T6SS: A Key to Pseudomonas's Success in Biocontrol?

Microbial inheritance through seed: a clouded area needs to be enlightened

Seed endophytes are actively used by the mother plant as both reservoir and vector of beneficial microbes. During seed dormancy endophytes experience significant physiochemical changes and only competent endophytes could colonise successfully in seeds and some of them act as obligate endophyte that are transmitted vertically across generations. The adaptive nature of endophytes allows them to switch lifestyles depending on environment and host conditions. In this review, instead of providing broad discussion on applicability of endophytes in plant growth improvement, the fundamental nature of endophytes, their survival strategies under stress conditions, transmittance, etc. have been broadly highlighted by collaborating recent discoveries and theories. We have also tried to differentiate endophyte with their pathogenic counterpart and their survival mechanism during seed dormancy stages. Critical analyses of physio-biochemical changes in seeds during maturation and parallel modifications of life styles of seed endophytes along with pathogens will enlighten the shaded part of seed-microbiome interactions. The mutualistic interrelations as well as their shipment towards pathogenic behaviour under stress are being discussed acutely. Finally, importances of conservation of seed microbiome to maintain seed quality and vigour have been pointed out. Throughout the manuscript, the knowledge gap on seed-microbiota have been mentioned, thus, in future, studies on these areas could help us to understand properly the actual role of endophytes for the betterment of maintaining seed quality and vigour.

Dispersal shapes compositional and functional diversity in aquatic microbial communities

Segregation and mixing shape the structure and functioning of aquatic microbial communities, but their respective roles are challenging to disentangle in field studies. We explored the hypothesis that functional differences and beta diversity among stochastically assembled communities would increase in the absence of dispersal. Contrariwise, we expected biotic selection during homogenizing dispersal to reduce beta and gamma diversity as well as functional variability. This was experimentally addressed by examining the compositional and functional changes of 20 freshwater bacterial assemblages maintained at identical conditions over seven growth cycles for 34 days and subjected to two consecutive dispersal regimes. Initial dispersal limitation generated high beta diversity and led to the repeated emergence of community types that were dominated by particular taxa. Compositional stability and evenness of the community types varied over successive growth cycles, reflecting differences in functional properties. Carbon use efficiency increased during cultivation, with some communities of unique composition outperforming the replicate community types. Homogenizing dispersal led to high compositional similarity and reduced gamma diversity. While a neutral and a competition-based (Elo-rating) model together largely explained community assembly, a pseudomonad disproportionally dominated across communities, possibly due to interaction-related genomic traits. In conclusion, microbial assemblages stochastically generated by dispersal limitation can be gradually "refined" into distinct community types by subsequent deterministic processes. Segregation of communities represented an insurance mechanism for highly productive but competitively weak microbial taxa that were excluded during community coalescence.

IMPORTANCE

We experimentally assessed the compositional and functional responses of freshwater bacterial assemblages exposed to two consecutive dispersal-related events (dispersal limitation and homogenizing dispersal) under identical growth conditions. While segregation led to a decreased local diversity, high beta diversity sustained regional diversity and functional variability. In contrast, homogenizing dispersal reduced the species pool and functional variability of the metacommunity. Our findings highlight the role of dispersal in regulating both diversity and functional variability of aquatic microbial metacommunities, thereby providing crucial insight to predict changes in ecosystem functioning.

Refined egoist: The toxin-antitoxin immune system of T6SS

The Type VI secretory system (T6SS) is a key regulatory network in the bacterial system, which plays an important role in host-pathogen interactions and maintains cell homeostasis by regulating the release of effector proteins in specific competition. T6SS causes cell lysis or competitive inhibition by delivering effector molecules, such as toxic proteins and nucleic acids, directly from donor bacterial cells to eukaryotic or prokaryotic targets. Additionally, it orchestrates synthesis of immune effectors that counteract toxins thus preventing self-intoxication or antagonistic actions by competing microbes. Even so, the mechanism of toxin-antitoxin regulation in bacteria remains unclear. In response, this review discusses the bacterial T6SS's structure and function and the mechanism behind toxin-antitoxin secretion and the T6SS's expression in order to guide the further exploration of the pathogenic mechanism of the T6SS and the development of novel preparations for reducing and replacing toxins and antitoxins.

A review on mechanisms and prospects of endophytic bacteria in biocontrol of plant pathogenic fungi and their plant growth-promoting activities

Endophytic bacteria, living inside plants, are competent plant colonizers, capable of enhancing immune responses in plants and establishing a symbiotic relationship with them. Endophytic bacteria are able to control phytopathogenic fungi while exhibiting plant growth-promoting activity. Here, we discussed the mechanisms of phytopathogenic fungi control and plant growth-promoting actions discovered in some major groups of beneficial endophytic bacteria such as Bacillus, Paenibacillus, and Pseudomonas. Most of the studied strains in these genera were isolated from the rhizosphere and soils, and a more extensive study of these endophytic bacteria is needed. It is essential to understand the underlying biocontrol and plant growth-promoting mechanisms and to develop an effective screening approach for selecting potential endophytic bacteria for various applications. We have suggested a screening strategy to identify potentially useful endophytic bacteria based on mechanistic phenomena. The discovery of endophytic bacteria with useful biocontrol and plant growth-promoting characteristics is essential for developing sustainable agriculture.

Magnesium oxide nanoparticles reduce clubroot by regulating plant defense response and rhizosphere microbial community of tumorous stem mustard (Brassica juncea var. tumida)

Clubroot, caused by Plasmodiophora brassicae, is a major disease that significantly impairs the yield of cruciferous crops and causes significant economic losses across the globe. The prevention of clubroot, especially in tumorous stem mustard (without resistant varieties), are is limited and primarily relies on fungicides. Engineered nanoparticles have opened up new avenues for the management of plant diseases, but there is no report on their application in the prevention of clubroot. The results showed that the control efficacy of 500 mg/L MgO NPs against clubroot was 54.92%. However, when the concentration was increased to 1,500 and 2,500 mg/L, there was no significant change in the control effect. Compared with CK, the average fresh and dry weight of the aerial part of plants treated with MgO NPs increased by 392.83 and 240.81%, respectively. Compared with the F1000 treatment, increases were observed in the content of soil available phosphorus (+16.72%), potassium (+9.82%), exchangeable magnesium (+24.20%), and water-soluble magnesium (+20.64%) in the 1,500 mg/L MgO NPs treatment. The enzyme-linked immune sorbent assay (ELISA) results showed that the application of MgO NPs significantly increased soil peroxidase (POD, +52.69%), alkaline protease (AP, +41.21%), alkaline phosphatase (ALP, +79.26%), urease (+52.69%), and sucrase (+56.88%) activities; And also increased plant L-phenylalanine ammonla-lyase (PAL, +70.49%), polyphenol oxidase (PPO, +36.77%), POD (+38.30%), guaiacol peroxidase (POX, +55.46%) activities and salicylic acid (SA, +59.86%) content. However, soil and plant catalase (CAT, -27.22 and - 19.89%, respectively), and plant super oxidase dismutase (SOD, -36.33%) activities were significantly decreased after the application of MgO NPs. The metagenomic sequencing analysis showed that the MgO NPs treatments significantly improved the α-diversity of the rhizosphere soil microbial community. The relative abundance of beneficial bacteria genera in the rhizosphere soil, including Pseudomonas, Sphingopyxis, Acidovorax, Variovorax, and Bosea, was significantly increased. Soil metabolic functions, such as oxidative phosphorylation (ko00190), carbon fixation pathways in prokaryotes (ko00720), indole alkaloid biosynthesis (ko00901), and biosynthesis of various antibiotics (ko00998) were significantly enriched. These results suggested that MgO NPs might control clubroot by promoting the transformation and utilization of soil nutrients, stimulating plant defense responses, and enriching soil beneficial bacteria.