SUBCELLULAR LOCATION OF AVOCADO SUNBLOTCH VIROID IN AVOCADO LEAVES

The subcellular accumulation site of avocado sunblotch viroid (ASBV) in avocado leaves has been investigated. By differential centrifugation techniques, ASBV was predominantly found in a form non-sedimentable at 100 000 x g for 2 h. A minor proportion of the viroid was also detected in association with larger cellular constituents. This association appeared to be mostly artefactual, since a very similar distribution pattern of ASBV was observed in control experiments with non-infected leaves processed with grinding medium containing purified viroid. Moreover, the ASBV contents of crude nuclear and chloroplastic preparations were considerably reduced after their purification, and small amounts of viroid still bound to the purified organella were also observed in controls with non-infected tissue and externally added ASBV. In all these experiments the distribution of the viroid paralleled that of the cytoplasmic 4S RNAs. Therefore, ASBV appears to have the same subcellular location as coconut cadang-cadang viroid, being the two only exceptions to the so far existing generalization pointing to the nucleus as the major accumulation site for viroids. © 1990.