Plants in terrestrial systems have evolved in direct association with microbes functioning as both agonists and antagonists of plant fitness and adaptability. As such, investigations that segregate plants and microbes provide only a limited scope of the biotic interactions that dictate plant community structure and composition in natural systems. Invasive plants provide an excellent working model to compare and contrast the effects of microbial communities associated with natural plant populations on plant fitness, adaptation, and fecundity. The last decade of DNA sequencing technology advancements opened the door to microbial community analysis, which has led to an increased awareness of the importance of an organism's microbiome and the disease states associated with microbiome shifts. Employing microbiome analysis to study the symbiotic networks associated with invasive plants will help us to understand what microorganisms contribute to plant fitness in natural systems, how different soil microbial communities impact plant fitness and adaptability, specificity of host-microbe interactions in natural plant populations, and the selective pressures that dictate the structure of above-ground and below-ground biotic communities. This review discusses recent advances in invasive plant biology that have resulted from microbiome analyses as well as the microbial factors that direct plant fitness and adaptability in natural systems.

The soil microbial community acts as a reservoir of microbes that directly influences the structure and composition of the aboveground plant community, promotes plant growth, increases stress tolerance and mediates local patterns of nutrient cycling. Direct interactions between plants and rhizosphere-dwelling microorganisms occur at, or near, the surface of the root. Upon introduction and establishment, invasive plants modify the soil microbial communities and soil biochemistry affecting bioremediation efforts and future plant communities. Here, we used tag-encoded FLX amplicon 454 pyrosequencing (TEFAP) to characterize the bacterial and fungal community diversity in the rhizosphere of Berberis thunbergii DC. (Japanese barberry) from invasive stands in coastal Maine to investigate effects of soil type, soil chemistry and surrounding plant cover on the soil microbial community structure. Acidobacteria, Actinobacteria, Proteobacteria and Verrucomicrobia were the dominant bacterial phyla, whereas fungal communities were comprised mostly of Ascomycota and Basidiomycota phyla members, including Agaricomycetes and Sordariomycetes. Bulk soil chemistry had more effect on the bacterial community structure than the fungal community. An effect of geographic location was apparent in the rhizosphere microbial communities, yet it was less significant than the effect of surrounding plant cover. These data demonstrate a high degree of spatial variation in the rhizosphere microbial communities of Japanese barberry with apparent effects of soil chemistry, location and canopy cover on the microbial community structure.

The phytophagous fruit fly Rhagoletis meigenii harbors the bacterium Wolbachia pipientis and, together with Japanese barberry, form a tri-partite symbiosis. R. meigenii is a seed predator of invasive Japanese barberry plants and is dependent on this insect-plant interaction for reproductive success. The endosymbiotic bacterium W. pipientis is a reproductive parasite known to alter the sex ratios of offspring and the fitness of infected host insects. We investigated Japanese barberry fruit for the degree of infestation by R. meigenii and characterized the Wolbachia strain infecting R. meigenii. Densities of R. meigenii in four naturalized stands of Japanese barberry revealed low numbers of fruit flies with high variability in the population densities observed among individual plants. Overall, R. meigenii infested roughly 10–20 % of the Japanese barberry fruits analyzed; fruit with two seeds (vs. one seed) were the most frequently infested. Approximately, 90 % of the R. meigenii tested positive for Wolbachia infection via PCR amplification of the Wolbachia surface protein (wsp) gene. No bacterial strain diversity was observed when comparing multi-locus sequence typing (MLST) profiles within or among five R. meigenii populations in Maine, although the MLST profile obtained from R. meigenii differed from three co-occurring Rhagoletis. The Wolbachia endosymbiont of R. meigenii is a member of the Wolbachia supergroup A and the ST-13 cluster complex.

A survey of the Maine landscape and nursery industry was conducted to identify industry views on invasive plant issues, attitudes towards potential regulation, and to estimate the potential economic costs of banning the sale of specific invasive plant species in Maine. Analysis of the 190 surveys returned (19% of 980 mailed) revealed that 76% of industry member respondents were genuinely concerned about invasive plant issues, and the same percentage felt the horticulture industry is responsible for educating customers about invasive plants. Industry members (68%) did not feel compelled to sell invasive plants merely on the basis of customer attraction to the plant, or due to competition with a neighboring business that sells the invasive plant. Self-reporting of sales indicated that Norway maple ($96K), burningbush ($68K), and Japanese barberry ($44K) constituted the largest portion of annual industry revenue (maximum values reported for 2006 to 2008) derived from the sale of seven identified invasive plants. Industry self-regulation was the most favored form of regulation, although the industry likely would not be significantly affected by legislated state-wide bans of at least purple loosestrife and oriental bittersweet. Bans on other popular invasive plants, including burningbush, Japanese barberry, and Norway maple likely would have a relatively small, short-term impact on the horticulture industry until alternative plants with similar properties were identified. The results of this survey demonstrated a need for identifying which plants are truly invasive or potentially invasive in Maine, as well as a need for open discussion of invasive plant issues among all interested parties in Maine.

Frankia strain CcI3 produces 2 truncated hemoglobins, HbN and HbO. Using ion-exchange chromatography, we characterized the expression of the relative amounts of HbN and HbO in −N (nitrogen-fixing) cultures and +N (nitrogen-supplemented) cultures. The −N cultures maintained an approximately constant ratio of HbO to HbN throughout the life of the culture, with HbO constituting 80%–85% of the total hemoglobin produced. In contrast, in +N cultures, HbN was observed to increase over time and HbO decreased. Total hemoglobin as a fraction of total protein was approximately constant throughout the growth phase in −N cultures, while it decreased somewhat in +N cultures. Subjecting −N cultures to a NO generator resulted in increased production of HbN, relative to the controls. Nitrite accumulated in +N cultures, but not in −N cultures. This suggests that the greater amount of HbN in +N cultures might be due to NO produced by the reduction of nitrite. The effects of O2 concentration were determined in +N cultures. Cultures grown in 1% O2 produced about 4 times more HbO than cultures grown in 20% O2. Overall, these results provide evidence for a role of HbN in NO oxidation and for a role of HbO in adaptation to low oxygen concentrations.