The inability of the non-phosphate-solubilizing, non-rhizosphere-competent isolate of M. olivasterospora to increase the
concentration of soil available P and to promote plant growth in
SP or PRP-amended soils compared to M. endolithica indicated
that it is likely that phosphate solubilization by M. endolithica
was responsible for the promotion of plant growth. It is
noteworthy that both M. endolithica and M. olivasterospora were
incapable ofproducing detectable level soft thePGRs in vitro and
both were also incapable of enhancing plant growth on their
own in the absence of added SP or PRP. This indicates clearly
that the major effect of M. endolithica plays in the greenhouse
studies is related to the solubilization of SP or PRP. This
contrasts with the result so fBarea et al.(1976),Datta et al.(1982)
and de Freitas et al. (1997) who found that their most promising
PSM also produced detectable levels of PGRs. Our study,
however, indicates while it may be desirable to have PGRs for
plant growth promotion, PGRs production is not a requirement
for effective P solubilization and plant growth promotion by M.
endolithica, however, PGRs other than those evaluated may also
be involved. Another aspect not included in our study was the
potential of the isolates we screened to suppress the activities of
microorganisms deleterious to plant growth.