Recent evidence suggests that plant-microbe interactions may play a large role in the success of biological invasions. Pink-pigmented facultative methylotrophic bacteria (PPFMs, Methylobacterium) are mutualists associated with the roots, leaves and seeds of most terrestrial plants. PPFMs utilize single carbon compounds (C₁) generated by respiring plants, such as methanol and formaldehyde. Plants harboring PPFMs benefit from the removal of C₁ compounds that can otherwise build up to damaging levels. Moreover, PPFMs enhance the germination, growth rates and productivity of plants by excreting plant growth hormones and can confer drought and pathogen resistance. To date, most work on plant-microbial mutualisms in exotic plant invasions has focused on N-fixers and mychorrizal fungi. Here our goal was to identify whether exotic invasion intensity and/or plant species correlated with PPFM abundance in California coastal sage scrub (CSS). CSS is home to over 100 endangered or threatened plant and animal species and does not recover well after invasion. We surveyed PPFMs from rhizospheres of five native plant species and five exotics along ten different invasion gradients in Southern California CSS. Each gradient extended from areas where natives were still intact to areas completely invaded. We assessed the abundance of PPFMs in rhizosphere soil using Most Probable Number analysis (MPN).

Results/Conclusions

Based on the MPN analysis, one particularly problematic invasive species in CSS, Hirschfeldia incana (mustard), appears to be associated with high levels of PPFMs. Our results suggest that some exotic plant species may often be associated with high PPFMs and that invasions may increase PPFMs at intermediate levels. The mutualism between PPFMs and exotics could accelerate invasion, and additional PPFMs may confer a relative disadvantage to the native species. Since H. incana (and other mustard species) is known not to form associations with mychorrizae, their strong mutualism with PPFMs may represent an alternative mechanism of mustard invasion.