In situ localization of Paenibacillus polymyxa HKA-15 in roots and root nodules of soybean (Glycine max.L.)

Abstract

Bacterial endophytes can colonize various plants and organs. However, endophytic bacteria (other than rhizobia) colonizing root nodules in legumes have been rarely analyzed. The present study aimed to examine the colonization and spread of gfp-tagged Paenibacillus polymyxa in soybean plants under gnobiotic conditions. Inoculation with gfp-tagged Paenibacillus. polymyxa HKA −15 alone and in combination with Bradyrhizobium japonicum were done on soybean seedlings. In situ localization was detected through confocal microscopy and PCR. Inoculation with P. polymyxa-gfp strain alone and in combination with B. japonicum DS-1 had a stimulatory effect on the plant growth. There was an increase in shoot (7.2 %) and root dry weights (14.5 %) when the two strains were co - inoculated over that of B. japonicum inoculation alone. In vivo simultaneous visualization using Confocal Laser Scanning Microscopy (CLSM) showed the localization of the gfp-tagged P. polymyxa cells in the root nodules and its spread in the root tissue, both tap as well as lateral roots. Systemic spread into aerial tissue did not occur as indicated by the absence of bacteria. CLSM observations of the presence of gfp-tagged P. polymyxa in the nodule and roots tissues was corroborated with PCR amplification of the gfp-gene from these tissues. CLSM and PCR methods confirmed that P. polymyxa invades roots and root nodules of soybean, but the spread is restricted to root tissue only. The strain improves plant growth when inoculated singly or in combination with B. japonicum.