Synergistic Plant-Microbe Interactions between Endophytic Actinobacteria and Their Role in Plant Growth Promotion and Biological Control of Cotton under Salt Stress

Metabonomic analysis to identify exometabolome changes underlying antifungal and growth promotion mechanisms of endophytic Actinobacterium Streptomyces albidoflavus for sustainable agriculture practice

In recent years, there has been an increasing focus on microbial ecology and its possible impact on agricultural production, owing to its eco-friendly nature and sustainable use. The current study employs metabolomics technologies and bioinformatics approaches to identify changes in the exometabolome of Streptomyces albidoflavus B24. This research aims to shed light on the mechanisms and metabolites responsible for the antifungal and growth promotion strategies, with potential applications in sustainable agriculture. Metabolomic analysis was conducted using Q Exactive UPLC-MS/MS. Our findings indicate that a total of 3,840 metabolites were identified, with 137 metabolites exhibiting significant differences divided into 61 up and 75 downregulated metabolites based on VIP >1, |FC| >1, and p < 0.01. The interaction of S. albidoflavus B24 monoculture with the co-culture demonstrated a stronger correlation coefficient. The Principal Component Analysis (PCA) demonstrates that PCA1 accounted for 23.36%, while PCA2 accounted for 20.28% distinction. OPLS-DA score plots indicate significant separation among different groups representing (t1) 24% as the predicted component (to1) depicts 14% as the orthogonal component. According to the findings of this comprehensive study, crude extracts from S. albidoflavus demonstrated varying abilities to impede phytopathogen growth and enhance root and shoot length in tested plants. Through untargeted metabolomics, we discovered numerous potential molecules with antagonistic activity against fungal phytopathogens among the top 10 significant metabolites with the highest absolute log2FC values. These include Tetrangulol, 4-Hydroxybenzaldehyde, and Cyclohexane. Additionally, we identified plant growth-regulating metabolites such as N-Succinyl-L-glutamate, Nicotinic acid, L-Aspartate, and Indole-3-acetamide. The KEGG pathway analysis has highlighted these compounds as potential sources of antimicrobial properties. The inhibitory effect of S. albidoflavus crude extracts on pathogen growth is primarily attributed to the presence of specific gene clusters responsible for producing cyclic peptides such as ansamycins, porphyrin, alkaloid derivatives, and neomycin. Overall, it is apparent that crude extracts from S. albidoflavus exhibited varying abilities to inhibit the growth of three phytopathogens and enhancement in both root and shoot length of tested plants. This research enhances our understanding of how secondary metabolites contribute to growth promotion and biocontrol, supporting ecosystem sustainability and resilience while boosting productivity in sustainable agriculture.

Optimisation of indole acetic acid production by Neopestalotiopsis aotearoa endophyte isolated from Thymus vulgaris and its impact on seed germination of Ocimum basilicum

BACKGROUND

Microbial growth during plant tissue culture is a common problem that causes significant losses in the plant micro-propagation system. Most of these endophytic microbes have the ability to propagate through horizontal and vertical transmission. On the one hand, these microbes provide a rich source of several beneficial metabolites.

RESULTS

The present study reports on the isolation of fungal species from different in vitro medicinal plants (i.e., Breynia disticha major, Breynia disticha, Duranta plumieri, Thymus vulgaris, Salvia officinalis, Rosmarinus officinalis, and Ocimum basilicum l) cultures. These species were tested for their indole acetic acid (IAA) production capability. The most effective species for IAA production was that isolated from Thymus vulgaris plant (11.16 µg/mL) followed by that isolated from sweet basil plant (8.78 µg/mL). On screening for maximum IAA productivity, medium, "MOS + tryptophan" was chosen that gave 18.02 μg/mL. The macroscopic, microscopic examination and the 18S rRNA sequence analysis indicated that the isolate that given code T4 was identified as Neopestalotiopsis aotearoa (T4). The production of IAA by N. aotearoa was statistically modeled using the Box-Behnken design and optimized for maximum level, reaching 63.13 µg/mL. Also, IAA extract was administered to sweet basil seeds in vitro to determine its effect on plant growth traits. All concentrations of IAA extract boosted germination parameters as compared to controls, and 100 ppm of IAA extract exhibited a significant growth promotion effect for all seed germination measurements.

CONCLUSIONS

The IAA produced from N. aotearoa (T4) demonstrated an essential role in the enhancement of sweet basil (Ocimum basilicum) growth, suggesting that it can be employed to promote the plant development while lowering the deleterious effect of using synthetic compounds in the environment.

Drought and salt stress mitigation in crop plants using stress-tolerant auxin-producing endophytic bacteria: a futuristic approach towards sustainable agriculture

Abiotic stresses, especially drought stress and salt stress in crop plants are accelerating due to climate change. The combined impact of drought and salt is anticipated to lead to the loss of up to 50% of arable land globally, resulting in diminished growth and substantial yield losses threatening food security. Addressing the challenges, agriculture through sustainable practices emerges as a potential solution to achieve Zero Hunger, one of the sustainable development goals set by the IUCN. Plants deploy a myriad of mechanisms to effectively address drought and salt stress with phytohormones playing pivotal roles as crucial signaling molecules for stress tolerance. The phytohormone auxin, particularly indole acetic acid (IAA) emerges as a paramount regulator integral to numerous aspects of plant growth and development. During both drought and salt stress conditions, auxin plays crucial roles for tolerance, but stress-induced processes lead to decreased levels of endogenous free auxin in the plant, leading to an urgent need for auxin production. With an aim to augment this auxin deficiency, several researchers have extensively investigated auxin production, particularly IAA by plant-associated microorganisms, including endophytic bacteria. These endophytic bacteria have been introduced into various crop plants subjected to drought or salt stress and potential isolates promoting plant growth have been identified. However, post-identification, essential studies on translational research to advance these potential isolates from the laboratory to the field are lacking. This review aims to offer an overview of stress tolerant auxin-producing endophytic bacterial isolates while identifying research gaps that need to be fulfilled to utilize this knowledge for the formulation of crop-specific and stress-specific endophyte bioinoculants for the plant to cope with auxin imbalance occurring during these stress conditions.

Bioprospecting of Metabolites from Actinomycetes and their Applications

Actinomycetes are present in various terrestrial and aquatic habitats, predominantly in the soil rhizosphere, encompassing marine and freshwater ecosystems. These microorganisms exhibit characteristics that resemble both bacteria and fungi. Numerous actinomycetes exhibit a mycelial existence and undergo significant morphological transformations. These bacteria are widely recognized as biotechnologically significant microorganisms utilized for the production of secondary metabolites. In all, over 45% of all bioactive microbial metabolites are produced by actinomycetes, which are responsible for producing around 10,000 of them. The majority of actinomycetes exhibit substantial saprophytic characteristics in their natural environment, enabling them to effectively decompose a diverse range of plant and animal waste materials during the process of decomposition. Additionally, these organisms possess a sophisticated secondary metabolic system, which enables them to synthesize almost two-thirds of all naturally occurring antibiotics. Moreover, they can create a diverse array of chemical compounds with medical or agricultural applications, including anticancer, antiparasitic, and antibacterial agents. This review aims to provide an overview of the prominent biotechnological domains in which actinobacteria and their metabolites demonstrate noteworthy applicability. The graphical abstract provides a preview of the primary sections covered in this review. This paper presents a comprehensive examination of the biotechnological applications and metabolites of actinobacteria, highlighting their potential for patent innovations.

Endophytic Streptomyces population induced by L-glutamic acid enhances plant resilience to abiotic stresses in tomato

Endophyte bacteria, which colonize plants including roots, stem, flower, and fruit, it can derive their nutrients from the host, are recognized for their mutualistic relationship with the host plant. They play a critical role in promoting host growth and modulating abiotic stress. Carbon and nitrogen have a significant impact on bacterial population and secondary metabolite production, which are highly specific in various categories such as bacterial growth regulation, anti-compounds production. Application of L-glutamic acid can significantly enhance Streptomyces globisporus population buildup in plants. However, the effectiveness of this population buildup against abiotic stresses such as salinity and drought has not been investigated. Therefore, in this study, we tested the bacteria and their prebiotic activity against salinity and drought stress in tomato plants. Three different amino acids were treated on the tomato plants, and it was observed that L-asparagine and L-proline had a negative effect on plant growth and phenotype, while L-glutamic acid promoted plant growth and increased bacteria population density. The bacteria were found to colonize the rhizosphere and root endosphere, with colonization being promoted by L-glutamic acid. Additionally, Streptomyces was found to have plant growth promotion effects and provided protection against abiotic stresses. Interestingly, L-glutamic acid reduced the damage caused by salinity stress, but not drought stress. These findings suggest that L-glutamic acid plays a role in providing tolerance to salinity stress with the core microbiota, thus the current study demonstrated their prebiotic activity in the agriculture system.

Inoculation with Actinobacteria spp. Isolated from a Hyper-Arid Environment Enhances Tolerance to Salinity in Lettuce Plants (Lactuca sativa L.)

Irrigated agriculture is responsible for a third of global agricultural production, but the overuse of water resources and intensification of farming practices threaten its sustainability. The use of saline water in irrigation has become an alternative in areas subjected to frequent drought, but this practice affects plant growth due to osmotic impact and excess of ions. Plant-growth-promoting rhizobacteria (PGPR) can mitigate the negative impacts of salinity and other abiotic factors on crop yields. Actinobacteria from the hyper-arid Atacama Desert could increase the plant tolerance to salinity, allowing their use as biofertilizers for lettuce crops using waters with high salt contents. In this work, rhizosphere samples of halophytic Metharme lanata were obtained from Atacama Desert, and actinobacteria were isolated and identified by 16S gene sequencing. The PGPR activities of phosphate solubilization, nitrogen fixation, and the production of siderophore and auxin were assessed at increasing concentrations of NaCl, as well as the enhancement of salt tolerance in lettuce plants irrigated with 100 mM of NaCl. Photosynthesis activity and chlorophyll content, proline content, lipid peroxidation, cation and P concentration, and the identification and quantification of phenolic compounds were assessed. The strains S. niveoruber ATMLC132021 and S. lienomycini ATMLC122021 were positive for nitrogen fixation and P solubilization activities and produced auxin up to 200 mM NaCl. In lettuce plants, both strains were able to improve salt stress tolerance by increasing proline contents, carotenoids, chlorophyll, water use efficiency (WUE), stomatal conductance (gs), and net photosynthesis (A), concomitantly with the overproduction of the phenolic compound dicaffeoylquinic acid. All these traits were positively correlated with the biomass production under saltwater irrigation, suggesting its possible use as bioinoculants for the agriculture in areas where the water resources are scarce and usually with high salt concentrations.