Towards enhancing phytoremediation: The effect of syringic acid, a plant secondary metabolite, on the presence of phenoxy herbicide-tolerant endophytic bacteria

Among emerging pollutants, residuals of phenoxy herbicides, including 2-chloro-4-methylphenoxy acid (MCPA), are frequently detected in non-targeted areas. MCPA can be removed from environmental matrices using biological remediation methods including endophyte-assisted phytoremediation. The interactions between selected plants excreting to the rhizosphere plant secondary metabolites (PSMs) and plant-associated bacteria (incl. endophytes) can speed up the removal of organics and increase the plants resistance to pollutants such as MCPA. The role of plant-associated bacteria in endophyte-assisted phytoremediation has been partially described, however neither MCPA-tolerant endophytic bacteria has been isolated nor characterized. So far, promising results were obtained by simultaneous cultivation of Cucurbita pepo (zucchini) and amendment of soil with structurally related PSM syringic acid (SA), which can substantially enhance removal of MCPA from soil. Hence, the main aim of this research was to study the effect of PSM (SA) on the presence of functional MCPA-tolerant endophytic bacteria using a culture-dependent and -independent approach. Comparison between the molecular and microbiological analysis revealed differences between applied methods. However, irrespectively of the genera identification methods, presence of phenolic compounds (MCPA or SA) favorized presence of potential MCPA-degraders. On the basis of MCPA tolerance tests of isolated bacteria, two Pseudomonas endophytic isolates from zucchini roots and three isolates from zucchini leaves i.e. Pseudomonas sp., Paenarthrobacter sp. and Acinetobacter sp. were selected for further screening of plant growth promoting properties (PGPP). MCPA-tolerant endophytic bacteria showed multiple PGPP. Therefore, these isolates can potentially contribute to an improved fitness of plants used for the purpose of enhancing phytoremediation of environments polluted with phenoxy herbicides.