Genetic and functional diversity of fluorescent Pseudomonas from rhizospheric soils of wheat crop

The Gene paaZ of the Phenylacetic Acid (PAA) Catabolic Pathway Branching Point and ech outside the PAA Catabolon Gene Cluster Are Synergistically Involved in the Biosynthesis of the Iron Scavenger 7-Hydroxytropolone in Pseudomonas donghuensis HYS

The newly discovered iron scavenger 7-hydroxytropolone (7-HT) is secreted by Pseudomonas donghuensis HYS. In addition to possessing an iron-chelating ability, 7-HT has various other biological activities. However, 7-HT's biosynthetic pathway remains unclear. This study was the first to report that the phenylacetic acid (PAA) catabolon genes in cluster 2 are involved in the biosynthesis of 7-HT and that two genes, paaZ (orf13) and ech, are synergistically involved in the biosynthesis of 7-HT in P. donghuensis HYS. Firstly, gene knockout and a sole carbon experiment indicated that the genes orf17-21 (paaEDCBA) and orf26 (paaG) were involved in the biosynthesis of 7-HT and participated in the PAA catabolon pathway in P. donghuensis HYS; these genes were arranged in gene cluster 2 in P. donghuensis HYS. Interestingly, ORF13 was a homologous protein of PaaZ, but orf13 (paaZ) was not essential for the biosynthesis of 7-HT in P. donghuensis HYS. A genome-wide BLASTP search, including gene knockout, complemented assays, and site mutation, showed that the gene ech homologous to the ECH domain of orf13 (paaZ) is essential for the biosynthesis of 7-HT. Three key conserved residues of ech (Asp39, His44, and Gly62) were identified in P. donghuensis HYS. Furthermore, orf13 (paaZ) could not complement the role of ech in the production of 7-HT, and the single carbon experiment indicated that paaZ mainly participates in PAA catabolism. Overall, this study reveals a natural association between PAA catabolon and the biosynthesis of 7-HT in P. donghuensis HYS. These two genes have a synergistic effect and different functions: paaZ is mainly involved in the degradation of PAA, while ech is mainly related to the biosynthesis of 7-HT in P. donghuensis HYS. These findings complement our understanding of the mechanism of the biosynthesis of 7-HT in the genus Pseudomonas.

Multifaceted Impacts of Plant-Beneficial Pseudomonas spp. in Managing Various Plant Diseases and Crop Yield Improvement

The modern agricultural system has issues with the reduction of agricultural productivity due to a wide range of abiotic and biotic stresses. It is also expected that in the future the entire world population may rapidly increase and will surely demand more food. Farmers now utilize a massive quantity of synthetic fertilizers and pesticides for disease management and to increase food production. These synthetic fertilizers badly affect the environment, the texture of the soil, plant productivity, and human health. However, agricultural safety and sustainability depend on an ecofriendly and inexpensive biological application. In contrast to synthetic fertilizers, soil inoculation with plant-growth-promoting rhizobacteria (PGPR) is one of the excellent alternative options. In this regard, we focused on the best PGPR genera, Pseudomonas, which exists in the rhizosphere as well as inside the plant's body and plays a role in sustainable agriculture. Many Pseudomonas spp. control plant pathogens and play an effective role in disease management through direct and indirect mechanisms. Pseudomonas spp. fix the amount of atmospheric nitrogen, solubilize phosphorus and potassium, and also produce phytohormones, lytic enzymes, volatile organic compounds, antibiotics, and secondary metabolites during stress conditions. These compounds stimulate plant growth by inducing systemic resistance and by inhibiting the growth of pathogens. Furthermore, pseudomonads also protect plants during different stress conditions like heavy metal pollution, osmosis, temperature, oxidative stress, etc. Now, several Pseudomonas-based commercial biological control products have been promoted and marketed, but there are a few limitations that hinder the development of this technology for extensive usage in agricultural systems. The variability among the members of Pseudomonas spp. draws attention to the huge research interest in this genus. There is a need to explore the potential of native Pseudomonas spp. as biocontrol agents and to use them in biopesticide development to support sustainable agriculture.

A Complex Mechanism Involving LysR and TetR/AcrR That Regulates Iron Scavenger Biosynthesis in Pseudomonas donghuensis HYS

7-Hydroxytropolone (7-HT) is a symmetrical seven-membered heteroatomic ring with a carboxyl group and two hydroxyl groups and was recently reported to be an iron scavenger of Pseudomonas donghuensis HYS. Cluster 1 includes 12 genes related to the synthesis of 7-HT; among these genes, those for two regulators, Orf1 and Orf12, were predicted to regulate 7-HT biosynthesis and to be LysR-type transcriptional regulators (LTTRs) and TetR/AcrR family transcriptional regulators, respectively. Data from real-time quantitative PCR and β-galactosidase and classical siderophore assays indicated that the transcription levels of orf1 and orf12, as well as those of crucial genes orf6 to orf9, were repressed under high-iron conditions. The deletion of orf1 and orf12 led to an absence of 7-HT and a decrease in orf6-orf9 expression. Orf1 and Orf12 were essential for the production of 7-HT through orf6-orf9 These two regulators are regulated by the Gac/Rsm system; Orf1 facilitates the expression of Orf12, and Orf12 concomitantly stimulates the expression of orf6-orf9 to synthesize 7-HT. The overexpression of Orf12 decreased 7-HT yields, possibly through decreased orf6-orf9 expression. This work thus outlines a complex mechanism regulating the biosynthesis of the iron scavenger 7-HT in P. donghuensis HYS. The synergy between Orf1 and Orf12 ensures that 7-HT acts as an iron chelator despite being toxic to bacteria and provides new ideas for the novel regulation of dual-functional secondary metabolism and research on 7-HT and its derivates in other bacteria.IMPORTANCE A complex regulation mechanism including two regulators, LysR and TetR/AcrR, in the biosynthesis of the novel iron scavenger 7-hydroxytropolone (7-HT) was verified in Pseudomonas donghuensis HYS. The coaction of LysR Orf1 and TetR/AcrR Orf12 may balance the toxicity and iron chelation of 7-HT in P. donghuensis HYS to overcome iron deficiency, as well as improve the bacterial competitiveness under iron-scarce conditions because of the toxicity of 7-HT toward other bacteria, making the accurate regulation of 7-HT biosynthesis indispensable. This regulation mechanism may be ubiquitous in the Pseudomonas putida group but may better explain the group's strong adaptability.

Alleviation of drought stress in pulse crops with ACC deaminase producing rhizobacteria isolated from acidic soil of Northeast India

The agricultural crops are often affected by the scarcity of fresh water. Seasonal drought is a major constraint on Northeast Indian agriculture. Almost 80% of the agricultural land in this region is acidic and facing severe drought during the winter period. Apart from classical breeding and transgenic approaches, the application of plant-growth-promoting bacteria (PGPB) is an alternative strategy for improving plant fitness under stressful conditions. The 1-aminocyclopropane-1-carboxylate (ACC) deaminase-producing PGPB offer drought stress tolerance by regulating plant ethylene levels. The aim of the present study was to evaluate the consortium effect of three ACC-deaminase producing rhizobacteria - Ochrobactrum pseudogrignonenseRJ12, Pseudomonas sp.RJ15 and Bacillus subtilisRJ46 on drought stress alleviation in Vigna mungo L. and Pisum sativum L. Consortium treatment significantly increase seed germination percentage, root length, shoot length, and dry weight of treated plants. An elevated production of reactive oxygen species scavenging enzymes and cellular osmolytes; higher leaf chlorophyll content; increase in relative water content and root recovery intension were observed after consortium treatment in comparison with the uninoculated plants under drought conditions. The consortium treatment decreased the ACC accumulation and down-regulated ACC-oxidase gene expression. This consortium could be an effective bio-formulator for crop health improvement in drought-affected acidic agricultural fields.

Phylogenetical coherence of Pseudomonas in unexplored soils of Himalayan region

Pseudomonas, an enormously diverse genus of the γ-Proteobacteria, is an important member of soil microbial communities. In this study, genetic heterogeneity and plant growth promotory property of Pseudomonas was compared within the group Pseudomonas sensu stricto isolated from the lesser explored niches of Himalayan region. A significant difference (P < 0.001 in total Pseudomonas count) was observed among the six types of soil samples collected from oak forest, fine forest, and agricultural soil. The highest numbers of bacteria were isolated from oak forest soil followed by pine forest soil and agricultural soil. 23.52 % of the total 238 isolates were siderophore producers and were identified as Pseudomonas on the basis of PCR amplification using 16S rDNA Pseudomonas specific primer. The molecular analysis by Jaccard's similarity coefficient resulted into eight different clusters and six outlying branches. Some of the clusters include Pseudomonas from forest as well as agricultural land. Among the 51 isolates 100 % were siderophore and IAA producers, 68.62 % were phosphate solubilizers and 62.74 % HCN producers. The results reveal that isolates from lesser explored area possess beneficial properties and show genetic heterogeneity among them.

Fluorescent Pseudomonas Strains with only Few Plant-Beneficial Properties Are Favored in the Maize Rhizosphere

Plant Growth-Promoting Rhizobacteria (PGPR) enhance plant health and growth using a variety of traits. Effective PGPR strains typically exhibit multiple plant-beneficial properties, but whether they are better adapted to the rhizosphere than PGPR strains with fewer plant-beneficial properties is unknown. Here, we tested the hypothesis that strains with higher numbers of plant-beneficial properties would be preferentially selected by plant roots. To this end, the co-occurrence of 18 properties involved in enhanced plant nutrition, plant hormone modulation, or pathogen inhibition was analyzed by molecular and biochemical methods in a collection of maize rhizosphere and bulk soil isolates of fluorescent Pseudomonas. Twelve plant-beneficial properties were found among the 698 isolates. Contrarily to expectation, maize preferentially selected pseudomonads with low numbers of plant-beneficial properties (up to five). This selection was not due to the predominance of strains with specific assortments of these properties, or with specific taxonomic status. Therefore, the occurrence of only few plant-beneficial properties appeared favorable for root colonization by pseudomonads.

Psychrotolerant Endophytic Pseudomonas sp. Strains OB155 and OS261 Induced Chilling Resistance in Tomato Plants (Solanum lycopersicum Mill.) by Activation of Their Antioxidant Capacity

Studies on chilling stress damage and its mitigation through microorganisms in members of family Solanaceae is limited, despite their economic importance. We studied chilling stress alleviation in tomato plants colonized by psychrotolerant bacterial strains Pseudomonas vancouverensis OB155-gfp and P. frederiksbergensis OS261-gfp. Log phase cultures of bacterial strains were coated on surface-sterilized seeds (bacterization) before sowing and nonbacterized (control) seeds were coated with sterile bacterial growth medium. All plants were grown at temperatures of 30 and 25°C and at the end of 4 weeks, chilling treatment (12 and 10°C) was imposed for 1 week on half of the bacterized and control plants. Under normal conditions (30 and 25°C), no significant difference was observed in antioxidant activity, proline accumulation, and expression of cold acclimation genes in tomato leaf tissues of both control and bacterized plants. However, plants exposed to temperatures of 12 and 10°C were found to decrease in robustness and nutrient uptake, accompanied by increased membrane damage. Chilling resistance in bacterized plants was evident from reduced membrane damage and reactive oxygen species levels, improved antioxidant activity in leaf tissues, and high expression of cold acclimation genes LeCBF1 and LeCBF3 compared with control plants. Confocal microscopy confirmed effective colonization and intercellular localization of cold-adapted bacterial strains OB155-gfp and OS261-gfp.