The transcription factor IbNAC29 positively regulates the carotenoid accumulation in sweet potato

Abstract

Carotenoid is a tetraterpene pigment beneficial for human health. Although the carotenoid biosynthesis pathway has been extensively studied in plants, relatively little is known about their regulation in sweet potato. Previously, we conducted the transcriptome database of differentially expressed genes between the sweet potato (<i>Ipomoea batatas</i>) cultivar 'Weiduoli' and its high-carotenoid mutant 'HVB-3'. In this study, we selected one of these candidate genes, <i>IbNAC29</i>, for subsequent analyses. IbNAC29 belongs to the plant-specific NAC (NAM, ATAF1/2, and CUC2) transcription factor family. Relative <i>IbNAC29</i> mRNA level in the HVB-3 storage roots was ~1.71-fold higher than Weiduoli. Additional experiments showed that the contents of α-carotene, lutein, β-carotene, zeaxanthin, and capsanthin are obviously increased in the storage roots of transgenic sweet potato plants overexpressing <i>IbNAC29</i>. Moreover, the levels of carotenoid biosynthesis genes in transgenic plants were also up-regulated. Nevertheless, yeast one-hybrid assays indicated that IbNAC29 could not directly bind to the promoters of these carotenoid biosynthesis genes. Furthermore, the level of <i>IbSGR1</i> was down-regulated, whose homologous genes in tomato can negatively regulate carotene accumulation. Yeast three-hybrid analysis revealed that the IbNAC29-IbMYB1R1-IbAITR5 could form a regulatory module. Yeast one-hybrid, electrophoretic mobility shift assay, quantitative PCR analysis of chromatin immunoprecipitation and dual-luciferase reporter assay showed that IbAITR5 directly binds to and inhibits the promoter activity of <i>IbSGR1</i>, up-regulating carotenoid biosynthesis gene <i>IbPSY</i>. Taken together, <i>IbNAC29</i> is a potential candidate gene for the genetic improvement of nutritive value in sweet potato.