The production of ACC by the target bacteria was also evaluated
because it is the precursor for ethylene synthesis in plant. Bacterial
ACC deaminase converts the ACC to ammonia and a-ketobutyrate,
thereby lowering ethylene levels in inoculated plants (Glick et al.,
1998). The lowering of ethylene levels is essential when plants are
exposed to environmental stressors as drought (Glick, 2004).
Bacillus sp. was the most drought sensitive bacteria and it produced
the highest ACC-deaminase and proline accumulation under stress
conditions. Both compounds would account for the compensation
of the bacterial lack of stress tolerance (40% of PEG addition).
However, this bacterial strain changed very little the APX and CAT
activities and lipid peroxidation (MDA) under the stress conditions
tested. These antioxidant bacterial activities play an important role
facilitating the removal of free radicals (Wang et al., 2007). Perhaps
in this bacterial strain the low reaction of these antioxidant
activities were compensated by the contribution of high PHB and/or ACC-deaminase production in alleviating cell osmotic stress.
Nevertheless, the low survival of this bacterial strain under 40%
PEG is contrasting with its abilities to synthesize these compounds.