β-Rhizobia from Mimosa pigra, a newly discovered invasive plant in Taiwan

A total of 191 rhizobial isolates from the root nodules of three geographically separate populations of the invasive plant Mimosa pigra in Taiwan were examined using amplified rDNA restriction analysis, 16S rDNA sequences, protein profiles and ELISA. Of these, 96% were identified as Burkholderia and 4% as Cupriavidus taiwanensis. The symbiosis-essential genes nodA and nifH were present in two strains of Burkholderia (PAS44 and PTK47), and in one of C. taiwanensis (PAS15). All three could nodulate M. pigra. Light and electron microscopy studies with a green fluorescent protein transconjugant variant of strain PAS44 showed the presence of fluorescent bacteroids in M. pigra nodules. These bacteroids expressed the nifH protein, hence this is the first confirmation that Burkholderia is a genuine symbiont of legume nodules. The predominance of Burkholderia in Taiwanese M. pigra suggests that this species may have brought its symbionts from its native South America, rather than entering into association with the Taiwanese Mimosa symbiont C. taiwanensis which so successfully nodulates Mimosa pudica and Mimosa diplotricha.