Cytokinins were first recognized as a class of phytohormones for their ability to promote cytokinesis in cultured plant cells and have since been shown to be involved in a wide range of physiological processes. Most recently, the availability of phytohorm one-specifying genes from Agrobacterium tumefaciens has allowed for direct in planta manipulation of cytokinin levels. Overexpression of the isopentenyl transferase (ipt) gene by constitutive promoters led to enhanced ability of plant cells to undergo shoot organogenesis but the high endogenous cytokinin levels almost completely suppressed root development. Transient overproduction of cytokinins using promoters regulated by environmental and/or developmental factors did not inhibit regeneration of rooted plants. Transgenic plants in which cytokinin levels can be modulated are being used to characterize the participation of cytokinins in fundamental regulatory mechanisms of morphogenesis, delayed senescence, disease resistance and directed nutrient transport. The potential for using reconstructed cytokinin biosynthesis genes in economically important crops is of tremendous agronomic significance.
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