. MtCEP1 was expressed in root tips, vascular tissue, and young lateral organs, and was up-regulated by low nitrogen levels and, independently, by elevated CO2. Overexpressing MtCEP1 or applying MtCEP1 peptide to roots elicited developmental phenotypes: inhibition of lateral root formation, enhancement of nodulation, and the induction of periodic circumferential root swellings, which arose from cortical, epidermal, and pericycle cell divisions and featured an additional cortical cell layer. MtCEP peptide addition to other legume species induced similar phenotypes. The enhancement of nodulation by MtCEP1 is partially tolerant to high nitrate, which normally strongly suppresses nodulation. These nodules develop faster, are larger, and fix more nitrogen in the absence and presence of inhibiting nitrate levels. At 25mM nitrate, nodules formed on pre-existing swelling sites induced by MtCEP1overexpression. RNA interference-mediated silencing of several MtCEPgenes revealed a negative correlation between transcript levels of MtCEP1and MtCEP2 with the number of lateral roots. MtCEP1 peptide-dependent phenotypes were abolished or attenuated by altering or deleting key residues in its 15 amino acid domain. RNA-Seq analysis revealed that 89 and 116 genes were significantly up- and down-regulated, respectively, by MtCEP1 overexpression, including transcription factors WRKY, bZIP, ERF, and MYB, homologues of LOB29, SUPERROOT2, and BABY BOOM. Taken together, the data suggest that the MtCEP1 peptide modulates lateral root and nodule development in M. truncatula.

Mycorrhizae application plays an important role regarding the yield, development and nutrient uptake of pepper plant (Capsicum annuum L.), especially in soils with high phosphorus fixation capacity. The objective of the study was to determine the effect of two mycorrhizal species and three different P doses on pepper growth in an iron oxide and carbonate rich Harran soil. Inan-3363 pepper variety was chosen for the experiment and inoculated with Glomus mosseaeand G. etunicatum mycorrhizae species at the seedling and transplanting stages. In the first stage, pepper seeds with about 1000 spores per viol in perlite+peat (1:1, V: V) mixture and without mycorrhizal inoculations were seeded and left to grow. At the second stage the seedlings were transplanted to the experimental plots. In addition to control (0), the plots have received 100 and 200 kg ha-1 P2O5 doses. Both mycorrhizal inoculations had statistically significant positive effect on the pepper yield; however, G. mosseae species caused more pepper yield than G. etunicatum inoculation. Inoculated seedlings resulted in 5.4 and 12.7% yield increase when compared to plots with the 200 kg ha-1P2O5doses only.However, yield increases in re-inoculated seedlings (at transplanting stage) were 6 and 20.9% with the 200 kg ha-1 P2O5 dose. The study confirms that the application of mycorrhizae is of great importance to increase the pepper production under field conditions in the region. Plant analyses showed that mycorrhizae inoculation has no statistically significant effect on the amount of plant nutrients.

We investigated the physiology that underlies the influence of arbuscular mycorrhizal (AM) colonization on outcomes of interactions between plants. We grew Medicago truncatula A17 and its AM-defective mutant dmi1 in intragenotypic (two plants per pot of the same genotype, x2) or intergenotypic (one plant of each genotype, 1+1) combinations, inoculated or not with Rhizophagus irregularis (formerlyGlomus intraradices) or Gigaspora margarita. We measured plant growth, colonization, contributions of AM and direct P uptake pathways using 32P, and expression of plant Pi transporter genes at two levels of P supply. A17 (x2) responded positively to inoculation only at Low P. The response was enhanced with 1+1 even at High P where colonization in A17 was reduced. With R. irregularis P uptake by the AM pathway was unaffected by P supply, whereas with G. margarita the AM pathway was lower at High P, and direct uptake higher. Gene expression varied and was unrelated to P uptake through the two pathways. There was no evidence of plant control of P uptake via R. irregularis at High P but there was via G. margarita. Importantly, growth responses of plant genotypes grown alone did not predict outcomes of intergenotypic interactions.

Leguminous plants have the ability to make their own nitrogen fertilizer by forming a root nodule symbiosis with nitrogen-fixing soil bacteria, collectively called rhizobia. This biological process plays a critical role in sustainable agriculture because it reduces the need for external nitrogen input. One remarkable property of legume–rhizobial symbiosis is its high level of specificity, which occurs at both inter- and intra-species levels and takes place at multiple phases of the interaction, ranging from initial bacterial infection and nodulation to late nodule development associated with nitrogen fixation. Knowledge of the molecular mechanisms controlling symbiotic specificity will facilitate the development of new crop varieties with improved agronomic potential for nitrogen-fixing symbiosis. In this report, we describe fine mapping of the Rj4 locus, a gene controlling nodulation specificity in soybean (Glycine max). The Rj4 allele prevents the host plant from nodulation with many strains ofBradyrhizobium elkanii, which are frequently present in soils of the southeastern USA. Since B. elkanii strains are poor symbiotic partners of soybean, cultivars containing an Rj4 allele are considered favorable. We have delimited the Rj4 locus within a 57-kb genomic region on soybean chromosome 1. The data reported here will facilitate positional cloning of the Rj4 gene and the development of genetic markers for marker-assisted selection in soybean.

Rhizobium sp. strain IRBG74 is the first known nitrogen-fixing symbiont in theAgrobacterium/Rhizobium clade that nodulates the aquatic legume Sesbania sp. and is also a growth-promoting endophyte of wetland rice. Here, we present the sequence of the IRBG74 genome, which is composed of a circular chromosome, a linear chromosome, and a symbiotic plasmid, pIRBG74a.

Plant–microbe interactions in the rhizosphere have important roles in biogeochemical cycling, and maintenance of plant health and productivity, yet remain poorly understood. Using RNA-based metatranscriptomics, the global active microbiomes were analysed in soil and rhizospheres of wheat, oat, pea and an oat mutant (sad1) deficient in production of anti-fungal avenacins. Rhizosphere microbiomes differed from bulk soil and between plant species. Pea (a legume) had a much stronger effect on the rhizosphere than wheat and oat (cereals), resulting in a dramatically different rhizosphere community. The relative abundance of eukaryotes in the oat and pea rhizospheres was more than fivefold higher than in the wheat rhizosphere or bulk soil. Nematodes and bacterivorous protozoa were enriched in all rhizospheres, whereas the pea rhizosphere was highly enriched for fungi. Metabolic capabilities for rhizosphere colonisation were selected, including cellulose degradation (cereals), H2 oxidation (pea) and methylotrophy (all plants). Avenacins had little effect on the prokaryotic community of oat, but the eukaryotic community was strongly altered in the sad1 mutant, suggesting that avenacins have a broader role than protecting from fungal pathogens. Profiling microbial communities with metatranscriptomics allows comparison of relative abundance, from multiple samples, across all domains of life, without polymerase chain reaction bias. This revealed profound differences in the rhizosphere microbiome, particularly at the kingdom level between plants.

Mycorrhiza formation represents a significant carbon (C) acquisition alternative for orchid species, particularly those that remain achlorophyllous through all life stages. While it is known that orchid mycorrhizas facilitate nutrient transfer (most notably of C), it has not been resolved if C transfer occurs only after lysis of mycorrhizal structures (fungal pelotons) or also across the mycorrhizal interface of pre-lysed pelotons. We used high-resolution secondary ion mass spectrometry (nanoSIMS) and labelling with enriched 13CO2 to trace C transfers, at sub-cellular scale, across mycorrhizal interfaces formed by Rhizanthella gardneri, an achlorphyllous orchid. Carbon was successfully traced in to the fungal portion of orchid mycorrhizas. However, we did not detect C movement across intact mycorrhizal interfaces up to 216 hours post 13CO2 labelling. Our findings provide support for the hypothesis that C transfer from the mycorrhizal fungus to orchid, at least for R. gardneri, likely occurs after lysis of the fungal peloton.

Aim

To develop coaggregated bacterial inoculant comprising of Methylobacterium oryzae CBMB20 / M. suomiense CBMB120 strains withAzospirillum brasilense (CW903) strain and testing their efficiency as inoculants for plant growth promotion (PGP).

Methods and Results

Biofilm formation and coaggregation efficiency was studied between A. brasilense CW903 and methylobacterial strains Methylobacterium oryzae CBMB20 and M. suomiense CBMB120. Survival and release of these coaggregated bacterial strains entrapped in alginate beads were assessed. PGP attributes of the coaggregated bacterial inoculant was tested in tomato plants under water stressed conditions. Results suggest that the biofilm formation efficiency of the CBMB20 and CBMB120 strains increased by 15% and 34%, respectively when co-cultivated with CW903. Coaggregation with CW903 enhanced the survivability of CBMB20 strain in alginate beads. Water stress index score showed least stress index in plants inoculated with CW903 and CBMB20 strains maintained as a coaggregated inoculant.

Conclusions

This study reports the development of coaggregated cell inoculants containing M. oryzae CBMB20 and A. brasilense CW903 strains conferred better shelf life and stress abatement in inoculated tomato plants.

Significance and impact of the study

These findings could be extended to other PGP bacterial species to develop multigeneric bioinoculants with multiple benefits for various crops.

Salinization of soil is one of the main threats for the development and maintenance of agricultural systems. Climate change will even increase soil salinity further. Soil salinity affects the establishment, growth, and development of crops and can result in severe yield reduction. Fortunately, the plant root contains beneficial microbes. These microbes, including plant growth-promoting rhizobacteria (PGPR) are able to promote plant growth and protect plants against various soilborne pathogens and can help plants to adapt to a number of environmental stresses. The mechanisms of alleviation of salt stress and plant growth promotion by PGPR include the production of phytohormones and the enzyme ACC deaminase, and competition for nutrient and niches. Increasing our understanding of the modes of action of these mechanisms will open new doors for proposing strategies to improve the efficacy of PGPR. For example, more detailed studies are needed on the role of abiotic factors in altering the activity of rhizobacteria and on managing plant–microbe interactions with respect to their adaptability to extreme conditions. This chapter provides a brief overview of our present knowledge of the alleviation of salt stress in plants by PGPR and the action modes of these PGPR under salt-stressed conditions.