The most common response of a host to pathogens is arguably the asymptomatic response. However, the genetic and molecular mechanisms responsible for asymptomatic responses to pathogens are poorly understood. Here we report on the genetic cloning of two genes controlling the asymptomatic response to tobacco mosaic virus (TMV) in cultivated tobacco (Nicotiana tabacum). These two genes are homologous to tobamovirus multiplication 2A (TOM2A) from Arabidopsis, which was shown to be critical for the accumulation of TMV. Expression analysis indicates that the TOM2A genes might play fundamental roles in plant development or in responses to stresses. Consistent with this hypothesis, a null allele of the TOM2A ortholog in tomato (Solanum lycopersicum) led to the development of bent branches and a high tolerance to both TMV and tomato mosaic virus (ToMV). However, the TOM2A ortholog in Nicotiana glauca did not account for the asymptomatic response to TMV in N. glauca. We showed that TOM2A family is plant-specific and originated from Chlorophyte, and the biological functions of TOM2A orthologs to promote TMV accumulation are highly conserved in the plant kingdom—in both TMV host and nonhost species. In addition, we showed that the interaction between tobacco TOM1 and TOM2A orthologs in plant species is conserved, suggesting a conserved nature of TOM1–TOM2A module in promoting TMV multiplication in plants. The tradeoff between host development, the resistance of hosts to pathogens, and their influence on gene evolution are discussed. Our results shed light on mechanisms that contribute to asymptomatic responses to viruses in plants and provide approaches for developing TMV/ToMV-resistant crops.

中文翻译：

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烟草花叶病毒无症状反应中烟草控制中的两种烟草病毒倍增 2A 同源物


宿主对病原体最常见的反应可以说是无症状反应。然而，人们对病原体无症状反应的遗传和分子机制知之甚少。在这里，我们报告了控制栽培烟草（Nicotiana tabacum）中对烟草花叶病毒（TMV）无症状反应的两个基因的基因克隆。这两个基因与来自拟南芥的烟草花叶病毒倍增 2A (TOM2A) 同源，该基因对于 TMV 的积累至关重要。表达分析表明 TOM2A 基因可能在植物发育或应激反应中发挥重要作用。与这一假设相一致的是，番茄（Solanum lycopersicum）中 TOM2A 直向同源物的无效等位基因导致了弯曲枝的发育以及对 TMV 和番茄花叶病毒（ToMV）的高度耐受性。然而，青烟草中的 TOM2A 直系同源物并不能解释青烟草中对 TMV 的无症状反应。我们发现 TOM2A 家族是植物特异性的，起源于叶绿，TOM2A 直系同源物促进 TMV 积累的生物学功能在植物界（TMV 宿主和非宿主物种中）高度保守。此外，我们发现植物物种中烟草 TOM1 和 TOM2A 直向同源物之间的相互作用是保守的，这表明 TOM1-TOM2A 模块在促进植物中 TMV 增殖方面具有保守性。讨论了宿主发育、宿主对病原体的抗性及其对基因进化的影响之间的权衡。我们的研究结果揭示了植物对病毒无症状反应的机制，并为开发 TMV/ToMV 抗性作物提供了方法。