Molecular characterization of a member of a new species of grapevine viroid

Abstract

Currently, viroids of five species are known to infect grapevine trees: Hop stunt viroid (HSVd-g) [1], Citrus exocortis viroid (CEVd-g) [2], Australian grapevine viroid (AGVd) [3], Grapevine yellow speckle viroid 1 (GYSVd-1) and Grapevine yellow speckle viroid 2 (GYSVd-2) [4–6]. These five grapevine viroid species have been divided into three genera based on sequence homologies of their central conserved regions (CCRs) [7]. GYSVd-1, GYSVd-2, and AGVd are classified in the genus Apscaviroid, whereas HSVd-g [8] and CEVd-g are classified in the genera Hostuviroid and Pospiviroid, respectively [9]. Mixed infections involving these viroids in cultivated grapevines are common, and in general, grapevine viroids produce very few, if any, disease symptoms. In the absence of manifested disease symptoms, these viroids are able to replicate in a host unnoticed. Of the five grapevine viroids identified, GYSVd-1 and GYSVd-2 are associated with disease symptoms such as yellow speckle, which was originally identified under hot greenhouse conditions in Australia [4, 5, 10]. Taylor and Woodham first described yellow speckle (YS) disease in Australia, and it is still the only grapevine disease known to be caused by viroids [10]. GYSVd-1, which is approximately 367 nucleotides (nts) in length, was first isolated from the grapevine cultivar ‘Cabernet Franc’, showing typical symptoms of YS disease. From this cultivar, a single circular RNA was isolated by two-dimensional electrophoresis [4]. GYSVd-2, which is approximately 363 nts in length, was first purified from the grapevine cultivar ‘Kyoto’, which was grafted onto the ‘Dogridge’ rootstock and also exhibited symptoms of YS disease [5]. So far, GYSVd-2 has only been reported in Australia and China [5, 6], and the molecular characterization of GYSVd-2 has been described [11]. Here, we report the characterization of a member of a species of grapevine viroid isolated from China. We call it Chinese grapevine viroid (CGVd) in this report and suggest tentatively naming it Grapevine yellow speckle viroid (GYSVd-3) based on the results of sequence, phylogenetic analysis and molecular characterization.