Phytohormones in Kappaphycus alvarezii: Genomic Insights, Activation Thresholds, and Implications for Seaweed-derived Biostimulants

The use of seaweed extracts in agriculture has gained significant attention in recent years, yet the genomic mechanisms underlying their activity remain poorly understood. Seaweeds such as Kappaphycus alvarezii synthesize diverse bioactive compounds, but their endogenous hormone pathways have been largely uncharacterized. Phytohormones, essential regulators of plant growth, reproduction, and stress responses, are central to the beneficial effects of these biostimulants. To uncover these mechanisms, we performed a genome-wide analysis of hormone biosynthesis and recognition pathways across thirteen algal species, focusing on nine classes of phytohormones. By integrating KEGG annotation, structural modeling, and eggNOG-based orthology, we assessed the completeness and conservation of hormone-associated pathways. Our results revealed consistent retention of key biosynthetic entry points, including IAM-related enzymes for auxin, TRIT1/CYP735A for cytokinins, GA oxidases for gibberellins, and pchA for salicylic acid, alongside divergent or absent canonical upstream enzymes. In parallel, partial but conserved signaling modules were identified for abscisic acid, ethylene, and other classes. This mosaic pattern, fragmented biosynthesis coupled with selectively retained signaling, suggests that algae employ noncanonical enzymatic routes or microbial complementation to sustain hormone activity. Expression profiling in K. alvarezii further revealed light-responsive regulation of auxin, cytokinin, GA, and ABA genes, highlighting their role in environmental adaptation. Together, these findings provide new genomic insights into algal hormone biology, establish the first structural and functional evidence for GA metabolism in red algae, and identify candidate phytohormones likely to contribute to the biostimulant activity of K. alvarezii extracts.