The capacity of transgenic tobacco to send a systemic RNA silencing signal depends on the nature of the inducing transgene locus

• Published in: The Plant journal : for cell and molecular biology 2003-07, Vol.35 (1), p.82-92
• Main Authors: Mallory, Allison C., Mlotshwa, Sizolwenkosi, Bowman, Lewis H., Vance, Vicki B.
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; DNA Methylation; Fundamental and applied biological sciences. Psychology; Gene expression; Gene Expression Regulation, Plant; Glucuronidase - genetics; HC‐Pro; inverted repeat; Molecular and cellular biology; Molecular genetics; Nicotiana - genetics; Plants, Genetically Modified; post‐transcriptional gene silencing (PTGS); RNA Interference; RNA interference (RNAi); RNA, Messenger - genetics; RNA, Messenger - metabolism; RNA, Plant - genetics; RNA, Plant - metabolism; siRNA; systemic silencing; Transgenes - genetics; Transplants
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1046/j.1365-313X.2003.01785.x

Summary RNA silencing is a conserved eukaryotic pathway in which double‐stranded RNA (dsRNA) triggers destruction of homologous target RNA via production of short‐interfering RNA (siRNA). In plants, at least some cases of RNA silencing can spread systemically. The signal responsible for systemic spread is expected to include an RNA component to account for the sequence specificity of the process, and transient silencing assays have shown that the capacity for systemic silencing correlates with the accumulation of a particular class of small RNA. Here, we report the results of grafting experiments to study transmission of silencing from stably transformed tobacco lines in the presence or absence of helper component‐proteinase (HC‐Pro), a viral suppressor of silencing. The studied lines carry either a tail‐to‐tail inverted repeat, the T4‐IR transgene locus, or one of two different amplicon transgene loci encoding replication‐competent viral RNA. We find that the T4‐IR locus, like many sense‐transgene‐silenced loci, can send a systemic silencing signal, and this ability is not detectably altered by HC‐Pro. Paradoxically, neither amplicon locus effectively triggers systemic silencing except when suppressed for silencing by HC‐Pro. In contrast to results from transient assays, these grafting experiments reveal no consistent correlation between capacity for systemic silencing and accumulation of any particular class of small RNA. In addition, although all transgenic lines used to transmit systemic silencing signals were methylated at specific sites within the transgene locus, silencing in grafted scions occurred without detectable methylation at those sites in the target locus of the scion.