Root colonizers

In some cases pectinolytic enzymes have been associated with virulence and it is generally accepted that pectinolysis by these bacteria facilitates their entry and spread in plant tissue. In Rhizohium, these enzymes may play a role in the root infection process that precedes nodule formation (Hubbell et al 1978). A. irakense has never been reported to be pathogenic on plants. It can therefore be speculated that moderate and strictly regulated pectinolysis of A. irakense facilitates entry in the outer cortex of plants roots, since A. irakense has been isolated from surface-sterilized roots. It is likely that breakdown of plant polysaccharides by root colonizing bacteria can provide them with extra carbon source. 

Kniper I, LV Kravchenko, GV Bloemberg, BJJ Lugtenberg (2002) Pseudomonas putida strain PC L1444, selected for efficient root colonization and naphthalene degradation, effectively utilizes root exudate components. Mol Plant-Microbe Interact 15 734-741. 

Sanchez-Contreras M, M Martin, M Villacieros, F O Gara, I Bonilla, R Rivalla (2002) Phenotropic selection and phase variation occur during alfalfa root colonization by Pseudomonas fluorescens F113. J Bacterial 184 1587-1596. 

Yee DC, JA Maynard, TK Wood (1998) Rhizoremediation of trichloroethyelene by a recombinant, root-colonizing Pseudomonas fluorescens strain expressing ort/jo-monooxygenase constitutively. Appl Environ Microbiol 64 112-118. 

As already mentioned, molecular cross talk seems to be the prerequisite mechanism for most of root microbial infections. Indeed the initial step of any root colonization involves the movement of microbes to the plant root surface bacterial movement can be passive, via soil water flux, or active, via specific induction of flagellar activity by plant released compounds (chemotaxis) (Chaps. 4 and 7). Other important steps are adsorption and anchoring to the root surface. 

The use of microbial siderophores by dicotyledonous plants appears to involve uptake of the entire metallated chelate (42-44), or an indirect process in which the siderophore undergoes degradation to release iron (45). As demonstrated in initial studies examining this question, there was concern that iron uptake from microbial siderophores may be an artifact of microbial iron uptake in which radiolabeled iron is accumulated by root-colonizing microorganisms (46). Consequently, evidence for direct uptake of iron from microbial siderophores has required the use of axenic plants. In experiments with cucumber, it was shown that the microbial siderophore ferrioxamine B could be used as an iron source at concentrations as low as 5 pM and that the siderophore itself entered the plant (42). 

These data strongly suggest that siderophore production by root-colonizing microorganisms is induced only at a level neeessary to supplement that which is not provided by phytosiderophores and organic acids released during the plant iron stress response. Thus, the plant iron stress response may control iron availability to microorganisms in the rhizosphere. 

Initial studies examining the function of siderophores in the rhizosphere have focused on practical problems related to agricultural biotechnology. Early research suggested the involvement of siderophores in plant disease suppression by certain root-colonizing pseudomonads (2). Since then, it has become apparent that siderophore-mediated interactions are only part of the story of how microor-... 

P. Marschner, D. E. Crowley, and B. Sattelmaeher, Root colonization and iron nutritional status of a Pseudomonas fluore.scens in different plant species. Plant Soil 796 311 (1997). 

M. Maurhofer, C. Hase, P. Meuwly, J. P. Metraus, and G. Defago, Induction of systemic resistance of tobacco necrosis virus by the root colonizing Pseudomonas fluorescens strain CHAO Influence of the gacA gene and of pyoverdine production. Phytopathology 54 139 (1994). 

Changes in cell-wall protein composition may regulate the molecular architecture of protein networks in a manner that allows new developmental outcomes for both fungal cell adhesion and root colonization. Further investigation of the structure and regulation of SRAP wall proteins will provide a more complete picture of their role in developing ectomycorrhizal tissues. Incompatibility between ectomycorrhizal hyphae and the host roots detected during the initial con-... 

Jumpponen A, Trappe JM, Dark septate endophytes A review of facultative biotrophic root-colonizing fungi. New Phytol 140 295-310, 1998. 

Spontaneous antibiotic-resistant mutants of these organisms are suitable for initial root colonization studies however, transposon mutants will be more suitable for detailed laboratory and field studies. This procedure is useful for sorting out the genetic relationships of these organisms and for determining the mechanisms controlling toxin production. 

The effect of plant root exudation and exudation patterns on root colonization and expression of toxin production must be considered. For example, it may be important to determine the effect of root exudates from cold-stressed plants on these organisms, since the exudates apparently first appear just after the plants break winter dormancy (34). These data should provide information on root colonization potential, possible stimulation or reduction of toxin production, and mechanisms of plant resistance to the organisms. 

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