Study On Host Factors Involved In Tomato Lethal Disease Caused By Cucumber Mosaic Virus Associated With Satellite RNA

Tomato(Solanum lycopersicum)is one of the main vegetable crops worldwide,while China is the country with the largest tomato production in recent years.However,virus diseases remain to be the important factor restricting tomato yield in China.Among all the viruses,Cucumber Mosaic Virus(CMV)is one of the most economically harmful viruses.CMV sometimes harbours the parasitic RNA molecule,the satellite RNA(satRNA).When it is associated with D satRNA(D-satRNA),the infected tomato plants would develop systemic necrosis,causing catastrophic lethal disease.It was previously reported that three nucleotides near its 3′ end determine its lethality and the lethal infection occurs specifically in tomato and its close relatives.So far no disease resistance or virus tolerance has been found in any known tomato cultivars,and little is known about the molecular mechanism underlying the lethal disease.This thesis is aimed at understanding the molecular mechanism of CMV/D-satRNA caused lethal disease in tomato,particularly the host factors that are involved.We found that rdr6 and sgs3 tomato mutants survived its infection.The accumulation of D-satRNA in these two mutants is similar to that in wild-type plants,indicating that D-satRNA can avoid virus resistance mediated by RDR6 and SGS3.Results from further analysis showed that the content of sRNA derived from D-satRNA(D-satRNA-sRNA)was reduced,especially the accumulation of the sRNA containing the three necrogenic nucleotides was drastically reduced in the mutants.These results suggest that RDR6 and SGS3 are involved in the generation of sRNA derived from D-satRNA,and the content of D-satRNA-sRNA is related to plant death.In the meantime,dcl4 and ago7 tomato mutants were tested for their response to CMV/D-satRNA infection,it was found that the mutants plants infected with CMV/D-satRNA all developed rapid systemic necrosis.The D-satRNA content in dcl4 mutant was largely increased and the sRNA was significantly reduced,while in ago7 mutant,the contents of D-satRNA and its sRNA were similar to those in wild-type tomato.Therefore,DCL4 that is involved in sRNA formation and AGO7 with a role in tasiRNA formation and mi RNA loading have no effect on the lethality of D-satRNA.In conclusion,the thesis presents the first discovery of key host factors involved in the lethal infection of D-satRNA in tomato plants,They are RDR6 and SGS3.The results also show that the death of tomato plants is related to the content of sRNA from the lethal region of D-satRNA,but is not regulated by the sRNA involving DCL4.