While many of the studies on PSM have concentrated on
the bulk soil, only a few (Reyes et al., 2002; Harris et al., 2006)
have focused on their potential for activity in the rhizosphere.
Rhizosphere competence is clearly an attribute necessary for
seedinoculationofanyPGPR(Benizrietal.,2001).Weller(1988)
described a good root colonizing strain as one which can
colonize the whole root system and survive during several
weeks in the presence of the natural microflora. Our
experiments were conducted in natural soil and revealed
that five of the nine phosphate-solubilizing isolates studied
were good root colonizers and showed variable levels of root
colonization, with M. endolithica being the strongest. These
isolates were found in the rhizosphere at all depths of the
roots, but population densities were greater in the first 8 cm of
the root system compared to other root depths. The relative
abundanceofactinomycetesin thefist8 cmsupportstheearly
observations (e.g. Waksman and Purvis, 1932) that actinomy-
cetes occur in greatest numbers in the top few inches of the
soil and decrease with depth.