Morphological studies of developing glandular trichomes and a novel HD-ZIP gene LjROC3 increasing glandular trichome density on leaves in honeysuckle (Lonicera japonica)

Glandular trichomes (GTs) of Lonicera japonica are the exocrine structure containing many medicinally active ingredients and essential oil. In this study, the GTs distribution in the shooting of honeysuckle was observed. On the leaf surface, GTs were mainly distributed along the prominent veins and decreased from the base to the tip, and their density gradually decreased with the development of leaves. However, GTs were evenly distributed on the stem surface, and their density first increased and then decreased as the stem continued to develop. Then, scanning electron microscopy (SEM) and paraffin sectioning (PS) were employed to identify the morphological structure and types of GTs. All GTs in L.japonica were first found to contain a disc-shaped base surrounding the basal cell and were newly named disc-pedestal capitate GT. Here, we also identified and compared the functions of two novel HD-ZIP genes designated as LjROC3 and LjHD-ZIP1. During GT development, these two HD-ZIP genes were both up-regulated. In addition, the heterologous expression and virus-induced gene silencing (VIGS) of LjROC3 increased and decreased the density of GTs, respectively, whereas the transformed lines of LjHD-ZIP1 showed no significant differences. Furthermore, the molecular mechanism of LjROC3 was analyzed using qRT-PCR in silencing lines. These results will provide valuable references for timely picking high metab-olite medicinal materials, exploring the regulatory mechanism of GT development, and breeding new varieties with high GT density in L. japonica.