The role of CitMYC3 in regulation of valencene synthesis-related CsTPS1 and CitAP2.10 in Newhall Sweet Orange

CsTPS1 (Cs5g12900) is a gene encoding a sesquiterpene synthase involved in generating valencene, a key volatile compound in sweet orange fruit. Here, a citrus basic helix-loop-helix (bHLH) transcription factor (CitMYC3), is characterized as a positive regulator of CsTPS1. Dual-luciferase assays showed that CitMYC3 transcriptionally activates its own promoter, also the CsTPS1 promoter and, in addition, the promoter of a second transcription factor, CitAP2.10, which in turn positively regulates the CsTPS1 expression. During the developmental period, CitMYC3 transcripts were detectable at 60 days after full bloom (DAFB) and increased gradually to 120 DAFB, while CsTPS1 mRNA rapidly increased from 90 to 150 DAFB. Exogenous ethylene accelerated the expression of CitMYC3 (peaking at 4 h) before CsTPS1 expression (at 24 h). Furthermore, CitMYC3 and CitAP2.10 were also shown to activate the promoter of Arabidopsis AtTPS11, a homolog of CsTPS1, which is in charge of the synthesis of over fifteen different sesquiterpenes, indicating that CitMYC3 and CitAP2.10 have a likely conserved function in the regulation of sesquiterpene genes across a variety of plant species.