The inability of the non-phosphate-solubilizing, non-rhizo-
sphere-competent isolate of M. olivasterospora to increase the
concentrationofsoilavailablePandtopromoteplantgrowthin
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 levelsofthePGRs 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
contrastswiththeresultsofBareaetal.(1976),Dattaetal.(1982)
and deFreitasetal. (1997)who found thattheirmostpromising
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
potentialoftheisolateswescreenedtosuppresstheactivitiesof
microorganisms deleterious to plant growth.