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Spinach curly top virus: A Newly Described Curtovirus Species from Southwest Texas with Incongruent Gene Phylogenies

First and fifth authors: Department of Biology, University of Texas at San Antonio, San Antonio 78249; second author: Department of Plant Pathology and Microbiology, Texas A&M University, Uvalde 78802; and third and fourth authors: U.S. Department of Agriculture-Agricultural Research Service, University of Nebraska, Lincoln 68583

A curtovirus associated with a disease of spinach was isolated in southwest Texas during 1996. Disease symptoms included severe stunting and chlorosis, with younger leaves curled, distorted, and dwarfed. Viral DNA was purified and an infectious clone obtained. Agroinoculation using a construct bearing full-length tandem repeats of the cloned viral genome resulted in systemic infection of species in six of seven plant families tested, indicating that the virus has a wide host range. Symptoms produced in spinach agroinoculated with cloned viral DNA were similar to those observed in the field. Viral single-stranded and double-stranded DNA forms typical of curtovirus infection were detected in host plants by Southern blot hybridization. The complete sequence of the infectious clone comprised 2,925 nucleotides, with seven open reading frames encoding proteins homologous to those of other curtoviruses. Complete genome comparisons revealed that the spinach curtovirus shared 64.2 to 83.9% nucleotide sequence identity relative to four previously characterized curtovirus species: Beet curly top virus, Beet severe curly top virus, Beet mild curly top virus, and Horseradish curly top virus. Phylogenetic analysis of individual open reading frames indicated that the evolutionary history of the three virion-sense genes was different from that of the four complementary-sense genes, suggesting that recombination among curtoviruses may have occurred. Collectively, these results indicate that the spinach curtovirus characterized here represents a newly described species of the genus Curtovirus, for which we propose the name Spinach curly top virus.

Additional keyword: geminivirus.