A link between RNA metabolism and silencing affecting Arabidopsis development.

MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) are abundant endogenous small RNAs (smRNAs) that control transcript expression through posttranscriptional gene silencing. Here, we show that concomitant loss of XRN4/EIN5, a 5'-3' exoribonuclease, and ABH1/CBP80, a subunit of the mRNA cap binding complex, results in Arabidopsis plants manifesting myriad developmental defects. We find that ABH1/CBP80 is necessary to obtain proper mature miRNA levels, which suggests this protein affects the miRNA-mediated RNA silencing pathway. Additionally, we show that XRN4/EIN5 affects the levels of a smRNA class that is processed from both sense and antisense strands of approximately 130 endogenous transcripts that apparently are converted to double-stranded RNA (dsRNA) and subsequently processed. We find that the parent transcripts of these smRNAs accumulate in an uncapped form upon loss of XRN4/EIN5, which suggests that uncapped endogenous transcripts can become smRNA biogenesis substrates. Overall, our results reveal unexpected connections between RNA metabolism and silencing pathways.