Unusual Accumulations of Cauliflower Mosaic Virus in Local Lesions, Dark Green Leaf Tissue, and Roots of Infected Plants. Nadia S Al-Kaff. Department of Virus Research, John Innes Centre, Colney Lane, Norwich NR4 7UH, U.K. Simon N. Covey Department of Virus Research, John Innes Centre, Colney Lane, Norwich NR4 7UH, U.K. MPMI 9:357-363. Accepted 19 February 1996. Copyright 1996 The American Phytopathological Society.

Macroscopic in situ nucleic acid hybridization was used to detect cauliflower mosaic virus (CaMV) in infected plant organs to investigate the relationship between virus distribution and pathogencsis. More foci of infection were detected by direct hybridization to inoculated host leaves than was suggested from local lesion symptoms. Older lesions had a complex structure with a focused ring of signal and a diffuse outer halo around a hollow center lacking virus. Virus that had systemically invaded expanded leaves and caused vein clearing was concentrated close to the vein borders and veinlets; less virus was in the in-terveinal tissues. In contrast, virus in younger systemically infected leaves accumulated in interveinal green islands rather than in the chlorotic vein borders, suggesting a different type of host-virus interaction from those in which green islands are virus free. Hybridization directly to cut surfaces of thicker organs showed that CaMV was present in most parts of the vegetative plant in varying concentrations but predominantly in the vascular tissue of petioles, stem, hypocotyl, and root. We also discovered a significant accumulation of virus in a relatively undifferentiated zone of the hypocotyl immediately below the stem apex which could act as a reservoir of virus supplying new leaves emerging from the apex.