Abstract: Provided are isolated polynucleotides which are at least 80% homologous to SEQ ID NO: 75, 1-74, 76-473, 783-1272, 1277-4139, 4142, 4146-5508, or 5509; and isolated polypeptides which are at least 80% homologous to 548, 474-547, 549-562, 564-620, 622-750, 752-782, 5510-5939, 5946-6856, 6858-7540, 7543, 7544, 7548-8735, or 8736, nucleic acid constructs comprising the isolated polynucleotides, transgenic plants expressing same and methods of using same for increasing abiotic stress tolerance, yield, biomass, growth rate, vigor, oil content, fiber yield, fiber quality, and/or nitrogen use efficiency of a plant.