Hijacking of the Plant Transcription Factor TCP24 by Geminiviruses for the Regulation of Late Gene Expression

The viral protein TrAP (AL2) regulates expression of the coat protein (CP) in many geminiviruses of the genus Begomovirus. AL2 does not efficiently bind double stranded DNA which suggests that it interacts with intermediate proteins of the host cell to activate AL2 responsive promoters. Two host proteins have been identified that may be recruited to the promoter to regulate coat protein (CP) expression, Arabidopsis PPD2 and TCP24. It is known that PPD2 can form a complex with AL2 on currently unidentified sequences within the CP promoter and in the current research we show that AtTCP24 interacts with conserved sequences within begomovirus CP promoters. Our hypothesis is that TCP24 represses CP promoter activity early in infection and that AL2 relieves that repression later in the infection cycle. In the current research we show through electrophoretic mobility shift assays, qPCR, plant infections and western blot, that AtTCP24 appears to play a critical role in CP promoter activity. We show that AtTCP24 binds to a conserved late element (CLE) sequence in Tomato golden mosaic virus and Cabbage leaf curl virus and in infectivity assays, symptom appearance is delayed in plants inoculated with viral constructs containing a mutation in the CLE. Furthermore, qPCR data show that there is a decrease in viral load in plants infused with cle mutant viruses and that CP mRNA levels are reduced. Based on these results we conclude that AtTCP24 binds the CP promoter and likely represses its activity.