PlantMetBench: A Multimodal Knowledge Graph Benchmark for Biosynthesis Prediction

Plant metabolism produces a vast diversity of molecules, yet identifying the enzymes that catalyse biosynthetic reactions remains a major open problem. Existing benchmarks typically reduce this problem to isolated enzyme-reaction pairs, overlooking pathway and metabolite context that determines biological plausibility, leaving no standardised way to evaluate approaches that exploit this richer multimodal and relational structure. We present PlantMetBench, a multimodal graph benchmark built on PlantMetWiki, a curated knowledge graph of plant metabolism spanning 424 organisms. The resulting benchmark graph comprises 13,682 proteins, 2,741 genes, 4,577 metabolites, and 11,980 experimentally curated enzyme-reaction pairs. PlantMetBench frames enzyme-reaction prediction as an organism-disjoint link-prediction task and provides graph-native dataset splits, precomputed multimodal node features, and evaluation protocols. Using PlantMetBench, we evaluate heterogeneous graph neural networks, standard architectures (HeteroSAGE, GAT, HGT, and R-GCN), alongside residual and jumping-knowledge variants, against non-graph models. Most heterogeneous graph architectures consistently outperform non-graph baselines on our rank-based evaluation, suggesting the value of relational context for enzyme-reaction prediction. Yet, performance remains far from saturated, indicating substantial room for improved representations and modelling strategies. Also, we show that benchmark design choices (shortcuts, splits, and metrics) are key to aligning evaluation with biologically meaningful tasks. PlantMetBench provides a standardised, reproducible framework to evaluate multimodal biosynthetic data, establishing a foundation for future work on graph-based architectures for enzyme-reaction prediction. Code and data are publicly available.