Biological control of plant bacterial pathogens by a beneficial Streptomyces sp.

In the present study a set of beneficial microorganisms were evaluated for
their in vitro antimicrobial activity on various phytopathogenic bacteria,
like Xanthomonas vesicatoria, Clavibacter michiganensis subsp.
michiganensis (Cmm), Clavibacter michiganensis subsp. sepedonicus
(Cms), Acidovorax citrulli and Ralstonia solanacearum. A prospective
antagonist, strain SA51 (Streptomyces sp.) was selected based on its remarkable
in vitro antibacterial activity and was further studied under
greenhouse conditions on tomato (Solanum lycopersicum) seedlings as
a model plant against X. vesicatoria. Fromin vitro results, it was clearly
evident that SA51 was very active against bacteria like Cmm, Cms, R.
solanacearum and X. vesicatoria compared to other antagonists. Tomato
plantlets were inoculated with SA51 at the roots and, thereafter, they were
sprayed with X. vesicatoria. Protection against X. vesicatoria by the bacterized
tomato plants was confirmed in the greenhouse: disease was reduced
by approximately 96%. Additionally, plants bacterized by strain SA51
showed significant plant growth, particularly in aerial parts as compared
to un-bacterized controls. Finally, benefit was seen in inoculated
healthy plants in terms of a significant increase in dry weight and length
of roots and shoots, as compared to the uninoculated controls. A GFP
mutant of strain SA51 was produced to study its endophytic colonisation
in tomato plants: results confirmed that SA51 was able to efficiently
colonise tomato endophytically, from the roots to the leaves. Field experiments
confirmed the ability of strain SA51 to act as plant growth promoting
agent: such promoting activity was also reflected into an increase
of fruit production by approximately 7%.