Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. Their effects were first discovered through the use of coconut milk in the 1940s by a scientist at the University of Wisconsin–Madison named Folke Skoog. [1]
There are two types of cytokinins: adenine-type cytokinins represented by kinetin, zeatin, and 6-benzylaminopurine, and phenylurea-type cytokinins like diphenylurea and thidiazuron (TDZ). The majority of adenine-type cytokinins are synthesized in the roots.[2] Cambium and other actively dividing tissues are also sites of cytokinin biosynthesis.[3] There is no evidence that the phenylurea cytokinins occur naturally in plant tissues.[4] Cytokinins are involved in both local and long distance signalling, the latter of which involves the same in planta transport mechanism as used for transport of purines and nucleosides.[5] Typically, cytokinins are transported within the xylem.[2]
Cytokinins act in concert with auxin, another plant hormone stimulating cell expansion.[2]
Contents |
The ratio of auxin to cytokinin plays an important role in the effect of cytokinin on plant growth. Parenchyma tissue cultured with auxin and without cytokinin have cells that grow large but do not divide. When cytokinin is added along with auxin, the cells both expand and differentiate. However, when the plant cells are cultured with only cytokinin, there is no effect. When cytokinin and auxin are present in equal levels, the parenchyma cells form a callus, or undifferentiated mass of cells. An increase in cytokinin will lead to the growth of shoot buds, while an increase in auxin induces root formation.[2]
Cytokinins are involved in many plant processes, including cell division and shoot and root morphogenesis. In particular, they are known to regulate axillary bud growth as well as affect apical dominance. These effects are a result of the cytokinin to auxin ratio, and termed the direct inhibition hypothesis. This theory states that the auxin, originating in the apical bud, travels down shoots to inhibit axillary bud growth. This promotes shoot growth, and restricts lateral branching. During this process, cytokinin moves from the roots and into the shoots, eventually signaling lateral bud growth. Simple experiments agree with this theory. When the apical bud—the major source of auxin—is removed, the axillary buds are liberated from inhibition. This allows the plant increased lateral growth, making the plant bushier. Applying auxin to the cleaved stem again inhibits lateral dominance.[2]
While cytokinin action in vascular plants is described as pleiotropic, this class of plant hormones specifically induces the transition from apical growth to growth via a three-faced apical cell in moss protonema. This bud induction can be pinpointed to differentiation of a specific single cell, and thus is a very specific effect of cytokinin. [6]
Cytokinins have also been known to slow the aging of plant organs. This process occurs by preventing protein breakdown, activating protein synthesis, and assembling nutrients from nearby tissues.[2] In a study that regulated leaf senescence in tobacco leaves, it was found that wild-type leaves showed yellowing of leaves, while the transgenic leaves remained mostly green. It was hypothesized that cytokinin may affect enzymes that regulate protein sythesis and degradation.[7]
Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis of isoprene cytokinins. It may use ATP, ADP, or AMP as substrates and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyl diphosphate (HMBDP) as prenyl donors.[8] This reaction is the rate-limiting step in cytokinin biosynthesis. DMAPP and HMBDP used in cytokinin biosynthesis are produced by the methylerythritol phosphate pathway (MEP).[8]
Cytokinins can also be produced by recycled tRNAs in plants and bacteria.[8][9] tRNAs with anticodons that start with a uridine and carrying an already-prenylated adenosine adjacent to the anticodon release on degradation the adenosine as a cytokinin.[8] The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.[9]
Auxin is known to regulate the biosynthesis of cytokinin.[10]
Because cytokinin promotes plant cell division and growth, it is commercially utilized by produce farmers to increase the yield of a crop. A study showed a 5–10% increase in cotton yield under drought conditions when cytokinin was applied to the seedlings. [11]
Cytokinins have recently been found to play a role in plant pathogenesis.
|
|
||||||||