Credit: BRAND X PICTURES

Extensive bidirectional transfer of mRNAs can occur between parasitic plant species and their hosts, according to the results of a new study published in Science. Although artificial constructs of inhibitory RNA in host plants have previously been shown to silence genes in their parasitic guests, this new work demonstrates, on a genome-wide scale, the extent to which RNA movement can occur between the parasitic plant Cuscuta pentagona and its host plants.

To facilitate the identification of parasite and host mRNA, the researchers grew C. pentagona on tomato and Arabidopsis thaliana host plants, as the genomes of these host plants have been sequenced previously. cDNA libraries were generated from tissue derived from the host stem above the region of C. pentagona attachment, the interface region where C. pentagona is attached to the host, and the C. pentagona stem. The team was then able to identify and compare host and parasite transcriptomes to determine whether mRNA transfer had occurred.

Analysis of the C. pentagona stems grown on A. thaliana hosts showed that they contained 1.1% A. thaliana transcriptome, whereas C. pentagona stems grown on tomato hosts contained 0.17% tomato transcriptome. Thus, mRNA from both host species was present in the parasite. Furthermore, A. thaliana and tomato host stems contained 0.6% and 0.38% C. pentagona transcriptome, respectively; thus, parasitic mRNA is also transmitted into both hosts. The interface region for A. thaliana contained half the parasite transcriptome and half the A. thaliana transcriptome, whereas the interface region for the tomato plants contained 86% tomato transcriptome and 14% C. pentagona transcriptome. Collectively, this work shows large-scale bidirectional transfer of mRNAs between the C. pentagona and two host plants. Moreover, the differences between the levels of transfer seen with tomato and A. thaliana indicate that mRNA mobility rates differ between different host species.

To further investigate differences in mRNA mobility, the researchers also examined the diversity of the mobile transcripts. Compared with 45% of expressed A. thaliana transcripts that were mobile, only 1.6% of expressed tomato transcripts were mobile. As the total number of genes in C. pentagona is unknown, the investigators used de novo transcriptome assembly to catalogue the number of expressed unique sequences ('unigenes') for transcripts derived from C. pentagona. A larger variety of C. pentagona unigenes (24% of all expressed C. pentagona unigenes) were found in A. thaliana than in tomato, where only 1% of expressed C. pentagona unigenes were detected. These data reinforce the idea that there are host-specific differences in mRNA transfer.

The function of the bidirectional mRNA transfer shown in this study is currently unclear, but the authors suggest that the parasite may use this approach to gather information regarding the health of the host, or to manipulate the host environment to create conditions that are favourable to parasitism.