Cytokinins are plant-specific hormones that play a central role in cell cycle regulation and various developmental processes. This study investigates the regulatory roles of cytokinins in plant growth and development by genetically engineering cytokinin oxidase (CKX) expression in transgenic tobacco plants to reduce their endogenous cytokinin levels. Cytokinin-deficient plants exhibited stunted shoots with smaller apical meristems and reduced leaf cell production, indicating an absolute requirement of cytokinins for leaf growth. In contrast, root meristems were enlarged and produced faster-growing, more branched roots, suggesting opposing roles of cytokinins in shoots and roots. Cytokinins were discovered in the 1950s for their ability to induce cell division. The auxin-cytokinin hypothesis proposed that cytokinins, along with auxins, are essential for plant morphogenesis. Cytokinins are N6-substituted purine derivatives, with isopentenyladenine (iP), zeatin (Z), and dihydrozeatin (DZ) being the main cytokinins in higher plants. Free bases and their ribosides are biologically active, while glycosidic conjugates play roles in transport and inactivation. The study reports the cloning of four putative CKX genes from Arabidopsis thaliana and their overexpression in transgenic tobacco plants. The results show that these genes have cytokinin oxidase activity and can be used to study cytokinin functions in plant development. Transgenic plants with reduced cytokinin content showed altered phenotypes, including reduced shoot growth, altered leaf development, and increased root growth. These findings support the role of cytokinins in regulating plant meristem activity and morphogenesis. The study also highlights the importance of cytokinins in cell cycle regulation, with different effects on shoot and root meristems. The findings suggest that cytokinins are crucial for maintaining cell division cycles and promoting cell differentiation. The results indicate that cytokinins are involved in controlling root growth rate and the generation of new root meristems. The study provides evidence that cytokinins are essential for plant growth and development, with opposing roles in shoots and roots. The research underscores the importance of cytokinins in plant development and their potential as a tool for studying plant growth and development.Cytokinins are plant-specific hormones that play a central role in cell cycle regulation and various developmental processes. This study investigates the regulatory roles of cytokinins in plant growth and development by genetically engineering cytokinin oxidase (CKX) expression in transgenic tobacco plants to reduce their endogenous cytokinin levels. Cytokinin-deficient plants exhibited stunted shoots with smaller apical meristems and reduced leaf cell production, indicating an absolute requirement of cytokinins for leaf growth. In contrast, root meristems were enlarged and produced faster-growing, more branched roots, suggesting opposing roles of cytokinins in shoots and roots. Cytokinins were discovered in the 1950s for their ability to induce cell division. The auxin-cytokinin hypothesis proposed that cytokinins, along with auxins, are essential for plant morphogenesis. Cytokinins are N6-substituted purine derivatives, with isopentenyladenine (iP), zeatin (Z), and dihydrozeatin (DZ) being the main cytokinins in higher plants. Free bases and their ribosides are biologically active, while glycosidic conjugates play roles in transport and inactivation. The study reports the cloning of four putative CKX genes from Arabidopsis thaliana and their overexpression in transgenic tobacco plants. The results show that these genes have cytokinin oxidase activity and can be used to study cytokinin functions in plant development. Transgenic plants with reduced cytokinin content showed altered phenotypes, including reduced shoot growth, altered leaf development, and increased root growth. These findings support the role of cytokinins in regulating plant meristem activity and morphogenesis. The study also highlights the importance of cytokinins in cell cycle regulation, with different effects on shoot and root meristems. The findings suggest that cytokinins are crucial for maintaining cell division cycles and promoting cell differentiation. The results indicate that cytokinins are involved in controlling root growth rate and the generation of new root meristems. The study provides evidence that cytokinins are essential for plant growth and development, with opposing roles in shoots and roots. The research underscores the importance of cytokinins in plant development and their potential as a tool for studying plant growth and development.