Bacterial endophytes: ecological and practical implications

It has long been known that tissues of healthy plants can be colonized inte rnally by microorganisms. The term "endophyte" is commonly used to describe such microorganisms. The best-characterized microbial endophytes are nonpa thogenic fungi, for which much compelling evidence of plant/microbe mutuali sm has been provided. Some endophytic fungi are thought to produce compound s that render plant tissues less attractive to herbivores, while other stra ins may increase host plant drought resistance. In return, fungal endophyte s are thought benefit from the comparatively nutrient rich, buffered enviro nment inside plants. However, endophytic fungi comprise only part of the no npathogenic microflora found naturally inside plant tissues. Bacterial popu lations exceeding 10(7) colony forming units (cfu) g(-1) plant matter have been reported within tissues of various plant species. Notwithstanding thei r discovery more than four decades ago, much less is known about bacterial endophytes compared to their fungal counterparts. Work with plant species o f agricultural and horticultural importance indicates that some endophytic bacterial strains stimulate host plant growth by acting as biocontrol agent s, either through direct antagonism of microbial pathogens or by inducing s ystemic resistance to disease-causing organisms. Other endophytic bacterial strains may protect crops from plant parasitic nematodes and insects. In B razil, the nitrogen-fixing bacterial endophytes of sugarcane (Saccharum off icinarun L.), Acetobacter diazotrophicus and Herbaspirillum spp., colonize internal root, stem and leaf tissues, and are thought to provide up to 80% of the host plant's nitrogen requirement. Other endophytic bacteria stimula te plant growth through mechanisms yet to be elucidated. In contrast to agricultural crop species, almost nothing is known about bac terial endophytes of trees. There have been occasional reports of endophyti c bacteria in asymptomatic angiosperm and gymnosperm species, but little is known about their influence on plant growth. We have found that lodgepole pine (Pinus contorta var, latifolia Engelm.) and white x Engelmann hybrid s pruce (Picea glauca x engelmannii) support bacterial endophyte populations naturally, and that such endophytes colonize internal root and stem tissues with up to 10(5) cfu g(-1) plant tissue. Furthermore, some of these strain s have been found to promote gymnosperm seedling growth. While the precise mechanism by which these bacterial endophytes enhance tree seedling growth is not completely understood, initial results suggest that biocontrol of in digenous soil microorganisms that inhibit plant growth is at least partly i nvolved. In addition, an endophytic Bacillus strain (Pw2), which was origin ally isolated from inside surface-sterilized pine root tissues, possesses n itrogenase activity and can colonize pine seedlings systemically after soil inoculation. These observations lead to the intriguing possibility that lo dgepole pine harbors an endophytic nitrogen-fixing bacterial population sim ilar to that of sugarcane, which would explain its ability to grow and even thrive, under nitrogen deficient conditions in the absence of significant rhizospheric nitrogen fixation. Bacterial endophytes may also be important in forest ecosystems by effectively increasing phenotypic plasticity of the ir long-lived tree hosts under variable or deleterious environmental condit ions (e.g., during periods of drought, nutrient deprivation, or pathogen at tack). Regardless of the mechanism(s) involved, bacterial endophytes appear to represent another type of mutualistic plant x microorganism symbiosis t hat warrants further study. In addition to the intriguing ecological questi ons regarding the diversity, evolution and effects on plant population biol ogy of bacterial endophytes, it may be fruitful to investigate their possib le practical applications in agriculture and forestry.