Provided are isolated polypeptides which are at least 80% homologous to SEQ ID NOs: 496-794, 2898-3645, and 3647-4855, isolated polynucleotides which are at least 80% identical to SEQ ID NOs: 1-495 and 795-2897, nucleic acid constructs comprising same, transgenic cells expressing same, transgenic plants expressing same and method of using same for increasing fertilizer use efficiency, nitrogen use efficiency, yield, growth rate, biomass, vigor, oil content, photosynthetic capacity, seed yield, fiber yield, fiber quality, fiber length, and/or abiotic stress tolerance of a plant.