Evolutionary Diversification of the Maize Str-like Gene Family Revealed Through Sequence, Structural and Functional Analyses

Strictosidine synthases (STRs) are catalytic enzymes involved in terpenoid indole alkaloid biosynthesis, whereas STR-like (STRL) genes in cereal crops remain poorly understood. Previous studies of the maize STR-like (STRL) gene family have mainly provided genome-wide identification, phylogenetic classification, structural annotation and expression profiling, but the evolutionary constraints and molecular mechanisms underlying STRL diversification remain insufficiently resolved. In this study, we investigated the maize STRL gene family from an evolutionary and structural perspective by integrating sequence divergence, codon usage bias, selection pressure, protein structural modelling, Gene Ontology (GO) enrichment and tissue-specific expression analysis. A total of 21 ZmSTRL genes were analyzed and their comparative and phylogenetic analyses revealed conserved lineages together with maize-associated expansion patterns. Codon usage and neutrality analyses indicated heterogeneous evolutionary constraints among ZmSTRL genes, suggesting that mutational pressure alone does not explain their sequence divergence. Protein conservation and three-dimensional structural modelling showed a generally conserved STR-related catalytic framework, while member-specific variation in terminal and loop regions suggested localized structural divergence. GO enrichment supported conserved catalytic and metabolic signatures, but these associations were interpreted as putative functional evidence rather than direct functional confirmation. Tissue-specific qRT-PCR analysis revealed divergent expression patterns among selected ZmSTRL genes in root, stem, leaf, and anther tissues, indicating possible regulatory specialization. Overall, this study provides an evolutionary-constraint-based framework for understanding STRL diversification in maize and identifies candidate genes and structural features for future functional validation.