Highlights d Phytophthora infection increases production of a pool of secondary siRNAs in Arabidopsis d Secondary siRNAs from a PPR gene cluster contribute to defense against Phytophthora d PPR-siRNAs potentially silence Phytophthora transcripts to confer resistance d Phytophthora effector PSR2 suppresses the biogenesis of PPR-siRNAs to promote infection

Small RNAs are central players in RNA silencing, yet their cytoplasmic compartmentalization and the effects it may have on their activities have not been studied at the genomic scale. Here we report that Arabidopsis microRNAs (miRNAs) and small interfering RNAs (siRNAs) are distinctly partitioned between the endoplasmic reticulum (ER) and cytosol. All miRNAs are associated with membrane-bound polysomes (MBPs) as opposed to polysomes in general. The MBP association is functionally linked to a deeply conserved and tightly regulated activity of miRNAs – production of phased siRNAs (phasiRNAs) from select target RNAs. The phasiRNA precursor RNAs, thought to be noncoding, are on MBPs and are occupied by ribosomes in a manner that supports miRNA-triggered phasiRNA production, suggesting that ribosomes on the rough ER impact siRNA biogenesis. This study reveals global patterns of cytoplasmic partitioning of small RNAs and expands the known functions of ribosomes and ER.DOI: http://dx.doi.org/10.7554/eLife.22750.001

Plants flower in an appropriate season to allow sufficient vegetative development and position flower development in favorable environments. In Arabidopsis, CONSTANS (CO) and FLAVIN-BINDING KELCH REPEAT F-BOX1 (FKF1) promote flowering by inducing FLOWER LOCUS T (FT) expression in the long-day afternoon. The CO protein is present in the morning but could not activate FT expression due to unknown negative mechanisms, which prevent premature flowering before the day length reaches a threshold. Here, we report that TARGET OF EAT1 (TOE1) and related proteins interact with the activation domain of CO and CO-like (COL) proteins and inhibit CO activity. TOE1 binds to the FT promoter near the CO-binding site, and reducing TOE function results in a morning peak of the FT mRNA. In addition, TOE1 interacts with the LOV domain of FKF1 and likely interferes with the FKF1-CO interaction, resulting in partial degradation of the CO protein in the afternoon to prevent premature flowering.

In , the MOS4-ASSOCIATED COMPLEX (MAC) is required for defense and development. The evolutionarily conserved, putative RNA helicase MAC7 is a component of the Arabidopsis MAC, and the human MAC7 homolog, Aquarius, is implicated in pre-mRNA splicing. Here, we show that, a partial loss-of-function mutant in , and two other MAC subunit mutants, and (), exhibit reduced microRNA (miRNA) levels, indicating that MAC promotes miRNA biogenesis. The mutant shows reduced primary miRNA (pri-miRNA) levels without affecting miRNA gene () promoter activity or the half-life of pri-miRNA transcripts. As a nuclear protein, MAC7 is not concentrated in dicing bodies, but it affects the localization of HYPONASTIC LEAVES1 (HYL1), a key protein in pri-miRNA processing, to dicing bodies. Immunoprecipitation of HYL1 retrieved 11 known MAC subunits, including MAC7, indicating association between HYL1 and MAC. We propose that MAC7 links transcription to pri-miRNA processing. RNA-seq analysis showed that downregulated genes in MAC subunit mutants are mostly involved in plant defense and stimulus responses, confirming a role of MAC in biotic and abiotic stress responses. We also discovered global intron retention defects in mutants in three subunits of MAC, thus linking MAC function to splicing in Arabidopsis.

Summary Unlike in metazoans, the stepwise biogenesis of microRNAs (miRNAs) occurs within the nucleus in plants. Whether or how the major steps in miRNA biogenesis are coordinated is largely unknown. Here, we show that the plant TREX-2 complex promotes multiple steps in miRNA biogenesis, including transcription, processing and nuclear export. Core subunits of TREX-2, THP1 and SAC3A, interact and co-localize with RNA Polymerase II to promote the transcription of MIR genes in the nucleoplasm. TREX-2 interacts with the microprocessor component SERRATE and promotes the formation of Dicing bodies in the nucleoplasm. THP1 also interacts and co-localizes with the nucleoporin protein NUP1 at the nuclear envelope. NUP1 and THP1 promote the nuclear export of miRNAs and AGO1. These results suggest that TREX-2 coordinates the transcription, processing and export steps in miRNA biogenesis to ensure efficient miRNA production.