Characterization of the Molecular Mechanism of Defective Interfering RNA-Mediated Symptom Attenuation in Tombusvirus-Infected Plants

Different tombusviruses were able to support the replication of either homologous or heterologous defective interfering (DI) RNAs, and those infected plants usually developed typical attenuated symptoms. However, in some helper virus-DI RNA combinations the inoculated plants were necrotized, although they contained a high level of DI RNA, suggesting that the accumulation of DI RNA and the resulting suppression of genomic RNA replication were not directly responsible for the symptom attenuation. Moreover, the 19-kDa protein product of ORF 5, which is known to play a crucial role in necrotic symptom development, accumulated at the same level in the infected plants in the presence of protective homologous DI RNA and in the presence of nonprotective heterologous DI RNA. It was also demonstrated, by chimeric helper viruses, that the ability of heterologous DI RNA to protect the virus-infected plants against systemic necrosis is determined by the 5′-proximal region of the helper virus genome. The results presented suggest that DI RNA-mediated protection did not operate via the specific inhibition of 19-kDa protein expression but, more likely, DI RNAs in protective DI-helper virus combinations specifically interacted with viral products, preventing the induction of necrotic symptoms.

不同番茄丛矮病毒组（tombusviruses）病毒均可支持同源或异源缺陷干扰RNA（defective interfering, DI RNA）的复制，感染此类病毒的植株通常会呈现典型的减毒症状。然而，在部分辅助病毒-DI RNA组合中，接种后的植株虽积累了高水平的DI RNA，却出现了坏死反应，这提示DI RNA的积累及其对基因组RNA复制的抑制作用，并非症状减毒的直接诱因。此外，已知在坏死症状发生过程中发挥关键调控作用的开放阅读框5（ORF 5）所编码的19 kDa蛋白产物，在携带保护性同源DI RNA与非保护性异源DI RNA的感染植株中，积累水平并无显著差异。研究人员还通过嵌合辅助病毒实验证实，异源DI RNA能否保护受病毒感染的植株免受系统坏死侵害，由辅助病毒基因组的5'近端区域决定。上述研究结果表明，DI RNA介导的保护作用并非通过特异性抑制19 kDa蛋白的表达来实现，更有可能的是，在保护性DI RNA-辅助病毒组合中，DI RNA可与病毒产物发生特异性相互作用，从而阻断坏死症状的诱导。