Mechanisms Of Plant Growth Promotion Research Articles

Multifarious plant growth-promoting traits of mangrove yeasts: growth enhancement in mangrove seedlings (Rhizophora mucronata) for conservation.

Plant Growth-Promoting Yeasts (PGPY) have garnered significant attention in recent years; however, research on PGPY from mangroves remains a largely unexplored frontier. This study, therefore, focused on exploring the multifaceted plant growth-promoting (PGP) capabilities of yeasts isolated from mangroves of Puthuvype and Kumbalam. The present work found that manglicolous yeasts exhibited diverse hydrolytic properties, with the predominance of lipolytic activity, in addition to other traits such as phosphate solubilization, and production of indole acetic acid, siderophore, ammonia, catalase, nitrate, and hydrogen cyanide. After screening for 15 PGP traits, three strains P 9, PV 23, and KV 35 were selected as the most potent ones. These strains also exhibited antagonistic activity against fungal phytopathogens and demonstrated resilience to abiotic stresses, making them not only promising biocontrol agents but also suited for field application. The potent strains P 9, PV 23, and KV 35 were molecularly identified as Candida tropicalis, Debaryomyces hansenii, and Aureobasidium melanogenum, respectively. The potential of these strains in enhancing the growth performance of mangrove seedlings of Rhizophora mucronata, was demonstrated using the pot-experiment. The results suggested that the consortium of three potent strains (P 9, PV 23, and KV 35) was more effective in increasing the number of shoot branches (89.2%), plant weight (87.5%), root length (83.3%), shoot height (57.9%) and total leaf area (35.1%) than the control seedlings. The findings of this study underscore the significant potential of manglicolous yeasts in contributing to mangrove conservation and restoration efforts, offering a comprehensive understanding of their diverse plant growth-promoting mechanisms and highlighting their valuable role in sustainable ecosystem management.

Speciation Features of Ferdinandcohnia quinoae sp. nov to Adapt to the Plant Host

The bacterial strain SECRCQ15T was isolated from seeds of Chenopodium quinoa in Spain. Phylogenetic, chemotaxonomic, and phenotypic analyses, as well as genome similarity indices, support the classification of the strain into a novel species of the genus Ferdinandcohnia, for which we propose the name Ferdinandcohnia quinoae sp. nov. To dig deep into the speciation features of the strain SECRCQ15T, we performed a comparative genomic analysis of the genome of this strain and those of the type strains of species from the genus Ferdinandcohnia. We found several genes related with plant growth-promoting mechanisms within the SECRCQ15T genome. We also found that singletons of F. quinoae SECRCQ15T are mainly related to the use of carbohydrates, which is a common trait of plant-associated bacteria. To further reveal speciation events in this strain, we revealed genes undergoing diversifying selection (e.g., genes encoding ribosomal proteins) and functions likely lost due to pseudogenization. Also, we found that this novel species contains 138 plant-associated gene-cluster functions that are unique within the genus Ferdinandcohnia. These features may explain both the ecological and taxonomical differentiation of this new taxon.

Peribacillus frigoritolerans T7-IITJ, a potential biofertilizer, induces plant growth-promoting genes of Arabidopsis thaliana.

This study aimed to isolate plant growth and drought tolerance-promoting bacteria from the nutrient-poor rhizosphere soil of Thar desert plants and unravel their molecular mechanisms of plant growth promotion. Among our rhizobacterial isolates, Enterobacter cloacae C1P-IITJ, Kalamiella piersonii J4-IITJ, and Peribacillus frigoritolerans T7-IITJ, significantly enhanced root and shoot growth (4-5-fold) in Arabidopsis thaliana under PEG-induced drought stress. Whole genome sequencing and biochemical analyses of the non-pathogenic bacterium T7-IITJ revealed its plant growth-promoting traits, viz., solubilization of phosphate (40-73µg/ml), iron (24±0.58mm halo on chrome azurol S media), and nitrate (1.58±0.01µg/ml nitrite), along with production of exopolysaccharides (125±20µg/ml) and auxin-like compounds (42.6±0.05µg/ml). Transcriptome analysis of A. thaliana inoculated with T7-IITJ and exposure to drought revealed the induction of 445 plant genes (log2fold-change>1, FDR<0.05) for photosynthesis, auxin and jasmonate signalling, nutrient uptake, redox homeostasis, and secondary metabolite biosynthesis pathways related to beneficial bacteria-plant interaction, but repression of 503 genes (log2fold-change < -1) including many stress-responsive genes. T7-IITJ enhanced proline 2.5-fold, chlorophyll 2.5-2.8-fold, iron 2-fold, phosphate 1.6-fold, and nitrogen 4-fold, and reduced reactive oxygen species 2-4.7-fold in plant tissues under drought. T7-IITJ also improved the germination and seedling growth of Tephrosia purpurea, Triticum aestivum, and Setaria italica under drought and inhibited the growth of two plant pathogenic fungi, Fusarium oxysporum, and Rhizoctonia solani. P. frigoritolerans T7-IITJ is a potent biofertilizer that regulates plant genes to promote growth and drought tolerance.

Whole-genome analysis revealed the growth-promoting and biological control mechanism of the endophytic bacterial strain Bacillus halotolerans Q2H2, with strong antagonistic activity in potato plants.

Endophytes are colonizers of healthy plants and they normally exhibit biocontrol activities, such as reducing the occurrence of plant diseases and promoting plant growth. The endophytic bacterium Bacillus halotolerans Q2H2 (Q2H2) was isolated from the roots of potato plants and was found to have an antagonistic effect on pathogenic fungi. Q2H2 was identified by morphological observations, physiological and biochemical identification, and 16S rRNA gene sequence analysis. Genes related to the anti-fungal and growth-promoting effects were analyzed using whole-genome sequencing and comparative genomic analysis. Finally, we analyzed the growth-promoting and biocontrol activities of Q2H2 in potato plants using pot experiments. Antagonism and non-volatile substance plate tests showed that Q2H2 had strong antagonism against Fusarium oxysporum, Fusarium commune, Fusarium graminearum, Fusarium brachygibbosum, Rhizoctonia solani and Stemphylium solani. The plate test showed that Q2H2 had the ability to produce proteases, cellulases, β-1,3-glucanase, dissolved organic phosphate, siderophores, indole-3-acetic acid (IAA), ammonia and fix nitrogen. The suitable growth ranges of Q2H2 under different forms of abiotic stress were pH 5-9, a temperature of 15-30°C, and a salt concentration of 1-5%. Though whole-genome sequencing, we obtained sequencing data of approximately 4.16 MB encompassed 4,102 coding sequences. We predicted 10 secondary metabolite gene clusters related to antagonism and growth promotion, including five known products surfactin, bacillaene, fengycin, bacilysin, bacillibactin, and subtilosin A. Average nucleotide identity and comparative genomic analyses revealed that Q2H2 was Bacillus halotolerans. Through gene function annotation, we analyzed genes related to antagonism and plant growth promotion in the Q2H2 genome. These included genes involved in phosphate metabolism (pstB, pstA, pstC, and pstS), nitrogen fixation (nifS, nifU, salA, and sufU), ammonia production (gudB, rocG, nasD, and nasE), siderophore production (fhuC, fhuG, fhuB, and fhuD), IAA production (trpABFCDE), biofilm formation (tasA, bslA, and bslB), and volatile compound production (alsD, ilvABCDEHKY, metH, and ispE), and genes encoding hydrolases (eglS, amyE, gmuD, ganB, sleL, and ydhD). The potato pot test showed that Q2H2 had an obvious growth-promoting effect on potato roots and better control of Fusarium wilt than carbendazim. These findings suggest that the strain-specific genes identified in bacterial endophytes may reveal important antagonistic and plant growth-promoting mechanisms.

Mechanism of Salt Tolerance and Plant Growth Promotion in Priestia megaterium ZS-3 Revealed by Cellular Metabolism and Whole-Genome Studies

Approximately one-third of agricultural land worldwide is affected by salinity, which limits the productivity and sustainability of crop ecosystems. Plant-growth-promoting rhizobacteria (PGPR) are a potential solution to this problem, as PGPR increases crop yield through improving soil fertility and stress resistance. Previous studies have shown that Priestia megaterium ZS-3(ZS-3) can effectively help plants tolerate salinity stress. However, how ZS-3 regulates its metabolic adaptations in saline environments remains unclear. In this study, we monitored the metabolic rearrangement of compatibilisers in ZS-3 and combined the findings with genomic data to reveal how ZS-3 survives in stressful environments, induces plant growth, and tolerates stress. The results showed that ZS-3 tolerated salinity levels up to 9%. In addition, glutamate and trehalose help ZS-3 adapt to osmotic stress under low NaCl stress, whereas proline, K+, and extracellular polysaccharides regulate the osmotic responses of ZS-3 exposed to high salt stress. Potting experiments showed that applying the ZS-3 strain in saline and neutral soils could effectively increase the activities of soil acid phosphatase, urease, and invertase in both soils, thus improving soil fertility and promoting plant growth. In addition, strain ZS-3-GFP colonised the rhizosphere and leaves of Cinnamomum camphora well, as confirmed by confocal microscopy and resistance plate count analysis. Genomic studies and in vitro experiments have shown that ZS-3 exhibits a variety of beneficial traits, including plant-promoting, antagonistic, and other related traits (such as resistance to saline and heavy metal stress/tolerance, amino acid synthesis and transport, volatile compound synthesis, micronutrient utilisation, and phytohormone biosynthesis/regulatory potential). The results support that ZS-3 can induce plant tolerance to abiotic stresses. These data provide important clues to further reveal the interactions between plants and microbiomes, as well as the mechanisms by which micro-organisms control plant health.

Deciphering the antifungal and plant growth-stimulating traits of the stress-tolerant Streptomyces achromogenes subsp. achromogenes strain UMAF16, a bacterium isolated from soils affected by underground fires

Crop plants are subjected to diverse abiotic and biotic stresses, which could affect their health and production. Therefore, searching for novel stress-resistant microbial agents with plant-beneficial activities is considered an urgent in agriculture. Here, we report the new stress tolerant bacterium Streptomyces achromogenes subsp. achromogenes UMAF16, isolated from a soil subjected to underground fires. Taxonomic and Genomic analysis derived from ANI (average nucleotide identity) species delimitation cut-off values, GGDC (genome-to-genome distance calculator), and phylogenetic analysis, corroborated the affiliation of UMAF16 to Streptomyces species. Genome mining of UMAF16 shows an excellent arsenal of stress tolerant genes (e.g. temperature, osmotic, and oxidative stress), as well as antifungal and plant growth-promoting mechanisms. Additionally, antiSMASH revealed multiple biosynthetic genes with antimicrobial (e.g. Achromosin, Livipeptin, Neocarazostatin A) and PGP activities (e.g. siderophores like Desferrioxamin B/Desferrioxamine E). To further confirmed previous predicted activities, it was biochemically evaluated the antifungal and plant growth-promoting mechanisms, including production of indole-3-acetic acid (IAA), proteases, siderophores, biofilm formation, and phosphate solubilization. Our results also show that strain UMAF16 grows well at temperature 50 °C, tolerates dehydration stress simulated by polyethylene glycol, and exert antifungal action against pathogens like Alternaria alternata, Mucor fragilis and Fusarium brachygibbosum. Strain UMAF16 also showed growth-stimulating activities when interacting with tomato plants, evaluated under three different plant-growth systems. Root and shoot dry weight, as well as chlorophyll content were increased in inoculated plants, compared to uninoculated controls. Finally, these results suggest a great potential of the strain UMAF16 as a bioinoculant candidate agent in stressful agriculture.

Plant endophytes: unveiling hidden applications toward agro-environment sustainability.

Endophytic microbes are plant-associated microorganisms that reside in the interior tissue of plants without causing damage to the host plant. Endophytic microbes can boost the availability of nutrient for plant by using a variety of mechanisms such as fixing nitrogen, solubilizing phosphorus, potassium, and zinc, and producing siderophores, ammonia, hydrogen cyanide, and phytohormones that help plant for growth and protection against various abiotic and biotic stresses. The microbial endophytes have attained the mechanism of producing various hydrolytic enzymes such as cellulase, pectinase, xylanase, amylase, gelatinase, and bioactive compounds for plant growth promotion and protection. The efficient plant growth promoting endophytic microbes could be used as an alternative of chemical fertilizers for agro-environmental sustainability. Endophytic microbes belong to different phyla including Euryarchaeota, Ascomycota, Basidiomycota, Mucoromycota, Firmicutes, Proteobacteria, and Actinobacteria. The most pre-dominant group of bacteria belongs to Proteobacteria including α-, β-, γ-, and δ-Proteobacteria. The least diversity of the endophytic microbes have been revealed fromBacteroidetes, Deinococcus-Thermus, and Acidobacteria. Among reported genera, Achromobacter, Burkholderia, Bacillus, Enterobacter, Herbaspirillum, Pseudomonas, Pantoea, Rhizobium, and Streptomyces were dominant in most host plants. The present review deals with plant endophytic diversity, mechanisms of plant growth promotion, protection, and their role for agro-environmental sustainability. In the future, application of endophytic microbes have potential role in enhancement of crop productivity and maintaining the soil health in sustainable manner.

Effect of Rhizobium mechanisms in improving tolerance to saline stress in lettuce plants

BackgroundSoils affected by salinity are a recurring problem that is continually increasing due to the impact of climate change on weather conditions and ineffective agricultural management practices. The use of plant growth promoting (PGP) Bacteria can alleviate its effects. In this regard, the genus Rhizobium has demonstrated excellent PGP capabilities through various plant growth promotion mechanisms and may therefore be a promising biofortifier under saline conditions. However, little is known about the production of volatile organic compounds (VOCs) by bacteria of this genus and their effects on plant development. Here, we aim to characterize the volatilome (the set of volatile metabolites synthesized by an organism) of Rhizobium for the first time and to further investigate the direct and VOC-mediated interaction between a strain of this genus and lettuce, a crop severely affected by salinity, both under saline and non-saline conditions.ResultsIn this study, it was shown that the use of Rhizobium sp. GPTR29 was able to increase the production of lettuce (Lactuca sativa L.) under normal and saline conditions. We analyzed the Rhizobium volatilome under non-saline (0 mM NaCl) and saline (100 mM NaCl) conditions by HS-SPME-GC‒MS and found a differential composition in response to salinity. We detected 20 different compounds, where 3-methyl-1-butanol, 2-methyl-1-butanol, and α-pinene were the backbone of the Rhizobium volatilome. Exposure to these compounds in bicameral plates under salt stress resulted in increases in plant development of 17.1%, 16.0% and 33.1% in aerial part size, number of leaves and root length, respectively. Under greenhouse conditions and salinity, the inoculation of Rhizobium sp. GPTR29 resulted in an increase of 17.8% and 27.4% in shoot fresh and dry weight, respectively. Phenolic compounds were analyzed by HPLC–DAD-MS, revealing an increase in total flavonoid content under salinity conditions (100 mM NaCl) and apigenin derivative, luteolin 7-O-glucoside and quercetin 3-O-glucuronide individually.ConclusionsThese results provide new avenues for the study of PGP mechanisms in this bacterial genus, such as VOCs and their effects on plant growth, which play an important role in mediating plant–microorganism interactions.Graphical abstract

Complete genome sequence of biocontrol strain Bacillus velezensis YC89 and its biocontrol potential against sugarcane red rot.

Sugarcane is one of the most important sugar crops worldwide, however, sugarcane production is seriously limited by sugarcane red rot, a soil-borne disease caused by Colletotrichum falcatum. Bacillus velezensis YC89 was isolated from sugarcane leaves and can significantly inhibited red rot disease caused by C. falcatum. In this study, the genome of YC89 strain was sequenced, its genome structure and function were analyzed using various bioinformatics software, and its genome was compared with those of other homologous strains. In addition, the effectiveness of YC89 against sugarcane red rot and the evaluation of sugarcane plant growth promotion were also investigated by pot experiments. Here, we present the complete genome sequence of YC89, which consists of a 3.95 Mb circular chromosome with an average GC content of 46.62%. The phylogenetic tree indicated that YC89 is closely related to B. velezensis GS-1. Comparative genome analysis of YC89 with other published strains (B. velezensis FZB42, B. velezensis CC09, B. velezensis SQR9, B. velezensis GS-1, and B. amyloliquefaciens DSM7) revealed that the strains had a part common coding sequences (CDS) in whereas 42 coding were unique of strain YC89. Whole-genome sequencing revealed 547 carbohydrate-active enzymes and identified 12 gene clusters encoding secondary metabolites. Additionally, functional analysis of the genome revealed numerous gene/gene clusters involved in plant growth promotion, antibiotic resistance, and resistance inducer synthesis. In vitro pot tests indicated that YC89 strain controlled sugarcane red rot and promoted the growth of sugarcane plants. Additionally, it increased the activity of enzymes involved in plant defense, such as superoxide dismutase, peroxidase, polyphenol oxidase, chitinase, and β-1,3-glucanase. These findings will be helpful for further studies on the mechanisms of plant growth promotion and biocontrol by B. velezensis and provide an effective strategy for controlling red rot in sugarcane plants.

Comparative Genomics and Physiological Investigations Supported Multifaceted Plant Growth-Promoting Activities in Two Hypericum perforatum L.-Associated Plant Growth-Promoting Rhizobacteria for Microbe-Assisted Cultivation.

Plants are no longer considered standalone entities; instead, they harbor a diverse community of plant growth-promoting rhizobacteria (PGPR) that aid them in nutrient acquisition and can also deliver resilience. Host plants recognize PGPR in a strain-specific manner; therefore, introducing untargeted PGPR might produce unsatisfactory crop yields. Consequently, to develop a microbe-assisted Hypericum perforatum L. cultivation technique, 31 rhizobacteria were isolated from the plant's high-altitude Indian western Himalayan natural habitat and in vitro characterized for multiple plant growth-promoting attributes. Among 31 rhizobacterial isolates, 26 produced 0.59 to 85.29 μg mL-1 indole-3-acetic acid and solubilized 15.77 to 71.43 μg mL-1 inorganic phosphate; 21 produced 63.12 to 99.92% siderophore units, and 15 exhibited 103.60 to 1,296.42 nmol α-ketobutyrate mg-1 protein h-1 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity. Based on superior plant growth-promoting attributes, eight statistically significant multifarious PGPR were further evaluated for an in planta plant growth-promotion assay under poly greenhouse conditions. Plants treated with Kosakonia cowanii HypNH10 and Rahnella variigena HypNH18 showed, by significant amounts, the highest photosynthetic pigments and performance, eventually leading to the highest biomass accumulation. Comparative genome analysis and comprehensive genome mining unraveled their unique genetic features, such as adaptation to the host plant's immune system and specialized metabolites. Moreover, the strains harbor several functional genes regulating direct and indirect plant growth-promotion mechanisms through nutrient acquisition, phytohormone production, and stress alleviation. In essence, the current study endorsed strains HypNH10 and HypNH18 as cogent candidates for microbe-assisted H. perforatum cultivation by highlighting their exclusive genomic signatures, which suggest their unison, compatibility, and multifaceted beneficial interactions with their host and support the excellent plant growth-promotion performance observed in the greenhouse trial. IMPORTANCE Hypericum perforatum L. (St. John's wort) herbal preparations are among the top-selling products to treat depression worldwide. A significant portion of the overall Hypericum supply is sourced through wild collection, prompting a rapid decline in their natural stands. Crop cultivation seems lucrative, although cultivable land and its existing rhizomicrobiome are well suited for traditional crops, and its sudden introduction can create soil microbiome dysbiosis. Also, the conventional plant domestication procedures with increased reliance on agrochemicals can reduce the diversity of the associated rhizomicrobiome and plants' ability to interact with plant growth-promoting microorganisms, leading to unsatisfactory crop production alongside harmful environmental effects. Cultivating H. perforatum with crop-associated beneficial rhizobacteria can reconcile such concerns. Based on a combinatorial in vitro, in vivo plant growth-promotion assay and in silico prediction of plant growth-promoting traits, here we recommend two H. perforatum-associated PGPR, Kosakonia cowanii HypNH10 and Rahnella variigena HypNH18, to extrapolate as functional bioinoculants for H. perforatum sustainable cultivation.

Biodiversity and Functional Attributes of Rhizospheric Microbiomes: Potential Tools for Sustainable Agriculture

The quest for increasing agricultural yield due to increasing population pressure and demands for healthy food has inevitably led to the indiscriminate use of chemical fertilizers. On the contrary, the exposure of the crops to abiotic stress and biotic stress interferes with crop growth further hindering the productivity. Sustainable agricultural practices are of major importance to enhance production and feed the rising population. The use of plant growth promoting (PGP) rhizospheric microbes is emerging as an efficient approach to ameliorate global dependence on chemicals, improve stress tolerance of plants, boost up growth and ensure food security. Rhizosphere associated microbiomes promote the growth by enhancing the uptake of the nutrients, producing plant growth regulators, iron chelating complexes, shaping the root system under stress conditions and decreasing the levels of inhibitory ethylene concentrations and protecting plants from oxidative stress. Plant growth-promoting rhizospheric microbes belong to diverse range of genera including Acinetobacter, Achromobacter, Aspergillus, Bacillus, Burkholderia, Flavobacterium, Klebsiella, Micrococcus, Penicillium, Pseudomonas, Serratia and Trichoderma. Plant growth promoting microbes are an interesting aspect of research for scientific community and a number of formulations of beneficial microbes are also commercially available. Thus, recent progress in our understanding on rhizospheric microbiomes along with their major roles and mechanisms of action under natural and stressful conditions should facilitate their application as a reliable component in the management of sustainable agricultural system. This review highlights the diversity of plant growth promoting rhizospheric microbes, their mechanisms of plant growth promotion, their role under biotic and abiotic stress and status of biofertilizers. The article further focuses on the role of omics approaches in plant growth promoting rhizospheric microbes and draft genome of PGP microbes.

Colonization Ability of Bacillus subtilis NCD-2 in Different Crops and Its Effect on Rhizosphere Microorganisms.

Bacillus subtilis strain NCD-2 is a promising biocontrol agent for soil-borne plant diseases and shows potential for promoting the growth of some crops. The purposes of this study were to analyze the colonization ability of strain NCD-2 in different crops and reveal the plant growth promotion mechanism of strain NCD-2 by rhizosphere microbiome analysis. qRT-PCR was used to determine the populations of strain NCD-2, and microbial communities' structures were analyzed through amplicon sequencing after application of strain NCD-2. Results demonstrated that strain NCD-2 had a good growth promotion effect on tomato, eggplant and pepper, and it was the most abundant in eggplant rhizosphere soil. There were significantly differences in the types of beneficial microorganisms recruited for different crops after application of strain NCD-2. PICRUSt analysis showed that the relative abundances of functional genes for amino acid transport and metabolism, coenzyme transport and metabolism, lipid transport and metabolism, inorganic ion transport and metabolism, and defense mechanisms were enriched in the rhizospheres of pepper and eggplant more than in the rhizospheres of cotton, tomato and maize after application of strain NCD-2. In summary, the colonization ability of strain NCD-2 for five plants was different. There were differences in microbial communities' structure in rhizosphere of different plants after application of strain NCD-2. Based on the results obtained in this study, it was concluded that the growth promoting ability of strain NCD-2 were correlated with its colonization quantity and the microbial species it recruited.

Potential of growth-promoting bacteria in maize (Zea mays L.) varies according to soil moisture

Climate change has caused irregularities in water distribution, which affect the soil drying-wetting cycle and the development of economically important agricultural crops. Therefore, the use of plant growth-promoting bacteria (PGPB) emerges as an efficient strategy to mitigate negative impacts on crop yield. We hypothesized that the use of PGPB (in consortium or not) had potential to promote maize (Zea mays L.) growth under a soil moisture gradient in both non-sterile and sterile soils. Thirty PGPB strains were characterized for direct plant growth-promotion and drought tolerance induction mechanisms and were used in two independent experiments. Four soil water contents were used to simulate a severe drought (30% of field capacity [FC]), moderate drought (50% of FC), no drought (80% of FC) and, finally, a water gradient comprising the three mentioned soil water contents (80%, 50%, and 30% of FC). Two bacteria strains (BS28–7 Arthrobacter sp. and BS43 Streptomyces alboflavus), in addition to three consortia (BC2, BC4 and BCV) stood out in maize growth performance in experiment 1 and were used in experiment 2. Overall, under moderate drought, inoculation with BS43 surpassed the control treatment in root dry mass and nutrient uptake. Considering the water gradient treatment (80–50–30% of FC), the greatest total biomass was found in the uninoculated treatment when compared to BS28–7, BC2, and BCV. The greatest development of Z. mays L. was only observed under constant water stress conditions in the presence of PGPB. This is the first report that demonstrated the negative effect of individual inoculation of Arthrobacter sp. and the consortium of this strain with Streptomyces alboflavus on the growth of Z. mays L. based on a soil moisture gradient; however, future studies are needed for further validation.

The Role of Plant Growth-Promoting Microorganisms (PGPMs) and Their Feasibility in Hydroponics and Vertical Farming

There are many reasons for the increase in hydroponics/soil-free systems in agriculture, and these systems have now advanced to the form of vertical farming. The sustainable use of space, the reduction in water use compared to soil-based agriculture, the lack of pesticides, the ability to control nutrient inputs, and the implementation of user-friendly technology for environmental control and harvesting are all factors that have made the global market for vertical farming predicted to reach more than USD 10.02 billion by 2027. By comparison, soil-based agriculture consumes 20 times more water, and some agricultural practices promote soil deterioration and cause environmental pollution. Plant growth-promoting microorganisms (PGPMs) have been used extensively in traditional agriculture to enhance plant growth, environmental stress tolerance, and the efficacy of phytoremediation in soil-based farming. Due to the controlled atmosphere in hydroponics and vertical farms, there is strong potential to maximize the use of PGPMs. Here, we review the leveraging of plant growth-promoting microorganism mechanisms in hydroponics and vertical farming. We recommend a synchronized PGPM treatment using a biostimulant extract added to the hydroponic medium while also pre-treating seeds or seedlings with a microbial suspension for aquaponic and aeroponic systems.

Plant Growth Promotion, Phytohormone Production and Genomics of the Rhizosphere-Associated Microalga, Micractinium rhizosphaerae sp. nov.

Microalgae are important members of the soil and plant microbiomes, playing key roles in the maintenance of soil and plant health as well as in the promotion of plant growth. However, not much is understood regarding the potential of different microalgae strains in augmenting plant growth, or the mechanisms involved in such activities. In this work, the functional and genomic characterization of strain NFX-FRZ, a eukaryotic microalga belonging to the Micractinium genus that was isolated from the rhizosphere of a plant growing in a natural environment in Portugal, is presented and analyzed. The results obtained demonstrate that strain NFX-FRZ (i) belongs to a novel species, termed Micractinium rhizosphaerae sp. nov.; (ii) can effectively bind to tomato plant tissues and promote its growth; (iii) can synthesize a wide range of plant growth-promoting compounds, including phytohormones such as indole-3-acetic acid, salicylic acid, jasmonic acid and abscisic acid; and (iv) contains multiple genes involved in phytohormone biosynthesis and signaling. This study provides new insights regarding the relevance of eukaryotic microalgae as plant growth-promoting agents and helps to build a foundation for future studies regarding the origin and evolution of phytohormone biosynthesis and signaling, as well as other plant colonization and plant growth-promoting mechanisms in soil/plant-associated Micractinium.

Recent Advances in the Bacterial Phytohormone Modulation of Plant Growth.

Phytohormones are regulators of plant growth and development, which under different types of stress can play a fundamental role in a plant's adaptation and survival. Some of these phytohormones such as cytokinin, gibberellin, salicylic acid, auxin, and ethylene are also produced by plant growth-promoting bacteria (PGPB). In addition, numerous volatile organic compounds are released by PGPB and, like bacterial phytohormones, modulate plant physiology and genetics. In the present work we review the basic functions of these bacterial phytohormones during their interaction with different plant species. Moreover, we discuss the most recent advances of the beneficial effects on plant growth of the phytohormones produced by PGPB. Finally, we review some aspects of the cross-link between phytohormone production and other plant growth promotion (PGP) mechanisms. This work highlights the most recent advances in the essential functions performed by bacterial phytohormones and their potential application in agricultural production.

Genome insights into the plant growth-promoting bacterium Saccharibacillus brassicae ATSA2T

Endophytes can facilitate the improvement of plant growth and health in agriculturally important crops, yet their genomes and secondary metabolites remain largely unexplored. We previously isolated Saccharibacillus brassicae strain ATSA2T from surface-sterilized seeds of kimchi cabbage and represented a novel species of the genus Saccharibacillus. In this study, we evaluated the plant growth-promoting (PGP) effect of strain ATSA2T in kimchi cabbage, bok choy, and pepper plants grown in soils. We found a significant effect on the shoot and root biomass, and chlorophyll contents following strain ATSA2T treatment. Strain ATSA2T displayed PGP traits such as indole acetic acid (IAA, 62.9 μg/mL) and siderophore production, and phosphate solubilization activity. Furthermore, genome analysis of this strain suggested the presence of gene clusters involved in iron acquisition (fhuABD, afuABC, fbpABC, and fepCDG) and phosphate solubilization (pstABCHS, phoABHLU, and phnCDEP) and other phytohormone biosynthesis genes, including indole-3-acetic acid (trpABCDEFG), in the genome. Interestingly, the secondary metabolites cerecidin, carotenoid, siderophore (staphylobactin), and bacillaene underlying plant growth promotion were found in the whole genome via antiSMASH analysis. Overall, physiological testing and genome analysis data provide comprehensive insights into plant growth-promoting mechanisms, suggesting the relevance of strain ATSA2T in agricultural biotechnology.

Relieving your stress: PGPB associated with Andean xerophytic plants are most abundant and active on the most extreme slopes.

Plants interact with plant growth-promoting bacteria (PGPB), especially under stress condition in natural and agricultural systems. Although a potentially beneficial microbiome has been found associated to plants from alpine systems, this plant- PGPB interaction has been scarcely studied. Nevados de Chillán Complex hold one of the southernmost xerophytic formations in Chile. Plant species living there have to cope with drought and extreme temperatures during the growing season period, microclimatic conditions that become harsher on equatorial than polar slopes, and where the interaction with PGPB could be key for plant survival. Our goal was to study the abundance and activity of different PGPB associated to two abundant plant species of Andean xerophytic formations on contrasting slopes. Twenty individuals of Berberis empetrifolia and Azorella prolifera shrubs were selected growing on a north and south slope nearby Las Fumarolas, at 2,050 m elevation. On each slope, microclimate based on temperature and moisture conditions were monitored throughout the growing period (oct. - apr.). Chemical properties of the soil under plant species canopies were also characterized. Bacterial abundance was measured as Log CFU g-1 from soil samples collected from each individual and slope. Then, the most abundant bacterial colonies were selected, and different hormonal (indoleacetic acid) and enzymatic (nitrogenase, phosphatase, ACC-deaminase) mechanisms that promote plant growth were assessed and measured. Extreme temperatures were observed in the north facing slope, recording the hottest days (41 vs. 36°C) and coldest nights (-9.9 vs. 6.6°C). Moreover, air and soil moisture were lower on north than on south slope, especially late in the growing season. We found that bacterial abundance was higher in soils on north than on south slope but only under B. empetrifolia canopy. Moreover, the activity of plant growth-promoting mechanisms varied between slopes, being on average higher on north than on south slope, but with plant species-dependent trends. Our work showed how the environmental heterogeneity at microscale in alpine systems (slope and plant species identity) underlies variations in the abundance and plant growth promoting activity of the microorganisms present under the plant canopy of the Andean xerophytic formations and highlight the importance of PGPB from harsh systems as biotechnological tools for restoration.

GROWTH PARAMETERS OF BLUEBERRY (&lt;i&gt;Vaccinium&lt;/i&gt; spp.) PLANTS INOCULATED WITH &lt;i&gt;Pseudomonas fluorescens&lt;/i&gt;

The production and consumption of blueberry have increased in Mexico owing to its health benefits. Symbiotic relationships have been shown to be crucial in blueberry plants. In particular, phytohormone production by Pseudomonas fluorescens is an important mechanism of plant growth promotion. However, there are only a few reports on the effects of plant growth-promoting bacteria in blueberries. Therefore, we aimed to evaluate the effects of four strains of P. fluorescens (UM16, UM240, UM256, and UM270) and two types of slow-release fertilizer (nitrophosphate and basacote) on the development of blueberry var. Biloxi under greenhouse conditions. Blueberry seedlings obtained from in vitro culture and adapted under greenhouse conditions were inoculated with 1 x 106 CFU with any of the four strains, depending on treatment. Plants inoculated showed increased average plant length, plant fresh weight, root length, and root fresh and dry weight, compared with those with the control treatment (non-inoculated plants). The plants inoculated and fertilized with nitrophosphate had a better development compared with those fertilized with basacote or the control plants (inoculated or fertilized). Inoculated plants fertilized with nitrophosphate also had greater plant length, higher fresh plant weight, longer roots, and greater root fresh and dry weight than the control (non-inoculated or non-fertilized plants). Our study could facilitate the sustainable propagation of blueberry plants.

Tracking the Biostimulatory Effect of Fractions from a Commercial Plant Protein Hydrolysate in Greenhouse-Grown Lettuce.

Protein hydrolysate biostimulants are environmentally friendly options for the reduction of nitrogen input, but their plant growth-promoting mechanisms are still not completely unveiled. Here, to put the "signaling peptide theory" to the test, a greenhouse experiment was undertaken using low (1 mM) and optimal (8 mM) NO3-treated butterhead lettuce and three molecular fractions (PH1 (&gt;10 kDa), PH2 (1-10 kDa) and PH3 (&lt;10 kDa) fractions), in addition to the whole product Vegamin®: PH, in a randomized block design. PH1 and PH3 significantly increased fresh yield (+8%) under optimal (lighter leaves), but not under low (darker leaves) NO3 conditions. Total ascorbic acid, lutein and β-carotene increased with PH3, and disinapoylgentobiose and kaempferol-3-hydroxyferuloyl-sophorosie-7-glucoside content increased with PH (whole/fractions) treatments, particularly under low NO3 conditions. The complete hydrolysate and analyzed peptide fractions have differential biostimulatory effects, enhancing the growth and nutritional quality of lettuce.