HEADLESS represses a CYP735 monooxygenase gene and promotes iP-type cytokinin accumulation in the <i>Medicago truncatula</i> shoot apex
Bibliographic record
Abstract
Cytokinins comprise a class of phytohormones that promote cell division and participate in diverse developmental processes. Cytokinin activity and steady-state levels in tissues are maintained through biosynthesis, signaling, degradation, side-chain modification, transport, and storage. Mutating HEADLESS (HDL), a Medicago truncatula homolog of Arabidopsis WUSCHEL (WUS), terminates the shoot apical meristem (SAM); thus, hdl mutants continuously generate leaves. In this study, we isolated a direct target of HDL [(HDL Target 1, HDT1)] based on RNA-seq transcriptome analyses using inducible HDL overexpression lines and the hdl mutant. HDT1 encodes a putative cytokinin hydroxylase homologous to Arabidopsis Cytochrome P450 mono-oxygenase CYP735A1 and CYP735A2. HDT1 trans-hydroxylates the prenyl side chain of cytokinins, converting the isopentenyl-adenine (iP)-type into trans-Zeatin (tZ)-type cytokinins. We show that HDL directly binds to the HDT1 promoter and represses its transcription, promoting the preferential accumulation of iP-type cytokinins. Overexpressing HDT1 slowed plant growth, leading to shorter plants caused by inhibited internode elongation, which is reminiscent of the hdl mutant phenotype albeit less severe. Both the hdl mutant and HDT1 overexpression lines accumulated lower levels of the iP-type but higher levels of the tZ-type cytokinins in shoots, indicating that HDT1 promotes tZ-type cytokinin biosynthesis and that HDL represses this process. Overall, this study uncovers the role of HDL in cytokinin biosynthesis and suggests that iP is preferred over tZ for SAM function in M. truncatula, opening avenues of research to better understand HDL function and the molecular mechanisms that regulate cytokinin biosynthesis.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".