Abstract
Viroids are small infectious pathogens, composed of a short single-stranded circular RNA. Hop (Humulus lupulus L.) plants are hosts to four viroids from the family Pospiviroidae. Hop latent viroid (HLVd) is spread worldwide in all hop-growing regions without any visible symptoms on infected hop plants. In this study, we evaluated the influence of HLVd infection on the content and the composition of secondary metabolites in maturated hop cones, together with gene expression analyses of involved biosynthesis and regulation genes for Saaz, Sládek, Premiant and Agnus cultivars. We confirmed that the contents of alpha bitter acids were significantly reduced in the range from 8.8% to 34% by viroid infection. New, we found that viroid infection significantly reduced the contents of xanthohumol in the range from 3.9% to 23.5%. In essential oils of Saaz cultivar, the contents of monoterpenes, terpene epoxides and terpene alcohols were increased, but the contents of sesquiterpenes and terpene ketones were decreased. Secondary metabolites changes were supported by gene expression analyses, except essential oils. Last-step biosynthesis enzyme genes, namely humulone synthase 1 (HS1) and 2 (HS2) for alpha bitter acids and O-methytransferase 1 (OMT1) for xanthohumol, were down-regulated by viroid infection. We found that the expression of ribosomal protein L5 (RPL5) RPL5 and the splicing of transcription factor IIIA-7ZF were affected by viroid infection and a disbalance in proteosynthesis can influence transcriptions of biosynthesis and regulatory genes involved in of secondary metabolites biosynthesis. We suppose that RPL5/TFIIIA-7ZF regulatory cascade can be involved in HLVd replication as for other viroids of the family Pospiviroidae.
Highlights
Viroids are non-encapsidated, covalently closed, non-coding circular RNA molecules consisting of 246 to 399 nucleotides
We aimed to evaluate the influence of Hop latent viroid (HLVd) infection on the content and composition of secondary metabolites in maturated hop cones
We evaluated the expression levels of genes involved in secondary metabolites biosynthesis and regulation to provide a complex understanding of transcriptomic and metabolic changes caused by viroid infection
Summary
Viroids are non-encapsidated, covalently closed, non-coding circular RNA molecules consisting of 246 to 399 nucleotides. Hop (Humulus lupulus L.) plants are hosts to several viroids from the family Pospiviroidae. Apple fruit crinkle viroid (AFCVd), of the genus Apscarviroid [3], and citrus bark cracking viroid (CBCVd), of the genus Cocadviroid [4], have been found in hop plants. CBCVd infection causes similar, more severe symptoms, which often lead to plant death [7,8]. The spreading of these viroids has been limited to local outbreaks but HLVd infection has been reported worldwide in hop-growing regions [2,7,9,10]
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