Polycomb-repressive complexes (PRCs) ensure the correct spatiotemporal expression of numerous key developmental regulators. Despite their pivotal role, how PRCs are recruited to specific targets remains largely unsolved, particularly in plants. Here we show that the Arabidopsis ASYMMETRIC LEAVES complex physically interacts with PRC2 and recruits this complex to the homeobox genes BREVIPEDICELLUS and KNAT2 to stably silence these stem cell regulators in differentiating leaves. The recruitment mechanism resembles the Polycomb response element-based recruitment of PRC2 originally defined in flies and provides the first such example in plants. Combined with recent studies in mammals, our findings reveal a conserved paradigm to epigenetically regulate homeobox gene expression during development.

Background: Translation initiation factors of the 4E and 4G protein families mediate resistance to several RNA plant viruses in the natural diversity of crops. Particularly, a single point mutation in melon eukaryotic translation initiation factor 4E (eIF4E) controls resistance to Melon necrotic spot virus (MNSV) in melon. Identification of allelic variants within natural populations by EcoTILLING has become a rapid genotype discovery method.

Mobile small RNAs serve as local positional signals in development and coordinate stress responses across the plant. Despite its central importance, an understanding of how the cell-to-cell movement of small RNAs is governed is lacking. Here, we show that miRNA mobility is precisely regulated through a gating mechanism polarised at defined cell–cell interfaces. This generates directional movement between neighbouring cells that limits long-distance shoot-to-root trafficking, and underpins domain-autonomous behaviours of small RNAs within stem cell niches. We further show that the gating of miRNA mobility occurs independent of mechanisms controlling protein movement, identifying the small RNA as the mobile unit. These findings reveal gate-keepers of cell-to-cell small RNA mobility generate selectivity in long-distance signalling, and help safeguard functional domains within dynamic stem cell niches while mitigating a ‘signalling gridlock’ in contexts where developmental patterning events occur in close spatial and temporal vicinity.

An analysis of nucleotide sequences in five coding and one non-coding genomic regions of 35 Cucurbit yellow stunting disorder virus (CYSDV) isolates collected on a local scale over an 8 year period is reported here. In total, 2277 nt were sequenced for each isolate, representing about 13 % of the complete virus genome. Mean nucleotide diversity for the whole population in synonymous positions in the coding regions was 0?00068, whilst in the 59 untranslated region (59 UTR) of genomic RNA2, it was 0?00074; both of these values are very small, compared with estimates of nucleotide diversity for populations of other plant viruses. Nucleotide diversity was also determined independently for each of the ORFs and for the 59 UTR of RNA2; the data showed that variability is not distributed evenly among the different regions of the viral genome, with the coat protein gene showing more diversity than the other four coding regions that were analysed. However, the low variability found precluded any inference of selection differences among gene regions. On the other hand, no evidence of selection associated with host adaptation was found. In contrast, at least a single amino acid change in the coat protein appears to have been selected with time. INTRODUCTIONRNA viruses are known to have a high variation potential, because of their high mutation and recombination rates (Drake & Holland, 1999;Bruyere et al., 2000;Malpica et al., 2002). However, this mutability does not necessarily result in a high degree of genetic variability of their populations, as the genetic composition of virus populations will depend on evolutionary forces, such as selection and random genetic drift. In fact, for most of the cases analysed, plant virus populations have been shown to be genetically stable (reviewed by García-Arenal et al., 2001). To explain this stability, population bottlenecks associated with ecological factors, such as colonization of new hosts or new geographical areas, are often invoked (Fraile et al., 1996(Fraile et al., , 1997a Sánchez-Campos et al., 2002). Also, selection pressures, including the maintenance of structural features of the virus or interaction with host and vector factors, may contribute to genetic stability (e.g. Moreno et al., 2004) (reviewed by García-Arenal et al., 2001). The relative contribution of these evolutionary forces can be studied by analysing the genetic structure of plant virus populations. Analysis through temporal series might be particularly informative, although studies of this type are scarce (Fraile et al., 1997b; Sánchez-Campos et al., 2002).We report here a local-scale analysis of the genetic diversity of a natural population of Cucurbit yellow stunting disorder virus (CYSDV), a whitefly-transmitted closterovirus that affects cucurbit crops extensively in many warm and temperate areas of production (Hassan & Duffus, 1991; Célix et al., 1996;Wisler et al., 1998; Abou-Jawdah et al., 2000;Desbiez et al., 2000;Kao et al., 2000;Louro et al., 2000). Whitefly-transmitted closteroviruses are respo...