Biosynthesized metal oxide nanoparticles are used as nano-fertilizers for sustainable agriculture as they have proven to be promising agents in increasing the germination rates and plant growth rate. Biosynthesis of copper oxide nanoparticles (CuO-NPs) was done for the first-time using extract of Mangifera indica leaves. Effects of as-synthesized CuO NPs on the seed germination of two legume seeds are investigated at different concentrations (0 - 2.5 mg/ml). UV-Vis and EDX analysis confirm the formation of CuO NPs & FESEM images revealed spherical shape of NPs with particle size ranging from 105nm to 155nm. CuO-NPs also revealed to be highly stable in aqueous suspension with zeta potential value -21.1mV. Germination rate, root /shoot growth and protein estimated of Cicer arietinum and Vigna radiata seeds found to be highest at 2.5mg/ml and 1mg/ml concentration, respectively. Negative impact on germination rate and root/shoot growth was observed due to toxic effects when CuO-NPs were applied at higher concentration 2.5mg/ml to Vigna radiata seeds. Thus, it is concluded that optimum concentration of biosynthesized CuO-NPs can be used to enhance the growth of leguminous seeds because of their possible interaction with the proteins and their up-regulation as confirmed by bioinformatics studies and molecular docking of protein.