Mutations in the ARGONAUTE gene ZIPPY(ZIP)/AGO7 in Arabidopsis accelerate the juvenile-to-adult transition. A screen for mutations that suppress this precocious phenotype yielded alleles of two auxin-related transcription factors known to be upregulated in zip: ETTIN (ETT)/ARF3 and ARF4. Mutations in ETT/ARF3 and ARF4 delay the expression of adult traits, demonstrating that these genes have non-redundant roles in shoot maturation. ZIP is not generally required for the production of trans-acting (ta) siRNAs, but is required for the production and/or stability of tasiR-ARF, a ta-siRNA that targets both ETT/ARF3 and ARF4. tasiR-ARF is absent in zip-2, and overexpression of a tasiR-ARF-insensitive form of ETT mimics the zip phenotype. We conclude that the precocious phenotype of zip is attributable to the absence of tasiR-ARF-mediated repression of ETT and ARF4. The abundance of tasiR-ARF, ETT/ARF3 and ARF4 RNA does not change during vegetative development. This result suggests that tasiR-ARF regulation establishes the threshold at which leaves respond to a temporal signal, rather than being a component of this signal.

Loss-of-function mutations of HASTY (HST) affect many different processes in Arabidopsis development. In addition to reducing the size of both roots and lateral organs of the shoot, hstmutations affect the size of the shoot apical meristem, accelerate vegetative phase change, delay floral induction under short days, adaxialize leaves and carpels, disrupt the phyllotaxis of the inflorescence, and reduce fertility. Double mutant analysis suggests that HST acts in parallel toSQUINT in the regulation of phase change and in parallel toKANADI in the regulation of leaf polarity. Positional cloning demonstrated that HST is the Arabidopsis ortholog of the importin β-like nucleocytoplasmic transport receptors exportin 5in mammals and MSN5 in yeast. Consistent with a potential role in nucleocytoplasmic transport, we found that HST interacts with RAN1 in a yeast two-hybrid assay and that a HST-GUS fusion protein is located at the periphery of the nucleus. HST is one of at least 17 members of the importin-βfamily in Arabidopsis and is the first member of this family shown to have an essential function in plants. The hst loss-of-function phenotype suggests that this protein regulates the nucleocytoplasmic transport of molecules involved in several different morphogenetic pathways, as well as molecules generally required for root and shoot growth.

Plants progress through a temporal sequence of juvenile, adult, and reproductive phases, each marked by the expression of phase-specific traits. Here we show that loss-of-function mutations in ZIPPY (ZIP) cause the premature expression of adult vegetative traits but do not accelerate the onset of reproductive competence or flowering time. ZIP encodes ARGONAUTE7 (AGO7), one of ten members of the ARGONAUTE family in Arabidopsis. In addition to playing developmental roles, some ARGONAUTE family members are required for RNAi-like phenomena, such as posttranscriptional gene silencing. In contrast to Arabidopsis ARGONAUTE1, ZIP has no significant role in transgene silencing; its primary function is in the regulation of developmental timing.

During cellularization, the Drosophila embryo undergoes a large-scale cytokinetic event that packages thousands of syncytial nuclei into individual cells, resulting in the de novo formation of an epithelial monolayer in the cortex of the embryo. The formation of adherens junctions is one of the many aspects of epithelial polarity that is established during cellularization: at the onset of cellularization, the Drosophila β-catenin homologue Armadillo (Arm) accumulates at the leading edge of the cleavage furrow, and later to the apicolateral region where the zonula adherens precursors are formed. In this paper, we show that the basal accumulation of Arm colocalizes with DE-cadherin and Dα-catenin, and corresponds to a region of tight membrane association, which we refer to as the basal junction. Although the two junctions are similar in components and function, they differ in their response to the novel cellularization protein Nullo. Nullo is present in the basal junction and is required for its formation at the onset of cellularization. In contrast, Nullo is degraded before apical junction formation, and prolonged expression of Nullo blocks the apical clustering of junctional components, leading to morphological defects in the developing embryo. These observations reveal differences in the formation of the apical and basal junctions, and offer insight into the role of Nullo in basal junction formation.