Probing into the unique relationship between a soil bacterium, Pseudomonas putida AKMP7 and Arabidopsis thaliana: A case of "conditional pathogenesis"

Pseudomonas putida configures Arabidopsis root architecture through modulating the sensing systems for phosphate and iron acquisition

Iron (Fe) and phosphate (Pi) are two essential nutrients that are poorly available in the soil and should be supplemented either as fertilizers or organic amendments to sustain crop production. Currently, determining how rhizosphere bacteria contribute to plant mineral nutrient acquisition is an area of growing interest regarding its potential application in agriculture. The aim of this study was to investigate the influence of root colonization by Pseudomonas putida for Arabidopsis growth through Fe and Pi nutritional signaling. We found that root colonization by the bacterium inhibits primary root elongation and promotes the formation of lateral roots. These effects could be related to higher expression of two Pi starvation-induced genes and AtPT1, the major Pi transporter in root tips. In addition, P. putida influenced the accumulation of Fe in the root and the expression of different elements of the Fe uptake pathway. The loss of function of the protein ligase BRUTUS (BTS), and the bHLH transcription factors POPEYE (PYE) and IAA-LEUCINE RESISTANT3 (ILR3) compromised the root branching stimulation triggered by bacterial inoculation while the leaf chlorosis in the fit1 and irt1-1 mutant plants grown under standard conditions could be bypassed by P. putida inoculation. The WT and both mutant lines showed similar Fe accumulation in roots. P. putida repressed the expression of the IRON-REGULATED TRANSPORTER 1 (IRT1) gene suggesting that the bacterium promotes an alternative Fe uptake mechanism. These results open the door for the use of P. putida to enhance nutrient uptake and optimize fertilizer usage by plants.

Functional Genome Analysis of a Conditionally Pathogenic Rhizobacterial Strain, Pseudomonas putida AKMP7

This manuscript reports the whole genome sequence of a conditionally pathogenic rhizobacterial strain, Pseudomonas putida AKMP7, which has been previously reported by us to be beneficial to Arabidopsis thaliana under well-watered conditions and pathogenic to the plant under water stress. As part of a study to understand this unique behavior, the whole genome sequence of this strain was analyzed. Based on the results, it was identified that the total length of the AKMP7 genome is 5,764,016 base pairs, and the total GC content of the genome is 62.93% (typical of P. putida). Using RAST annotation pipeline, it was identified that the genome has 5605 coding sequences, 80 repeat regions, 71 tRNA genes, and 22 rRNA genes. A total of 4487 functional proteins and 1118 hypothetical proteins were identified. Phylogenetic analysis has classified it as P. putida species, with a P value of 0.03. In order to identify close relatives of this strain, comparative genomics was performed with 30 other P. putida strains, taken from publicly available genome databases, using Average Nucleotide Identity (ANI) analysis. Whole genome comparison with these strains reveals that AKMP7 possesses Type-IV Secretion System (T4SS) with conjugative transfer functionality. Interestingly, the T4SS feature is absent in all the beneficial/harmless strains of P. putida that we analyzed. All the plant pathogenic bacteria that were analyzed had the T4SS feature in their genome, indicating its role in pathogenesis. This study aims to address important gaps in understanding the molecular mechanisms involved in the conditional/opportunistic pathogenesis of plant-associated, beneficial soil bacteria, using genomics approaches.

The rhizobacterial strain, Pseudomonas putida AKMP7, causes conditional pathogenesis in Arabidopsis thaliana via negative regulation of salicylic acid signaling, under water stress

We have previously reported a phenomenon of "conditional pathogenesis", in which, a drought-tolerant rhizobacterium, Pseudomonas putida AKMP7, promotes plant growth under well-watered conditions, while, deteriorating plant health under water-stressed conditions, in Arabidopsis thaliana seedlings. To understand the molecular mechanisms behind this phenomenon, we studied the modulation of salicylic acid (SA) biosynthesis as well as SA-responsive gene expression, involved in systemic acquired resistance (SAR), in A. thaliana, by AKMP7, under well-watered and water-stressed conditions. We found that, the plant SA levels were upregulated by AKMP7, both under, well-watered as well as water-stressed conditions. However, the SA signaling gene, Non-expressor of Pathogenesis Related gene 1 (NPR1) and Pathogenesis Related gene 1 (PR1) were upregulated under well-watered conditions and suppressed under water-stress, in AKMP7 inoculated seedlings. To understand the reason for this, we studied the expression of NPR4, a negative regulator of NPR1, and, NPR3, a negative regulator of PR1. We observed that, AKMP7 suppresses NPR1 and, consequently, PR1 genes, by upregulating NPR4 under water stress. To understand the potential role of NPR4 in conditional-pathogenesis, we performed physiological studies with NPR4 knockout mutants of A. thaliana and found that the NPR4 mutants did not exhibit any signs of the characteristic growth retardation caused by AKMP7 inoculation, under water stress. Preliminary studies with the model pathogen, Pseudomonas syringae, indicate that AKMP7 may lead to enhanced disease suppression under well-watered conditions, but not under water-stress. Taken together, our data suggest that, AKMP7 causes conditional pathogenesis by an overall compromise in plant immune response under water-stress.

Native bio-control agents from the rice fields of Telangana, India: characterization and unveiling the potential against stem rot and false smut diseases of rice

The stem rot caused by Sclerotium hydrophilum and false smut caused by Ustilaginoidea virens are two of the major production constraints in rice cultivation in India and other countries. Stem rot and false smut can be effectively controlled with synthetic fungicides. However, the indiscriminate use of chemical fungicides may cause development of resistance among the pathogens. In addition to this, synthetic fungicides also exhibit harmful impacts on the environment. Exploiting microbe-based alternatives for managing plant diseases diminishes public concerns about the ill effects of pesticide usage in crops. In this regard, the present study was designed to investigate the potential of native microbial biocontrol agents (BCAs) from rice rhizosphere for the sustainable management of stem rot and false smut diseases in rice. Potential BCAs and pathogens were identified and characterized through morphological, biochemical, and sanger sequencing techniques. Bio-efficacy tests of potential BCAs against stem rot and false smut diseases on rice under glasshouse conditions indicated higher seed vigour index of the treated seeds, significant improvement in the growth of the seedling, increased dry weight, reduction in percentage disease index viz., 70.03% (stem rot) and 69.24% (false smut) over the control plants. Phytohormones indole acetic acid (IAA), abscisic acid (ABA), gibberellic acid (GA), salicylic acid (SA), and zeatin (tZ) were detected and quantified in the four potential BCAs using liquid chromatography- tandem mass spectrometry (LC-MS/MS). Scanning electron microscopy (SEM) studies revealed the endophytic nature of the strains in rice. The study indicated a positive correlation between the diversity and concentration of phytohormones released by the bioagents and enhanced plant growth promotion and disease suppression in rice.

Pseudomonas spp. can help plants face climate change

Climate change is increasingly affecting agriculture through droughts, high salinity in soils, heatwaves, and floodings, which put intense pressure on crops. This results in yield losses, leading to food insecurity in the most affected regions. Multiple plant-beneficial bacteria belonging to the genus Pseudomonas have been shown to improve plant tolerance to these stresses. Various mechanisms are involved, including alteration of the plant ethylene levels, direct phytohormone production, emission of volatile organic compounds, reinforcement of the root apoplast barriers, and exopolysaccharide biosynthesis. In this review, we summarize the effects of climate change-induced stresses on plants and detail the mechanisms used by plant-beneficial Pseudomonas strains to alleviate them. Recommendations are made to promote targeted research on the stress-alleviating potential of these bacteria.