Cytokinins, are a class of plant hormones that plays important roles in plant development and growth (i.e., promotion of cell division, release of lateral buds from apical dominance, delay of senescence etc.). Cytokinin homeostasis is regulated by the rate of biosynthesis, import, conjugation (mainly in the form of glycosides) and degradation. Cytokinins are irreversibly degraded by cytokinin oxidases which catalyse cleavage of the N6-isopentenyl-side chain resulting in formation of adenine or adenosine. Cytokinins are N6-substituted purine derivatives, such as isopentenyladenine, cis-zeatin, trans-zeatin, and their ribosides that are cleaved in a single enzymatic step by oxidative side chain cleavage. This activity is conserved in many plants and is important in regulating cytokinin levels in specific cells, tissues and organs and has consequences for their development. For example, Cytokinins OXIDASE/DEHYDROGENASE (OsCKX3) is highly expressed in lamina joint, and the encodes an endoplasmic reticulum localized enzyme OsCKX3 (Huang et al., 2023). It preferentially catabolizes trans-zeatin and isopentenyladenine that leads to depletion of the cytokinins level. The reduction in cytokinin levels results in larger leaf angles. In the osckx3 knockout mutants, levels of cytokinins in lamina joint remain high resulting in smaller leaf angles. In contrast, OsCKX3 overexpressing transgenic plants have larger leaf angles. Also, osckx3 mutants have increase in primary branches, grain size, and leaf size. Thus, OsCKX3 is negative regulator of leaf angle by mediating the cytokinins degradation in lamina joint (Huang et al., 2023).
