Serratia sp. CP-13 alleviates Cd toxicity by morpho-physio-biochemical improvements, antioxidative potential and diminished Cd uptake in Zea mays L. cultivars differing in Cd tolerance

Cadmium (Cd) is highly toxic for plant metabolic processes even in low concentration due to higher retention rates, longer half-life and non-biodegradable nature. The current study was designed to assess the bioremediation potential of Cd tolerant PGPR, Serratia sp. CP-13 together with two differentially Cd tolerant maize cultivars (MMRI-Yellow, Sahiwal-2002) selected amongst ten cultivars after screening. The maize cultivars were grown under different Cd treatments (0, 6, 12, 18, 24, 30 µM) in Petri plates both with and without Serratia sp. CP-13 inoculation. Treated plants were analyzed for their biomass accumulation, chlorophylls, carotenoids, proline, anthocyanin, protein, malondialdehyde (MDA), H₂O₂ as well as for antioxidants (POD, SOD, CAT) and mineral elements (Ca, Mg, Zn, K, Fe, Na, Cd). The maize cultivar MMRI-Yellow (tolerant) and Sahiwal-2002 (sensitive) exhibited significant reduction in leaf area, nutrient contents, plant biomass, activity of antioxidants, total proteins, photosynthetic pigments as well as flavonoids with increased production of H₂O₂, proline, MDA and relative membrane permeability (RMP) under Cd stress. However, this reduction was cultivar specific and recorded higher in cv. Sahiwal-2002 as compared to MMRI-Yellow. Application of Serratia sp. CP-13 significantly augmented plant biomass, photosynthetic pigments, antioxidative machinery, as well as flavonoids and proline while diminishing H₂O₂, RMP MDA production even under Cd stress in studied cultivars. Furthermore, CP-13 inoculation assisted the Cd stressed plants to sustain an optimal level of essential nutrients (Ca, Mg, Zn, K, Fe) except for Na and Cd which responded antagonistically. It was inferred that both inoculated maize cultivars exhibited better health and metabolism but substantial Cd tolerance was acquired by the sensitive cv. Sahiwal-2002 than the tolerant cv. MMRI-Yellow under applied Cd regimes. Furthermore, studied maize cultivars depicted maximum Cd tolerance in order of 30 < 24 < 18 < 12 < 6 < 0 µM Cd treatments under Serratia sp. CP-13 inoculation. Findings of current work highlighted the importance of Serratia sp. CP-13 and its inoculation impact on morpho-physio-biochemical attributes of maize growth under Cd dominant environment, which is likely an addition towards efficient approaches for bacterially-assisted Cd bioremediation and minimal Cd retention in edible plant parts.