Cell cycle specific isopentenyl transferase expression led to coordinated enhancement of cell division, cell growth and plant development in transgenic Arabidopsis

Cytokinin is an essential plant developmental regulator that requires precise temporal and spatial control for synergistic action on morphogenesis. To identify the cellular target for proper cytokinin function, the bacterial gene IPT, encoding an isopentenyl transferase for de novo cytokinin biosynthesis, was expressed in transgenic plants using promoters with different specificities. Analysis of the transgenic plants revealed that ectopic IPT expression was detrimental to plant development, whereas exclusive expression of IPT in cycling cells led to normal plant development with increased growth and final organ size. The enlarged organ size was a result of increase in both cell number and cell size, which was accompanied by increased expression of CycD3 and CycB1, indicating that cytokinin controls organ size by regulating cyclin expression. The cell cycle-specific cyclin promoters were active in multiple organs, including root, leaf and flower, suggesting the biological significance of the locally produced cytokinin on morphogenesis, and the amplified cytokinin biosynthesis in the pre-existing dividing cells of these organs is necessary and sufficient for coordinated cell division, cell growth, pattern formation and organ development. To our knowledge, this is the first case that cytokinin was genetically manipulated in transgenic plants to produce dramatically enhanced phenotypes without noticeable negative effect, providing a promising opportunity for crop improvement.