In situ hybridization and promoter analysis reveal that cotton leaf curl Multan betasatellite localizes in the phloem

Begomoviruses (the family Geminiviridae) have either amonopartite or abipartite (DNA Aand DNA B) single-stranded DNA genome. DNA B contains the open reading frame for movement protein and enables some bipartite begomoviruses to invade non-phloem tissues, whereas monopartite begomoviruses are limited to the phloem. Betasatellite DNA replicates in association with some monopartite begomoviruses to produce severe symptoms and enhance replication of helper virus in some hosts. The cotton leaf curl Multan betasatellite (CLCuMuB) has been shown to substitute for the DNA B of abipartite begomovirus by permitting systemic infection, but the tissue tropism and the role of betasatellites in releasing monopartite begomoviruses from the phloem to adjacent tissue has not been described. In this study, the tissue tropism of CLCuMuB and amonopartite helper virus, tomato leaf curl virus (ToLCV), has been investigated using in situ hybridization, promoter localization and analysis of transgenic 2 beta plants, which contain adimer of the CLCuMuB genome. In situ hybridization showed that CLCuMuB and ToLCV were localized into the vascular tissue of infected plants. In addition, CLCuMuB was detected only in the vascular tissue of 2 beta transgenic plants after infection with ToLCV. beta-glucuronidase (GUS) expression under the beta C1 promoter was also limited to the phloem tissues. In conclusion, we showed for the first time that CLCuMuB localizes only in the phloem tissues and unlike the DNA B component, CLCuMuB is unable to release the monopartite helper virus out of phloem tissues.