Establishment of a Genetic Transformation System for Hippophae gyantsensis and the Regulatory Role of Hgfw2.2 and Hgfw3.2 in Fruit Size

Hippophae gyantsensis Lian is an important native tree species in the “One River, Two Streams” valley of Tibet, valued for its ecological restoration potential and nutrient-rich fruits. However, this species has several limitations, including a long fruiting cycle (3–5 years to flowering and 10–15 years to reach peak fruit production), small fruit size, and numerous branch thorns. These traits hinder large-scale cultivation and mechanized harvesting, creating an urgent need for improved varieties with larger fruit and higher yield. In this study, we established an efficient Agrobacterium-mediated genetic transformation system for H. gyantsensis using hypocotyls as explants. Under optimized conditions (OD600 = 0.5, AS = 200 μmol/L, infection time = 15 min), the transformation efficiency reached 36.67% (calculated as the number of PCR-positive plants divided by the total number of explants initially inoculated with Agrobacterium). A rooting rate of 12.5% was achieved using 100 mg/L rooting powder (ABT1) for 40 min, resulting in an overall success rate of approximately 4–5%. Furthermore, we identified and cloned two fruit-size-related genes, Hgfw2.2 and Hgfw3.2, from H. gyantsensis. Heterologous expression of Hgfw2.2 and Hgfw3.2 in tomato decreased and increased fruit size, respectively, consistent with their regulatory roles in fruit development. Given the positive regulatory effect of Hgfw3.2, this gene was further transformed into H. gyantsensis. This study represents the first report of a stable genetic transformation platform for H. gyantsensis, providing a robust technical foundation for future molecular breeding and the development of improved, large-fruited varieties.