Submitted to: Phytoparasitica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 13, 2001
Publication Date: October 10, 2001
Citation: SCHNELL II, R.J., KUHN, D.N., OLANO, C.T., QUINTANILLA, W.E. SEQUENCE DIVERSITY AMONG AVOCADO SUNBLOTCH VIROIDS ISOLATED FROM SINGLE AVOCADO TREES. PHYTOPARASITICA. 2001. Interpretive Summary: Avocado Sunblotch Viroid (ASBVd) is a persistent, often undetected pathogen of the common avocado. ASBVd is self reproducing and similar to plant viruses. We developed a molecular detection method for ASBVd and screened the avocado collection for infection. Around 20% of the trees in the collection were found to be infected with the viroid. In this investigation we were looking at differences in the genetic code among different strains occurring in individual trees. We determined that many strains co-exist in an individual tree and that these evolved within the individual tree rather than occurring from cross-infection from related trees. The strains that cause the bleaching condition differ from other strains that do not seem to cause symptoms. These bleaching strains have specific changes in the right terminal loop of the viroid.
Technical Abstract: In previous studies, we developed an avocado sunblotch viroid (ASBVd) specific reverse transcription-polymerase chain reaction (RT-PCR) protocol & tested all avocado germplasm accessions in the U.S. National Germplasm Repository (NGR) for ASBVd. We found that 19% of the accessions were infected with the viroid. To characterize the population of sequence variants, we selected single trees of seven accessions with a history of infection. RT-PCR products were cloned & sequenced from these trees & a total of 60 sequence variants were identified from 122 clones. Most of the variants were found to be unique to the individual tree from which they were isolated. The percentage of unique variants was 81% in Lima Late, 81% in Aycock Red, 67% in Hawaii, & 60% in Young Special. Analysis of the sequence data suggests that variants evolved within a tree rather than arising from multiple infection events, supporting the quasispecies concept for ASBVd. Changes in the right terminal loop (RTL), previously associated with leaf variegation & bleaching, were found in all variants isolated from bleached tissue; however, 16 clones isolated in non- symptomatic tissue also had changes in the RTL. The addition of a U between bases 115-118 was found in 21 of the 23 clones isolated from the bleached tissue but not found in the 99 clones isolated from green tissue. These data suggest that the insertion of the U between bases 115-118 is responsible for the bleaching condition.