Co-evolution between hosts' and parasites' genomes shapes diverse pathways of acquired immunity based on silencing small (s)RNAs. In plants, sRNAs cause heterochromatinization, sequence degeneration, and, ultimately, loss of autonomy of most transposable elements (TEs). Recognition of newly invasive plant TEs, by contrast, involves an innate antiviral-like silencing response. To investigate this response's activation, we studied the single-copy element EVAD E (EVD), one of few representatives of the large Ty1/ Copia family able to proliferate in Arabidopsis when epigenetically reactivated. In Ty1/Copia elements, a short subgenomic mRNA (shGAG) provides the necessary excess of structural GAG protein over the catalytic components encoded by the full-length genomic flGAG-POL. We show here that the predominant cytosolic distribution of shGAG strongly favors its translation over mostly nuclear flGAG-POL. During this process, an unusually intense ribosomal stalling event coincides with mRNA breakage yielding unconventional 5'OH RNA fragments that evade RNA quality control. The starting point of sRNA production by RNA-DEPENDENT-RNA-POLYMERASE-6 (RDR6), exclusively on shGAG, occurs precisely at this breakage point. This hitherto-unrecognized "translationdependent silencing" (TdS) is independent of codon usage or GC content and is not observed on TE remnants populating the Arabidopsis genome, consistent with their poor association, if any, with polysomes. We propose that TdS forms a primal defense against EVD de novo invasions that underlies its associated sRNA pattern.

7Understory plant species take on different functional strategies, whereby some exploit periods 1 8 of available light in springtime before the canopy closes, and others also benefit from sunlight 1 9 later in autumn when the canopy opens again. These strategies involve understory species 2 0 coordinating phenological events to pre-empt canopy leaf out and to extend their growing 2 1 season beyond canopy leaf senescence, meanwhile accumulating photo-protective pigments 2 2 which mitigate periods of high-light exposure. Canopy closure brings shade to the understory, 2 3 but also causes drastic changes in light quality. Whilst many experiments manipulating spectral 2 4 quality have revealed understory plant responses to the changing R:FR ratio in shade, effect of 2 5 the blue and UV regions have been examined very little. We installed filters attenuating short 2 6 wavelength regions of the solar spectrum in a forest understory in southern Finland, creating the 2 7 following treatments: a transparent control filter, and filters attenuating UV radiation < 350 nm, 2 8 all UV radiation, and both UV and blue light. In eight understory species, representing different 2 9 plant functional types, we repeatedly assessed leaf optical properties to obtain epidermal 3 0 flavonol and anthocyanin contents from leaf emergence in spring to leaf senescence in autumn, 3 1 during both 2017 and 2018. Flavonols responded more to seasonal changes in light quality in 3 2 relatively light-demanding species than in shade-tolerant and wintergreen species; and were 3 3 particularly responsive to blue light. However, anthocyanins were largely unaffected by our 3 4 filter treatments, suggesting that other cues such as cold temperatures govern their seasonal 3 5 variation. UV radiation only accelerated leaf senescence in Acer platanoides seedlings, but blue 3 6 light accelerated leaf senescence in all species measured apart from Quercus robur. In 3 7 summary, seasonal changes in understory solar radiation in the blue and UV regions affected 3 8 leaf pigments and leaf phenology; particularly for more light-demanding species. An increase in 3 9 canopy duration under climate change will extend the period of shade in the understory, with 4 0 consequences for the spectral cues available to understory plants. The resultant reduction in 4 1 blue and UV radiation in shade, could delay leaf senescence in the understory even further. 4 2