To elucidate the biodiversity of plant growth-promoting
rhizobacteria (PGPR) in Korea, 7,638 bacteria isolated
from the rhizosphere of plant species growing in many
different regions were screened. A large number of PGPR
were identified by testing the ability of each isolate
to promote the growth of cucumber seedlings. After
redundant rhizobacteria were removed via amplified
rDNA restriction analysis, 90 strains were finally selected
as PGPR. On the basis of 16S ribosomal RNA sequences,
68 Gram-positive (76%) and 22 Gram-negative (24%)
isolates were assigned to 21 genera and 47 species. Of
these genera, Bacillus (32 species) made up the largest
complement, followed by Paenibacillus (19) and Pseudomonas
(11). Phylogenetic analysis showed that most of the Grampositive
PGPR fell into two categories: low- and high-
G+C (Actinobacteria) strains. The Gram-negative PGPR
were distributed in three categories: α-proteobacteria, β-
proteobacteria, and γ-proteobacteria. To our knowledge,
this is the largest screening study designed to isolate
diverse PGPR. The enlarged understanding of PGPR genetic
diversity provided herein will expand the knowledge base
regarding beneficial plant-microbe interactions. The
outcome of this research may have a practical effect on
crop production methodologies.