Transposable elements (TEs) or transposons, are mobile DNA sequences capable of replicate within host genomes. TEs are silenced by numerous epigenetic mechanisms, being the deposition of DNA methylation one of the most relevant in plants. In angiosperms, the RNA-directed DNA methylation (RdDM) pathway plays an important role in depositing de novo DNA methylation on TEs through the production of 24nt-siRNAs. However, the duckweed family (Lemnaceae) appears to be an exception. Duckweeds represent the smallest and fastest growing flowering plants known to date, thanks to rapid asexual clonal propagation. During clonal propagation, several of its members show no expression of RdDM components, low levels of 24nt-siRNAs, and low DNA methylation associated with RdDM activity although TEs remain silenced. To fully understand TE silencing mechanisms in duckweeds, we have expanded our investigation to one of the most recent lineages of the family: Wolffia brasiliensis.

Wolffia brasiliensis has a relatively high TE content and show recent bursts of transposition compared to the other duckweeds investigated. Like other duckweeds, Wolffia exhibits no RdDM expression and associated DNA methylation signatures. However, in contrast to the other species, 22nt-siRNAs can be found arising from TE loci alongside 24nt-siRNA in several instances. In plants, 22nt siRNAs are associated with Post-Transcriptional Gene Silencing (PTGS) while 24nt are involved in RdDM mediated Transcriptional Gene Silencing (TGS), two pathways considered as mutually exclusive. Whether both siRNA classes share a common source of siRNA precursors, or the different silencing pathways in Wolffia follow a tissue specific expression pattern, remains unknown so far. These unique features of duckweeds, and Wolffia in particular, offer an exceptional opportunity to investigate TE silencing and their interplay with their hosts in non-model plant organisms.