| Popis: |
Techniques for enumeration and specific detection using DNA and immunological probes have shown their efficiency for the study of colonization of bacterial populations on plant roots. However, direct observation is helpful for understanding the biophysical processes of their attachment to plant root surface and the resulting colonization. These observations are also used to evaluate the specifiCity of the interaction between plant roots and their associated micro-organisms. In this work, electronic microscope (EM) observations were used, besides plate counts, to evaluate root colonization of rice, maize and wheat by strains belonging to the enterobacterial species, Pantoea agglomerans and Rahnella aquatilis. The most interesting observations were obtained with the colonization of rice roots by P. agglomerans. Most of the strains belonging to this species are able to form cell aggregates named symplasmata when grown in a liquid medium. The nitrogen-fixing P. agglomerans strain NO30, isolated from the rhizosphere of rice, was inoculated into roots of axenically grown wheat and rice seedlings and were able to colonize the roots of both plants. The ability of NO30 cells to colonize plant roots seemed comparable in host and non-host plants, as far as colony forming unit (cfu) measurements were concerned. Nevertheless, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that, in the case of rice, the host plant for NO30, colonization was characterized by the formation of symplasmata, whereas only individual cells were detected on wheat roots. Symplasmata formation seems to be specific for colonization of the host plant (rice). This result also means that colonization of the host plant may be largely underestimated when measured by conventional techniques. Symplasmata formed in liquid medium or on roots of rice were stained using Thiery’s and Swift’s technique, and the presence of polysaccharides and proteins was revealed in the extracellular matrix as well as in fibrils anchoring symplasmata to other symplasmata or to plant cells. |
