Small RNA Isolation and Library Construction for Expression Profiling of Small RNAs from Neurospora crassa and Fusarium oxysporum and Analysis of Small RNAs in Fusarium oxysporum-Infected Plant Root Tissue.

Due to crucial roles in gene regulation, noncoding small RNAs (sRNAs) of 20-30 nucleotides (nt) have been intensively studied in mammals and plants and are implicated in significant diseases and metabolic disorders. Elucidation of biogenesis mechanisms and functional characterization of sRNAs is often achieved using tools such as separation of small-sized RNA and deep sequencing. Although RNA interference pathways, such as quelling and meiotic silencing, have been well-described in Neurospora crassa, knowledge of sRNAs in other filamentous fungi is still limited compared to other eukaryotes. As a prerequisite for study, isolation and sequence analysis of sRNAs is necessary. We developed a protocol for isolation and library construction of sRNAs of 20-30 nt for deep sequencing in two filamentous fungi, N. crassa and Fusarium oxysporum f.sp. lycopersici. Using 200-300 µg total RNA, sRNA was isolated by size-fractionation and ligated with adapters and amplified by RT-PCR for deep sequencing. Sequence analysis of several cDNA clones showed that the cloned sRNAs were not tRNAs and rRNAs and were fungal genome-specific. In order to validate fungal miRNAs that were imported into the host cell, we developed a straightforward method to isolate protoplasts from tomato roots infected by Fusarium oxysporum f.sp. lycopersici using enzymatic digestion.