PROGRESS IN NATURAL SCIENCE V ol .15 , No .6 , June 2005 REVIEW ARTICLE Progress and prospects of studies on Polymyxa graminis and its transmitted cereal viruses in China＊ CH EN Jianping ＊＊ (Virology and Biotechnology Institute , Zhejiang Academy of Agricultural Sciences, Key Laboratory of Plant Virology , Ministry of Agri- culture and Zhejiang Province, Hangzhou 310021, China) Received October 8 , 2004 ;revised October 25 , 2004 Abstract Polymyxa gram inis is a eukaryotic obligate biotrophic parasite of plant roots that belongs to a poorly studied discrete taxonomic unit informally called “ plasmodiophorids” .P .graminis is nonpathogenic , but has the ability to acquire and transmit nine plant viruses w hich belong to genera Bymovirus and Furovirus and cause serious diseases in cereal crop speciesand also result in significant yield reductions in China and elsew here.Genus Bymovirus contains barley yellow mosaic virus (BaYMV), barley mild mosaic virus (BaMMV), w heat yellow mosaic virus (WYMV), w heat spindle streak mosaic virus (WSS MV), and oat mosaic virus (OMV), and genus F urovirus contains soil-borne w heat mosaic virus(S BW MV), oat golden stripe virus(OGSV), and new ly identified Chinese w heat mosaic virus(CWM V)and soil-borne cereal mosaic virus(S BCMV).All these viruses have been sequenced and their w orldw ide distribu- tions have been studied .The viruses are protected by the environment w ithin P .gra minis resting spores that may remain dormant but vi- able for decades(probably until a suitable host plant is encountered).S pontaneous deletion mutants of S BW MV , OGS V and OMV are de- tected , and these deletion mutants are not transmissible by the fungus.The persistent, soil-borne nature of these diseases makes the use of virus-resistant crop varieties cu rren tly the only practical and environmentally friendly means to control them , and a large number of disease resistant germ plasms have been screened . Keywords: Polymyxa graminis, cereal viruses, genus Bymovirus, genus Furovirus . As early as in the 1920s , a mosaic or rosette disease gal vector are ex tremely tolerant to a poo r environ- w as first reported on winter wheat in America , and then ment and chemicals , and they can survive in the soil it w as found that the pathogen w as transmitted via soil. fo r decades ;(2)there are v arious species of viruses In 1925 , Mckinney successfully transmitted the pathogen and strains , and the continuous appearance of new from the infected plants to healthy plants by mechanical pathogenic types and strains breaks the resistance of inoculation and proved that the pathogen w as a virus , the virus-resistant cultivars and creates difficulties in named soil-bo rne w heat mosaic virus (SBWMV).In cereals breeding .Therefore , to understand the mech- 1969 , Rao and Brakke found that SBWMV w as anism of interaction among the viruses , fungal vector [ 1] transmitted by Polymy xa graminis in soil .In later and cereal crops at the molecular level , and to estab- years , similar diseases were also repo rted in Japan , I- lish new sy stem s and methods fo r disease control are taly , France , Germany , Brazil and Argentina , w hich significantly important .Since 1985 , w e have studied attracted people' s attention because of their serious the fungal vector P .graminis , virus species , ge- effects on production of cereal crops . nomic organization , disease epidemiology , fungal In China , Poly myxa graminis transmitted cere- transmission, as w ell as screening of resistant al viruses w ere first identified in the 1970s[ 2] .The germ plasms .In this review , I summarize the re- diseases caused by these viruses have seriously oc- search prog ress and prospects of the fung al transmit- curred in estimated 200 million ha of successive barley ted cereal viruses in China . and w heat crops in Henan , Sichuan , Hubei , Shaanx- 1 Polymyxa graminis transmitted cereal i , Shandong , Jiangsu , Zhejiang , Anhui provinces and Shanghai City and resulted in a loss of 1 .5 million viruses tons of yield .The rapid spread of the diseases and se- 1 .1 Taxonomy and genomic organization rious damage of crops are caused by the follow ing rea- sons :(1)the thick-w alled resting spo res of the fun- All of the Polymy xa graminis transmitted cereal ＊ Supported by the National Natural S cience Foundation of China (G rant No .30225031) ＊＊ E-mail:Jpchen2001 @Hzcnc.com 482 w ww .tandf.co .uk/journals Prog ress in Natural Science Vol .15 No .6 2005 viruses contain tw o single-stranded positive-sense reading frame (ORF)w hich encodes a 257 —271 kD RNA genomes , and belong to genera By mov irus and single poly peptide .From this single large poly protein Furovirus respectively . precursor , 8 mature functional proteins , P3 , 7K , CI , 14K , NIa-Vpg , NIa-Pro , NIb and CP from N- M embers of genus By mov irus are mainly dis- terminus to C-terminus , are derived by proteolytic tributed in Asia , Europe and No rth America , and are cleavage (Fig .1(a)).Among these pro teins , NIb is barley yellow mosaic virus (BaYM V), barley mild related to genome replication , C I is possibly associat- mosaic virus (BaMM V), w heat yellow mosaic virus ed to virus movement , NIa-Pro encodes main viral (WYM V), w heat spindle streak mosaic virus (WSS- proteinase , N Ia -VPg encodes genome-linked pro- M V)and oat mosaic virus (OMV).These viruses se- tein , and CP is the only structure protein for RNA riously infect winter barley , w heat , oat and rye crops encapsidation .RNA2 also contains a large ORF and and are mostly serologically related , but have sig nifi- encodes a 98 —101 kD polypeptide w hich produces cant differences in nucleotide sequences as w ell as host two functional proteins (P1 and P2)after proteolytic ranges[ 3 —8] .Recently , w e have identified that those cleavage .Of these two pro teins , P1 is a proteinase , viruses that occurred in China are BaYM V and BaM- and P2 is possibly associated to fungal transmission . M V for barley and WYMV for w heat .The genomic Both RNA1 and RNA2 contain a poly(A)tail at their o rganization has been determined fo r BaYMV and 3′-terminus[ 9 , 10] .After infection by these bymovirus- W YMV and partial for BaMM V , showing a similari- es, a large number of membranous bodies connected ty among them and some homology to the insect w ith endaplasmic reticulum and cylindrical o r pin- transmitted poty viruses .BaYM V and WYMV , the w heel inclusion bodies fo rm in the infected cells of the w ell studied bymoviruses , have bipartite genomes cereal crops .This cytopatholog ical change can be w ith a larger RNA 1 (about 7 .5 kb)and a smaller used as a diagnostic character for infection of by- RNA 2 (about 3 .5 kb)in the longer and shorter par- [ 4 , 5 , 11] moviruses . ticles, respectively .RNA1 contains one large open Fig .1 . Genomic organizations of genera Bymovirus(a)and F urovirus(b). Genus Furov irus has not been classified to any bers and structures of RNA genome , Torrnace and family of plant virus .This genus used to be consid- M ayo re-classified the previous genus Furov irus into ered to contain all fungus transmitted rod-shaped four new genera including Benyv irus , Pomov irus , viruses.According to species of fungal vector , num- Pecluv irus and Furovirus . The new genus Prog ress in Natural Science Vol.15 No .6 2005 w ww .tandf.co .uk/journals 483 Furovirus is characterized by rod-shaped virus parti- w as w idely adopted because its short g row ing period cles, bipartite RNA genomes containing CP-RT and hig h yield made it particularly suitable for use in a gene , but not TGB genes and poly (A) tail , and cropping system having three crops each year (rice transmitted by P .graminis .The definite members from M ay-August and again August-November , fol- of this genus are SBWMV and sorghum chlo rotic spot low ed by either barley , w heat o r oilseed rape from virus (SrCMV), and oat golden stripe virus (OGSV) November-May).This system became widely estab- w as classified as a strain of SBWM V[ 12] .Recently , lished during the 1960s in the middle and lower re- our studies demonstrate that OGSV is an independent gions of the Yang tze River basin and other parts of member[ 13] , and tw o new identified species, Chinese Eastern China .How ever , Zhaoshu 3 proved to be w heat mosaic virus (CWMV)and soil-bo rne cereal very susceptible to BaYM V , and by the mid-1970s, mosaic virus (SBCMV ), are different from SB- BaYM V had become serious in all the areas w here the WMV , and are also members of this genus[ 14, 15] . cultivar w as regularly grow n[ 17, 18] . Virus particles of genus Furov irus consist of two We have determined the complete sequence of single-stranded positive-sense RNA genomes (Fig .1 BaYM V Yancheng isolate[ 9] .Sequence com parison a- (b))and a coat protein .Both RNA1 and RNA2 are mong Chinese , Germ an and Japanese BaYMV isolates necessary for virus infection and replication , and con- indicated that the 5′-UTR regions have the most sig- tain a cap structure (M 7GpppG)at their 5′-termini , nificant variation , and P1 , P3 , CI , NIa , 5′-part of but no poly(A)tail at the 3′-termini .RNA1 contains CP and 3′-terminus of RNA1 also present different three ORFs .The first one encodes a 149 —153 kD variations .In general, RNA2 show s a g reater varia- polypeptide and the opal termination codon (UGA) tion than RNA1 .The P2 fragment w as more variable can be partially suppressed to ex tend to the second than the CP and phylogenetic analysis of both regions ORF which produces a 208 —212 kD read-throug h show ed that Asian and European isolates fo rm distinct protein .The motifs for methyl-transferase and N TP- clusters, indicating that molecular evolution of binding helicase activity are identified in the 149 — BaYM V isolates is linked to their geog raphical distri- 153 kD protein whereas that for RNA-dependent bution[ 9] .