Cotton fibres are the fastest growing and among the longest single cells in the plant kingdom. In the space of about16 days, these single cells can expand from a few micrometers to 3 cm in length. When the fibres stop growing they then thicken with cellulose, becoming over 90% cellulose by the end of maturation. As long fibres are important for cotton fibre quality, we set out to study cotton fibre elongation using biochemistry and molecular genetics. We identified several potential gene targets for improving this process of elongation. We have shown that 2 key processes are important in controlling fibre growth. 1. Sucrose metabolism in the fibre and 2. Solute transport into the fibre. Using transgenic cotton we have shown that early in fibre growth, an enzyme of sucrose metabolism (sucrose synthase; SuSy) limits fibre elongation. Also, in fuzz fibre, SuSy activity is low and delayed. Later in elongation, the movement of solutes into the fibre control fibre elongation. The fibre must inflate analogous to a filling balloon. We have shown that solute (sucrose and potassium) transporters play a key role in this process and that the pores (plasmodesmata or PD) between the fibre and the seed coat must close to allow the fibre to inflate with solutes pumped into the cell and expand to full length. Importantly, we have shown that in short fibre cultivars and in fuzz fibres, the timing of this process of PD closure is not optimal. The longer the period of PD closure, the more the fibre expands. We have identified candidate genes which could be manipulated to improve fibre length through the control of this process. We have also demonstrated that during cell wall thickening the enzyme SuSy also plays a key role in supplying sugars for cellulose biosynthesis and that a small decrease in the level of this enzyme has a large negative effect on fibre cellulose content. We have produced transgenic cotton plants with increased seed SuSy and will analyse fibre properties in these plants. We hope to use the genes we have identified to produce cultivars with longer fibres and more consistent cellulose content.