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Cuscuta spp. are obligate parasites that connect to host vascular tissue using a haustorium. In addition to water, nutrients, and metabolites, a large number of mRNAs are bidirectionally exchanged between Cuscuta spp. and their hosts. This trans-specific movement of mRNAs raises questions about whether these molecules function in the recipient species. To address the possibility that mobile mRNAs are ultimately translated, we built upon recent studies that demonstrate a role for transfer RNA (tRNA)-like structures (TLSs) in enhancing mRNA systemic movement. C. campestris was grown on Arabidopsis that expressed a β-glucuronidase (GUS) reporter transgene either alone or in GUS-tRNA fusions. Histochemical staining revealed localization in tissue of C. campestris grown on Arabidopsis with GUS-tRNA fusions, but not in C. campestris grown on Arabidopsis with GUS alone. This corresponded with detection of GUS transcripts in Cuscuta on Arabidopsis with GUS-tRNA, but not in C. campestris on Arabidopsis with GUS alone. Similar results were obtained with Arabidopsis host plants expressing the same constructs containing an endoplasmic reticulum localization signal. In C. campestris, GUS activity was localized in the companion cells or phloem parenchyma cells adjacent to sieve tubes. We conclude that host-derived GUS mRNAs are translated in C. campestris and that the TLS fusion enhances RNA mobility in the host-parasite interactions.more » « less
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Societal Impact Statement Rafflesia is a genus of parasitic plants with the largest flowers in the world, unique to the threatened forest habitats of tropical Asia. Here, we report on genes that are active (the transcriptome) inRafflesia seeds as part of a larger effort to understandRafflesia .Rafflesia has never been grown successfully outside of its native range. Consequently, seed banking is not yet possible, precluding a critical management strategy for conservation. The study ofRafflesia seed biology is a critical step to improve its cultivation, which will educate the public about unique species and the importance of conserving their habitats.Summary Rafflesia is of great interest as one of the only two plants known to have completely lost its chloroplast genome.Rafflesia is a holoparasite and an endophyte that lives inside the tissues of its host, a tropical grape vine (Tetrastigma ), emerging only to bloom—with the largest flower of any plant. Here, we report the firstRafflesia seed transcriptome and compare it with those of other plants to deepen our understanding of its extraordinary life history.We assembled a transcriptome from RNA extracted from seeds of the Philippine endemic
Rafflesia speciosa and compared this with those of other plants, includingArabidopsis , parasitic plantsStriga andCuscuta , and the mycoheterotrophic orchidAnoectochilus .Genetic and metabolic seed pathways in
Rafflesia were generally similar to the other plant species. However, there were some notable exceptions. We found evidence of horizontal transfer of a gene potentially involved in circumventing host defenses. Moreover, we identified a possible convergence among parasitic plants becauseRafflesia ,Striga , andCuscuta shared important similarities. We were unable to find evidence of genes involved in mycorrhizal symbiosis, suggesting that mycoheterotrophy is unlikely to play a role inRafflesia parasitism.To date, ex situ propagation of
Rafflesia by seed has been mostly unsuccessful. Our research is a bold step forward in understanding the fundamentals ofRafflesia seed biology that will inform the continued propagation and seed‐banking efforts concerning this recalcitrant plant. We discuss our findings in the broader context of the conservation of a genus in peril.