Plant growth-promoting bacteria in the rhizo- and endosphere of plants: Their role, colonization, mechanisms involved and prospects for utilization

In both managed and natural ecosystems, beneficial plant-associated bacteria play a key role in supporting and/or increasing plant health and growth. Plant growth-promoting bacteria (PGPB) can be applied in agricultural production or for the phytoremediation of pollutants. However, because of their capacity to confer plant beneficial effects, efficient colonization of the plant environment is of utmost importance. The majority of plant-associated bacteria derives from the soil environment. They may migrate to the rhizosphere and subsequently the rhizoplane of their hosts before they are able to show beneficial effects. Some rhizoplane colonizing bacteria can also penetrate plant roots, and some strains may move to aerial plant parts, with a decreasing bacterial density in comparison to rhizosphere or root colonizing populations. A better understanding on colonization processes has been obtained mostly by microscopic visualisation as well as by analysing the characteristics of mutants carrying disfunctional genes potentially involved in colonization. In this review we describe the individual steps of plant colonization and survey the known mechanisms responsible for rhizosphere and endophytic competence. The understanding of colonization processes is important to better predict how bacteria interact with plants and whether they are likely to establish themselves in the plant environment after field application as biofertilisers or biocontrol agents

Control of T-2 toxin in Fusarium langsethiae and geotrichum candidum co-culture

Due to contamination of barley grains by Fusarium langsethiae, T-2 toxin can be present in the brewing process. It has been observed that the presence of the yeast Geotrichum candidum during malting can reduce the final concentration of this mycotoxin in beer. In this work, a co-culture method was carried out for both microorganisms in order to evaluate the effect on T-2 mycotoxin concentration in comparison with the pure culture of F. langsethiae in the same conditions. The microbial growth of both microorganisms was assessed using three different methods: dry weight, DOPE-FISH, and DNA quantification. In co-culture, both microorganisms globally developed less than in pure cultures but G. candidum showed a better growth than F. langsethiae. The concentration of T-2 was reduced by 93 % compared to the pure culture. Hence, the interaction between G. candidum and F. langsethiae led to a drastic mycotoxin reduction despite the only partial inhibition of fungal growth

Climate change effects on beneficial plant-microorganism interactions

It is well known that beneficial plant-associated microorganisms may stimulate plant growth and enhance resistance to disease and abiotic stresses. The effects of climate change factors such as elevated CO2, drought and warming on beneficial plant-microorganism interactions are increasingly being explored. This now makes it possible to test whether some general patterns occur and whether different groups of plant-associated microorganisms respond differently or in the same way to climate change. Here, we review the results of 135 studies investigating the effects of climate change factors on beneficial microorganisms and their interaction with host plants. The majority of studies showed that elevated CO2 had a positive influence on the abundance of arbuscular and ectomycorrhizal fungi, whereas the effects on plant growth-promoting bacteria and endophytic fungi were more variable. In most cases, plant-associated microorganisms had a beneficial effect on plants under elevated CO2. The effects of increased temperature on beneficial plant-associated microorganisms were more variable, positive and neutral, and negative effects were equally common and varied considerably with the study system and the temperature range investigated. Moreover, numerous studies indicated that plant growth-promoting microorganisms (both bacteria and fungi) positively affected plants subjected to drought stress. Overall, this review shows that plant-associated microorganisms are an important factor influencing the response of plants to climate chang

Metabolic potential of endophytic bacteria

The bacterial endophytic microbiome promotes plant growth and health and beneficial effects are in many cases mediated and characterized by metabolic interactions. Recent advances have been made in regard to metabolite production by plant microsymbionts showing that they may produce a range of different types of metabolites. These substances play a role in defense and competition, but may also be needed for specific interaction and communication with the plant host. Furthermore, few examples of bilateral metabolite production are known and endophytes may modulate plant metabolite synthesis as well. We have just started to understand such metabolic interactions between plants and endophytes, however, further research is needed to more efficiently make use of beneficial plant-microbe interactions and to reduce pathogen infestation as well as to reveal novel bioactive substances of commercial interest

The 125th anniversary of the first postulation of the soil origin of endophytic bacteria – a tribute to M.L.V. Galippe

In both managed and natural ecosystems, a wide range of various non-nodulating bacteria can thrive as endophytes in the plant interior, and some can be beneficial to their hosts (Hallmann and Berg 2007; Reinhold-Hurek and Hurek 2011). Colonizationmechanisms, the ecology and functioning of these endophytic bacteria as well as their interactions with plants have been investigated (Hardoim et al. 2008; Compant et al. 2010). Although the source of colonization can also be the spermosphere, anthosphere, caulosphere, and the phyllosphere,most endophytic bacteria are derived from the soil environment (Hallmann and Berg 2007; Compant et al. 2010)

In vitro and in planta fungicide properties of ozonated water against the esca-associated fungus Phaeoacremonium aleophilum

Ozone could be used as a sanitary agent in the food and agricultural industries. The present study investigates whether ozonated water could be used to control conidia dispersal of the esca-associated fungus Phaeoacremonium aleophilum. The fungicide properties of ozonated water was firstly assessed in vitro by exposing spores to several concentrations of ozonated water (2.2, 4.5, or 13.5g/m3 of ozone dissolved into water) and observing the germination rate on plate. Secondarily an in planta approach was conducted on grapevine cuttings of cv. Cabernet Sauvignon clone 15. P. aleophilum conidia were inoculated in pruning wounds, which then received ozonated (4.5g/m3) or sterile water. P. aleophilum DNA was quantified by a quantitative polymerase chain reaction (qPCR) 4 and 9 weeks post-inoculation. The effect of ozonated water on plant-defense gene expression was monitored by reverse-transcriptase qPCR (RT-qPCR) 48h post treatment. The results indicate that ozonated water totally suppresses spore germination in vitro. In addition, at 9 weeks post-inoculation, fungal development was significantly reduced by 50% in planta. RT-qPCR analysis shows that ozonated water did not induce plant-defense-related genes 48h post treatment. The fungicide properties of ozonated water and the absence of gene induction in planta make however ozonated water a promising candidate for limiting grapevine infection by P. aleophilum in nurseries

Beneficial insects deliver plant growth-promoting bacterial endophytes between tomato plants

Beneficial insects and mites, including generalist predators of the family Miridae, are widely used in biocontrol programs against many crop pests, such as whiteflies, aphids, lepidopterans and mites. Mirid predators frequently complement their carnivore diet by feeding plant sap with their piercing–sucking mouthparts. This implies that mirids may act as vectors of phytopathogenic and beneficial microorganisms, such as plant growth-promoting bacterial endophytes. This work aimed at understanding the role of two beneficial mirids (Macrolophus pygmaeus and Nesidiocoris tenuis) in the acquisition and transmission of two plant growth-promoting bacteria, Paraburkholderia phytofirmans strain PsJN (PsJN) and Enterobacter sp. strain 32A (32A). Both bacterial strains were detected on the epicuticle and internal body of both mirids at the end of the mirid-mediated transmission. Moreover, both mirids were able to transmit PsJN and 32A between tomato plants and these bacterial strains could be re-isolated from tomato shoots after mirid-mediated transmission. In particular, PsJN and 32A endophytically colonised tomato plants and moved from the shoots to roots after mirid-mediated transmission. In conclusion, this study provided novel evidence for the acquisition and transmission of plant growth-promoting bacterial endophytes by beneficial mirid

In Planta Colonization and Role of T6SS in Two Rice Kosakonia Endophytes.

Endophytes live inside plants and are often beneficial. Kosakonia is a novel bacterial genus that includes many diazotrophic plant-associated isolates. Plant–bacteria studies on two rice endophytic Kosakonia beneficial strains were performed, including comparative genomics, secretome profiling, in planta tests, and a field release trial. The strains are efficient rhizoplane and root endosphere colonizers and localized in the root cortex. Secretomics revealed 144 putative secreted proteins, including type VI secretory system (T6SS) proteins. A Kosakonia T6SS genomic knock-out mutant showed a significant decrease in rhizoplane and endosphere colonization ability. A field trial using rice seed inoculated with Kosakonia spp. showed no effect on plant growth promotion upon nitrogen stress and microbiome studies revealed that Kosakonia spp. were significantly more present in the inoculated rice. Comparative genomics indicated that several protein domains were enriched in plant-associated Kosakonia spp. This study highlights that Kosakonia is an important, recently classified genus involved in plant–bacteria interaction

Colonization of Vitis vinifera L. by the Endophyte Trichoderma sp. Strain T154: Biocontrol Activity Against Phaeoacremonium minimum

[EN] Trichoderma strains used in biological control products usually exhibit high efficiency in the control of plant diseases. However, their behavior under field conditions is difficult to predict. In addition, the potential of indigenous strains has been poorly assayed as well as their possible behavior as endophytes. Hence, niche colonization is a key feature for an effective protection. In this study, we aimed to: (i) explore the possibility of using a new Trichoderma strain isolated from vine to control pathogens, (ii) study the in planta interaction with the pathogen Phaeoacremonium minimum W. Gams, Crous, M.J. Wingf. & L. Mugnai (formerly Phaeoacremonium aleophilum), a pioneer fungus involved in Grapevine Trunk Diseases (GTDs) such as esca. For this purpose, fluorescently tagged Trichoderma sp. T154 and a P. minimum strain were used for scanning electron microscopy and confocal scanning laser microscopy analyses. Data showed that the Trichoderma strain is able to colonize plants up to 12 weeks post inoculation and is located in xylem, fibers, as well as in parenchymatic tissues inside the wood. The beneficial fungus reduced colonization of the esca-related pathogen colonizing the same niches. The main observed mechanism involved in biocontrol of Trichoderma against the esca pathogen was spore adhesion, niche exclusion and only few typical hypha coiling was found between Trichoderma and the pathogen. These results suggest that the Trichoderma strain has potential for reducing the colonization of Phaeoacremonium minimum and thus, an inoculation of this biological control agent can protect the plant by limiting the development of GTD, and the strain can behave as an endophyte.SIThe grant awarded to GC-H (FPU15/04681) comes from the Ministry of Education, Culture, and Sport (Spain). We thank Pago de Carraovejas winery for the project “GLOBALVITI IDI- 20120746” “Solució n global para mejorar la producció n vitivinı́ cola frente al cambio climá tico basada en robó tica, tecnologı́ a IT y en estrategias biotecnoló gicas y de manejo del viñedo” (Global solution for enhancing viticulture production against: climatic change based on: robotics, IT technology, biotechnological strategies, and vineyard management) that was granted by the Centro para el Desarrollo Tecnoló gico Industrial –CDTI-. SC and MG received funding via DaFNE Project Nr. 101384 from the Austrian Federal Ministry for Sustainability and Tourism (BMNT)

Visualization of grapevine root colonization by the Saharan soil isolate Saccharothrix algeriensis NRRL B-24137 using DOPE-FISH microscopy

Background and aim There is currently a gap of knowledge regarding whether some beneficial bacteria isolated from desert soils can colonize epi- and endophytically plants of temperate regions. In this study, the early steps of the colonization process of one of these bacteria, Saccharothrix algeriensis NRRL B-24137, was studied on grapevine roots to determine if this beneficial strain can colonize a non-natural host plant. An improved method of fluorescence in situ hybridization (FISH), the double labeling of oligonucleotide probes (DOPE)-FISH technique was used to visualize the colonization behavior of such bacteria as well as to determine if the method could be used to track microbes on and inside plants. Methods A probe specific to Saccharothrix spp. was firstly designed. Visualization of the colonization behavior of S. algeriensis NRRL B-24137 on and inside roots of grapevine plants was then carried out with DOPE-FISH microscopy. Results The results showed that 10 days after inoculation, the strain could colonize the root hair zone, root elongation zone, as well as root emergence sites by establishing different forms of bacterial structures as revealed by the DOPE-FISH technique. Further observations showed that the strain could be also endophytic inside the endorhiza of grapevine plants. Conclusions Taking into account the natural niches of this beneficial strain, this study exemplifies that, in spite of its isolation from desert soil, the strain can establish populations as well as subpopulations on and inside grapevine plants and that the DOPE-FISH tool can allow to detect it