Plant-Growth-Promoting Bacteria Can Impact Zinc Uptake in Zea mays: An Examination of the Mechanisms of Action Using Functional Mutants of Azospirillum brasilense

Among the PGPB, the genus Azospirillum—with an emphasis on A. brasilense—is likely the most studied microorganism for mitigation of plant stress. Here, we report the investigation of functional mutants HM053, ipdC and FP10 of A. brasilense to understand how the biological functions of these microorganisms can affect host Zn uptake. HM053 is a Nif (+) constitutively expressed strain that hyper-fixes N(2) and produces high levels of the plant’s relevant hormone auxin. FP10 is a Nif(-) strain deficient in N(2)-fixation. ipdC is a strain that is deficient in auxin production but fixes N(2). Zn uptake was measured in laboratory-based studies of 3-week-old plants using radioactive (65)Zn(2+) (t(½) 244 days). Principal Component Analysis was applied to draw out correlations between microbial functions and host (65)Zn(2+) accumulation. Additionally, statistical correlations were made to our prior data on plant uptake of radioactive (59)Fe(3+) and (59)Fe(2+). These correlations showed that low microbial auxin-producing capacity resulted in the greatest accumulation of (65)Zn. Just the opposite effect was noted for (59)Fe where high microbial auxin-producing capacity resulted in the greatest accumulation of that tracer.