Cytoplasmic Arabidopsis AGO7 accumulates in membrane-associated siRNA bodies and is required for ta-siRNA biogenesis

Formation of trans-acting small interfering RNAs (ta-siRNAs) from the TAS3 precursor is triggered by the AGO7/miR390 complex, which primes TAS3 for conversion into double-stranded RNA by the RNA-dependent RNA polymerase RDR6 and SGS3. These ta-siRNAs control several aspects of plant development. The mechanism routing AGO7-cleaved TAS3 precursor to RDR6/SGS3 and its subcellular organization are unknown. We show that AGO7 accumulates together with SGS3 and RDR6 in cytoplasmic siRNA bodies that are distinct from P-bodies. siRNA bodies colocalize with a membrane-associated viral protein and become positive for stress-granule markers upon stress-induced translational repression, this suggests that siRNA bodies are membrane-associated sites of accumulation of mRNA stalled during translation. AGO7 congregates with miR390 and SGS3 in membranes and its targeting to the nucleus prevents its accumulation in siRNA bodies and ta-siRNA formation. AGO7 is therefore required in the cytoplasm and membranous siRNA bodies for TAS3 processing, revealing a hitherto unknown role for membrane-associated ribonucleoparticles in ta-siRNA biogenesis and AGO action in plants.     

RNA Silencing Is Resistant to Low-Temperature in Grapevine

RNA silencing is a natural defence mechanism against viruses in plants, and transgenes expressing viral RNA-derived sequences were previously shown to confer silencing-based enhanced resistance against the cognate virus in several species. However, RNA silencing was shown to dysfunction at low temperatures in several species, questioning the relevance of this strategy in perennial plants such as grapevines, which are often exposed to low temperatures during the winter season. Here, we show that inverted-repeat (IR) constructs trigger a highly efficient silencing reaction in all somatic tissues in grapevines. Similarly to other plant species, IR-derived siRNAs trigger production of secondary transitive siRNAs. However, and in sharp contrast to other species tested to date where RNA silencing is hindered at low temperature, this process remained active in grapevine cultivated at 4°C. Consistently, siRNA levels remained steady in grapevines cultivated between 26°C and 4°C, whereas they are severely decreased in Arabidopsis grown at 15°C and almost undetectable at 4°C. Altogether, these results demonstrate that RNA silencing operates in grapevine in a conserved manner but is resistant to far lower temperatures than ever described in other species.     

Deficiency of a Glycogen Synthase-associated Protein,Epm2aip1, Causes Decreased Glycogen Synthesis and Hepatic Insulin Resistance

Glycogen synthesis is a major component of the insulin response, and defective glycogen synthesis is a major portion of insulin resistance. Insulin regulates glycogen synthase (GS) through incompletely defined pathways that activate the enzyme through dephosphorylation and, more potently, allosteric activation. We identify Epm2aip1 as a GS-associated protein. We show that the absence of Epm2aip1 in mice impairs allosteric activation of GS by glucose 6-phosphate, decreases hepatic glycogen synthesis, increases liver fat, causes hepatic insulin resistance, and protects against age-related obesity. Our work identifies a novel GS-associated GS activity-modulating component of insulin resistance.     

Evaluation in tobacco of the organ specificity and strength of therolD promoter,domain A of the 35S promoter and the 35S2 promoter

In order to study the expression in plants of therolD promoter ofAgrobacterium rhizogenes, we have constructed chimaeric genes placing the coding region of thegusA (uidA) marker gene under control of tworolD promoter fragments of different length. Similar results were obtained with both genes. Expression studies were carried out in transformed R₁ progeny plants. In mature transformed tobacco plants, therolD-gus genes were expressed strongly in roots, and to much lower levels in stems and leaves. This pattern of expression was transmitted to progeny, though the ratio of the level of expression in roots relative to that in leaves was much lower in young seedlings. The degree of root specificity inrolD-gus transformants was less than that of a gene constructed with domain A of the CaMV 35S promoter,domA-gus, but the level of root expression was much higher than with the latter gene. However, the level of expression of therolD-gus genes was less than that of agus gene with a 35S promoter with doubled domain B, 35S²-gus. TherolD-gus genes had a distinctive pattern of expression in roots, compared to that of the two other genes, with the strongest GUS activity observed in the root elongation zone and in vascular tissue, and much less in the root apex.