A small RNA targets pokeweed antiviral protein transcript.

Ribosome-inactivating proteins (RIPs) are a class of plant defense proteins with N-glycosidase activity (EC 3.2.2.22). Pokeweed antiviral protein (PAP) is a Type I RIP isolated from the pokeweed plant, Phytolacca americana, thought to confer broad-spectrum virus resistance in this plant. Through a combination of standard molecular techniques and RNA sequencing analysis, we report here that a small RNA binds and cleaves the open reading frame of PAP mRNA. Additionally, sRNA targeting of PAP is dependent on jasmonic acid (JA), a plant hormone important for defense against pathogen infection and herbivory. Levels of small RNA increased with JA treatment, as did levels of PAP mRNA and protein, suggesting that the small RNA functions to moderate the expression of PAP in response to this hormone. The association between JA and PAP expression, mediated by sRNA299, situates PAP within a signaling pathway initiated by biotic stress. The consensus sequence of sRNA299 was obtained through bioinformatic analysis of pokeweed small RNA sequencing. To our knowledge, this is the first account of a sRNA targeting a RIP gene.

RNA toxins: mediators of stress adaptation and pathogen defense.

RNA toxins are a group of enzymes primarily synthesized by bacteria, fungi, and plants that either cleave or depurinate RNA molecules. These proteins may be divided according to their RNA substrates: ribotoxins are nucleases that cleave ribosomal RNA (rRNA), ribosome inactivating proteins are glycosidases that remove a base from rRNA, messenger RNA (mRNA) interferases are nucleases that cleave mRNAs, and anticodon nucleases cleave transfer RNAs (tRNAs). These modifications to the RNAs may substantially alter gene expression and translation rates. Given that some of these enzymes cause cell death, it has been suggested that they function mainly in defense, either to kill competing cells or to elicit suicide and thereby limit pathogen spread from infected cells. Although good correlations have been drawn between their enzymatic functions and toxicity, recent work has shown that some RNA toxins cause apoptosis in the absence of damage to RNA and that defense against pathogens can be achieved without host cell death. Moreover, a decrease in cellular translation rate, insufficient to cause cell death, allows some organisms to adapt to stress and environmental change. Although ascribing effects observed in vitro to the roles of these toxins in nature has been challenging, recent results have expanded our understanding of their modes of action, and emphasized the importance of these toxins in development, adaptation to stress and defense against pathogens.