Plant Growth Promotion and Induction of Systemic Tolerance to Drought and Salt Stress of Plants by Quorum Sensing Auto-Inducers of the N-acyl-homoserine Lactone Type: Recent Developments

Rhizobial, passenger nodule endophytes and phyllosphere bacteria in combination with acyl homoserine lactones enhances the growth and yield of groundnut

Quorum sensing (QS) mechanisms play an essential role in mediating several signals and plant-bacteria interactions, promoting plant growth. This study demonstrated production of multiple Homoserine lactone (HSL) molecules like C6 HSL, C7 HSL, C8 HSL, 3-Hydroxy-C8-HSL and 3-oxo-C14 HSL in rhizobial and passenger endophytes and phyllospheric bacteria which regulated production of plant growth promoting traits viz., indole acetic acid and exo-polysaccharide production, biofilm formation, and motility. Quorum quenching (QQ) molecules like salicylic acid, gallic acid, and disalicylic acid impaired these traits, but exogenous addition of QS molecules (C7HSL and 3-oxo-C14 HSL) restored these inhibitory effects of QQ compounds. The pot culture experiment revealed that the treatment involving Methylobacterium populi TMV7-4 or Enterobacter cloacae S23 with salicylic acid, C7HSL and 3-oxo-C14 HSL significantly enhanced plant growth including root length, nodulation, pod formation, soil available nutrients and plant nutrients uptake. In future field validation is required for the use of QS molecules in improving groundnut production.

Transformative strategies for saline soil restoration: Harnessing halotolerant microorganisms and advanced technologies

Soil salinity is a critical global challenge that severely impairs crop productivity and soil health by disrupting water uptake, nutrient acquisition, and ionic balance in plants, thereby posing a significant threat to food security. This review underscores innovative strategies to mitigate salinity stress, focusing on the pivotal role of halotolerant microorganisms and their synergistic interactions with plants. Halotolerant microorganisms enhance plant resilience through diverse mechanisms under salinity, including exopolysaccharide production, sodium sequestration, and phytohormone regulation. It improves ionic balance, nutrient uptake, and root development, facilitated by osmoregulatory and genetic adaptations. In this discussion, we explored emerging technologies, including genome editing (e.g., CRISPR-Cas9), synthetic biology, and advanced omics-based tools such as metagenomics and metatranscriptomics. These cutting-edge approaches offer profound insights into microbial diversity and their functional adaptations to saline environments. By leveraging these technologies, it is possible to design targeted bioremediation strategies through the customization of microbial functionalities to address specific environmental challenges effectively. Advanced methodologies, such as microbial volatile organic compounds (mVOCs), nanotechnology, and stress-tolerant microbial consortia, significantly enhance plant stress tolerance and facilitate soil restoration. Moreover, integrating digital technologies, including machine learning and artificial intelligence (AI), optimizes bioremediation processes by providing precise, scalable, and adaptable solutions tailored to diverse agricultural ecosystems. The synergistic application of halotolerant microbe-mediated approaches with advanced biotechnological and digital innovations presents a transformative strategy for saline soil restoration. Future research should focus on harmonizing these technologies and methodologies to maximize plant-microbe interactions and establish resilient, sustainable agricultural systems.

Rhizosphere bacterial communities of Namib Desert plant species: Evidence of specialised plant-microbe associations

Rhizosphere microbial communities are intimately associated with plant root surfaces. The rhizosphere microbiome is recruited from the surrounding soil and is known to impact positively on the plant host via enhanced resistance to pathogens, increased nutrient availability, growth stimulation and increased resistance to desiccation. Desert ecosystems harbour a diversity of perennial and annual plant species, generally exhibiting considerable physiological adaptation to the low-water environment. In this study, we explored the rhizosphere bacterial microbiomes associated with selected desert plant species. The rhizosphere bacterial communities of 11 plant species from the central Namib Desert were assessed using 16S rRNA gene-dependent phylogenetic analyses. The rhizosphere microbial community of each host plant species was compared with control soils collected from their immediate vicinity, and with those of all other host plants. Rhizosphere and control soil bacterial communities differed significantly and were influenced by both location and plant species. Rhizosphere-associated genera included 67 known plant growth-promoting taxa, including Rhizobium, Bacillus, Microvirga, Kocuria and Paenibacillus. Other than Kocuria, these genera constituted the 'core' rhizosphere bacterial microbiome, defined as being present in > 90 % of the rhizosphere communities. Nine of the 11 desert plant species harboured varying numbers and proportions of species-specific microbial taxa. Predictive analyses of functional pathways linked to rhizosphere microbial taxa showed that these were significantly enriched in the biosynthesis or degradation of a variety of substances such as sugars, secondary metabolites, phenolic compounds and antimicrobials. Overall, our data suggest that plant species in the Namib Desert recruit unique taxa to their rhizosphere bacterial microbiomes that may contribute to their resilience in this extreme environment.

Diversity and activity of AHL-lactonases in Bacillus spp. from various environments

Disrupting quorum sensing (QS) pathways in animal and plant pathogenic bacteria is an effective strategy to mitigate infections without promoting antibiotic and pesticide resistance. This approach inhibits the production of virulence factors, biofilm formation, and toxin production, reducing bacterial pathogenicity. In plant health protection, Bacillus spp. are extensively researched and utilized as biocontrol agents; however, the potential of their AHL-lactonase-producing ability, which plays a key role as a QS inhibitor of Gram-negative pathogens, remains largely unexplored. This study examined the activity and diversity of QQ enzymes from Bacillus spp. isolates obtained from various natural sources, confirming their presence in previously unreported environments associated with agricultural fields (straw and manure). Our findings show that AiiA lactonase is the most dominant and highly conserved AHL-lactonase among Bacillus isolates from bulk soil, manure, and straw. Despite its sequence conservation, we observed significant variation in AiiA lactonase activities toward the N-hexanoyl-DL-homoserine lactone (C6-HSL) substrate. Furthermore, in silico analysis suggested that the Bacillus sp. YtnP lactonase may have a lower affinity for C6-HSL compared to AiiA lactonase. Finally, this research presents a selection of Bacillus isolates with high AiiA lactonase activity for potential testing against plant pathogens.

The role of microbial partners in heavy metal metabolism in plants: a review

Heavy metal pollution threatens plant growth and development as well as ecological stability. Here, we synthesize current research on the interplay between plants and their microbial symbionts under heavy metal stress, highlighting the mechanisms employed by microbes to enhance plant tolerance and resilience. Several key strategies such as bioavailability alteration, chelation, detoxification, induced systemic tolerance, horizontal gene transfer, and methylation and demethylation, are examined, alongside the genetic and molecular basis governing these plant-microbe interactions. However, the complexity of plant-microbe interactions, coupled with our limited understanding of the associated mechanisms, presents challenges in their practical application. Thus, this review underscores the necessity of a more detailed understanding of how plants and microbes interact and the importance of using a combined approach from different scientific fields to maximize the benefits of these microbial processes. By advancing our knowledge of plant-microbe synergies in the metabolism of heavy metals, we can develop more effective bioremediation strategies to combat the contamination of soil by heavy metals.

Stimulation of PGP Bacteria on the Development of Seeds, Plants and Cuttings of the Obligate Halophyte Arthrocaulon (Arthrocnemum) macrostachyum (Moric.) Piirainen & G. Kadereit

The Earth is undergoing alterations at a high speed, which causes problems such as environmental pollution and difficulty in food production. This is where halophytes are interesting, due to their high potential in different fields, such as remediation of the environment and agriculture. For this reason, it is necessary to deepen the knowledge of the development of halophytes and how plant growth-promoting bacteria (PGP) can play a fundamental role in this process. Therefore, in this work were tested the effects of five PGP bacteria on its rhizosphere and other endophytic bacteria at different concentrations of NaCl on seed germination, plant growth (0 and 171 mM) and cutting growth (0 mM) of Arthrocaulon macrostachyum. The growth promotion in this strict halophyte is highlighted due to the presence of PGP bacteria and the fact that no salt is needed. Thus, without salt, the bacterial strains Kocuria polaris Hv16, Pseudarthrobacter psychrotolerans C58, and Rahnella aceris RTE9 enhanced the biomass production by more than 60% in both stems and roots. Furthermore, germination was encouraged by more than 30% in the presence of both R. aceris RTE9 and K. polaris Hv16 at 171 mM NaCl; the latter also had a biocontrol effect on the fungi that grew on the seeds. Additionally, for the first time in cuttings of this perennial species, the root biomass was improved thanks to the consortium of K. polaris Hv16 and P. psychrotolerans C58. Finally, this study demonstrates the potential of PGPs for optimising the development of halophytes, either for environmental or agronomic purposes.

Impaired microbial N-acyl homoserine lactone signalling increases plant resistance to aphids across variable abiotic and biotic environments

Beneficial bacteria interact with plants using signalling molecules, such as N-acyl homoserine-lactones (AHLs). Although there is evidence that these molecules affect plant responses to pathogens, few studies have examined their effect on plant-insect and microbiome interactions, especially under variable soil conditions. We investigated the effect of the AHL-producing rhizobacterium Acidovorax radicis and its AHL-negative mutant (does not produce AHLs) on modulating barley (Hordeum vulgare) plant interactions with cereal aphids (Sitobion avenae) and earthworms (Dendrobaena veneta) across variable nutrient soils. Acidovorax radicis inoculation increased plant growth and suppressed aphids, with stronger effects by the AHL-negative mutant. However, effects varied between barley cultivars and the presence of earthworms altered interaction outcomes. Bacteria-induced plant defences differed between cultivars, and aphid exposure, with pathogenesis-related and WRKY pathways partly explaining the ecological effects in the more resistant cultivars. Additionally, we observed few but specific indirect effects via the wider root microbiome where the AHL-mutant strain influenced rare OTU abundances. We conclude that bacterial AHL-signalling disruption affects plant-microbial interactions by inducing different plant pathways, leading to increased insect resistance, also mediated by the surrounding biotic and abiotic environment. Understanding the mechanisms by which beneficial bacteria can reduce insect pests is a key research area for developing effective insect pest management strategies in sustainable agriculture.

Diversity and functions of quorum sensing bacteria in the root environment of the Suaeda glauca and Phragmites australis coastal wetlands

The quorum sensing (QS) system plays a significant role in the bacteria-bacteria or plant-bacteria relationships through signal molecules. However, little is known about the distribution and functional diversity of QS bacteria in the root environment of Suaeda glauca and Phragmites australis in coastal wetlands. We explored the bacterial community by amplicon sequencing and isolated 1050 strains from the rhizosphere soil and root tissues of S. glauca and P. australis in northern China to investigate the bacterial community and AHL producers. AHL activity was found in 76 isolates, and 22 distinct strains were confirmed by 16S rRNA gene sequencing. A substantial number of AHL producers clustered in rhizobiales and sphingomonadale, which derived from the root tissues. AHL producers in the rhizosphere soil mostly belonged to rhodobacterales. The different taxa of AHL producers in the rhizosphere soil and root tissues resulted in a variation of AHL profiles that C6-HSL dominated the AHL profiles in root bacteria compared to the C8-HSL in rhizobacteria, implying different ecological roles for AHL producers in the rhizosphere soil and root tissues. Many AHL producers may form biofilms, and some can degrade DMSP and oil, demonstrating that QS bacteria in the root environment have a wide ecological roles. In our study, for one of the first times here, we explore the distribution and functional variety of AHL producers in the root environment of S. glauca-P. australis. This study expands current knowledge of the relationship between QS bacteria and coastal plants (S. glauca and P. australis), and vital roles of QS bacterial in maintaining the health of coastal wetlands.

Novel Pseudomonas sp. SCA7 Promotes Plant Growth in Two Plant Families and Induces Systemic Resistance in Arabidopsis thaliana

Pseudomonas sp. SCA7, characterized in this study, was isolated from roots of the bread wheat Triticum aestivum. Sequencing and annotation of the complete SCA7 genome revealed that it represents a potential new Pseudomonas sp. with a remarkable repertoire of plant beneficial functions. In vitro and in planta experiments with the reference dicot plant A. thaliana and the original monocot host T. aestivum were conducted to identify the functional properties of SCA7. The isolate was able to colonize roots, modify root architecture, and promote growth in A. thaliana. Moreover, the isolate increased plant fresh weight in T. aestivum under unchallenged conditions. Gene expression analysis of SCA7-inoculated A. thaliana indicated a role of SCA7 in nutrient uptake and priming of plants. Moreover, confrontational assays of SCA7 with fungal and bacterial plant pathogens revealed growth restriction of the pathogens by SCA7 in direct as well as indirect contact. The latter indicated involvement of microbial volatile organic compounds (mVOCs) in this interaction. Gas chromatography-mass spectrometry (GC-MS) analyses revealed 1-undecene as the major mVOC, and octanal and 1,4-undecadiene as minor abundant compounds in the emission pattern of SCA7. Additionally, SCA7 enhanced resistance of A. thaliana against infection with the plant pathogen Pseudomonas syringae pv. tomato DC3000. In line with these results, SA- and JA/ET-related gene expression in A. thaliana during infection with Pst DC3000 was upregulated upon treatment with SCA7, indicating the ability of SCA7 to induce systemic resistance. The thorough characterization of the novel Pseudomonas sp. SCA7 showed a remarkable genomic and functional potential of plant beneficial traits, rendering it a promising candidate for application as a biocontrol or a biostimulation agent.

Research Progress and Hopeful Strategies of Application of Quorum Sensing in Food, Agriculture and Nanomedicine

Quorum sensing (QS) regulates the expression of several genes including motility, biofilm development, virulence expression, population density detection and plasmid conjugation. It is based on "autoinducers", small molecules that microorganisms produce and release in the extracellular milieu. The biochemistry of quorum sensing is widely discussed and numerous papers are available to scientists. The main purpose of this research is to understand how knowledge about this mechanism can be exploited for the benefit of humans and the environment. Here, we report the most promising studies on QS and their resulting applications in different fields of global interest: food, agriculture and nanomedicine.

Bacterial Modulation of the Plant Ethylene Signaling Pathway Improves Tolerance to Salt Stress in Lettuce (Lactuca sativa L.)

Salinity has extensive adverse effects on plant growth and the development of new agronomic strategies to improve crop salt tolerance is becoming necessary. Currently, the use of plant growth promoting rhizobacteria (PGPR) to mitigate abiotic stress in crops is of increasing interest. The most analyzed mechanism is based on ACC deaminase activity, an enzyme that decreases the ethylene synthesis, an important phytohormone in plant stress response. We aimed to identify other PGPR mediated mechanisms involved in the regulation of salt stress in plant. We used three PGPR strains (ESL001, ESL007, SH31), of which only ESL007 demonstrated ACC deaminase activity, to evaluate their effect on lettuce plants under salt stress (100 mM NaCl). We measured growth and biochemical parameters (e.g., proline content, lipid peroxidation and ROS degradation), as well as expression levels of genes involved in ethylene signaling (CTR1, EBF1) and transcription factors induced by ethylene (ERF5, ERF13). All bacterial strains enhanced growth on salt-stressed lettuce plants and modulated the proline levels. Strains ESL007 and SH31 triggered a higher catalase and ascorbate-peroxidase activity, compared to non-stressed plants. Differential expression of ethylene-related genes in inoculated plants subjected to salinity was observed. We gained consistent evidence for the existence of alternative mechanisms to ethylene modulation, which probably rely on bacterial IAA production and other chemical signals. These mechanisms modify the expression of genes associated with ethylene signaling and regulation, complementarily to the ACC deaminase model to diminish abiotic stress responses.

Reactive Oxygen Species, Antioxidant Responses and Implications from a Microbial Modulation Perspective

Simple Summary

Environmental conditions are subject to unprecedented changes due to recent progressive anthropogenic activities on our planet. Plants, as the frontline of food security, are susceptible to these changes, resulting in the generation of unavoidable byproducts of metabolism (ROS), which eventually affect their productivity. The response of plants to these unfavorable conditions is highly intricate and depends on several factors, among them are the species/genotype tolerance level, intensity, and duration of stress factors. Defensive mechanisms in plant systems, by nature, are concerned primarily with generating enzymatic and non-enzymatic antioxidants. In addition to this, plant-microbe interactions have been found to improve immune systems in plants suffering from drought and salinity stress.

Abstract

Plants are exposed to various environmental stresses in their lifespan that threaten their survival. Reactive oxygen species (ROS), the byproducts of aerobic metabolism, are essential signalling molecules in regulating multiple plant developmental processes as well as in reinforcing plant tolerance to biotic and abiotic stimuli. However, intensified environmental challenges such as salinity, drought, UV irradiation, and heavy metals usually interfere with natural ROS metabolism and homeostasis, thus aggravating ROS generation excessively and ultimately resulting in oxidative stress. Cellular damage is confined to the degradation of biomolecular structures, including carbohydrates, proteins, lipids, pigments, and DNA. The nature of the double-edged function of ROS as a secondary messenger or harmful oxidant has been attributed to the degree of existing balance between cellular ROS production and ROS removal machinery. The activities of enzyme-based antioxidants, catalase (CAT, EC 1.11.1.6), monodehydroascorbate reductase (MDHAR, E.C.1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), and guaiacol peroxidase (GPX, EC 1.11.1.7); and non-enzyme based antioxidant molecules, ascorbate (AA), glutathione (GSH), carotenoids, α-tocopherol, prolines, flavonoids, and phenolics, are indeed parts of the defensive strategies developed by plants to scavenge excess ROS and to maintain cellular redox homeostasis during oxidative stress. This review briefly summarises current knowledge on enzymatic and non-enzymatic antioxidant machinery in plants. Moreover, additional information about the beneficial impact of the microbiome on countering abiotic/biotic stresses in association with roots and plant tissues has also been provided.

Molecular mechanisms of N-acyl homoserine lactone signals perception by plants

N-acyl homoserine lactones (AHLs) belong to the class of bacterial quorum sensing signal molecules involved in distance signal transduction between Gram-negative bacteria colonizers of the rhizosphere, as well as bacteria and plants. AHLs synchronize the activity of genes from individual cells, allowing the bacterial population to act as a multicellular organism, and establish a symbiotic or antagonistic relationship with the host plant. Although the effect of AHLs on plants has been studied for more than ten years, the mechanisms of plant perception of AHL signals are not fully understood. The specificity of the reactions caused by AHL indicates the existence of appropriate mechanisms for their perception by plants. In the current review, we summarize available data on the molecular mechanisms of AHL-signal perception in plants, its effect on plant growth, development, and stress resistance. We describe the latest research demonstrating direct (on plants) and indirect (on rhizosphere microflora) effects of AHLs, as well as the prospects of using these compounds in biotechnology to increase plant resistance to biotic and abiotic stresses.