Dual nanozyme characteristics of iron oxide nanoparticles alleviate salinity stress and promote the growth of an agroforestry tree, Eucalyptus tereticornis Sm.

Abstract

Nanomaterials possessing enzyme-like activities have shown immense potential to improve plants' performance under unfavorable conditions such as abiotic stresses. Although several strategies for evaluating nano–bio interactions have been evidenced to enhance the productivity of crop/fruit-based plant species, no such attempts have been made on agroforestry trees. Herein, an abiotic stress management approach using iron oxide nanoparticles (IONPs) is demonstrated for the first time in Eucalyptus tereticornis against a very high salinity concentration, 300 mM NaCl, known to cause inhibitory effects. Supplementing the microshoots of E. tereticornis with an optimized, 25 ppm IONP dose resulted in distinct biochemical changes in superoxide dismutase (∼3.8-fold rise in activity), malondialdehyde (lowered by ∼33%) concentration, and total soluble sugar (∼1.9-fold rise) and proline (11.8-fold rise) content, the biomarkers known to circumvent the stress response. The molecular mechanism of salt-responsive genes encoding HKT1, SOS1 and NHX1 indicated that IONPs can up-regulate their transcript levels either by the efflux of Na⁺ ions from the cell or by their sequestration in vacuole under stress conditions. Systematic analysis of morpho/physiological growth parameters also showed a remarkable increase in shoot length and the chlorophyll content by ∼2.5-fold and 128.5%, respectively, as compared to the control, under similar test conditions. Even under non-stressed conditions, IONPs acted as nano-supplements for promoting the shoot growth of E. tereticornis by increasing the gene expression of various antioxidant enzymes and synergistically improved the activity of catalase and peroxidase enzymes. This study strengthens our belief that nanomaterials could potentially be employed as a tool to enhance the performance of agroforestry trees under abiotic stress conditions. Furthermore, the impact of IONPs to replenish iron deficiency in soils concurrently with a superior salt stress tolerance could pave new pathways for managing the abiotic stresses in agroforestry.