Alleviating Effects of Serendipita indica and Nitric Oxide in Proso Millet Plants (Panicum miliaceum L.) Exposed to Copper Toxicity

This study focuses on potential role of Serendipita indica colonization and exogenous nitric oxide (NO) in abating copper (Cu) toxicity in proso millet (Panicum miliaceum L.) plants. S. indica-inoculated seedlings were subjected to 200 mg Cu kg−1 soil and spray of 150 µM sodium nitroprusside (SNP, as NO donor) under greenhouse conditions. Biomass production, photosynthetic parameters, copper content, antioxidant enzyme activities, proline content, and oxidative stress biomarkers were determined. The root and shoot Cu concentrations were significantly increased by supply of Cu to the soil. The root Cu content was higher than the shoot in all treatments. Application of S. indica and exogenous NO increased Cu retention in the roots and decreased its content in the shoots. Proso millet plants subjected to Cu toxicity showed reduced chlorophyll content, retarded photosynthetic activity, and stunted growth. Compared with control plants, the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) enzymes were elevated in Cu-exposed plants. Applied NO and S. indica inoculation further boosted SOD, CAT, and POD activities. The oxidative damage in stressed plants was signposted by the elevated content of hydrogen peroxide (H2O2) and malondialdehyde (MDA). The presence of fungus or exogenous application of NO led to a decrease in H2O2 and MDA content of Cu-treated plants, which was enhanced up by the combination of NO + S. indica. The Cu-stressed plants treated with NO and S. indica showed greater chlorophyll content, elevated photosynthetic activity, improved biomass, and enhanced metal tolerance index under Cu-toxic condition. The results provide evidence that colonization of S. indica along with exogenous NO synergistically interact to alleviate Cu toxicity in proso millet plants.