Dynamics of Small RNA Profiles of Virus and Host Origin in Wheat Cultivars Synergistically Infected by Wheat Streak Mosaic Virus and Triticum Mosaic Virus: Virus Infection Caused a Drastic Shift in the Endogenous Small RNA Profile

Co-infection of wheat (Triticum aestivum L.) by Wheat streak mosaic virus (WSMV, a Tritimovirus) and Triticum mosaic virus (TriMV, a Poacevirus) of the family Potyviridae causes synergistic interaction. In this study, the effects of the synergistic interaction between WSMV and TriMV on endogenous and virus-derived small interfering RNAs (vsiRNAs) were examined in susceptible (‘Arapahoe’) and temperature-sensitive resistant (‘Mace’) wheat cultivars at 18°C and 27°C. Single and double infections in wheat caused a shift in the profile of endogenous small RNAs from 24 nt being the most predominant in healthy plants to 21 nt in infected wheat. Massive amounts of 21 and 22 nt vsiRNAs accumulated in singly and doubly infected Arapahoe at both temperatures and in Mace at 27°C but not 18°C. The plus- and minus-sense vsiRNAs were distributed throughout the genomic RNAs in Arapahoe at both temperature regimens and in Mace at 27°C, although some regions served as hot-spots, spawning an excessive number of vsiRNAs. The vsiRNA peaks were conserved among cultivars, suggesting that the Dicer-like enzymes in susceptible and resistant cultivars similarly accessed the genomic RNAs of WSMV or TriMV. Accumulation of large amounts of vsiRNAs in doubly infected plants suggests that the silencing suppressor proteins encoded by TriMV and WSMV do not prevent the formation of vsiRNAs; thus, the synergistic effect observed is independent from RNA-silencing mediated vsiRNA biogenesis. The high-resolution map of endogenous and vsiRNAs from WSMV- and/or TriMV-infected wheat cultivars may form a foundation for understanding the virus-host interactions, the effect of synergistic interactions on host defense, and virus resistance mechanisms in wheat.

小麦（Triticum aestivum L.）被小麦条纹花叶病毒（Wheat streak mosaic virus, WSMV，隶属于Tritimovirus属）与马铃薯Y病毒科（Potyviridae）的小麦花叶病毒（Triticum mosaic virus, TriMV，隶属于Poacevirus属）共同侵染时，会引发协同互作。本研究针对18℃与27℃两个温度条件下，感病小麦品种‘Arapahoe’及温敏抗病小麦品种‘Mace’中，WSMV与TriMV的协同互作对内源小RNA以及病毒来源小干扰RNA（virus-derived small interfering RNAs, vsiRNAs）的影响展开了探究。小麦单侵染与双侵染均会导致内源小RNA的谱型发生显著偏移：健康植株中占比最高的24 nt小RNA，在侵染后的小麦中转变为以21 nt为主。在两种温度条件下的Arapahoe单、双侵染植株，以及27℃条件下的Mace单、双侵染植株中，均积累了大量21 nt与22 nt的vsiRNAs；而18℃条件下的Mace植株未出现该积累现象。正、负义链的vsiRNAs在两种温度条件下的Arapahoe，以及27℃条件下的Mace的病毒基因组RNA上均有分布，尽管部分区域存在vsiRNA生成热点，可产生过量的vsiRNAs。不同小麦品种间的vsiRNA峰值具有保守性，这表明感病与抗病品种中的类Dicer酶（Dicer-like enzymes）均可相似地靶向WSMV或TriMV的基因组RNA。双侵染植株中大量vsiRNA的积累表明，TriMV与WSMV编码的沉默抑制蛋白并不会阻碍vsiRNA的生成，因此观测到的协同效应并不依赖于RNA沉默介导的vsiRNA生物发生过程。本研究获得的WSMV和/或TriMV侵染小麦品种的内源小RNA与vsiRNAs高分辨率图谱，可为解析病毒-宿主互作、协同互作对宿主防御的影响，以及小麦病毒抗性机制奠定研究基础。