Plant growth and productivity is negatively affected by soil salinity. However, it is predicted that plant
growth-promoting bacterial (PGPB) endophytes that contain 1-aminocyclopropane-1-carboxylate (ACC)
deaminase (E.C. 4.1.99.4) can facilitate plant growth and development in the presence of a number of
different stresses. In present study, the ability of ACC deaminase containing PGPB endophytes Pseudomonas
fluorescens YsS6, Pseudomonas migulae 8R6, and their ACC deaminase deficient mutants to
promote tomato plant growth in the absence of salt and under two different levels of salt stress (165 mM
and 185 mM) was assessed. It was evidence that wild-type bacterial endophytes (P. fluorescens YsS6 and
P. migulae 8R6) promoted tomato plant growth significantly even in the absence of stress (salinity).
Plants pretreated with wild-type ACC deaminase containing endophytic strains were healthier and grew
to a much larger size under high salinity stress compared to plants pretreated with the ACC deaminase
deficient mutants or no bacterial treatment (control). The plants pretreated with ACC deaminase containing
bacterial endophytes exhibit higher fresh and dry biomass, higher chlorophyll contents, and a
greater number of flowers and buds than the other treatments. Since the only difference between wildtype
and mutant bacterial endophytes was ACC deaminase activity, it is concluded that this enzyme is
directly responsible for the different behavior of tomato plants in response to salt stress. The use of PGPB
endophytes with ACC deaminase activity has the potential to facilitate plant growth on land that is not
normally suitable for the majority of crops due to their high salt contents