This review chronicles the development of the cytokinin research during the last 30 years. Cytokinin and auxin are the two major plant growth hormones that control virtually all aspects of growth and development in higher plants. The pathways for cytokinin biosynthesis and metabolism have been characterized by the identification of isopentenyl pyrophosphate transferase, cytokinin oxidases, cytokinin hydroxylase, zeatin cis-/trans-isomerase, cytokinin phosphoribosyl hydrolases, cytokinin-specific riboside phosphorylase, and others enzymes. Loss-of function mutant phenotypes of cytokinin degradation/activating enzymes indicate the regulation of concentration and spatial distribution of bio-active cytokinin plays a pivotal role in the increase in panicle size, in the numbers of floral organs, and eventually in seed yield. One of the most fundamental questions in the cytokinin field is one concerning the prevalence of cis-zeatin in monocotyledonous crops (rice and maize) and in dicotyledonous legumes (pea, chickpea) and potato/sweet potato. A hypothesis is that cis-zeatin is synthesized by the cis-specific hydroxylation of the terminal methyl group of N6-isopentenyl side chain of N6-isopentenyl adenosine (i6Ado) or of their mono-, di-, or tri-phosphates catalyzed by the cis-specific hydroxylase. A second potential pathway is the isomerization of trans-zeatin to cis-zeatin by zeatin cis-/trans-isomerase. A second fundamental question to be addressed is the physiological role of cis-zeatin. Some have argued for a special function of cis-zeatin to account for the prevalence of the cis-zeatin in the plant kingdom from algae to higher plants.
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