Identification of two novel poleroviruses and the occurrence of Tobacco bushy top disease causal agents in natural plants

Abstract

Tobacco bushy top disease (TBTD) is a devastating tobacco disease in the southwestern region of China. TBTD in the Yunnan Province is often caused by co-infections of several plant viruses: tobacco bushy top virus (TBTV), tobacco vein distorting virus (TVDV), tobacco bushy top virus satellite RNA (TBTVsatRNA) and tobacco vein distorting virus-associated RNA (TVDVaRNA). Through this study, two new poleroviruses were identified in two TBTD symptomatic tobacco plants and these two novel viruses are tentatively named as tobacco polerovirus 1 (TPV1) and tobacco polerovirus 2 (TPV2), respectively. Analyses of 244 tobacco samples collected from tobacco fields in the Yunnan Province through RT-PCR showed that a total of 80 samples were infected with TPV1 and/or TPV2, and the infection rates of TPV1 and TPV2 were 8.61% and 29.51%, respectively. Thirty-three TPV1 and/or TPV2-infected tobacco samples were selected for further test for TBTV, TVDV, TBTVsatRNA and TVDVaRNA infections. The results showed that many TPV1 and/or TPV2-infected plants were also infected with two or more other assayed viruses. In this study, we also surveyed TBTV, TVDV, TBTVsatRNA and TVDVaRNA infections in a total of 1713 leaf samples collected from field plants belonging to 29 plant species in 13 plant families and from 11 provinces/autonomous regions in China. TVDV had the highest infection rates of 37.5%, while TVDVaRNA, TBTV and TBTVsatRNA were found to be at 23.0%, 12.4% and 8.1%, respectively. In addition, TVDV, TBTV, TBTVsatRNA and TVDVaRNA were firstly detected of co-infection on 10 plants such as broad bean, pea, oilseed rape, pumpkin, tomato, crofton weed etc., and 1 to 4 of the TBTD causal agents were present in the samples collected from Guizhou, Hainan, Henan, Liaoning, Inner mongolia and Tibet autonomous regions. The results indicated that TBTD causal agents are expanding its host range and posing a risk to other crop in the field.

Figure 1

Symptoms of TBTD-affected flue-cured tobacco cultivar K326 in field. Symptoms of an early infection on tobacco plant including chlorosis, internode shortening and serious stunting (A). Early infected plant (indicated by the red arrow) and healthy looking tobacco plants (B). Late infection symptoms on tobacco plants showing lateral branches proliferation, small leaves, foliar yellowing or chlorosis, stunting (C). Late infected plant (left) and healthy looking tobacco plant (right) (D).

Figure 2

Genome structures of TPV1, TPV2 and PLRV.

Figure 3

Neighbor joining tree (NJ) of the RdRp nt sequences of TPV1, TPV2 as well as those of other viruses in the family Solemoviridae. The phylogenetic trees are based on alignments of the nt sequences. The sequences were aligned with Clustal W and NJ trees constructed with MEGA 5.0. The scale bar indicates the genetic distance. *Two novel poleroviruses.

Figure 4

Detection rate of TBTV, TVDV, TBTVsatRNA and TVDVaRNA in field collected samples.