SKI2 mediates degradation of RISC 5'-cleavage fragments and prevents secondary siRNA production from miRNA targets in Arabidopsis.

Small regulatory RNAs are fundamental in eukaryotic and prokaryotic gene regulation. In plants, an important element of post-transcriptional control is effected by 20-24 nt microRNAs (miRNAs) and short interfering RNAs (siRNAs) bound to the ARGONAUTE1 (AGO1) protein in an RNA induced silencing complex (RISC). AGO1 may cleave target mRNAs with small RNA complementarity, but the fate of the resulting cleavage fragments remains incompletely understood. Here, we show that SKI2, SKI3 and SKI8, subunits of a cytoplasmic cofactor of the RNA exosome, are required for degradation of RISC 5', but not 3'-cleavage fragments in Arabidopsis. In the absence of SKI2 activity, many miRNA targets produce siRNAs via the RNA-dependent RNA polymerase 6 (RDR6) pathway. These siRNAs are low-abundant, and map close to the cleavage site. In most cases, siRNAs were produced 5' to the cleavage site, but several examples of 3'-spreading were also identified. These observations suggest that siRNAs do not simply derive from RDR6 action on stable 5'-cleavage fragments and hence that SKI2 has a direct role in limiting secondary siRNA production in addition to its function in mediating degradation of 5'-cleavage fragments.

Stars and symbiosis: microRNA- and microRNA*-mediated transcript cleavage involved in arbuscular mycorrhizal symbiosis.

The majority of plants are able to form the arbuscular mycorrhizal (AM) symbiosis in association with AM fungi. During symbiosis development, plant cells undergo a complex reprogramming resulting in profound morphological and physiological changes. MicroRNAs (miRNAs) are important components of the regulatory network of plant cells. To unravel the impact of miRNAs and miRNA-mediated mRNA cleavage on root cell reprogramming during AM symbiosis, we carried out high-throughput (Illumina) sequencing of small RNAs and degradome tags of Medicago truncatula roots. This led to the annotation of 243 novel miRNAs. An increased accumulation of several novel and conserved miRNAs in mycorrhizal roots suggest a role of these miRNAs during AM symbiosis. The degradome analysis led to the identification of 185 root transcripts as mature miRNA and also miRNA*-mediated mRNA cleavage targets. Several of the identified miRNA targets are known to be involved in root symbioses. In summary, the increased accumulation of specific miRNAs and the miRNA-mediated cleavage of symbiosis-relevant genes indicate that miRNAs are an important part of the regulatory network leading to symbiosis development.

Expression pattern suggests a role of MiR399 in the regulation of the cellular response to local Pi increase during arbuscular mycorrhizal symbiosis.

Many plants improve their phosphate (Pi) availability by forming mutualistic associations with arbuscular mycorrhizal (AM) fungi. Pi-repleted plants are much less colonized by AM fungi than Pi-depleted plants. This indicates a link between plant Pi signaling and AM development. MicroRNAs (miR) of the 399 family are systemic Pi-starvation signals important for maintenance of Pi homeostasis in Arabidopsis thaliana and might also qualify as signals regulating AM development in response to Pi availability. MiR399 could either represent the systemic low-Pi signal promoting or required for AM formation or they could act as counter players of systemic Pi-availability signals that suppress AM symbiosis. To test either of these assumptions, we analyzed the miR399 family in the AM-capable plant model Medicago truncatula and could experimentally confirm 10 novel MIR399 genes in this species. Pi-depleted plants showed increased expression of mature miR399 and multiple pri-miR399, and unexpectedly, levels of five of the 15 pri-miR399 species were higher in leaves of mycorrhizal plants than in leaves of nonmycorrhizal plants. Compared with nonmycorrhizal Pi-depleted roots, mycorrhizal roots of Pi-depleted M. truncatula and tobacco plants had increased Pi contents due to symbiotic Pi uptake but displayed higher mature miR399 levels. Expression levels of MtPho2 remained low and PHO2-dependent Pi-stress marker transcript levels remained high in these mycorrhizal roots. Hence, an AM symbiosis-related signal appears to increase miR399 expression and decrease PHO2 activity. MiR399 overexpression in tobacco suggested that miR399 alone is not sufficient to improve mycorrhizal colonization supporting the assumption that, in mycorrhizal roots, increased miR399 are necessary to keep the MtPho2 expression and activity low, which would otherwise increase in response to symbiotic Pi uptake.

Distribution pattern of small RNA and degradome reads provides information on miRNA gene structure and regulation.

Plant microRNAs (miRNAs) have an impact in the regulation of several biological processes such as development, growth and metabolism by negatively controlling gene expression at the post-transcriptional level. However, the role of these small molecules in the symbiotic interaction of plant roots and arbuscular mycorrhizal (AM) fungi remained elusive. To elucidate the role of miRNAs during AM symbiosis we used a deep sequencing approach to analyze the small RNA and degradome sequence tags of Medicago truncatula non-mycorrhizal and mycorrhizal roots. We identified 243 novel Medicago microRNAs and 118 mRNA cleavage targets of miRNA mature and star sequences. Several AM symbiosis-relevant genes were identified as miRNA targets. The transcript of MtNsp2, encoding a GRAS transcription factor involved in the nodule and mycorrhizal signaling pathway, is cleaved by a novel member of the miR171 gene family, namely miR171h. Here, we carried out a detailed analysis of the genomic structure of the MIR171h gene comprising our deep sequencing data. The results suggest a feedback circuit between mature miR171h and its own primary transcript showing the ability of this miRNA regulating itself.

Evidence that SNF1-related kinase and hexokinase are involved in separate sugar-signalling pathways modulating post-translational redox activation of ADP-glucose pyrophosphorylase in potato tubers.

We recently discovered that post-translational redox modulation of ADP-glucose pyrophosphorylase (AGPase) is a powerful new mechanism to adjust the rate of starch synthesis to the availability of sucrose in growing potato tubers. A strong correlation was observed between the endogenous levels of sucrose and the redox-activation state of AGPase. To identify candidate components linking AGPase redox modulation to sugar supply, we used potato tuber discs as a model system. When the discs were cut from growing wild-type potato tubers and incubated for 2 h in the absence of sugars, redox activation of AGPase decreased because of a decrease in internal sugar levels. The decrease in AGPase redox activation could be prevented when glucose or sucrose was supplied to the discs. Both sucrose uptake and redox activation of AGPase were increased when EDTA was used to prepare the tuber discs. However, EDTA treatment of discs had no effect on glucose uptake. Feeding of different glucose analogues revealed that the phosphorylation of hexoses by hexokinase is an essential component in the glucose-dependent redox activation of AGPase. In contrast to this, feeding of the non-metabolisable sucrose analogue, palatinose, leads to a similar activation as with sucrose, indicating that metabolism of sucrose is not necessary in the sucrose-dependent AGPase activation. The influence of sucrose and glucose on redox activation of AGPase was also investigated in discs cut from tubers of antisense plants with reduced SNF1-related protein kinase activity (SnRK1). Feeding of sucrose to tuber discs prevented AGPase redox inactivation in the wild type but not in SnRK1 antisense lines. However, feeding of glucose leads to a similar activation of AGPase in the wild type and in SnRK1 transformants. AGPase redox activation was also increased in transgenic tubers with ectopic overexpression of invertase, containing high levels of glucose and low sucrose levels. Expression of a bacterial glucokinase in the invertase-expressing background led to a decrease in AGPase activation state and tuber starch content. These results show that both sucrose and glucose lead to post-translational redox activation of AGPase, and that they do this by two different pathways involving SnRK1 and an endogenous hexokinase, respectively.

Starch synthesis in potato tubers is regulated by post-translational redox modification of ADP-glucose pyrophosphorylase: a novel regulatory mechanism linking starch synthesis to the sucrose supply.

Transcriptional and allosteric regulation of ADP-Glc pyrophosphorylase (AGPase) plays a major role in the regulation of starch synthesis. Analysis of the response after detachment of growing potato tubers from the mother plant revealed that this concept requires extension. Starch synthesis was inhibited within 24 h of tuber detachment, even though the catalytic subunit of AGPase (AGPB) and overall AGPase activity remained high, the substrates ATP and Glc-1-P increased, and the glycerate-3-phosphate/inorganic orthophosphate (the allosteric activator and inhibitor, respectively) ratio increased. This inhibition was abolished in transformants in which a bacterial AGPase replaced the potato AGPase. Measurements of the subcellular levels of each metabolite between Suc and starch established AGPase as the only step whose substrates increase and mass action ratio decreases after detachment of wild-type tubers. Separation of extracts on nonreducing SDS gels revealed that AGPB is present as a mixture of monomers and dimers in growing tubers and becomes dimerized completely in detached tubers. Dimerization led to inactivation of the enzyme as a result of a marked decrease of the substrate affinity and sensitivity to allosteric effectors. Dimerization could be reversed and AGPase reactivated in vitro by incubating extracts with DTT. Incubation of tuber slices with DTT or high Suc levels reduced dimerization, increased AGPase activation, and stimulated starch synthesis in vivo. In intact tubers, the Suc content correlated strongly with AGPase activation across a range of treatments, including tuber detachment, aging of the mother plant, heterologous overexpression of Suc phosphorylase, and antisense inhibition of endogenous AGPase activity. Furthermore, activation of AGPase resulted in a stimulation of starch synthesis and decreased levels of glycolytic intermediates.