| Topic | Key points | Quantitative data | Organism/background | Supporting citation IDs |
|---|---|---|---|---|
| Identity/locus | OsCKX2 is the rice cytokinin oxidase/dehydrogenase 2 gene; the grain-number QTL **Gn1a** was molecularly cloned as **OsCKX2** on chromosome 1. Reduced-expression or null alleles increase cytokinin in inflorescence meristems and raise grain number. This matches the UniProt target CKX2/OsCKX2/Gn1a from *Oryza sativa*. | Habataki Gn1a allele increased grain number by **~92 grains per main panicle** and explained **44%** of the grain-number difference versus Koshihikari; a natural 11-bp deletion allele in 5150 was associated with **>400 grains per main panicle**. | Rice (*Oryza sativa*), especially japonica Koshihikari and indica Habataki/5150 backgrounds | (pqac-00000000, pqac-00000005) |
| Enzymatic function | OsCKX2 encodes a CKX-family enzyme that **irreversibly degrades cytokinins**, thereby lowering endogenous CK abundance in reproductive meristems and other tissues. Functional expression in yeast confirmed CKX catalytic activity for OsCKX2 protein variants. | Reduced OsCKX2 activity in Gn1a-associated lines caused accumulation of cytokinin nucleotides such as **tZRMP** and **iPRMP** relative to Koshihikari. | Rice Gn1a/OsCKX2 lines; yeast heterologous expression for catalytic confirmation | (pqac-00000014, pqac-00000001) |
| Substrates & mechanism | Plant CKX enzymes are **FAD-dependent flavoenzymes** that cleave the unsaturated N6 isoprenoid side chain of cytokinin nucleobases/ribosides, yielding adenine/adenosine plus an aldehyde. Family-level evidence indicates preference for **isoprenoid CKs** such as **iP** and **tZ** (and their ribosides), supporting inference for OsCKX2. Rice-specific knockdown evidence indicates OsCKX2 normally restrains **tZ, 2-iP, kinetin, and DHZ** in inflorescence meristems. | CKX enzymes have **low-micromolar Km** for iP/tZ-type substrates in family-level studies; OsCKX2-RNAi/knockdown plants showed elevated **tZ, 2-iP, kinetin, DHZ** in IM tissue. | General plant CKX enzymology with rice OsCKX2 functional inference; rice inflorescence meristem knockdown lines | (pqac-00000015, pqac-00000020, pqac-00000013, pqac-00000017) |
| Localization | Direct OsCKX2 localization evidence was not retrieved in the gathered texts. However, OsCKX2 has a signal peptide in UniProt and CKX family proteins localize to distinct compartments including **extracellular/apoplastic, ER-associated, vacuolar, and cytosolic** pools depending on isoform. OsCKX2 function is experimentally tied to the **inflorescence meristem/panicle tissue** where its transcript abundance affects CK levels. | No direct OsCKX2 compartment quantification retrieved; OsCKX3 localizes predominantly to **ER**, OsCKX4 is **cytosolic**, illustrating family divergence. | Rice CKX family context; OsCKX2 expression/function studied in inflorescence meristems and young panicles | (pqac-00000019, pqac-00000013, pqac-00000012) |
| Pathway/regulation | OsCKX2 is a core node in cytokinin-homeostasis pathways controlling inflorescence meristem activity. **DST** directly activates OsCKX2; **MED25** acts as a DST coactivator; phosphorylation by the **OsMKKK10–OsMKK4–OsMPK6** cascade promotes DST-mediated OsCKX2 expression. **FZP** represses **DST**, thereby indirectly limiting OsCKX2 and sustaining CK in young panicles. | In the severe **fzp/abp1** allele, **grain number per main panicle fell ~66%**; in fzp young panicles, CKs decreased: **IP ~60% lower**, **IPA ~25% lower**, **zeatin ~27% lower**. | Rice young panicles/inflorescence meristems; FZP-DST-OsCKX2 regulatory module | (pqac-00000010, pqac-00000009) |
| Phenotypes/yield data | Lower OsCKX2 expression increases sink strength by enhancing meristem activity, panicle branching, tillering, and grain number; overexpression/activation has the opposite effect. OsCKX2 is therefore a **negative regulator** of grain number and tiller formation through cytokinin catabolism. | shRNA suppression caused **27–81% more tillers**, **24–67% more grains per plant**, and **5–15% higher 1000-grain weight** in field tests; insertional activation reduced tiller number in a dosage-dependent manner. | Transgenic rice, including *O. sativa* cv. TNG67; greenhouse and field experiments | (pqac-00000001, pqac-00000007) |
| Recent 2023–2024 developments | 2023 work refined the upstream transcriptional network controlling OsCKX2 via **FZP→DST→OsCKX2** and linked altered OsCKX2 expression to reduced cytokinin and defective spikelet development. 2024 CRISPR work showed that **Osckx2** loss-of-function in indica rice increases panicle branching and grain yield while also improving drought-related performance. | 2023: **66% reduction** in grain number per main panicle in severe fzp background; CK decreases of **~60% IP**, **~25% IPA**, **~27% zeatin**. 2024: qualitative increases in secondary branching, grain number, and drought survival traits; provided pages did not include exact numeric effect sizes. | 2023 BMC Plant Biology study in rice panicles; 2024 CRISPR/Cas9 study in indica rice cv. MTU1010 | (pqac-00000009, pqac-00000010, pqac-00000008, pqac-00000011) |
| Applications | OsCKX2/Gn1a is a validated target for **yield breeding**, **QTL introgression**, **gene pyramiding**, RNAi knockdown, and **CRISPR/Cas9 editing**. Practical use cases include exploiting weak/null alleles to raise grain number and using edited alleles for high-yield, climate-resilient rice. | Gn1a introgression delivered major sink-size effects (e.g., **~92 grains/panicle**, **44%** difference component); recent CRISPR mutants increased grain number and drought tolerance qualitatively; OsCKX2 knockdown also reduced yield penalty under salinity in earlier work. | Rice breeding in japonica and indica backgrounds; near-isogenic lines, transgenics, and CRISPR-edited plants | (pqac-00000000, pqac-00000003, pqac-00000008, pqac-00000005) |


*Table: This table compiles the main functional-annotation evidence for rice OsCKX2/Gn1a, including identity, enzymatic role, regulatory pathway, quantitative phenotypes, and 2023–2024 updates. It is useful as a compact evidence map for gene-function interpretation and breeding relevance.*