Metal-tolerant morganella morganii isolates can potentially mediate nickel stress tolerance in Arabidopsis by upregulating antioxidative enzyme activities

Abstract

Plant growth-promoting rhizobacteria (PGPRs) have been utilized to immobilize heavy metals, limiting their translocation in metal contaminated settings. However, studies on the mechanisms and interactions that elucidate how PGPRs mediate Nickel (Ni) tolerance in plants are rare. Thus, in this study we investigated how two pre-characterized heavy metal tolerant isolates of Morganella morganii (ABT9 and ABT3) improve Ni stress tolerance in Arabidopsis while enhancing its growth and yield. Arabidopsis seedlings were grown for five weeks in control/Ni contaminated (control, 1.5 mM and 2.5 mM) potted soil, in the presence or absence of PGPRs. Plant growth characteristics, quantum yield, and antioxidative enzymatic activities were analyzed to assess the influence of PGPRs on plant physiology. Oxidative stress tolerance was quantified by measuring MDA accumulation in Arabidopsis plants. As expected, Ni stress substantially reduced plant growth (shoot and root fresh weight by 53.25% and 58.77%, dry weight by 49.80% and 57.41% and length by 47.16% and 64.63% over control), chlorophyll content and quantum yield (by 40.21% and 54.37% over control). It also increased MDA content by 84.28% at higher (2.5 mM) Ni concentrations. In contrast, inoculation with M. morganii led to significant improvements in leaf chlorophyll,­quantum­yield,­and­Arabidopsis­biomass­production.­The­mitigation­of­adverse­effects­of­Ni­stresson­biomass­observed­in­M. morganii-inoculated­plants­was­attributed­to­the­enhancement­of­antioxidative­ enzyme­ activities­ compared­ to­ Ni-treated­ plants.­ This­ upregulation­ of­ the­ antioxidative­ defensemechanism­ mitigated­ Ni-induced­ oxidative­stress,­ leading­to­ improved­ performance­ of­the­ photosynthetic­machinery,­which,­ in­turn,­enhanced­ chlorophyll­ content­ and­quantum­yield.­Understanding­theunderlying­mechanisms­of­these­tolerance-inducing­processes­will­help­to­complete­the­picture­of­PGP s-­ mediated­defense­signaling.­Thus,­it­suggests­that­M. morganii PGP s­candidate­can­potentially­be­utilizedfor­plant­growth­promotion­by­reducing­oxidative­stress­via­upregulating­antioxidant­defense­systems­inNi-contaminated­soils­and­reducing­Ni­metal­uptake.