Purification and partial characterization of isometric virus-like particles in Kalanchoë species.
Isometric virus-like particles (IVLP) were detected in crude sap from K. pinnata, K. daigremontiana and K. tubiflora plants showing a mild mosaic on the leaves. These particles of 35 nm in diam. were transmitted mechanically to several test plants but not to healthy Kalanchoë. Air temp. >30°C hindered the infection process. The IVLP were purified from systematically infected Nicotiana benthamiana using Triton X-100 as clarifying agent followed by PEG precipitation. IVLP were degraded by organic solvents and formed aggregates in the presence of 2 mmol/litre CaCl2. The particles occurred in relatively low concn in plant sap and lost infectivity in leaves frozen at -70° for one week and in purified preparations kept at 4°. In buffer crude sap of N. benthamiana IVLP had a thermal inactivation point between 45 and 50° and a longevity in vitro of 20 h at 25°. Particles contained 1 nucleoprotein component with a molecular weight of 46 000 daltons and a ssRNA species which, when denatured, had a molecular weight of 1.2 × 106. IVLP purified preparations exhibited a typical nucleoprotein absorption spectrum with a max. at 254-260 nm and a min. at 240-243 nm and a A 260/280 ratio of 1.56. The buoyant density of the IVLP was 1.32 g/ml calculated by isopycnic centrifugation on CsCl. Ultrastructural studies of infected leaves of K. pinnata indicated that IVLP caused an increase in chloroplast volume, distortion of the grana and reduced the number of thylakoids per grana. IVLP infection also impaired the diurnal pattern of synthesis and hydrolysis of starch, characteristic of CAM plants. The non-serological reaction of the IVLP with antisera specific to members of 7 different groups of spherical viruses as well as the combination of physicochemical properties and host range exhibited by these particles impeded their taxonomic location. In nature, young Kalanchoë plantlets acquire the IVLP through their physical connections with the infected mature leaves.