Developmental study of thorns in tissue culture clone of Lycium ruthenicum

Abstract

Lycium ruthenicum is a thorny shrub species that possesses both economic and ecological benefits; however, the processes and mechanisms underlying its thorn development remain inadequately understood. This study found that clonal line 11 of L. ruthenicum exhibited no thorns prior to transplantation. However, 40 days after transplantation, two distinct phenotypes emerged: thorny and thornless. Thorny plants exhibited significantly higher thorn frequency, larger stem diameters (both at the apex and base), increased leaf density, greater leaf width, and elevated photosynthetic rates in the apical buds and leaves compared to thornless plants. Histological analysis of paraffin sections demonstrated that the meristem tissues located at the axils of thornless stems progressively differentiated into axillary buds, without giving rise to thorn primordia. In contrast, the meristem tissues at the axils of thorny stems developed into thorn primordia, which subsequently matured into branch-thorns, while the tissues beneath the thorn primordia differentiated into axillary buds. The developmental stages of the apical bud and the upper four stem nodes were identified as critical periods determining the presence of thorns on the branch. RNA fluorescence in situ hybridization analysis demonstrated that a L. ruthenicum sucrose synthase gene (LrSUS) was highly expressed in leaflets and leaf primordia, as well as in glandular hairs, but not in thorn structures. Additionally, real-time quantitative PCR analysis revealed that the expression level of LrSUS in the leaves of thorny plants was significantly higher than that observed in the leaves of thornless plants. These suggest that the LrSUS gene does not directly promote thorn formation through high expression in thorn primordia; instead, it may indirectly promote thorn development by its elevated expression in leaves and enhancing overall photosynthesis. This study elucidates the developmental process of thorn in L. ruthenicum line 11, emphasizes the role of LrSUS in thorn development, and lays a foundation for future gene screening and mechanistic investigations.