Isolated polynucleotides are provided. Each of the isolated polynucleotides comprise a nucleic acid sequence encoding a polypeptide having an amino acid sequence at least 80% homologous to SEQ ID NO: 121, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 122, 123, 124, 125, 126, 95 or 96, wherein the polypeptide is capable of regulating cotton fiber development. Also provided are methods of using such polynucleotides for improving fiber quality and/or yield of a fiber producing plant, as well as methods of using such polynucleotides for producing plants having increased biomass/vigor/yield.