Conclusions and Outlook

When studying endophytic bacteria, the microscopic proof of the endophytic localization, together with an in situ identification of the endophytic microbial agent either by immunological, molecular biological (FISH), or gene marker (GFP) techniques, is absolutely fundamental. The fact that certain bacteria were isolated from surface sterilized specimen is not sufficient and can be regarded only as a single step towards the characterization of a true endophytic association. The direct proof of endophytic localization becomes even more important, as endophytes may turn to unculturable but physiologically active forms inside the plant host as a result of a supposedly efficient adaptation to the plant environment. For this reason, many more yet unknown endophytic diazotrophs may exist inside plant tissues which await discovery by culture independent molecular methods, and finally, isolation as pure freeliving cultures.

In some bacterial species, a plant endophytic life style is quite common or even the rule (e.g., in some species of the genera Gluconacetobacter, Herbaspirillum or Azoarcus), while in other cases only specific strains have endophytic potential (like in Azospirillum). Therefore, it is important to specify clearly the bacterial strain and the matching plant partner down to the cultivar level. It becomes apparent that the plant partner plays a major role in allowing or prohibiting the entry and distribution of a bacterium within its organs. The recognition processes in bacteria-plant interactions are quite well understood in the case of plant pathogens. Possibly, similar key determinants such as the lipopolysaccharide and exopolysaccharide structures, the flagellin type, and the type of signaling molecules produced by the bacteria, e.g., for quorum sensing, are potential elicitors of the plant's innate immune response.

Productive endophytic interactions of procaryotic and eucaryotic organisms are called symbiotic systems, although the detailed mechanisms of the interactions remain unclear in many cases so far. Furthermore, it must be closely observed whether or not, in some cases or under certain circumstances, a usually advantageous endophytic interaction may turn out to be harmful to the plant host. The presence of potentially pathogenic factors in endophytic bacteria is certainly one important issue which has to be researched. Genomic information in major endophytic diazotrophs such as Azoarcus sp. strain BH72, Herbaspirillum seropedicae Z87, Klebsiella pneumoniae 342, Gluconacetobacter diazotrophicus PAL5, Azospirillum brasilense Sp245, A. lipoferum 4B and Pseudomonas stutzeri A1501 is either complete or well on its way to becoming complete. The genomic information will also provide information about potential human health risks of endophytic bacteria and about their extent of relatedness to opportunistic human pathogens (e.g., Burkholderia cepacia and Ochrobactrum antropi). The rhi-zosphere is known as a habitat which harbors a diversity of bacteria which are closely related to or even undistinguishable from human pathogens (Berg et al. 2005). Although biotechnological applications are very tempting in many cases, the risks of endophytes to develop into or act as plant or human pathogens under certain circumstances need to be evaluated very carefully.

Acknowledgements This chapter is dedicated to Dr. Johanna Döbereiner in honor of her great achievements concerning diazotrophic bacteria in non-leguminous plants, and her pathfinding ideas about endophytic diazotrophs in Gramineae.

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