Exploring the Potential of Purple Non-Sulfur Bacteria Strains A3-5 and F3-3 in Sustainable Agriculture: A Study on Nutrient Solubilization, Plant Growth Promotion, and Acidic Stress Tolerance

22 - Plant growth-promoting potential of endophytic bacteria for sustainable agriculture

The native microbial flora and fauna are replaced by commercial chemical fertilizers and pesticides, in the current agricultural system. Imbalance of beneficial microbial diversity and natural competitors increases the severity of plant diseases. Hence, sustainable agricultural practices like bio-inoculant, stress tolerant consortium, crop rotation and mix cropping sequences is only the solution of recharging the microbial population in soils to make healthier for crop productivity and suppression of soil borne phytopathogen. Microorganisms use several direct mechanism activities, e.g. production of plant hormones (indole-3-acetic acid), ammonium, siderophore and nutrient solubilization, and indirect mechanism activities, e.g. hydrogen cyanide, chitinase, protease and antibiotic for plant growth promotion. The plant growth-promoting effect of bacteria, fungi, mycorrhizal fungi and algae is widely explored. Yeast is a single-celled microbe classified as members of the kingdom fungi. Yeast and their product use in the food industry, medical science and biotechnological research purpose but very few literatures reported that yeasts have the ability to produce a group of plant growth-promoting activities and biocontrolling activity. Therefore, the main aim of this mini review is to highlight the application of yeasts as biological agents in different sectors of sustainable farming practices.

Simple SummarySalt stress is an important environmental problem that negatively affects agricultural and food production in the world. Currently, the use of plant beneficial bacteria for plant growth promotion is attractive due to the demand for eco-friendly and sustainable agriculture. In this study, salt tolerant deep-sea actinobacterium, Dermacoccus abyssi MT1.1T was investigated plant growth promotion and salt stress mitigation in tomato seedlings. In addition, D. abyssi MT1.1T whole genome was analyzed for plant growth promoting traits and genes related to salt stress alleviation in plants. We also evaluated the biosafety of this strain on human health and organisms in the environment. Our results highlight that the inoculation of D. abyssi MT1.1T could reduce the negative effects of salt stress in tomato seedlings by growth improvement, total soluble sugars accumulation and hydrogen peroxide reduction. Moreover, this strain could survive and colonize tomato roots. Biosafety testing and genome analysis of D. abyssi MT1.1T showed no pathogenicity risk. In conclusion, we provide supporting evidence on the potential of D. abyssi MT1.1T as a safe strain for use in plant growth promotion under salt stress.Salt stress is a serious agricultural problem threatens plant growth and development resulted in productivity loss and global food security concerns. Salt tolerant plant growth promoting actinobacteria, especially deep-sea actinobacteria are an alternative strategy to mitigate deleterious effects of salt stress. In this study, we aimed to investigate the potential of deep-sea Dermacoccus abyssi MT1.1T to mitigate salt stress in tomato seedlings and identified genes related to plant growth promotion and salt stress mitigation. D. abyssi MT1.1T exhibited plant growth promoting traits namely indole-3-acetic acid (IAA) and siderophore production and phosphate solubilization under 0, 150, 300, and 450 mM NaCl in vitro. Inoculation of D. abyssi MT1.1T improved tomato seedlings growth in terms of shoot length and dry weight compared with non-inoculated seedlings under 150 mM NaCl. In addition, increased total soluble sugar and total chlorophyll content and decreased hydrogen peroxide content were observed in tomato inoculated with D. abyssi MT1.1T. These results suggested that this strain mitigated salt stress in tomatoes via osmoregulation by accumulation of soluble sugars and H2O2 scavenging activity. Genome analysis data supported plant growth promoting and salt stress mitigation potential of D. abyssi MT1.1T. Survival and colonization of D. abyssi MT1.1T were observed in roots of inoculated tomato seedlings. Biosafety testing on D. abyssi MT1.1T and in silico analysis of its whole genome sequence revealed no evidence of its pathogenicity. Our results demonstrate the potential of deep-sea D. abyssi MT1.1T to mitigate salt stress in tomato seedlings and as a candidate of eco-friendly bio-inoculants for sustainable agriculture.

Exploration of diverse environmental samples for plant growth-promoting microbes to fulfill the increasing demand for sustainable agriculture resulted in increased use of bacterial biofertilizer. We aimed for the isolation of plant growth-promoting as well as antibiotic sensitive bacteria from bovine manure samples. The basic theme of our study is to highlight potentials of bacteria in manure and the unchecked risk associated with the application of manure i.e. introducing antibiotic-resistant microbial flora, as fertilizer. Fifty-two, morphologically distinct isolates; from eight different manure samples, were subjected to plant growth-promoting parametric tests along with antibiotic resistance. Thirteen antibiotic sensitive bacterial strains with potentials of plant growth promotion further characterized by 16S rRNA ribotyping and the identified genera were Stenotrophomonas, Achromobacter, Pseudomonas, and Brevibacillus. Successful radish seeds germination under sterile in-vitro conditions showed the potential of selected bacterial isolates as plant growth-promoting bacteria. The results of this study confirmed plant growth-promoting characteristics of bovine manures’ bacterial strains along with an alarming antibiotic resistance load which comprises 75% of bacterial isolated population. Our study showed distinct results of un-explored manure bacterial isolates for plant growth promotion and flagged ways associated with unchecked manure application in agriculture soil through high load of antibiotic resistant bacteria.

Unveiling the significance of rhizosphere: Implications for plant growth, stress response, and sustainable agriculture

The plant growth-promoting rhizobacteria (or PGPR) are the beneficial microorganism that colonizes rhizosphere and help in promoting plant growth, protecting from biotic and abiotic stresses, and significantly increasing soil fertility. For the effective ways of developing sustainable agriculture for improving crop productivity with a minimal disturbance to the environment is the exploration of plant growth-promoting rhizobacteria and some other microbe-based symbioses in plants. For increasing crop yields, the use of PGPR has been well proven for its eco-friendly sound by promoting plant growth either direct or indirect mechanism. The mechanisms of plant growth-promoting rhizobacteria include resistance against plant pathogens, solubilizing nutrients for easy uptake, and maintaining the plant growth regulator hormone. This chapter emphasizes an eco-friendly approach to increase crop production and health, the development of sustainable agriculture, the mechanism of PGPR for agricultural sustainability, and the role in different major crop plant varieties along with their mechanism of action.

Novel plant growth promoting rhizobacteria—Prospects and potential

The aim of this study was to analyze if cofactor pyrroquinoline quinone from Serratia sp. S119 is involved in the inorganic phosphate solubilization mechanism and in its ability to promote the plant growth. Site directed mutagenesis was performed to obtain a pqqE- minus mutant of strain Serratia sp. S119. The phosphate solubilization ability, gluconate and PQQ production of the mutant Serratia sp. RSL (pqqE-) was analyzed. Mutant RSL (pqqE-) showed significant decrease in P soluble and gluconic acid levels produced and undetectable levels of PQQ cofactor compared with wild-type strain. Complementation with synthetic PQQ cofactor restored P solubilization and gluconate production reaching the levels produced by wild-type strain. PqqE gene sequence indicated that it is highly conserved within Serratia strains and its product shows conserved motifs found in other PqqE proteins of several bacteria. The effect of the inoculation of the PQQ- mutant on peanut and maize plants was evaluated in pot assays. Plants growth parameters showed no differences among the different treatments indicating that PQQ from Serratia sp. S119 is not involved in the growth promotion of these plants. PQQ cofactor is essential for phosphate solubilization ability of Serratia sp. S119 but is not required for growth promotion of peanut and maize plants.

The aim of the study was to demonstrate the potential of the promotion and regulation of plant physiology and growth under control and copper stress conditions, and the impact of the exogenous application of methyl jasmonate on this potential. Runner bean plants were treated with methyl jasmonate (1 or 10 µM) (J; J1 or J10) and Cu (50 µM), and inoculated with a bacterial isolate (S17) originating from Spitsbergen soil, and identified as Pseudomonas luteola using the analytical profile index (API) test. Above- and under-ground plant parts were analyzed. The growth parameters; the concentration of the photosynthetic pigments, elements, flavonoids (FLAVO), phenolics (TPC), allantoin (ALLA), and low molecular weight organic acids (LMWOAs); the activity of antioxidant enzymes and enzymes of resistance induction pathways (e.g., superoxide dismutase (SOD), catalase (CAT), ascorbate (APX) and guaiacol (GPX) peroxidase, glucanase (GLU), and phenylalanine (PAL) and tyrosine ammonia-lyase (TAL)), and the antioxidant capacity (AC) were studied. The leaves exhibited substantially higher ALLA and LMWOA concentrations as well as PAL and TAL activities, whereas the roots mostly had higher activities for a majority of the enzymes tested (i.e., SOD, CAT, APX, GPX, and GLU). The inoculation with S17 mitigated the effect of the Cu stress. Under the Cu stress and in the presence of J10, isolate S17 caused an elevation of the shoot fresh weight, K concentration, and TAL activity in the leaves, and APX and GPX (also at J1) activities in the roots. In the absence of Cu, isolate S17 increased the root length and the shoot-to-root ratio, but without statistical significance. In these conditions, S17 contributed to a 236% and 34% enhancement of P and Mn, respectively, in the roots, and a 19% rise of N in the leaves. Under the Cu stress, S17 caused a significant increase in FLAVO and TPC in the leaves. Similarly, the levels of FLAVO, TPC, and AC were enhanced after inoculation with Cu and J1. Regardless of the presence of J, inoculation at Cu excess caused a reduction of SOD and CAT activities, and an elevation of GPX. The effects of inoculation were associated with the application of Cu and J, which modified plant response mainly in a concentration-dependent manner (e.g., PAL, TAL, and LMWOA levels). The conducted studies demonstrated the potential for isolate S17 in the promotion of plant growth.

With the intensification of the problems encountered in agriculture, the use of microorganisms shows promise. Fungi of the Trichoderma genus are found naturally in the soil and have been; they are used as an active ingredient in biofungicides and have activity as promoters of plant growth. This work aimed to evaluate the efficiency of the commercial product TrichoPlus, containing Trichoderma asperellum (Samuels, Lieckf & Nirenberg) as a promoter of soybean plant growth. Four independent experiments were carried out in different periods between 2019 and 2020. The experiments were conducted in a greenhouse, and biomass and nutrient contents were examined. In the first experiment, among the different rates of TrichoPlus tested (3, 4, 5, and 6 g per kg of seeds), the best dose for most of the characteristics evaluated was 5 g per kg of seed. Hence, the inoculation of TrichoPlus at 5 g per kg of seed in the other experiments showed positive results in the accumulation of biomass, nitrogen, and phosphorus content. Based on the study's results, the product TrichoPlus, composed of T. asperellum, can be used as an efficient inoculant for promoting soybean plant growth.

Plant growth-promoting rhizobacteria (PGPR) are the bacterial strains that are associated with the rhizospheric part of plants and are known to positively influence plant growth. These microorganisms induce plant growth promotion directly through production of phytohormones, solubilization of phosphates, biological fixation of nitrogen, inhibition of ethylene biosynthesis, and indirectly by imparting resistance to combat pathogenic attacks by the production of secondary metabolites such as antibiotics, siderophores, and many enzymes, and also by inducing tolerance to abiotic stress conditions. PGPR are especially used as bioinoculants due to their role in increasing crop production. PGPR are influenced, inter alia, by plant genotype, environmental conditions (both biotic and abiotic), and various biotic interactions (plant-microbe interactions and microbe-microbe interactions) taking place in the rhizosphere. Besides, agricultural activities, including use of chemical fertilizers, pesticides, and soil tillage, also have substantial influence not only on the PGPR population but also on physical, chemical, and biological characteristics of soil. In view of their profound growth-promoting characteristics and ever-increasing demand for adoption of ecologically compatible and eco-friendly practices in agriculture, PGPR could be an important tool in sustainable agriculture and can be safely relied upon for increasing productivity. It is in this backdrop, an attempt has been made in the present chapter to emphasize the importance of rhizobacteriain the promotion of plant development and sustainable agriculture.

Chapter 11 - Role of plant growth–promoting rhizobacteria in sustainable agriculture

Plants are intimately associated with diverse bacterial communities inhabiting distinct ecological niches, namely the rhizosphere, endosphere, and phyllosphere, all of which contribute to plant health and development. Among these microorganisms, plant growth-promoting bacteria (PGPB) play a pivotal role in enhancing plant growth, resilience, and productivity, particularly under abiotic stress conditions. These bacteria are increasingly being used in sustainable agriculture and phytoremediation strategies because of their ability to modulate plant physiology and improve stress tolerance. PGPB must successfully colonize and persist in plant-associated environments such as the rhizosphere, where they initiate interactions with plant roots before migrating to internal root tissues (endosphere) or aerial plant parts (phyllosphere), where they exert their effects through a variety of mechanisms. This review systematically examines the diversity, ecological distribution, and functional traits of PGPB that associate with the plant rhizosphere, endosphere, and phyllosphere, with a focus on their modes of action, which include phytohormone modulation, nutrient solubilization, and stress alleviation. The current review reveals that the rhizosphere and endosphere have a higher abundance and diversity of beneficial bacterial taxa than the phyllosphere, with genera such as Bacillus sp. and Pseudomonas sp. being particularly common. This review offers a comprehensive overview of the bacterial community and its interactions with plants, focusing on the various microbial types and their roles in the rhizosphere, endosphere, and phyllosphere. This is critical for a better understanding of plant–microbe interactions and choosing the right bacterial taxa and genera to promote plant growth and productivity in the face of environmental stresses and challenges.

Klebsiella is a bacterial genus widely recognized in the medical field but with underexplored potential in agriculture. This study employed a systematic review and meta-analysis to investigate scientific articles on plant growth promotion effects associated with endophytic bacteria Klebsiella species. A total of 39 relevant studies, published between 2012 and 2024, were identified based on strict inclusion and exclusion criteria. The analysis revealed that Klebsiella sp., K. pneumoniae, and K. variicola are cosmopolitan species that have functional versatility in phytohormone production, nutrient solubilization, and pathogen control in agricultural systems in both tropical and temperate zones. The data showed a significant correlation between the use of Klebsiella sp. and plant growth, highlighting the positive impact of these species in controlling aggressive pathogens. These findings underscore the potential of Klebsiella as a biotechnological tool for sustainable agricultural practices, enhancing plant growth and reducing the reliance on chemical inputs. The study further emphasizes the need for future research to deepen genomic characterization and expand the agricultural applications of these bacteria.

Research related to the isolation and characterization of rhizobacteria and their potential as plant growth promoters was carried out to support the development of environmentally friendly and sustainable agriculture. This research aims to isolate and characterize rhizobacteria from saline soil which have the potential to promote plant growth. This research was carried out for 2 months at the Agrotechnology Laboratory, Agronomy Unit, Faculty of Agriculture, Halu Oleo University, Kendari. The research was conducted using an experimental design. Research on testing rhizobacteria isolates as plant growth promoters was carried out using a completely randomized design with 3 replications. The research was carried out by isolating and characterization of rhizobacteria both morphologically (elevation, appearance, margins, color, shape, and size of microbes) and biochemically (nitrogen fixation test, phosphate solubilization, and IAA hormone synthesis) and continued with germination tests of local tomato seeds. Data from research on morphological, physiological, and biochemical tests of endophytic microbes were analyzed using descriptive analysis, while rhizobacteria tests on germination (maximum growth potential) were analyzed using analysis of variance. The results of the analysis showing a real effect were followed by the Duncan test at a confidence level of 95%. Based on the research results, 20 isolates were obtained. The isolates with the highest ability to fix nitrogen (isolates Ab01, Ab03, Ab05, and Bb07), phosphate solubilizers (Ab10 and Ab03), and the highest ability of bacteria to produce IAA were obtained from isolate Ab04 at 124.91 ppm and isolate Bb05 at 115.27 ppm. Meanwhile, the highest ability of rhizobacteria isolates to germinate seeds (maximum growth potential) was obtained in isolate Bb05 at 88.89% and isolate Ab04 at 82.22% when compared with other isolates and controls.