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DOI: 10.3923/pjbs.2012.212.214
URL: https://scialert.net/abstract/?doi=pjbs.2012.212.214
Agriculture is the main source of global food production and its production has been limitized by various factors, which mainly consists of plant pathogens and environmentally induced stresses. New strains of plant pathogens are emerging with the passage of time and most of the currently used resistant cultivars are becoming susceptible to these strains (Singh et al., 2008). The rate of evolution in plant pathogens is higher under agricultural ecosystem as compared with natural conditions (Stukenbrock and McDonald, 2008). In order to compete with evolving plant pathogens a continuous effort is required to find new sources of resistance and strategies against these pathogens. Molecular biology has opened new avenues in agriculture field to cope with the harmful effects of biotic and a-biotic stresses. Transgenic crops have been developed that are resistant to plant insects, pathogens and other environmental stresses (Hernandez-Campuzano et al., 2009; Wu et al., 2009; Parkhi et al., 2010). Recent research shows that there are some key factors of metabolism that mainly contribute in the virulence of a pathogen (Wahl et al., 2010) and their better understanding could generate new options to deal with them. Hence, to develop effective and long lasting strategy against plant pathogens the detail understanding of their metabolites and regulatory activities at molecular level is inevitable.
In our ecosystem, bacteria interact with majority of species in both positive and negative manner but their harmful effects are so influential that they are the pivot of current research activities. Virulence of bacteria is a part of its metabolic activities and a number of factors like different metabolites, regulatory genes and proteins etc. are known to have important role in its regulation (Persson et al., 2009; Poncet et al., 2009; Hann and Rathjen, 2010). For the acquisition of essential nutrients these bacteria mainly relay on host resources. Iron is one of the essential micronutrient and is known to have its potential role in bacterial pathogenesis (Dill et al., 2009; Sharma et al., 2011). Siderophores are high affinity iron scavenging compounds used by these bacteria to fetch the iron from host environment. These are also known to have significant role in their virulence (Miethke et al., 2006) but this concept is only well developed in mammals. To check the role of these siderophores in plant pathogenic bacteria (Jones and Wildermuth, 2011) conducted a study on Pseudomonas syringae pv. tomato DC3000 and tried to elaborate the potential involvement of siderophores in its virulence.
It has been reported that under iron limiting conditions Pseudomonas syringae pv. tomato DC3000 produces two siderophores named as yersiniabactin (Ybt) and pyoverdin (Pvd) (Jones et al., 2007). Mutant lacking the gene encoding Ybt showed no defect in growth and led to a conclusion that other siderophore (Pvd) could be serving as iron transporter for bacterial cell. To confirm this hypothesis (Jones and Wildermuth, 2011) conducted a study to access the effect of Pvd in a Pseudomonas syringae pv. tomato DC3000 lacking pvd (P¯). They tested its activity in both low and severally iron limited conditions, no growth defect was observed under low iron conditions but the difference between wild type (wt) and P¯ was significant under severally iron limited growth conditions. On comparison of both P¯ and Y¯ (lacking ybt) on severally iron limited media, Pvd was found to be more important than Ybt for growth under such conditions. This fact was further confirmed by employing double mutant (Y¯ P¯) through chrome azurol S (CAS) assay, Y¯ P¯ and P¯ showed significant reduction in iron scavenging activity as compared with Y¯ alone. On analysis of double mutant Y¯ P¯, through CAS assay the existence of new siderophores was hypothesized, because double mutant was still showing some iron scavenging activity.
Based on the pre-existing facts that citrate is also involved in high affinity iron transport authors explored different possible reasons for iron scavenging activity in Y¯ P¯. A homolog of FecA (an outer membrane transporter known for its citrate mediated iron import) was found in DC3000 genome and suspected to be responsible for iron scavenging activity of Y¯ P¯. A third mutant F- (lacking fecA) was developed and found to have no harmful effects on bacterial growth. But when a triple mutant (Y¯ P¯ F¯) was compared with Y¯ P¯ under iron limiting conditions, a severe growth reduction was observed. On further analysis it was found that this bacterium produce citrate endogenously under iron limiting conditions and without FecA citrate mediated iron transport couldnt occur. By combining all of the results, authors concluded that under iron limiting conditions high affinity iron scavenging activity by Ybt Pvd or citrate have a growth determinant factor.
Leaf pathogenesis assay was conducted to determine the probable involvement of these siderophores in bacterial virulence. All strains Y¯, P¯ and Y¯ P¯ developed normal disease symptoms (as in wt) when inoculated through vacuum infiltration on tomato leaves. To obtain results that correspond to the natural conditions, standard leaf dipping bioassay was also performed and similar results were obtained. Triple mutant strain produced disease symptoms similar to the wt, when inoculated on tomato leaves either through leaf infiltration or dipping. The fact, that triple mutant still posses the virulence activity led the investigators to find other possible sources of iron responsible for its virulence activity. So, researchers focused their investigation on other sources like plant iron carriers (heme/hemin and nicotianamine); some components of high-affinity heme/hemin scavenging system were also detected in the genome of DC3000. Authors compared the growth of Y¯ P¯ and Y¯ P¯ F¯ when supplemented with different concentrations of hemin and nicotianamine. There was no improvement in the growth of Y¯ P¯ F¯ when supplemented with low concentration of these agents but only nicotianamine complemented the Y¯ P¯ F¯ growth to Y¯ P¯ when provided at 100 μM concentration. To determine the effect of lower affinity iron scavenging activity on its growth, different iron sources were provided and growth of Y¯ P¯ F¯ and Y¯ P¯ was compared. Authors conclude that if the concentration of available iron is more than 1 μM then DC3000 can fulfill its iron requirements of growth through low affinity iron import.
Iron is an essential component, required for various metabolic activities. Plants and animals have developed specialized systems to acquire iron for their metabolic activities, so do the bacteria. Siderophores plays important role to fulfill the growth related iron requirements of Pseudomonas syringae pv. tomato DC3000 but unlike other mammalian pathogenic bacteria no significant link between siderophores and their virulence was found by Jones and Wildermuth (2011). Though there are many factors, which are common for pathogenesis of bacteria infecting different species but Jones and Wildermuth (2011) have shown that Pseudomonas syringae pv. tomato DC3000 is not dependent on siderophores to acquire iron for its pathogenic activities. This finding will be of significant importance in order to develop new strategies specifically against Pseudomonas syringae and generally against bacterial plant pathogens.
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