The core idea is elegant in its simplicity. In the canonical mitotic cell cycle, a cell passes through G1, replicates its DNA in S phase, prepares in G2, and then enters mitosis, the M phase, before dividing. Endoreduplication severs the link between the first half of this cycle and the second. G1/S progression, the machinery that licenses and executes DNA replication, continues to run, but M-phase entry is suppressed. The result is a cell that doubles its nuclear DNA content with each pass, cycling from 2C to 4C to 8C and beyond, a state called endopolyploidy. Ishida emphasizes that this is distinct from endomitosis, where a cell enters but fails to complete mitosis, and from chemically induced polyploidization; endoreduplication arises from an intrinsic reconfiguration of the cell cycle control network itself.

Much of the conceptual scaffolding for this field was built in animal systems, particularly in the fruit fly Drosophila, whose larval tissues are famously polytene. Studies there established that a functional endocycle requires an oscillator: a set of feedback loops that alternately activate S phase and suppress mitosis, period after period. The critical question the review poses is how far these principles map onto plants, whose cell cycle machinery, while conserved in outline, is regulated by a distinctly plant-specific cast of characters. The available evidence suggests the logic is transferable, but the implementation in angiosperms has its own architecture, organized around two pillars: repeated S-phase entry and robust mitotic suppression.

The first pillar depends on DNA replication licensing. Before any genome can be copied, the origins of replication must be loaded with the pre-replicative complex, a process involving the origin recognition complex (ORC), CDC6, and CDT1. In Arabidopsis, the expression and stability of these licensing factors are tightly tied to endoreduplication competence. CDC6, for example, is an E2F transcription factor target whose accumulation correlates with endocycling cells, and the ORC1 genes are expressed differently in proliferating versus endoreduplicating tissues. F-box protein FBL17 drives the degradation of the licensing inhibitor CDT1a in early S phase, preventing genome instability, while Cullin 4-based ubiquitin ligases shape the timing of licensing cycles in trichomes. On the enzymatic side, DNA topoisomerase VI, along with accessory components such as BIN4, RHL1, and MIDGET, is essential; mutants in this complex cannot sustain endoreduplication, presumably because untangling and decatenating ever-larger replicated genomes becomes impossible without it. Together these findings portray repeated genome duplication as a carefully staged licensing problem that the endocycle must solve anew with every round.

The second pillar is the suppression of mitosis, and here the anaphase-promoting complex/cyclosome (APC/C) takes center stage. This E3 ubiquitin ligase, when activated by the CCS52A proteins, targets mitotic cyclins for destruction and locks cells out of M phase. Loss-of-function mutations in CCS52A1 restore mitotic cycling in cells that should have become endocycling, underscoring its role as a gatekeeper. In parallel, the B-type cyclin-dependent kinases, CDKB1 complexes paired with A-type cyclins such as CYCA2;3, actively suppress endocycle onset; their inactivation permits the transition to endoreduplication. The plant-specific CDK inhibitor SIAMESE (SIM), named for the multi-headed trichomes produced by its mutants, targets both CDKA and CDKB complexes to establish the endocycle in trichomes, while its relatives, the SIAMESE-RELATED (SMR) proteins, extend this control into stress responses. Another layer of mitotic braking comes from DEL1, a DP-E2F-like transcriptional repressor that keeps the APC/C activator CCS52A1 in check, and from APC/C inhibitors UVI4 and GIGAS CELL1, which preserve mitotic competence where division is still needed. Deubiquitinating enzymes add further tuning: the UBP14–CDKB1;1–CDKG2 cascade illustrates how a proteolytic regulator, a mitotic kinase, and a CDK-like protein can be wired into a single pathway governing ploidy and growth.

What makes the endocycle oscillator concept useful is that it frames these components as elements of a periodic system rather than a one-way switch. For a cell to cycle through repeated S phases, the machinery must reset: licensing factors must be reloaded, inhibitors cleared, and mitotic suppressors re-established in the right sequence. Feedback loops generating this periodicity are well defined in Drosophila, but in plants, Ishida argues, they remain incompletely mapped. Identifying the loops that generate oscillation within the plant endocycle is one of the field’s central open problems, and resolving it will likely require moving beyond static measurements of ploidy toward dynamic, system-level analyses that track the cell cycle machinery in living tissues over time.

Remarkably, this core oscillator does not operate in isolation. A diverse array of upstream signals converges on it, allowing plants to tune ploidy in response to their development and environment. Light is a classic example: phytochrome photoreceptors control the number of endoreduplication cycles in the Arabidopsis hypocotyl, linking photomorphogenesis directly to ploidy. Phytohormones provide another route. Cytokinin promotes endocycle onset in roots by inducing expression of the APC/C activator CCS52A1, while auxin modulates the transition from the mitotic cycle to the endocycle, and single-cell RNA sequencing has revealed auxin fluctuation during the endocycle itself. D-type cyclins of the CYCD3 class, which respond to cytokinin, act as rate-limiting factors that keep cells in the mitotic cycle, so their regulation effectively gates entry into endoreduplication.

Stress and DNA damage add yet another dimension. When Arabidopsis experiences DNA double-strand breaks, it can respond by deliberately inducing endoreduplication, a programmed shift mediated by the DNA damage response. The ATM kinase phosphorylates the transcription factor SOG1, which can trigger early onset of endoreduplication by inducing CCS52A1 expression in roots, while the SMR5 and SMR7 CDK inhibitors regulate the DNA damage checkpoint in response to reactive oxygen species. Mechanical signals participate as well; recent work shows that endoreplication mediates cell size control through mechanochemical signaling from the cell wall, and DELLA proteins, the growth-repressing components of gibberellin signaling, mediate stress-induced cell differentiation by modulating APC/C activity. The transcriptional layer is equally rich: TCP family transcription factors, including TCP14 and TCP15, modulate cell-cycle gene expression and are themselves regulated by ubiquitin receptors DA1, DAR1, and DAR2, while the trihelix factor GTL1 and the fluctuating epidermal regulator ATML1 shape ploidy-dependent patterns of giant cells in leaves and sepals.

Why does all this matter beyond the mechanics? Endoreduplication is increasingly recognized as a flexible mechanism supporting developmental plasticity. The relationship between ploidy and cell size is one of the oldest observations in plant biology, with the karyoplasmic ratio theory invoked to explain fruit growth in tomato, and it underlies the striking mosaics of giant and small pavement cells that pattern the leaf surface. Because endoreduplication allows cells to grow large without division, it provides a rapid, energetically efficient way to build organs, and its responsiveness to light, hormones, and stress suggests it functions as an integration point where environmental information is converted into cellular architecture. Polyploid cells may also buffer genome stress, offering a survival strategy under adverse conditions.

The review closes with a forward-looking agenda. Two priorities stand out: first, experimentally identifying the feedback loops that generate periodicity in the plant endocycle oscillator, work that will benefit from live-cell reporters of cell cycle phase and from quantitative imaging approaches capable of inferring spatial ploidy patterns across tissues; and second, advancing from static ploidy snapshots to dynamic, context-resolved analyses spanning developmental, environmental, and stress-related conditions. Such tools, including fluorescent cell cycle sensors and time-series single-cell transcriptomics, are already available in Arabidopsis and are poised to reveal how the oscillator behaves in real time. As Ishida’s synthesis makes clear, plants have turned an apparent cell cycle accident into a precision instrument, and decoding its internal clock promises to explain how one of biology’s most unusual cell cycles is truly run.

Cell division progression hinges critically on the timely activation and deactivation of cyclin-dependent kinases (CDKs), enzymes that coordinate cell cycle transitions through phosphorylation. In plants, CDKB1 plays a pivotal role, yet the regulatory phosphatases responsible for resetting CDKB1 activity had remained elusive. Through a cutting-edge global quantitative phosphoproteomics approach, the research team dissected phosphorylation dynamics in synchronized wild-type and mutant bsl1-1 cells at four meticulously chosen cell cycle stages: newborn cells, mid-G1, early S/M, and mid S/M.

Microscopy revealed bsl1-1 mutant cells undergo delayed division, corroborated by mass spectrometry data showing accumulation of S phase and mitotic hallmark proteins such as MCM3, RIR1, CDKA, CYCB1, and CDKB1 at later timepoints. Intriguingly, CDKB1 phosphorylation at threonine 14 (Thr14) and tyrosine 15 (Tyr15) was markedly elevated in the mutant at 8 and 10 hours, indicating BSL1’s role as a critical phosphatase targeting these sites.

Employing parallel reaction monitoring mass spectrometry, the researchers demonstrated a 30- to 60-fold increase in the doubly phosphorylated CDKB1 peptide harboring pThr14 and pTyr15 modifications within bsl1-1 cells relative to wild type. This contrasted sharply with a moderate twofold increase in the singly phosphorylated pTyr15 form, underscoring BSL1’s specificity in dephosphorylating both residues. Control peptides from proteins like PEPC2 and CDPKK exhibited no significant phosphorylation changes, pointing to the selectivity of BSL1-mediated dephosphorylation.

In established model organisms including yeast and animals, phosphorylation of the CDK catalytic subunit on Thr14 and Tyr15 is a canonical response to DNA replication stress, halting the cell cycle to maintain genome integrity. Exploiting this conceptual framework, the team induced replication stress in Chlamydomonas cells using hydroxyurea (HU), a ribonucleotide reductase inhibitor. HU treatment intensified CDKB1 phosphorylation without altering total protein abundance, mirroring stress-activated checkpoint phenomena known in other eukaryotes.

To parse the biochemical activity of BSL1, the investigators immunoprecipitated CDKB1 fused with a fluorescent mCherry tag from HU-treated cells and incubated it with recombinant BSL1 phosphatase domains. The pronounced reduction in phosphorylated CDKB1 species after treatment — comparable to the effect of lambda phosphatase and greater than that observed with Toxoplasma gondii PPKL — established BSL1 as a direct and efficient CDKB1 phosphatase. Negative controls, including buffer alone or unrelated proteins, failed to elicit such dephosphorylation, affirming assay specificity.

Further biophysical analysis via bio-layer interferometry uncovered a direct physical interaction between recombinant GST-tagged CDKB1 and purified BSL1, with a dissociation constant (K_d) of approximately 900 nM. This nanomolar affinity interaction strengthens the model wherein BSL1 engages its CDKB1 substrate in vivo, facilitating precise modulation of CDKB1 phosphorylation state and functional activity.

Confocal microscopy provided compelling spatial insight, demonstrating that BSL1 and CDKB1 colocalize near spindle poles during metaphase. Such subcellular localization is consistent with the previously reported spindle pole enrichment of CYCB1—a key cyclin partner of CDKB1—suggesting a coordinated regulatory complex modulating mitosis at the microtubule organizing centers.

The team also engineered a CDKB1 mutant variant harboring non-phosphorylatable alanine and phenylalanine substitutions at Thr14 and Tyr15, respectively. This CDKB1(AF) construct abolished phosphorylation signals upon HU exposure, confirming these residues as critical regulatory phosphorylation sites controllable by BSL1. Parallel proximity labeling experiments using the BSL1–BioIDG3 fusion protein revealed an enrichment of CYCB1 labeling, implicating BSL1 within a broader mitotic complex that integrates cyclin and kinase signaling pathways.

Taken together, these comprehensive in vivo and in vitro data compellingly position BSL1 as an indispensable phosphatase directing the removal of inhibitory phosphate marks on CDKB1 to promote mitotic entry in plant cells. The intricate temporal control of CDKB1 phosphorylation by BSL1 ensures proper cell cycle progression and faithful chromosome segregation.

This discovery opens new avenues for manipulating plant growth and cell division, with implications for agriculture, bioengineering, and synthetic biology. By decoding how cyclin-dependent kinase activity is finely tuned through specific phosphatase action, scientists gain a powerful tool for modulating developmental timing, stress responses, and tissue regeneration in crops.

Moreover, the findings resonate beyond plants, as phosphorylation-dependent regulation of CDKs is a conserved feature across eukaryotes. Comparative analyses can now explore whether functionally analogous phosphatases operate in other kingdoms, potentially enriching our understanding of mitotic regulation and cell cycle checkpoints universally.

Future research can build on this work to elucidate the regulatory networks upstream of BSL1 that control its activity and subcellular localization, as well as downstream consequences on mitotic machinery and genome stability. Understanding how environmental signals and cellular stress integrate with phosphatase-mediated CDK control will deepen insight into plant adaptability and resilience.

The elegant combination of phosphoproteomics, molecular genetics, live-cell imaging, biochemical assays, and biophysical techniques exemplifies a multidisciplinary approach at the forefront of cell biology research. This integrative methodology serves as a blueprint for dissecting complex regulatory mechanisms in diverse biological systems.

In sum, the revelation that BSL1 dephosphorylates CDKB1 at Thr14 and Tyr15 represents a critical advance in unraveling plant-specific controls of cell division. This study not only fills a pivotal knowledge gap but also sets the stage for transformative applications in plant science and beyond.

Subject of Research:
The study investigates the regulation of mitotic entry in plants by elucidating the role of the plant-specific phosphatase BSL1 in dephosphorylating cyclin-dependent kinase B1 (CDKB1) at key inhibitory sites.

Article Title:
Mitotic entry is controlled by the plant-specific phosphatase BSL1 and cyclin-dependent kinase B.

Article References:
Tulin, F., Aizezi, Y., Reyes, A.V. et al. Mitotic entry is controlled by the plant-specific phosphatase BSL1 and cyclin-dependent kinase B. Nat. Plants (2025). https://doi.org/10.1038/s41477-025-02145-z

DOI:
https://doi.org/10.1038/s41477-025-02145-z