Cytokinins and Cell Division

A cytokinin is a small adenine-derived molecule — chemically, an N⁶-substituted purine — that promotes cytokinesis (the completion of cell division after mitosis) in the presence of auxin. The name reflects this history: "cytokinin" was coined to describe the ability to induce cytoplasmic division, distinguishing it from auxins, which alone can drive DNA synthesis and nuclear division but cannot finish the job of splitting one cell into two. Naturally occurring cytokinins include zeatin (the most abundant and physiologically important form in most plants), isopentenyladenine (iP), and their ribosides and glucosides, which serve as transport and storage forms. Kinetin, the molecule that led to the whole discovery, does not occur naturally in plants — it is a degradation artifact — but it is a fully active synthetic cytokinin, and its discovery opened the door to finding the real thing. Cytokinins are synthesized mainly in root tips, in developing seeds and fruits, and in shoot apical and cambial tissues, from isopentenyl diphosphate condensed onto AMP by isopentenyltransferase enzymes; the immediate precursors are then converted to active zeatin-type cytokinins by cytochrome P450 enzymes of the CYP735A family. Roots export cytokinins upward through the xylem, so cytokinin functions physiologically as a long-distance "good news from below" signal, complementing auxin's predominantly root-ward, cell-to-cell polar transport described in the Auxin and Polar Transport article. The core biological principle this article develops is that cytokinin rarely acts alone. Almost every classic cytokinin phenotype — cell division, shoot regeneration, delayed senescence, release of axillary buds from apical dominance — is properly described as a response to the relative concentration of auxin and cytokinin, not to either hormone in isolation. This ratio-dependence is one of the cleanest examples in all of plant biology of combinatorial hormonal control of cell fate, and it was demonstrated with unusual rigor in a small number of classic tissue-culture experiments.

Step 1 — What a cytokinin is and where it comes from. Cytokinins are derivatives of adenine bearing a substituent at the N⁶ position of the purine ring. Zeatin carries an isoprenoid side chain (a modified 5-carbon isoprene unit) at that position; kinetin, isolated originally from autoclaved herring sperm DNA, carries a furfuryl group produced by heat-induced breakdown of the deoxyribose-phosphate backbone. Biosynthesis begins when isopentenyltransferase (IPT) enzymes attach an isopentenyl group from dimethylallyl diphosphate onto the N⁶ position of ATP or AMP, generating iP-type cytokinins; CYP735A hydroxylases then convert these to the more active trans-zeatin type. Cytokinin levels are also controlled by degradation: cytokinin oxidase/dehydrogenase (CKX) enzymes irreversibly cleave the side chain, and this degradative step is itself a major regulatory target — overexpressing CKX shrinks meristems and roots, while knocking it out enlarges them.

Step 2 — Perception: a bacterial-style two-component relay repurposed in plants. Unlike animal steroid or peptide hormone receptors, the cytokinin receptor is a transmembrane histidine kinase, homologous to bacterial two-component sensor systems — a striking case of an ancient prokaryotic signaling architecture being retained and adapted in plants (a relic of the cyanobacterial ancestry of the chloroplast lineage's regulatory toolkit, though the receptors themselves are nuclear/plasma-membrane-localized plant proteins, not organellar). In Arabidopsis the receptors are called AHK2, AHK3, and AHK4/CRE1, located mainly on the endoplasmic reticulum membrane. Cytokinin binding to the extracellular-facing sensor domain triggers autophosphorylation of a conserved histidine residue on the receptor's own kinase domain.

Step 3 — Phosphorelay to the nucleus. The phosphoryl group is passed, in a multistep His-Asp phosphorelay, from the receptor histidine to an aspartate on the receptor itself, then to small mobile phosphotransfer proteins (AHPs), which shuttle into the nucleus. There they transfer the phosphate to type-B response regulators (ARR-Bs), a family of transcription factors. Phosphorylated ARR-Bs activate transcription of downstream genes, prominently including type-A response regulators (ARR-As), which feed back to dampen the pathway — a classic negative-feedback loop that keeps cytokinin signaling from running away with itself.

Step 4 — From signal to cell division. Cytokinin-activated transcription upregulates D-type cyclins and other cell-cycle components that drive cells through the G1-to-S and G2-to-M transitions, and it promotes expression of genes needed for cell plate formation during cytokinesis. Because auxin independently promotes progression through S phase (DNA synthesis) via its own signaling pathway, a cell generally needs both hormones to complete a full division cycle: auxin licenses DNA replication, cytokinin licenses the completion of division. This is the molecular basis for why pith tissue cultured with auxin alone enlarges its nuclei (endoreduplication, repeated DNA synthesis without division) but never actually divides — a phenomenon Skoog's group observed directly.

Step 5 — Ratio-dependent organogenesis. Once cells are dividing, the relative levels of auxin and cytokinin in the tissue determine what kind of organ, if any, differentiates from the dividing mass. High auxin relative to cytokinin favors root primordium initiation (consistent with auxin's central role in lateral root formation); high cytokinin relative to auxin favors shoot apical meristem formation and leaf primordium initiation; roughly balanced ratios favor undifferentiated callus proliferation. Mechanistically, high cytokinin promotes expression of the shoot-meristem identity gene WUSCHEL and SHOOT MERISTEMLESS, establishing a new stem cell niche, while high auxin — via local auxin maxima generated by polar transport — recruits PLETHORA and PIN-dependent programs that specify root pole identity. The ratio essentially acts as a molecular switch between two alternative developmental programs built from the same pool of totipotent cells.

Step 6 — Apical dominance and bud release. In the intact plant, the shoot apex produces auxin that moves basipetally down the stem and suppresses outgrowth of axillary buds below it — classical apical dominance. Root-derived cytokinin, arriving via the xylem, opposes this suppression: locally applied cytokinin can release a dormant axillary bud from apical dominance even while the apex (and its auxin) is still present, as shown directly by painting cytokinin onto lateral buds. Current understanding (itself refined since the simple "two-hormone" model) holds that apical auxin does not act directly on the bud; rather it regulates cytokinin biosynthesis and transport locally in the stem, and also controls a second hormone, strigolactone, which independently suppresses bud outgrowth. Removing the apex (decapitation) cuts off the auxin supply, cytokinin synthesis rises near the buds, strigolactone signaling changes, and buds break dormancy — the biological basis of pruning and hedge-trimming.

The chemical hunt (Skoog, Miller, Strong, Folke Skoog's lab, 1940s–1955). Folke Skoog, working first with Philip White and later with graduate student Carlos O. Miller, had shown by the 1940s that fragments of tobacco pith parenchyma could be induced to divide in culture, and that unidentified factors from vascular tissue, coconut milk, and — crucially — aged or autoclaved samples of herring sperm DNA all promoted this division far better than any known hormone. Adding purified, fresh DNA did nothing; only DNA that had been heated or stored so that it degraded was active. This was the key clue: the active factor was a breakdown product of DNA, not DNA itself. Miller, Skoog, and coworkers fractionated autoclaved herring sperm DNA and in 1955 isolated and crystallized the active compound, naming it kinetin (6-furfurylaminopurine) for its ability to promote cytokinesis. Kinetin does not occur naturally in living plant tissue — an important and often-missed point — but its discovery told researchers exactly what chemical class to search for in real plants. That search, led by Skoog's group and by D. S. Letham's laboratory in New Zealand and Australia, identified the natural, endogenous cytokinin zeatin in 1963, isolated from immature maize (Zea mays) kernels, whose structure closely resembled kinetin — an N⁶-substituted adenine, but with a natural isoprenoid rather than a furfuryl side chain.

The defining experiment: Skoog and Miller's tobacco pith bioassay (1957). To determine what controlled organ formation in cultured tissue, Skoog and Miller grew sterile explants of tobacco pith parenchyma on defined agar medium and systematically varied the concentrations of auxin (indole-3-acetic acid, IAA) and the new factor, kinetin, factorially against each other. The results, published in their now-classic 1957 paper in the Symposia of the Society for Experimental Biology, were unambiguous:

Critically, doubling both hormones together while keeping the ratio constant reproduced the same developmental outcome, while shifting the ratio — even at similar absolute concentrations — flipped the tissue from rooting to shooting. This demonstrated that the plant tissue was responding to relative, not absolute, hormone levels, a genuinely quantitative rule rare in 1950s developmental biology. The experiment also showed that auxin alone can drive nuclear DNA replication and enlargement (endoreduplication) but cannot complete cytokinesis — cytokinin was required specifically for the cell to finish dividing, matching its Greek-rooted name.

Independent confirmation and generalization. The auxin:cytokinin ratio principle was subsequently reproduced across dozens of species and tissue types — carrot phloem explants (used by F.C. Steward in classic totipotency work), Arabidopsis root and hypocotyl explants, and woody species used in forestry micropropagation — establishing it as a general rule of plant tissue culture rather than a tobacco-specific quirk. Molecular work in the 1990s–2010s (notably from the labs of Jen Sheen, Joseph Kieber, and others) then traced the mechanism behind the phenotype, identifying the AHK receptors, AHP phosphotransfer proteins, and ARR transcription factors described above, and showing that auxin signaling represses cytokinin signaling components (and vice versa) at the transcriptional level — giving a molecular basis for why the two hormones behave as mutual antagonists in organ specification.

Apical dominance evidence. The role of cytokinin in releasing axillary buds was shown by a complementary line of experiments: decapitating a plant's shoot apex releases lateral buds from dormancy; replacing the removed apex with auxin in lanolin paste restores suppression (classic experiments going back to Kenneth Thimann and Folke Skoog in the 1930s, extending auxin's role); but painting cytokinin directly onto a still-dormant lateral bud, even in the presence of the intact, auxin-producing apex, causes that bud to break dormancy and grow out. This directly demonstrated that cytokinin acts locally at the bud to counteract auxin-mediated apical dominance, independent of whether the apex itself is removed.

Example 1 — Designing a tissue-culture medium from the ratio rule. A horticulturist wants to regenerate whole plantlets from callus culture of a woody ornamental species. Baseline callus-maintenance medium contains auxin (as NAA) at 1.0 mg/L and cytokinin (as BAP) at 1.0 mg/L — a 1:1 ratio that keeps the tissue as undifferentiated, proliferating callus, matching Skoog and Miller's "balanced ratio → callus" result. To induce shoot formation, the protocol calls for shifting the ratio roughly tenfold in cytokinin's favor: reduce auxin to 0.1 mg/L and raise cytokinin to 1.0 mg/L, giving an auxin:cytokinin ratio of 1:10. Shoot buds differentiate from the callus mass within two to four weeks. Once small shoots are excised, they are transferred to a rooting medium in which the ratio is inverted — auxin raised to 1.0–2.0 mg/L (often as IBA) and cytokinin dropped to zero or near-zero — inducing adventitious root primordia at the cut stem base. The plantlet, now with both shoots and roots, can be transferred ("hardened off") to soil. This two-step protocol — high cytokinin:auxin for shoots, then high auxin:cytokinin for roots — is exactly the logic validated in the 1957 tobacco experiments, scaled to industrial micropropagation of millions of genetically identical clones (bananas, oil palm, ornamental orchids, virus-free potato seed stock).

Example 2 — Predicting bud outgrowth after pruning, quantitatively reasoned. A fruit-tree grower has a young apple tree with a single dominant leader (main vertical shoot) and eight lateral buds along the trunk below it, all currently dormant due to apical dominance. The leader is producing auxin that moves basipetally at a typical rate of about 1 cm/hour through the phloem-adjacent tissue, maintaining strigolactone signaling and suppressing cytokinin biosynthesis in the buds nearest the tip. The grower cuts off the top 15 cm of the leader (decapitation). Reasoning through the mechanism: removing the apex eliminates the auxin source; within roughly 24–48 hours auxin levels in the upper stem fall sharply because auxin already in transit is used up and not replenished; falling auxin derepresses cytokinin biosynthesis genes locally in the nodal tissue; rising local cytokinin (reinforced by root-derived cytokinin arriving via the xylem) activates the buds' own cell-division machinery via the AHK–ARR pathway; the two or three buds closest to the cut, which experience the fastest and largest drop in auxin, break dormancy first and become new competing leaders, while buds farther down the trunk, still under residual auxin influence from those newly growing shoots, may remain suppressed. This explains the standard pruning outcome — a "crotch" of two or three new co-dominant branches near the cut rather than uniform bud break down the whole trunk — and shows why pruning height, not just pruning per se, controls the resulting branching architecture.

Folke K. Skoog (1908–2001) was a Swedish-American plant physiologist who spent most of his career at the University of Wisconsin–Madison. Trained originally in the tradition of auxin physiology under Kenneth Thimann and James Bonner in the 1930s, Skoog's early work already probed how removing the shoot apex changed hormone-dependent growth in stem segments — planting the seed of his later interest in what, besides auxin, controls cell division and organ formation. From the 1940s onward, working with a long line of graduate students and postdocs including Carlos O. Miller, Skoog turned to tobacco pith parenchyma as a simple, reproducible bioassay system, because these fully differentiated cells could be induced back into division and even into forming new organs under the right chemical conditions — a striking demonstration of plant cell totipotency that paralleled Frederick Steward's carrot work. Skoog's insistence on rigorous, quantitative, factorial dose-response experiments (varying two hormones simultaneously across a grid of concentrations, rather than one at a time) was itself methodologically important; it is precisely what allowed the ratio effect, rather than either hormone's absolute concentration, to be identified as the causal variable. Carlos Miller, who did much of the hands-on chemistry of isolating and characterizing kinetin, went on to lead his own influential research program on cytokinin physiology at Indiana University.

Micropropagation, built directly on the auxin:cytokinin ratio rule, is now a multibillion-dollar global industry that clonally propagates disease-free banana, potato, sugarcane, oil palm, and ornamental plants from tiny explants, allowing rapid multiplication of elite or virus-free genotypes that would take years to bulk up conventionally. Genetic transformation protocols for crops (including Agrobacterium-mediated transformation used to make many transgenic and gene-edited crop varieties) rely on the same shoot-induction media, using cytokinin-rich regeneration steps to recover whole transgenic plants from single transformed cells. In agriculture, cytokinin-modulating treatments and breeding for altered cytokinin oxidase activity are used to engineer "stay-green" cultivars with delayed leaf senescence (since cytokinin famously delays senescence, the original discovery that gave the hormone its second major application after Richard and Helen Mothes' classic 1960s experiments showing cytokinin-treated leaf patches stay green while surrounding tissue yellows). Breeders manipulating strigolactone and cytokinin pathways together are developing crop varieties with altered branching architecture — more axillary shoots for higher yield in cereals, or fewer for easier mechanical harvesting in trees and shrubs — a direct descendant of the apical-dominance logic described here. In basic and applied plant science, cytokinin receptor mutants and CKX-overexpression lines are standard tools for engineering root system architecture (larger cytokinin-deficient roots forage soil more effectively under drought or low-nutrient stress), directly informing efforts to breed climate-resilient crops. Conservation and restoration biology also use cytokinin-based tissue culture to propagate rare or endangered plant species from limited founder material, preserving genetic diversity that could not otherwise be bulked up fast enough for reintroduction programs.

Ask the tutor: How does the auxin:cytokinin ratio interact with the newer strigolactone pathway to fine-tune shoot branching in real crop plants?