The MYB transcription factor CiMYB42 regulates limonoids biosynthesis in citrus.

Background: Limonoids are major bioactive compounds that are produced by the triterpenoid metabolic pathway. The detailed biochemical process of limonoid biosynthesis and the mechanism of its molecular regulation remain elusive. The identification of transcription factors that regulate limonoid biosynthetic pathways is very important for understanding the underlying regulatory mechanisms. This information could also provide tools for manipulating biosynthesis genes to modulate limonoid production.

Results: In this study, the CiMYB42 transcription factor was isolated to identify its role in limonoid biosynthesis. Multiple alignment analysis and phylogenetic analysis demonstrated that CiMYB42 is a typical R2R3MYB transcription factor that shares high similarity of its amino acid sequence with AtMYB42. Limonoids contents were higher in Citrus sinensis and Citrus grandis than in other species. Limonoid accumulation during leaf development also showed diverse trends in different genotypes. The expression of CiMYB42 was significantly related to the limonoid content and the expression of CiOSC in some citrus accessions. The overexpression of CiMYB42 in sweet orange resulted in significant accumulation of limonin, whereas the downregulation of CiMYB42 by RNAi resulted in a dwarf phenotype and less nomilin accumulation. Furthermore, the results of a yeast one-hybrid assay and EMSA indicated that CiMYB42 binds exclusively to the TTGTTG sequence (type II MYB core) in the promoter of CiOSC. Together, these results suggest that CiMYB42 positively regulates limonoid biosynthesis by regulating the expression of CiOSC by binding to the TTGTTG sequence (type II MYB core) of its promoter.

Conclusions: CiMYB42 is an important transcription activator involved in limonoid biosynthesis that regulates the expression of CiOSC by binding to the TTGTTG sequence (type II MYB core).