Age-dependent starch-sucrose metabolic transition during bulb development of Fritillaria unibracteata

The bulb of Fritillaria unibracteata is a valuable traditional Chinese medicinal material whose bioactive compound accumulation is closely linked to carbon metabolism. As a perennial herb, its bulbs undergo multi-year developmental cycles, with starch and sucrose serving as key storage and mobilizable carbohydrates, respectively. However, the age-dependent dynamics of starch-sucrose interconversion during bulb development and its expression patterns remain poorly understood, limiting our comprehension of the carbon allocation dynamics that support biomass accumulation and medicinal metabolite production.

Starch content increased progressively from the first to fourth year, peaked in the fourth year, and then significantly declined in the fifth year. By contrast, sucrose levels exhibited an opposite trend, decreasing in the fourth year and rising sharply in the fifth year, indicating a marked shift in carbohydrate allocation during the late developmental stage. Transcriptome analysis identified 21,964 differentially expressed genes, among which the starch and sucrose metabolic pathway was significantly enriched. Notably, key genes involved in starch biosynthesis, including ADP-glucose pyrophosphorylase and soluble starch synthase/granule-bound starch synthase, showed their highest expression levels in the fourth year, coinciding with maximum starch accumulation. Conversely, the expression of genes associated with sucrose biosynthesis, particularly sucrose phosphate synthase, was significantly upregulated from the fourth to fifth year, corresponding to the observed sucrose increase. Enzyme activity assays mirrored these transcriptional patterns. Weighted gene co-expression network analysis further identified modules associated with starch-sucrose metabolic traits, providing evidence for coordinated transcriptional networks in carbon allocation. Collectively, these findings reveal a pronounced metabolic transition from starch accumulation to sucrose synthesis during bulb development.

The transition from the fourth to fifth year represents a critical developmental stage, during which the carbon allocation pattern shifts from starch-dominated storage to enhanced sucrose accumulation and mobilization. This study provides evidence for an age-dependent starch-sucrose metabolic pattern during bulb development in F. unibracteata, offering a theoretical basis for understanding carbon allocation dynamics in Fritillaria bulbs.