Abstract
Cassava brown streak disease (CBSD) is a destructive disease of cassava in Eastern and Central Africa. Because there was no source of resistance in African varieties to provide complete protection against the viruses causing the disease, we searched in South American germplasm and identified cassava lines that did not become infected with the cassava brown streak viruses. These findings motivated further investigations into the mechanism of virus resistance. We used RNAscope® in situ hybridization to localize cassava brown streak virus in cassava germplasm lines that were highly resistant (DSC 167, immune) or that restricted virus infections to stems and roots only (DSC 260). We show that the resistance in those lines is not a restriction of long-distance movement but due to preventing virus unloading from the phloem into parenchyma cells for replication, thus restricting the virus to the phloem cells only. When DSC 167 and DSC 260 were compared for virus invasion, only a low CBSV signal was found in phloem tissue of DSC 167, indicating that there is no replication in this host, while the presence of intense hybridization signals in the phloem of DSC 260 provided evidence for virus replication in companion cells. In neither of the two lines studied was there evidence of virus replication outside the phloem tissues. Thus, we conclude that in resistant cassava lines, CBSV is confined to the phloem tissues only, in which virus replication can still take place or is arrested.
Highlights
Cassava brown streak disease (CBSD) is the most devastating disease of cassava (Manihot esculenta Crantz) that strongly affects the quality and yield of this important food crop’s storage roots
The disease is caused by cassava brown streak virus (CBSV) and Uganda cassava brown streak virus (U/CBSV), both being distinct species of the genus Ipomovirus in the family Potyviridae [1]
We identified several lines immune to CBSV and we found others in which the virus infection was confined to the roots and no virus was detectable in leaf tissues
Summary
Cassava brown streak disease (CBSD) is the most devastating disease of cassava (Manihot esculenta Crantz) that strongly affects the quality and yield of this important food crop’s storage roots. In the “immune” cassava, accession DSC 167 (CIAT Col 2182), we previously found no evidence for virus replication and neither of the two viruses U/CBSV were traceable in leaves or tuberous roots [4] This observation led us to speculate on the fate of the virus in a resistant line, in particular when the virus is continuously delivered from an infected susceptible source through a vascular tissue junction established by grafting. Prior to biochemical and molecular studies that would lead to a mechanistic description of resistance, we were interested to further study virus translocation and replication in susceptible cassava and in the resistant lines DSC 167 and DSC 260, to shed light on the tissues and cell types that may be involved in restricting virus movement and replication. We employed cryo-sectioning to localize the virus in invaded cells which we later want to combine with RNA isolation and RNA-seq to elucidate the molecular basis of the resistance types we have identified
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