Root colonization by host-specific rhizobacteria alters indigenous root endophyte and rhizosphere soil bacterial communities and promotes the growth of mandarin orange

Abstract

We assessed whether colonization of mandarin orange roots by host-specific plant growth promoting rhizobacteria (PGPR) can influence the composition of root endophyte, rhizosphere and non-rhizosphere bacterial communities and plant growth. The ability to colonize root by four host-specific PGPR, namely Enterobacter hormaechei RCE1, Enterobacter asburiae RCE2, Enterobacter ludwigii RCE5 and Klebsiella pneumoniae RCE7, was tested in axenically grown mandarin orange seedlings. The combined results of scanning and transmission electron microscopic images showed that PGPR inoculants successfully colonised on surface and within root tissues of axenic mandarin orange seedlings. The effect of root inoculation of 90 days old mandarin orange seedlings grown in natural soil with RCE1, RCE2, RCE5 and RCE7 either individually or as a consortium was evaluated in terms of plant biomass yield at 90, 180 and 360 days of post inoculation (dpi). Inoculation thus enhanced plant biomass yield over the uninoculated control. NMDS and hierarchical clustering of PCR DGGE fingerprints indicated that both the PGPR inoculation and plant age affected bacterial communities in root tissues and rhizosphere soils. Based on sequencing, the majority of the root endophyte and rhizosphere PCR-DGGE bands were from gammaproteobacteria, and the other were from Firmicutes and Deinococcus-Thermus. In conclusion, host-specific PGPR successfully colonized mandarin orange root tissue and rhizosphere soil and enhanced plant growth.