Geminiviruses (family Geminiviridae) are among the most devastating plant pathogens worldwide,causing heavy losses on food and cash crops and representing a threat to global food security and sustainability.These viruses have very compact genomes comprising one (monopartite) or two (bipartite) circular singlestranded DNA (ssDNA) molecules,each ranging from 2.6 to 2.8 kb.Amplification of geminiviral genome is achieved through a combination of rolling circle and recombination-mediated replication,producing double-stranded (ds) DNA replication form intermediates.Because of the very compact nature of geminiviral genomes,viral proteins have multiple functions.The smallest geminivirus open reading frame (ORF) codes for C4 in monopartite viruses or AC4 in bipartite viruses and is embedded within the coding region of the replication initiation protein (Rep),which is in a different ORF.C4 has long been demonstrated to be required for monopartite geminivirus infection.More recently,AC4 of African cassava mosaic virus (ACMV) (Hipp et al.,2016) and of Mungbean yellow mosaic virus (MYMV) (Carluccio et al.,2018) were shown to be required for virus infection.