Phosphate as a Limiting Factor for the Cell Division of Tobacco BY-2 Cells

The re-addition of phosphate to tobacco BY-2 cells deprived of phosphate for 3 d induced cells to semi-synchronously re-enter the cell cycle from a static state. Though the addition of auxin to auxin-starved tobacco BY-2 cells also induced cell division (Ishida et al. 1993), some major differences were observed between these two systems. BY-2 cells lost the ability to re-enter the cell cycle after prolonged periods of auxin deprivation, but in contrast retained this ability after longer periods in the absence of phosphate. By differential cDNA screening we identified a phosphate-induced gene phi-1. phi-1 was rapidly induced by the addition of phosphate with transcript levels starting to decrease by the start of DNA synthesis. phi-1 does not share any significant homology with any gene with known functions over its full length. However, the N-terminus shared some homology with plasma membrane ATPases suggesting that it may be involved in some process of phosphorylation. Immunolocalization of the phi-1 gene product revealed that it rapidly accumulated in the cytoplasm prior to the start of plastid and nuclear DNA synthesis. These results are discussed in relation to the role of phosphate in inducing plant cell division.