Identification and expression analysis of chlorogenic acid biosynthesis key gene PpHCT in peach

Shikimic acid/quinic acid hydroxy cinnamyl transferase (HCT) is one of the key enzymes in the phenylpropanoid pathway. However, the role of the HCT gene in chlorogenic acid (CGA) biosynthesis in peach fruit remains unclear. For this, we identified the accumulation pattern of CGA in four peach cultivars, cloned and characterized 11 PpHCT gene members, and further analyzed the expression patterns of these PpHCT genes during fruit development. The contents of CGAs in the four peach cultivars all exhibited a trend of increasing and then decreasing during the fruit growth and development. Moreover, the contents of CGAs in the peel and flesh were tissue-specific. Gene structure analysis indicated that the PpHCT genes were highly conserved, containing two exons and one intron. The protein structure analysis demonstrated that the PpHCT proteins contained two conserved motifs (HXXXD, DFGWG) and a transferase domain (PF02458), which belonged to the BAHD acyltransferase family. The cis-acting element analysis suggested that the promoters of PpHCT genes contained many light-related, hormone-related, stress-related, tissue-specific, and circadian-related elements, and they could participate in a variety of biological processes. Phylogenetic analysis showed that the HCT proteins of peach were closely related to the HCT proteins of plum and had a close evolutionary relationship. The qRT-PCR analysis indicated that the expression levels of PpHCT1 and PpHCT2 showed an opposite trend to the accumulation of CGA, whereas the expression levels of PpHCT4, PpHCT5, PpHCT7, PpHCT8, and PpHCT11 demonstrated the same trend as CGA accumulation. It was worth noting that only PpHCT4 and PpHCT5 were highly expressed in the two high-CGA cultivars but showed low levels of expression in the two low-CGA cultivars. Therefore, it was hypothesized that these two genes might be key genes to the synthesis of CGA in peach fruit. Those findings provide a theoretical basis for further study on the biological functions of the HCT gene and help to reveal the molecular mechanism of CGA.