Background: Hydroxycinnamoyl-CoA quinate hydroxycinnamoyl-transferase (HQT) is an essential enzyme for chlorogenic acid (CGA) biosynthesis in plants. Multiple HQT-encoding genes potentially involved in CGA synthesis in sweet potato (Ipomoea batatas) have been predicted. However, the functions of these genes have not been verified. Methods: In this study, the gene IbHQT1 was isolated from the sweet potato cultivar ‘Nanshu-88’ and functionally characterized using transgenic technology. Results: IbHQT1 encodes a protein comprising 431 amino acids, with conserved HXXXD and DFGWG motifs characteristic of BAHD acyltransferase family members. A phylogenetic analysis indicated that IbHQT1 has a close evolutionary relationship with StHQT in Solanum tuberosum. According to qPCR data, IbHQT1 is highly expressed in young leaves, and its expression is affected by exogenous MeJA (100 µM), ABA (100 µM), GA₃ (50 µM), and SA (100 µM). Analyses of cis-acting regulatory elements indicated that the IbHQT1 promoter contains multiple elements responsive to MeJA, ABA, SA, GA₃, and light. In plants overexpressing IbHQT1, CGA contents in mature leaves and storage roots increased 1.30- to 1.44-fold and 1.28- to 1.43-fold, respectively. Conversely, in IbHQT1-RNAi lines, CGA contents in mature leaves and storage roots decreased by 16–38% and 18–40%, respectively. Conclusions: These findings indicate that IbHQT1 positively regulates CGA biosynthesis in sweet potato plants.

Stomata are essential for photosynthesis and water-use efficiency in plants. When expressed in transgenic Arabidopsis thaliana plants, the potato (Solanum tuberosum) proteins EPIDERMAL PATTERNING FACTOR 2 (StEPF2) and StEPF-LIKE9 (StEPFL9) play antagonistic roles in regulating stomatal density. Little is known, however, about how these proteins regulate stomatal development, growth, and response to water deficit in potato. Transgenic potato plants overexpressing StEPF2 (E2 plants) or StEPFL9 (ST plants) were generated, and RT-PCR and Western blot analyses were used to select two lines overexpressing each gene. E2 plants showed reduced stomatal density, whereas ST plants produced excessive stomata. Under well-watered conditions, ST plants displayed vigorous growth with improved leaf gas exchange and also showed increased biomass/yields compared with non-transgenic and E2 plants. E2 plants maintained lower H₂O₂ content and higher levels of stomatal conductance and photosynthetic capacity than non-transgenic and ST plants, which resulted in higher water-use efficiency and biomass/yields during water restriction. These results suggest that StEPF2 and StEPFL9 functioned in pathways regulating stomatal development. These genes are thus promising candidates for use in future breeding programs aimed at increasing potato water-use efficiency and yield under climate change scenarios.