Abstract
The sequence of the genome of Pseudomonas fluorescens F113 has shown the presence of multiple traits relevant for rhizosphere colonization and plant growth promotion. Among these traits are denitrification and chemotactic motility. Besides aerobic growth, F113 is able to grow anaerobically using nitrate and nitrite as final electron acceptors. F113 is able to perform swimming motility under aerobic conditions and under anaerobic conditions when nitrate is used as the electron acceptor. However, nitrite can not support swimming motility. Regulation of swimming motility is similar under aerobic and anaerobic conditions, since mutants that are hypermotile under aerobic conditions, such as gacS, sadB, kinB, algU and wspR, are also hypermotile under anaerobic conditions. However, chemotactic behavior is different under aerobic and denitrification conditions. Unlike most pseudomonads, the F113 genome encode three complete chemotaxis systems, Che1, Che2 and Che3. Mutations in each of the cheA genes of the three Che systems has shown that the three systems are functional and independent. Mutation of the cheA1 gene completely abolished swimming motility both under aerobic and denitrification conditions. Mutation of the cheA2 gene, showed only a decrease in swimming motility under both conditions, indicating that this system is not essential for chemotactic motility but is necessary for optimal motility. Mutation of the cheA3 gene abolished motility under denitrification conditions but only produced a decrease in motility under aerobic conditions. The three Che systems proved to be implicated in competitive rhizosphere colonization, being the cheA1 mutant the most affected.
Highlights
The Pseudomonas fluorescens complex comprises a group of related pseudomonads that are often found saprophytically associated with plants
It is interesting to note that opposed to growth under aerobic conditions, the siderophore pyoverdine was not produced in the iron limited medium SA under denitrification conditions
The presence in the F113 genome of complete sets of nor and nos genes [4], suggests that nitric oxide and nitrous oxide are putative electron acceptors for this strain and that molecular dinitrogen is the final product of denitrification
Summary
The Pseudomonas fluorescens complex comprises a group of related pseudomonads that are often found saprophytically associated with plants. A large number of strains belonging to this group have been found to colonize the rhizosphere and the endophytic compartments and to positively influence plant growth, either directly through biofertilization or manipulation of PLOS ONE | DOI:10.1371/journal.pone.0132242. A series of recent phylogenomic studies [2,3,4] have shown that the P. fluorescens complex contains up to five different phylogenetic subgroups and that many traits putatively responsible of the plant growth promoting abilities are phylogenetically distributed. F113 is a plant growth promoting rhizobacteria (PGPR) that is able to colonize the rhizosphere [5], influencing plant growth directly, by phosphate mobilization [6] and degradation of the plant hormone ethylene [4], and indirectly by the production of siderophores, fungicides (DAPG) and competition for niche with plant pathogens [7]. The full genomic sequence of F113 is available and analysis of the genome has shown the presence of numerous traits that are likely to be involved in its rhizosphere colonization and PGPR abilities [4]
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