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.