Probable mechanical transmission of a virus-like agent from rose rosette disease-infected multiflora rose to Nicotiana species

As part of efforts to identify the causal agent of the rose rosette disease (RRD) of multiflora rose (Rosa multiflora Thunb.), root tip extracts from both symptomatic and nonsymptomatic roses were used to mechanically inocula te leaves of Nicotiana glutinosa. Pale green spots were observed along the margins of the major leaf veins only on leaves inoculated with extracts pre pared from symptomatic rose plants. Light microscopy revealed abnormal deve lopment of the palisade and spongy mesophyll cells in the symptomatic tissu e, although no virus-like particles (VLPs) were observed by electron micros copy. However, VLPs were observed in cells from tissue adjacent to the leaf veins and bordered by the pale green spots. Inoculation of N. benthamiana with extracts from symptomatic N. glutinosa initially did not result in vis ible symptoms on N. benthamiana-inoculated leaves. However, approximately 4 wk post inoculation, splitting of leaf tissue across and along major leaf veins in expanding leaves occurred. In later stages of leaf expansion some leaves split in regions not associated with veins. Light microscopy of thic k sections revealed separation between palisade cells and groups of small d ead cells in the mesophyll tissue of expanding systemically infected leaf b lades. Electron microscopy revealed crystalline arrays in the cytoplasm of mesophyll cells. No abnormal cellular changes were observed in plants inocu lated with extracts prepared from nonsymptomatic rose plants.