A Ca²⁺/Calmodulin-Interacting IQD Hub in Tartary Buckwheat: Genome-Wide FtIQD Analysis and Characterization of FtIQD19

IQ67-domain (IQD) proteins are plant-specific calmodulin (CaM)/calmodulin-like (CML) targets implicated in the spatial organization of Ca²⁺ signaling, yet their roles in tartary buckwheat (Fagopyrum tataricum) remain largely unexplored. Here, we identified 24 FtIQD genes and classified them into six phylogenetic subfamilies. FtIQDs show uneven chromosomal distribution and mainly arise from segmental duplication under purifying selection. Promoter analysis revealed the enrichment of MYB-, light-, and ABA-related cis-elements. To link FtIQDs with rutin variation, we performed an FtIQD-focused association analysis using whole-genome resequencing data from altitude-stratified panels of up to 220 accessions. Under additive, dominant, and recessive models, multiple significant SNPs (p < 1 × 10^-5) were detected near a subset of FtIQD loci, showing clear model- and environment-dependent patterns. Recurrent loci included FtIQD22, FtIQD02, FtIQD16, and FtIQD19. RNA-seq under PEG-induced drought stress, tissue expression patterns, pathway co-expression, and qRT-PCR further prioritized FtIQD19. FtIQD19-GFP showed predominant nuclear localization with additional filamentous/peripheral signals, and yeast two-hybrid assays identified FtCaM7.2 as the strongest interactor among representative CaMs. Structural modeling of the FtIQD19-FtCaM7.2 complex suggested testable residue-level interaction features. Collectively, this work provides a foundational FtIQD resource and highlights candidate Ca²⁺/CaM-IQD modules potentially associated with altitude-dependent rutin variation in tartary buckwheat.