Embryonic regulators LEC2 (LEAFY COTYLEDON2) and FUS3 (FUSCA3) are involved in multiple aspects of Arabidopsis (Arabidopsis thaliana) seed development, including repression of leaf traits and premature germination and activation of seed storage protein genes. In this study, we show that gibberellin (GA) hormone biosynthesis is regulated by LEC2 and FUS3 pathways. The level of bioactive GAs is increased in immature seeds of lec2 and fus3 mutants relative to wild-type level. In addition, we show that the formation of ectopic trichome cells on lec2 and fus3 embryos is a GA-dependent process as in true leaves, suggesting that the GA pathway is misactivated in embryonic mutants. We next demonstrate that the GA-biosynthesis gene AtGA3ox2, which encodes the key enzyme AtGA3ox2 that catalyzes the conversion of inactive to bioactive GAs, is ectopically activated in embryos of the two mutants. Interestingly, both b-glucuronidase reporter gene expression and in situ hybridization indicate that FUS3 represses AtGA3ox2 expression mainly in epidermal cells of embryo axis, which is distinct from AtGA3ox2 pattern at germination. Finally, we show that the FUS3 protein physically interacts with two RY elements (CATGCATG) present in the AtGA3ox2 promoter. This work suggests that GA biosynthesis is directly controlled by embryonic regulators during Arabidopsis embryonic development.Higher plant embryogenesis is divided into two major phases: embryo development (or morphogenesis) and seed maturation (West and Harada, 1993). During embryo development, early morphogenetic processes occur that give rise to embryonic cell types, tissues, and organs. During seed maturation, the fully developed embryo undergoes maturation, during which food reserves accumulate and dormancy and desiccation tolerance develop.Seed development has been extensively studied in Arabidopsis (Arabidopsis thaliana) using mutants defective either in morphogenesis, such as GNOM (Mayer et al., 1991;Shevell et al., 1994) or KNOLLE (Mayer et al., 1991;Lukowitz et al., 1996), or in maturation, such as ABI (ABSCISIC ACID-INSENSITIVE) loci that were initially identified on the basis of the abscisic acid (ABA) hormone-resistant germination of mutants at these loci (Koornneef et al., 1984;Giraudat et al., 1992;Finkelstein et al., 1998;Finkelstein and Lynch, 2000). A particular set of mutants exhibiting the lec phenotype, which consists of a partial transformation of cotyledons into leaves, has allowed the identification of an important network of regulatory genes. The LEC1 (LEAFY COTYLEDON1; Meinke, 1992), LEC2 (Meinke et al., 1994), and FUS3 (FUSCA3;Keith et al., 1994;Meinke et al., 1994) genes, which are defined as the LEC genes hereafter, are the only known regulators required for normal development during both the morphogenesis and the maturation phases (Holdsworth et al., 1999;Harada, 2001 Article, publication date, and citation information can be found at www.plantphysiol.org/cgi

Phytohormones are signal molecules produced within the plant that control its growth and development through the regulation of gene expression. Interaction between different phytohormone pathways is essential in coordinating tissue outgrowth in response to environmental changes, such as the adaptation of root development to water deficit or the initiation of seed germination during imbibition. Recently, microRNAs (miRNAs) have emerged as key regulators of phytohormone response pathways in planta by affecting their metabolism, distribution, and perception. Here we review current knowledge on the miRNA-mediated regulations involved in phytohormone crosstalk. We focus on the miRNAs exhibiting regulatory links with more than one phytohormone pathway and discuss their possible implication in coordinating multiple phytohormone responses during specific developmental processes.

In Arabidopsis, genetic evidence demonstrates that RNA-dependent RNA polymerase 6 (RDR6) plays a fundamental role in at least four RNA silencing pathways whose functions range from defense against transgenes or viruses to endogene regulation in development and in stress responses. Despite its critical role in RNA silencing, the biochemical activities of RDR6 have yet to be characterized. In this study, we transiently expressed Arabidopsis RDR6 in Nicotiana benthamiana and investigated the biochemical activities of immunopurified RDR6 in vitro. We showed that RDR6 possesses terminal nucleotidyltransferase activity as well as primer-independent RNA polymerase activity on single-stranded RNAs. We found that RDR6 cannot distinguish RNAs with or without a cap or poly(A) tail. We also demonstrated that RDR6 has strong polymerase activity on singlestranded DNA. All these activities require the conserved catalytic Asp 867 residue. Our findings have important implications on the processes involving RDR6 in vivo and provide new biochemical insights into the mechanisms of RNA silencing in Arabidopsis.

BackgroundThe transitions from juvenile to adult and adult to reproductive phases of growth are important stages in the life cycle of plants. The regulators of these transitions include miRNAs, in particular miR156 and miR172 which are part of a regulatory module conserved across the angiosperms. In Arabidopsis miR171 represses differentiation of axillary meristems by repressing expression of SCARECROW-LIKE(SCL) transcription factors, however the role of miR171 has not been examined in other plants.ResultsTo investigate the roles of mir171 and its target genes in a monocot, the Hvu pri-miR171a was over-expressed in barley (Hordeum vulgare L. cv. Golden promise) leading to reduced expression of at least one HvSCL gene. The resulting transgenic plants displayed a pleiotropic phenotype which included branching defects, an increased number of short vegetative phytomers and late flowering. These phenotypes appear to be the consequence of changes in the organisation of the shoot meristem. In addition, the data show that miR171 over-expression alters the vegetative to reproductive phase transition by activating the miR156 pathway and repressing the expression of the TRD (THIRD OUTER GLUME) and HvPLA1 (Plastochron1) genes.ConclusionsOur data suggest that some of the roles of miR171 and its target genes that have been determined in Arabidopsis are conserved in barley and that they have additional functions in barley including activation of the miR156 pathway.