Diversity of auxin-producing bacteria associated toPseudomonas savastanoi-induced olive knots

Plant Growth-Promoting Rhizobacteria as a Green Alternative for Sustainable Agriculture

Environmental stress is a major challenge for sustainable food production as it reduces yield by generating reactive oxygen species (ROS) which pose a threat to cell organelles and biomolecules such as proteins, DNA, enzymes, and others, leading to apoptosis. Plant growth-promoting rhizobacteria (PGPR) offers an eco-friendly and green alternative to synthetic agrochemicals and conventional agricultural practices in accomplishing sustainable agriculture by boosting growth and stress tolerance in plants. PGPR inhabit the rhizosphere of soil and exhibit positive interaction with plant roots. These organisms render multifaceted benefits to plants by several mechanisms such as the release of phytohormones, nitrogen fixation, solubilization of mineral phosphates, siderophore production for iron sequestration, protection against various pathogens, and stress. PGPR has the potential to curb the adverse effects of various stresses such as salinity, drought, heavy metals, floods, and other stresses on plants by inducing the production of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. Genetically engineered PGPR strains play significant roles to alleviate the abiotic stress to improve crop productivity. Thus, the present review will focus on the impact of PGPR on stress resistance, plant growth promotion, and induction of antioxidant systems in plants.

A review on the consumption of vended fruits: microbial assessment, risk, and its control

Fruits are very beneficial to health and their consumption has been recommended in daily diets. However, when fresh fruits are harvested and processed into sliced ready-to-eat or vended forms for sale, hygienic procedures are neglected. Thus, they are easily infested by pathogens. In addition, uneducated vendors who sell these fruits often display or hawk them on the streets in contaminated containers, utensils, or dirty environments. This poses a great threat to the health of consumers. In the light of this realization, some microbial assessments have been carried out to ascertain the safety of these vended fruits, thus making it a necessity to exploit the outcomes of some of these microbial assessments on vended fruits in order to sensitize the consumers on the effect of their consumption and mitigate risk by improving their quality. This will assist to lower certain health concerns, an outbreak of diseases, and death.

Role of Phytohormones in Piriformospora indica-Induced Growth Promotion and Stress Tolerance in Plants: More Questions Than Answers

Phytohormones play vital roles in the growth and development of plants as well as in interactions of plants with microbes such as endophytic fungi. The endophytic root-colonizing fungus Piriformospora indica promotes plant growth and performance, increases resistance of colonized plants to pathogens, insects and abiotic stress. Here, we discuss the roles of the phytohormones (auxins, cytokinin, gibberellins, abscisic acid, ethylene, salicylic acid, jasmonates, and brassinosteroids) in the interaction of P. indica with higher plant species, and compare available data with those from other (beneficial) microorganisms interacting with roots. Crosstalks between different hormones in balancing the plant responses to microbial signals is an emerging topic in current research. Furthermore, phytohormones play crucial roles in systemic signal propagation as well as interplant communication. P. indica interferes with plant hormone synthesis and signaling to stimulate growth, flowering time, differentiation and local and systemic immune responses. Plants adjust their hormone levels in the roots in response to the microbes to control colonization and fungal propagation. The available information on the roles of phytohormones in beneficial root-microbe interactions opens new questions of how P. indica manipulates the plant hormone metabolism to promote the benefits for both partners in the symbiosis.

The date palm tree rhizosphere is a niche for plant growth promoting bacteria in the oasis ecosystem

In arid ecosystems environmental factors such as geoclimatic conditions and agricultural practices are of major importance in shaping the diversity and functionality of plant-associated bacterial communities. Assessing the influence of such factors is a key to understand (i) the driving forces determining the shape of root-associated bacterial communities and (ii) the plant growth promoting (PGP) services they provide. Desert oasis environment was chosen as model ecosystem where agriculture is possible by the microclimate determined by the date palm cultivation. The bacterial communities in the soil fractions associated with the root system of date palms cultivated in seven oases in Tunisia were assessed by culture-independent and dependent approaches. According to 16S rRNA gene PCR-DGGE fingerprinting, the shapes of the date palm rhizosphere bacterial communities correlate with geoclimatic features along a north-south aridity transect. Despite the fact that the date palm root bacterial community structure was strongly influenced by macroecological factors, the potential rhizosphere services reflected in the PGP traits of isolates screened in vitro were conserved among the different oases. Such services were exerted by the 83% of the screened isolates. The comparable numbers and types of PGP traits indicate their importance in maintaining the plant functional homeostasis despite the different environmental selection pressures.

The olive knot disease as a model to study the role of interspecies bacterial communities in plant disease

There is an increasing interest in studying interspecies bacterial interactions in diseases of animals and plants as it is believed that the great majority of bacteria found in nature live in complex communities. Plant pathologists have thus far mainly focused on studies involving single species or on their interactions with antagonistic competitors. A bacterial disease used as model to study multispecies interactions is the olive knot disease, caused by Pseudomonas savastanoi pv. savastanoi (Psv). Knots caused by Psv in branches and other aerial parts of the olive trees are an ideal niche not only for the pathogen but also for many other plant-associated bacterial species, mainly belonging to the genera Pantoea, Pectobacterium, Erwinia, and Curtobacterium. The non-pathogenic bacterial species Erwinia toletana, Pantoea agglomerans, and Erwinia oleae, which are frequently isolated inside the olive knots, cooperate with Psv in modulating the disease severity. Co-inoculations of these species with Psv result in bigger knots and better bacterial colonization when compared to single inoculations. Moreover, harmless bacteria co-localize with the pathogen inside the knots, indicating the formation of stable bacterial consortia that may facilitate the exchange of quorum sensing signals and metabolites. Here we discuss the possible role of bacterial communities in the establishment and development of olive knot disease, which we believe could be taking place in many other bacterial plant diseases.

Bacterial multispecies studies and microbiome analysis of a plant disease

Although the great majority of bacteria found in nature live in multispecies communities, microbiological studies have focused historically on single species or competition and antagonism experiments between different species. Future directions need to focus much more on microbial communities in order to better understand what is happening in the wild. We are using olive knot disease as a model to study the role and interaction of multispecies bacterial communities in disease establishment/development. In the olive knot, non-pathogenic bacterial species (e.g. Erwinia toletana) co-exist with the pathogen (Pseudomonas savastanoi pv. savastanoi); we have demonstrated cooperation among these two species via quorum sensing (QS) signal sharing. The outcome of this interaction is a more aggressive disease when co-inoculations are made compared with single inoculations. In planta experiments show that these two species co-localize in the olive knot, and this close proximity most probably facilitates exchange of QS signals and metabolites. In silico recreation of their metabolic pathways showed that they could have complementing pathways also implicating sharing of metabolites. Our microbiome studies of nine olive knot samples have shown that the olive knot community possesses great bacterial diversity; however. the presence of five genera (i.e. Pseudomonas, Pantoea, Curtobacterium, Pectobacterium and Erwinia) can be found in almost all samples.

Diversity and antimicrobial properties of lactic acid bacteria isolated from rhizosphere of olive trees and desert truffles of Tunisia

A total of 119 lactic acid bacteria (LAB) were isolated, by culture-dependant method, from rhizosphere samples of olive trees and desert truffles and evaluated for different biotechnological properties. Using the variability of the intergenic spacer 16S-23S and 16S rRNA gene sequences, the isolates were identified as the genera Lactococcus, Pediococcus, Lactobacillus, Weissella, and Enterococcus. All the strains showed proteolytic activity with variable rates 42% were EPS producers, while only 10% showed the ability to grow in 9% NaCl. In addition, a low rate of antibiotic resistance was detected among rhizospheric enterococci. Furthermore, a strong antibacterial activity against plant and/or pathogenic bacteria of Stenotrophomonas maltophilia, Pantoea agglomerans, Pseudomonas savastanoi, the food-borne Staphylococcus aureus, and Listeria monocytogenes was recorded. Antifungal activity evaluation showed that Botrytis cinerea was the most inhibited fungus followed by Penicillium expansum, Verticillium dahliae, and Aspergillus niger. Most of the active strains belonged to the genera Enterococcus and Weissella. This study led to suggest that environmental-derived LAB strains could be selected for technological application to control pathogenic bacteria and to protect food safety from postharvest deleterious microbiota.

Pseudomonas savastanoi pv. savastanoi: some like it knot

Pseudomonas savastanoi pv. savastanoi is the causal agent of olive (Olea europaea) knot disease and an unorthodox member of the P. syringae complex, causing aerial tumours instead of the foliar necroses and cankers characteristic of most members of this complex. Olive knot is present wherever olive is grown; although losses are difficult to assess, it is assumed that olive knot is one of the most important diseases of the olive crop. The last century witnessed a large number of scientific articles describing the biology, epidemiology and control of this pathogen. However, most P. savastanoi pv. savastanoi strains are highly recalcitrant to genetic manipulation, which has effectively prevented the pathogen from benefitting from the scientific progress in molecular biology that has elevated the foliar pathogens of the P. syringae complex to supermodels. A number of studies in recent years have made significant advances in the biology, ecology and genetics of P. savastanoi pv. savastanoi, paving the way for the molecular dissection of its interaction with other nonpathogenic bacteria and their woody hosts. The selection of a genetically pliable model strain was soon followed by the development of rapid methods for virulence assessment with micropropagated olive plants and the analysis of cellular interactions with the plant host. The generation of a draft genome of strain NCPPB 3335 and the closed sequence of its three native plasmids has allowed for functional and comparative genomic analyses for the identification of its pathogenicity gene complement. This includes 34 putative type III effector genes and genomic regions, shared with other pathogens of woody hosts, which encode metabolic pathways associated with the degradation of lignin-derived compounds. Now, the time is right to explore the molecular basis of the P. savastanoi pv. savastanoi-olive interaction and to obtain insights into why some pathovars like it necrotic and why some like it knot.

SYNONYMS

Pseudomonas syringae pv. savastanoi.

TAXONOMY

Kingdom Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Family Pseudomonadaceae; Genus Pseudomonas; included in genomospecies 2 together with at least P. amygdali, P. ficuserectae, P. meliae and 16 other pathovars from the P. syringae complex (aesculi, ciccaronei, dendropanacis, eriobotryae, glycinea, hibisci, mellea, mori, myricae, phaseolicola, photiniae, sesami, tabaci, ulmi and certain strains of lachrymans and morsprunorum); when a formal proposal is made for the unification of these bacteria, the species name P. amygdali would take priority over P. savastanoi.

MICROBIOLOGICAL PROPERTIES

Gram-negative rods, 0.4-0.8 × 1.0-3.0 μm, aerobic. Motile by one to four polar flagella, rather slow growing, optimal temperatures for growth of 25-30 °C; oxidase negative, arginine dihydrolase negative; elicits the hypersensitive response on tobacco; most isolates are fluorescent and levan negative, although some isolates are nonfluorescent and levan positive.

HOST RANGE

P. savastanoi pv. savastanoi causes tumours in cultivated and wild olive and ash (Fraxinus excelsior). Although strains from olive have been reported to infect oleander (Nerium oleander), this is generally not the case; however, strains of P. savastanoi pv. nerii can infect olive. Pathovars fraxini and nerii are differentiated from pathovar savastanoi mostly in their host range, and were not formally recognized until 1996. Literature before about 1996 generally names strains of the three pathovars as P. syringae ssp. savastanoi or P. savastanoi ssp. savastanoi, contributing to confusion on the host range and biological properties.

DISEASE SYMPTOMS

Symptoms of infected trees include hyperplastic growths (tumorous galls or knots) on the stems and branches of the host plant and, occasionally, on leaves and fruits.

EPIDEMIOLOGY

The pathogen can survive and multiply on aerial plant surfaces, as well as in knots, from where it can be dispersed by rain, wind, insects and human activities, entering the plant through wounds. Populations are very unevenly distributed in the plant, and suffer drastic fluctuations throughout the year, with maximum numbers of bacteria occurring during rainy and warm months. Populations of P. savastanoi pv. savastanoi are normally associated with nonpathogenic bacteria, both epiphytically and endophytically, and have been demonstrated to form mutualistic consortia with Erwinia toletana and Pantoea agglomerans, which could result in increased bacterial populations and disease symptoms.

DISEASE CONTROL

Based on preventive measures, mostly sanitary and cultural practices. Integrated control programmes benefit from regular applications of copper formulations, which should be maintained for at least a few years for maximum benefit. Olive cultivars vary in their susceptibility to olive knot, but there are no known cultivars with full resistance to the pathogen.

USEFUL WEBSITES

http://www.pseudomonas-syringae.org/; http://genome.ppws.vt.edu/cgi-bin/MLST/home.pl; ASAP access to the P. savastanoi pv. savastanoi NCPPB 3335 genome sequence https://asap.ahabs.wisc.edu/asap/logon.php.

Erwinia oleae sp. nov., isolated from olive knots caused by Pseudomonas savastanoi pv. savastanoi

Three endophytic bacterial isolates were obtained in Italy from olive knots caused by Pseudomonas savastanoi pv. savastanoi. Phenotypic tests in combination with 16S rRNA gene sequence analysis indicated a phylogenetic position for these isolates in the genera Erwinia or Pantoea, and revealed two other strains with highly similar 16S rRNA gene sequences (>99 %), CECT 5262 and CECT 5264, obtained in Spain from olive knots. Rep-PCR DNA fingerprinting of the five strains from olive knots with BOX, ERIC and REP primers revealed three groups of profiles that were highly similar to each other. Multilocus sequence analysis (MLSA) based on concatenated partial atpD, gyrB, infB and rpoB gene sequences indicated that the strains constituted a single novel species in the genus Erwinia. The strains showed general phenotypic characteristics typical of the genus Erwinia and whole genome DNA-DNA hybridization data confirmed that they represented a single novel species of the genus Erwinia. The strains showed DNA G+C contents ranging from 54.7 to 54.9 mol%. They could be discriminated from phylogenetically related species of the genus Erwinia by their ability to utilize potassium gluconate, l-rhamnose and d-arabitol, but not glycerol, inositol or d-sorbitol. The name Erwinia oleae sp. nov. (type strain DAPP-PG 531(T)= LMG 25322(T) = DSM 23398(T)) is proposed for this novel taxon.