Heavy metal induced oxidative damage and root morphology alterations of maize (Zea mays L.) plants and stress mitigation by metal tolerant nitrogen fixing Azotobacter chroococcum

Abstract

Heavy metals are one of the major abiotic stresses that adversely affect the quantity and nutritive value of maize. Microbial management involving the use of plant growth promoting rhizobacteria (PGPR) is a promising inexpensive strategy for metal clean up from polluted soils. Considering these, metal tolerant plant growth promoting nitrogen fixing rhizobacterial strain CAZ3 identified by 16SrRNA gene sequence analysis as Azotobacter chroococcum was recovered from metal polluted chilli rhizosphere. When exposed to varying levels of metals, A. chroococcum survived up to 1400 and 2000 µg mL−1 of Cu and Pb, respectively and expressed numerous plant growth promoting activities even under metal stress. Strain CAZ3 secreted 65.5 and 60.8 µg mL−1 IAA at 400 µg mL−1 each of Cu and Pb, respectively and produced siderophores, ammonia and ACC deaminase under metal pressure. The melanin extracted from A. chroococcum revealed metal chelating ability under EDX. Following application, strain CAZ3 enhanced growth and yield of maize grown both in the presence of Cu and Pb. The dry biomass of roots of inoculated plants grown with 2007 mg Cu kg−1 and 585 mg Pb kg−1 was increased by 28% and 20%, respectively. At 585 mg Pb kg−1, the bioinoculant also increased the kernel attributes. At 2007 mg Cu kg−1 strain CAZ3 enhanced the number, yield and protein of kernels by 10%, 45% and 6%, respectively. Interestingly, strain CAZ3 significantly reduced the levels of proline, malondialdehyde and antioxidant enzymes in foliage. The roots of inoculated plants accumulated greatest amounts of metals compared to other organs. In kernels, the concentration of Pb was more as compared to Cu. The metal concentrations in roots, shoots and kernels, however, declined following CAZ3 inoculation. Copper and lead had substantial distortive impact on root and leaf morphology while cell death were visible under CLSM and SEM. Conclusively, A. chroococcum CAZ3 could be a most suitable and promising option to increase maize production in metal polluted soils despite the soils being contaminated with heavy metals.