Effects of Exogenous Indole-3-Acetic Acid on the Regeneration from Sargassum fusiforme Holdfasts

Optimization of holdfast-based asexual propagation is critical for scalable, the genetically uniform seedling production in Sargassum fusiforme aquaculture owing to its capacity to preserve genetic uniformity. Although exogenous auxin can be applied to promote juvenile regeneration, the optimal concentration and associated physiological mechanisms remain insufficiently characterized. In this study, fine strains of S. fusiforme were selected to systematically investigate the concentration-dependent effects and regulatory mechanisms of exogenous indole-3-acetic acid (IAA) on holdfast regeneration. Results revealed that treatment with 2.0 mg·L−1 exogenous IAA significantly enhanced the photosynthetic performance relative to untreated controls. Specifically, this treatment resulted in increased accumulation of chlorophyll a, chlorophyll c, and fucoxanthin as well as elevated maximum relative electron transport rate, maximum quantum yield of PSI, light utilization efficiency, and net photosynthetic rate. Concurrently, nitrogen assimilation and metabolic utilization were markedly stimulated, as evidenced by significant increases in the activities of nitrogen synthesis-related enzymes and substantial accumulation of soluble proteins and total carbohydrates. These coordinated physiological improvements collectively contributed to a 185.4% increase in relative growth rate and 417.1% increase in holdfast regeneration rate as compared with the control group, thereby supplying abundant energy and biosynthetic precursors essential for photosynthesis and juvenile development. Ultimately, the holdfasts exhibited accelerated differentiation into morphologically mature juvenile thalli. Overall, application of 2.0 mg·L−1 exogenous IAA represent a highly effective strategy for enhancing holdfast regeneration efficiency and improving the viable seedling output. This study provides a mechanistic foundation and an expandable solution for addressing the supply of S. fusiforme seedlings.