Carica papaya MicroRNAs Are Responsive to Papaya meleira virus Infection

MicroRNAs are implicated in the response to biotic stresses. Papaya meleira virus (PMeV) is the causal agent of sticky disease, a commercially important pathology in papaya for which there are currently no resistant varieties. PMeV has a number of unusual features, such as residence in the laticifers of infected plants, and the response of the papaya to PMeV infection is not well understood. The protein levels of 20S proteasome subunits increase during PMeV infection, suggesting that proteolysis could be an important aspect of the plant defense response mechanism. To date, 10,598 plant microRNAs have been identified in the Plant miRNAs Database, but only two, miR162 and miR403, are from papaya. In this study, known plant microRNA sequences were used to search for potential microRNAs in the papaya genome. A total of 462 microRNAs, representing 72 microRNA families, were identified. The expression of 11 microRNAs, whose targets are involved in 20S and 26S proteasomal degradation and in other stress response pathways, was compared by real-time PCR in healthy and infected papaya leaf tissue. We found that the expression of miRNAs involved in proteasomal degradation increased in response to very low levels of PMeV titre and decreased as the viral titre increased. In contrast, miRNAs implicated in the plant response to biotic stress decreased their expression at very low level of PMeV and increased at high PMeV levels. Corroborating with this results, analysed target genes for this miRNAs had their expression modulated in a dependent manner. This study represents a comprehensive identification of conserved miRNAs inpapaya. The data presented here might help to complement the available molecular and genomic tools for the study of papaya. The differential expression of some miRNAs and identifying their target genes will be helpful for understanding the regulation and interaction of PMeV and papaya.

微RNA（microRNAs）参与植物对生物胁迫的应答反应。木瓜梅尔病毒（Papaya meleira virus, PMeV）是木瓜黏胶病的致病原，该病害是木瓜产业中极具经济重要性的病害，目前尚无有效的抗病品种。PMeV存在诸多异常特征，例如定位于受侵染植株的乳管组织内，而木瓜对PMeV侵染的应答机制目前尚不明确。PMeV侵染木瓜的过程中，20S蛋白酶体亚基的蛋白表达水平显著升高，这表明蛋白水解通路可能是植物防御应答机制的关键组成部分。截至目前，植物微RNA数据库（Plant miRNAs Database）中共收录了10598条已鉴定的植物微RNA，但其中仅2条来源于木瓜，分别为miR162与miR403。本研究以已知的植物微RNA序列为参照，在木瓜基因组中搜寻潜在的微RNA，最终共鉴定出462条木瓜微RNA，分属于72个微RNA家族。本研究采用实时荧光定量PCR（real-time PCR）技术，对比了11个微RNA在健康与PMeV侵染的木瓜叶片组织中的表达差异；这11个微RNA的靶基因均参与20S与26S蛋白酶体降解通路，以及其他胁迫应答途径。研究结果显示，参与蛋白酶体降解通路的微RNA，其表达量在PMeV滴度极低时上调，而随着病毒滴度升高逐渐下调。与之相反，参与植物生物胁迫应答的微RNA，其表达量在PMeV滴度极低时下调，而在病毒滴度较高时上调。与上述研究结果一致，本研究分析的这些微RNA的靶基因，其表达量也呈现出相应的依赖性调控模式。本研究实现了对木瓜保守微RNA的系统性鉴定。本研究所得数据可进一步完善木瓜研究现有的分子与基因组学研究工具。部分微RNA的差异表达特征及其靶基因的鉴定，将有助于解析PMeV与木瓜之间的调控互作机制。