Transcriptional analysis reveals formation of axillary solitary hook in vine plant Uncaria rhynchophylla

The hook of Uncaria rhynchophylla is an important external phenotype representing the quality of Gou-Teng. The hook formation pattern of U. rhynchophylla displays a unique feature that solitary hook and opposite hooks alternately occur in the leaf axils forming a liner series. The solitary hook only grows on the lower side of the leaf axil, resulting from the decline in the total yield of hooks. However, the reasons for solitary hook formation in U. rhynchophylla are not clear. Therefore, a survey on the molecular mechanisms of solitary hook formation needs to be performed urgently. We obtained 250.67 million high-quality clean reads from the lower side of axillary stem segments (HS) and the upper side of axillary stem segments (HSCK) libraries. We detected 42 DEGs (differentially expressed genes) between HS and HSCK libraries; 11 were down-regulated, and 31 were up-regulated. Gene ontology functional classification of the DEGs indicated that they included two genes that encoded LAZY1 related to the regulation of polar auxin transport, two genes that encoded AUX/IAA and AP2/ERF related to plant hormone signaling transduction, and two genes that encoded F3H related to the flavone synthase activity. These six genes were associated with lateral polar auxin transport (PAT) regulation, and their expression level had a high correlation with the pattern of IAA distribution upon gravity stimulation. And the inhibition of solitary hook development was observed after treatment with N -1-naphthylphthalamic acid (NPA) or quercetin, which was known as the polar auxin inhibitor. The above findings might provide new information about the molecular mechanisms of regulation of lateral PAT in the axillary bud development process under gravity.