Down Regulation of Virulence Factors of Pseudomonas aeruginosa by Salicylic Acid Attenuates Its Virulence on Arabidopsis thaliana and Caenorhabditis elegans

Department of Horticulture and Landscape Architecture, Colorado State University, 217 Shepardson Building, Fort Collins, CO 80523, USA.
Infection and Immunity (Impact Factor: 3.73). 10/2005; 73(9):5319-28. DOI: 10.1128/IAI.73.9.5319-5328.2005
Source: PubMed


Salicylic acid (SA) is a phenolic metabolite produced by plants and is known to play an important role in several physiological
processes, such as the induction of plant defense responses against pathogen attack. Here, using the Arabidopsis thaliana-Pseudomonas aeruginosa pathosystem, we provide evidence that SA acts directly on the pathogen, down regulating fitness and virulence factor production
of the bacteria. Pseudomonas aeruginosa PA14 showed reduced attachment and biofilm formation on the roots of the Arabidopsis mutants lox2 and cpr5-2, which produce elevated amounts of SA, as well as on wild-type Arabidopsis plants primed with exogenous SA, a treatment known to enhance endogenous SA concentration. Salicylic acid at a concentration
that did not inhibit PA14 growth was sufficient to significantly affect the ability of the bacteria to attach and form biofilm
communities on abiotic surfaces. Furthermore, SA down regulated three known virulence factors of PA14: pyocyanin, protease,
and elastase. Interestingly, P. aeruginosa produced more pyocyanin when infiltrated into leaves of the Arabidopsis transgenic line NahG, which accumulates less SA than wild-type plants. This finding suggests that endogenous SA plays a role in down regulating
the synthesis and secretion of pyocyanin in vivo. To further test if SA directly affects the virulence of P. aeruginosa, we used the Caenorhabiditis elegans-P. aeruginosa infection model. The addition of SA to P. aeruginosa lawns significantly diminished the bacterium's ability to kill the worms, without affecting the accumulation of bacteria
inside the nematodes' guts, suggesting that SA negatively affects factors that influence the virulence of P. aeruginosa. We employed microarray technology to identify SA target genes. These analyses showed that SA treatment affected expression
of 331 genes. It selectively repressed transcription of exoproteins and other virulence factors, while it had no effect on
expression of housekeeping genes. Our results indicate that in addition to its role as a signal molecule in plant defense
responses, SA works as an anti-infective compound by affecting the physiology of P. aeruginosa and ultimately attenuating its virulence.

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Available from: Balakrishnan Prithiviraj, Dec 18, 2013
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    • "Beneficial effects by application of lower concentrations (less than 1 mM) has been reported for plant growth (Rivas-San Vicente and Plasencia, 2011). SA caused enhancement (Kang et al., 2004; Kiddle et al., 1994; Wang et al., 2007) or decline (D'Onofrio et al., 2009; Prithiviraj et al., 2005) in specific secondary metabolites of the plant. In sweet basil, increase in the content of essential oil in response to sprays of SA at 0.1 mM is reported (Gharib, 2006). "
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    ABSTRACT: The experiment aimed to understand how these organic acids are going to affect on essential oil production in sweet basil. Three concentrations of each citric acid (CA; 0, 4, or 7mM) and salicylic acid (SA; 0.5 or 1mM) were applied. We added a standard control for comparison. The first spray was started at 2-leaf stage that was continued with four subsequent sprays in a 15 day interval. The plants were harvested 75 days after sowing the seeds, when they had produced seed. SA in 1mM concentration caused the maximum of the total essential oil production along with the highest essential oil production by both leaves and stems with an increase of 32.8, 38.3, and 25.8% (respectively) when compared to control treatment. A synergism between 7mM CA and 1mM SA was observed in flowering parameters, which yielded the tallest inflorescence in CA7SA1 along with the highest inflorescence count per plant and floret count in main inflorescence. Despite these quantitative improvements in flowering parameters, the thousand seed weight was still at its maximum in this combination, as well. The seed mucilage, on the other hand, increased significantly by foliar application of 0.5mM SA. Seed oil content responded positively to applied CA, while SA effect was negligible. The results show the potential for using these compounds in manipulation of plant growth and metabolism toward the intended final use.
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    • "Phenolic acids are also known to regulate the infection efficiency during the exchange of signals between legumes and rhizobia (Mandal et al., 2010). However, despite the known effects of phenolic compounds on bacterial virulence, the mechanisms of action behind them are, as yet, unresolved (Chow et al., 2011; Mirzoeva et al., 1997; Prithiviraj et al., 2005). "
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    ABSTRACT: Several studies have reported effects of the plant phenolic acids cinnamic acid (CA) and salicylic acid (SA) on virulence of soft rot enterobacteria. However, the mechanisms involved in these processes are not yet fully understood. Here, we investigated whether CA and SA interfere with the quorum-sensing (QS) system of two Pectobacterium species, P. aroidearum and P. carotovorum subsp brasiliense, which are known to produce N-acyl-homoserine lactone (AHL) QS signals. Our results clearly indicate that both phenolic compounds affect the QS machinery of the two species, consequently altering the expression of bacterial virulence factors. While in control treatments, expression of QS-related genes increased over time, exposure of bacteria to nonlethal concentrations of CA or SA inhibited the expression of QS genes, including expI, expR, PC1_1442 (luxR transcriptional regulator) and luxS (a component of the AI-2 system). Other virulence genes known to be regulated by the QS system, such as pecS, pel, peh and yheO, were also down-regulated relative to the control. In agreement with the low levels of expression of expI and expR, CA and SA also reduced the level of AHL signal. The effects of CA and SA on AHL signaling were confirmed in compensation assays, in which exogenous application of N-(β-ketocaproyl)-L-homoserine lactone (eAHL) led to the recovery of the reduction in virulence caused by the two phenolic acids. Collectively, the results of gene expression studies, bioluminescence assays, virulence assays and compensation assays with eAHL clearly support a mechanism by which CA and SA interfere with Pectobacterium virulence via the QS machinery. This article is protected by copyright. All rights reserved.
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    • "It is not well-understood which rhizodeposit compounds recruit or influence which groups of microbes and how. An effective approach is to examine microbial associations with plant mutants deficient in the biosynthesis and rhizodeposition of specific groups of compounds (Prithiviraj et al. 2005; Rudrappa et al. 2008). It is worth noting that composition of rhizodeposits varies substantially among different plant species (Czarnota et al. 2003; Warembourg et al. 2003). "
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    ABSTRACT: High bacterial density and diversity near plant roots has been attributed to rhizodeposit compounds that serve as both energy sources and signal molecules. However, it is unclear if and how specific rhizodeposit compounds influence bacterial diversity. We silenced the biosynthesis of isoflavonoids, a major component of soybean rhizodeposits, using RNA interference in hairy root composite plants, and examined changes in rhizosphere bacteriome diversity. We used successive sonication to isolate soil fractions from different rhizosphere zones at two different time points, and analyzed denaturing gradient gel electrophoresis profiles of 16S rRNA gene amplicons. Extensive diversity analysis of the resulting spatio temporal profiles of soybean bacterial communities indicated that indeed isoflavonoids significantly influenced soybean rhizosphere bacterial diversity. Our results also suggested a temporal gradient effect of rhizodeposit isoflavonoids on the rhizosphere. However, the hairy root transformation process itself significantly altered rhizosphere bacterial diversity, necessitating appropriate additional controls. Gene silencing in hairy root composite plants combined with successive sonication is a useful tool to determine the spatio temporal effect of specific rhizodeposit compounds on rhizosphere microbial communities.
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