BACTERIAL ENDOPHYTES IN AGRICULTURAL CROPS

Endophytic bacteria are ubiquitous in most plant species, residing lat ently or actively colonizing plant tissues locally as well as systemic ally. Several definitions have been proposed for endophytic bacteria; in this review endophytes will be defined as those bacteria that can b e isolated from surface-disinfested plant tissue or extracted from wit hin the plant, and that do not visibly harm the plant. While this defi nition does not include nonextractable endophytic bacteria, it is a pr actical definition based on experimental limitations and is inclusive of bacterial symbionts, as well as internal plant-colonizing nonpathog enic bacteria with no known beneficial or detrimental effects on colon ized plants. Historically, endophytic bacteria have been thought to be weakly virulent plant pathogens but have recently been discovered to have several beneficial effects on host plants, such as plant growth p romotion and increased resistance against plant pathogens and parasite s. In general, endophytic bacteria originate from the epiphytic bacter ial communities of the rhizosphere and phylloplane, as well as from en dophyte-infested seeds or planting materials. Besides gaining entrance to plants through natural openings or wounds, endophytic bacteria app ear to actively penetrate plant tissues using hydrolytic enzymes like cellulase and pectinase. Since these enzymes are also produced by path ogens, more knowledge on their regulation and expression is needed to distinguish endophytic bacteria from plant pathogens. In general, endo phytic bacteria occur at lower population densities than pathogens, an d at least some of them do not induce a hypersensitive response in the plant, indicating that they are not recognized by the plant as pathog ens. Evolutionarily, endophytes appear to be intermediate between sapr ophytic bacteria and plant pathogens, but it can only be speculated as to whether they are saprophytes evolving toward pathogens, or are mor e highly evolved than plant pathogens and conserve protective shelter and nutrient supplies by not killing their host. Overall, the endophyt ic microfloral community is of dynamic structure and is influenced by biotic and abiotic factors, with the plant itself constituting one of the major influencing factors. Since endophytic bacteria rely on the n utritional supply offered by the plant, any parameter affecting the nu tritional status of the plant could consequently affect the endophytic community. This review summarizes part of the work being done on endo phytic bacteria, including their methodology, colonization, and establ ishment in the host plant, as well as their role in plant-microbe inte ractions. In addition, speculative conclusions are raised on some poin ts to stimulate thought and research on endophytic bacteria.