Growth Interactions during Bacterial Colonization of Seedling Rootlets

Abstract

Rootlet elongation and bacterial growth on rootlets were determined after inoculation of cucumber and spinach seedlings with Pseudomonas strains differing in production of siderophores and HCN. Siderophore producers grew more profusely than nonproducers on both species and promoted rootlet elongation on cucumber. Coinoculation of siderophore producers and nonproducers resulted in restricted growth of the latter. The total populations of nonproducers of HCN in the presence of HCN producers were not decreased, but the tenacity of their association with the rootlet surface was altered.Germinating seeds and growing plants influence the activities of soil microorganisms in the adjoining volumes of soil known as the spermosphere and the rhizosphere, respectively (24). Conversely, microorganisms in these settings condition the seeds and plants in a number of ways. Some of the microorganisms (e.g., the so-called plant-growth-promoting rhizobacteria) may enhance plant health and productivity by synthesizing phytohormones, increasing the local availability of nutrients, facilitating the uptake of nutrients by the plants, decreasing heavy metal toxicity in the plants, antagonizing plant pathogens, and inducing systemic resistance in the plants to pathogens (5, 8, 9). Detrimental effects are produced by other organisms, such as the so-called deleterious rhizosphere microorganisms, and these effects include release of toxic products of microbial metabolism, alteration of nutrient cycling, impairment of uptake of nutrients, competition for nutrients, and retardation of root growth (28). Among the factors involved in plant-microbe interactions, as well as in microbemicrobe interactions, in the rhizosphere, siderophores and hydrocyanic acid (HCN) have received special attention. Siderophores are high-affinity Fe 3ϩ chelators that are synthesized and released extracellularly under iron limitation conditions, where they make otherwise inaccessible supplies of insoluble iron available to organisms with specific membrane-bound siderophore receptors (14,17). Such receptors are present in the microorganisms that produce the corresponding siderophores but can also be found in plants. The iron nutrition of these plants is thus enhanced. In addition, since plant-growth-promoting rhizobacteria produce siderophores with higher Fe 3ϩ affinity than the siderophores produced by deleterious rhizosphere microorganisms, the latter microorganisms are outcompeted in their quest for iron. HCN is released as product of secondary metabolism by several microorganisms and affects sensitive organisms by inhibiting the synthesis of ATP mediated by cytochrome oxidase (22). Therefore, depending on the target organisms, HCN-producing microorganisms are regarded as harmful when they impair plant health and beneficial when they suppress unwanted components of the microbial community (23, 29). The significance of siderophore and HCN generation in rhizosphere management and engineering has been studied and reviewed extensively (11,13,16,21,26,36), but there is...

Requirement of Siderophore Biosynthesis for Plant Colonization by Salmonella enterica

“…The expression of genes that are required for the biosynthesis of each Salmonella siderophore, enterobactin and salmochelin, was downregulated by the addition of iron to alfalfa root exudates. Our results support the conclusion that S. enterica encounters an iron-poor environment requiring siderophore production during alfalfa root colonization, similar to other plant-associated bacteria (15).…”

“…Phytopathogenic pseudomonads, such as P. syringae and P. aeruginosa, can import either plantor bacterium-derived iron(III) dicitrate through FecA (28, 35); however, unlike Pseudomonas or Escherichia coli, Salmonella is devoid of a citrate-dependent iron transport system (56). S. enterica relies on its own siderophore biosynthesis during root colonization, similar to plant-associated pseudomonads (15). However, Salmonella's dependency on its own siderophore biosynthesis during leaf colonization differs from phytobacterial pathogens, such as Pseudomonas and Xanthomonas, but may be similar to the enteric Dickyea dadantii (21).…”

Isolation and characterization of bacteria from the rhizosphere of wheat

“…Plants are able to use bacterial iron-siderophore complexes as a means of obtaining iron from soil (Glick 1995). For example, Pseudomonas siderophore producers promoted rootlet elongation on cucumber grew under gnotobiotic conditions (Bellis and Ercolani 2001).…”

Interactions between plants and beneficial Pseudomonas spp.: exploiting bacterial traits for crop protection

“…strain (Sharma et al 2003). Promotion of rootlet elongation was achieved when cucumber seedlings were inoculated with siderophore-producer Pseudomonas strains (De Bellis and Ercolani 2001). Finally, siderophore producing pseudomonads can also be influenced by plant phytosiderophores.…”