BACTERIAL ENDOPHYTES: ECOLOGICAL AND PRACTICAL IMPLICATIONS
Department of Forest Sciences, Vancouver, Canada
It has long been known that tissues of healthy plants can be colonized internally by microorganisms . The term 'endophyte' is commonly used to describe such microorganisms . The best-characterized microbial endophytes are nonpathogenic fungi, for which much compelling evidence of plant-microbe mutualism has been provided. Some endophytic fungi are thought to produce compounds that render plant tissues less attractive to herbivores, while other strains may increase host plant drought resistance. In return, fungal endophytes are thought to benefit from the comparatively nutrient rich, buffered environment inside plants. However, endophytic fungi comprise only part of the nonpathogenic microflora found naturally inside plant tissues. Bacterial populations exceeding 107 colony forming units (cfu) per gram of plant matter have been reported within tissues of various plant species. Notwithstanding their discovery more than four decades ago, much less is known about bacterial endophytes compared to their fungal counterparts. Work with plant species of agricultural and horticultural importance indicates that some endophytic bacterial strains stimulate host plant growth by acting as biocontrol agents, either through direct antagonism of microbial pathogens or by inducing systemic resistance to disease-causing organisms. Other endophytic bacterial strains may protect crops from plant parasitic nematodes and insects. In Brazil, the nitrogen-fixing bacterial endophytes of sugarcane (Saccharum officinarum 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 stimulate plant growth through mechanisms yet to be elucidated.
In contrast to agricultural crop species, almost nothing is known about bacterial endophytes of trees. Occasional reports of endophytic bacteria in asymptomatic angiosperm and gymnosperm species have been made, 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 spruce (Picea glauca X engelmannii) support bacterial endophyte populations naturally, and that such endophytes colonize internal root and stem tissues with up to 105 cfu per gram of plant tissue. Furthermore, some of these strains have been found to promote gymnosperm seedling growth. While the precise mechanism by which these bacterial endophytes enhance seedling growth is not completely understood, initial results suggest that biocontrol of indigenous soil microorganisms that inhibit plant growth is at least partly involved. In addition, an endophytic Bacillus strain (Pw2), which was originally isolated from inside surface-sterilized pine root tissues, possesses nitrogenase activity and can colonize pine seedlings systemically after soil inoculation. These observations lead to the intriguing possibility that lodgepole pine harbours an endophytic nitrogen-fixing bacterial population similar 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 their long-lived tree hosts under variable or deleterious environmental conditions (e.g., during periods of drought, nutrient deprivation, or pathogen attack). Regardless of the mechanism(s) involved, bacterial endophytes appear to represent another type of mutualistic plant-microorganism symbiosis that warrants further study. In addition to the intriguing ecological questions regarding the diversity, evolution and effects on plant population biology of bacterial endophytes, it may be fruitful to investigate their possible practical applications in agriculture and forestry.