Phytosterols play an important role in plant growth and development,including cell division,cell elongation,embryogenesis,cellulose biosynthesis,and cell wall formation.Cotton fiber,which undergoes synchronous cell elongation and a large amount of cellulose synthesis,is an ideal model for the study of plant cell elongation and cell wall biogenesis.The role of phytosterols in fiber growth was investigated by treating the fibers with tridemorph,a sterol biosynthetic inhibitor.The inhibition of phytosterol biosynthesis resulted in an apparent suppression of fiber elongation in vitro or in planta.The determination of phytosterol quantity indicated that sitosterol and campesterol were the major phytosterols in cotton fibers;moreover,higher concentrations of these phytosterols were observed during the period of rapid elongation of fibers.Furthermore,the decrease and increase in campesterol:sitosterol ratio was associated with the increase and decease in speed of elongation,respectively,during the elongation stage.The increase in the ratio was associated with the transition from cell elongation to secondary cell wall synthesis.In addition,a number of phytosterol biosynthetic genes were down-regulated in the short fibers of ligon lintless-1mutant,compared to its near-isogenic wild-type TM-1.These results demonstrated that phytosterols play a crucial role in cotton fiber development,and particularly in fiber elongation. Phytosterols play an important role in plant growth and development, including cell division, cell elongation, embryogenesis, cellulose biosynthesis, and cell wall formation. Cotton fiber, which undergoes synchronous cell elongation and a large amount of cellulose synthesis, is an ideal model for the study of plant cell elongation and cell wall biogenesis. The role of phytosterols in fiber growth was investigated by treating the fibers with tridemorph, a sterol biosynthetic inhibitor. inhibition by phytosterol biosynthesis resulted in an apparent suppression of fiber elongation in vitro or in planta. The determination of phytosterol quantity indicated that sitosterol and campesterol were the major phytosterols in cotton fibers; moreover, higher concentrations of these phytosterols were observed during the period of rapid elongation of fibers. Furthermore, the decrease and increase in campesterol: sitosterol ratio was associated with the increase and decease in speed of elongation, respectively, durin g the elongation stage. increase in the ratio was associated with the transition from cell elongation to secondary cell wall synthesis. In addition, a number of phytosterol biosynthetic genes were down-regulated in the short fibers of ligon lintless-1 mutant, compared to its near-isogenic wild-type TM-1.These results of that phytosterols play a crucial role in cotton fiber development, and particularly in fiber elongation.