Arabidopsis inflorescence architecture requires the activities of KNOX-BELL homeodomain heterodimers

In flowering plants, post-embryonic development is mediated by the activity of shoot and root apical meristems. Shoot architecture results from activity of the shoot apical meristem (SAM), which initiates primordia, including leaves, internodes and axillary meristems, repetitively from its flanks. Axillary meristems can develop into secondary shoots or flowers. In Arabidopsis, two paralogous BEL1-like (BELL) homeobox genes, PENNYWISE (PNY) and POUND-FOOLISH (PNF), expressed in the SAM, encode DNA-binding proteins that are essential for specifying floral primordia and establishing early internode patterning events during inflorescence development. Biochemical studies show that PNY associates with the knotted1-like homeobox (KNOX) proteins, SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS (BP). PNY-BP heterodimers are essential for establishing early internode patterning events, while PNY-STM heterodimers are critical for SAM function. In this report, we examined the role of PNY, PNF and STM during development. First, we show that PNF interacts with STM and BP indicating that PNY and PNF are redundant functioning proteins. Inflorescence development, but not vegetative development, is sensitive to the dosage levels of PNY, PNF and STM. Characterization of stm-10, a weak allele in the Columbia ecotype, indicates that STM is also involved in floral specification and internode development. Our examination of the genetic requirements for PNY, PNF and STM demonstrates that these KNOX–BELL heterodimers control floral specification, internode patterning and the maintenance of boundaries between initiating floral primordia and the inflorescence meristem.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Overproduction of OsSLRL2 alters the development of transgenic Arabidopsis plants

SLR1 (SLENDER RICE 1) was thought to be the sole DELLA protein in rice considering the constitutive GA response phenotype of slr1 mutants. There were two other SLR1 homologous SLRL1 and SLRL2 (SLR1 like 1 and 2) which did not have DELLA domain but still shared high level similarity to the C-terminal region of SLR1 found after searching the whole rice genome. SLRL2 specially expressed in the embryo of immature rice seeds and the expression of SLRL2 was increased when treated with GA(3). The SLRL2 over-expressed transgenic Arabidopsis plants were semi-dwarfed, late flowering, and insensitive to GA. Moreover, the expression of AtGA20ox1 and AtGA3ox1 was increased and the expression of AtGA2ox1 decreased, indicating SLRL2 was a repressor of GA signaling. We suggested SLRL2 might function to overcome too strong GA responses and maintained a basic repression. Furthermore, a different form of DELLA family in monocots against dicots was discussed.     

Autonomy of cell proliferation and developmental programs during Arabidopsis aboveground organ morphogenesis

Elaboration of size and shape in multicellular organisms involves coordinated cell division and cell growth. In higher plants, continuity of cell layer structures exists from the shoot apical meristem (SAM), where organ primordia arise, to mature aboveground organs. To unravel the extent of inter-cell layer coordination during SAM and aboveground organ development, cell division in the epidermis was selectively restricted by expressing two cyclin-dependent kinase inhibitor genes, KRP1/ICK1 and KRP4, driven by the L1 layer-specific AtML1 promoter. The transgenes conferred reduced plant size with striking, distorted lateral organ shape. While epidermal cell division was severely inhibited with compensatory cell size enlargement, the underlying mesophyll/cortex layer kept normal cell numbers and resulted in small, packed cells with disrupted cell files. Our results demonstrate the autonomy of cell number checkpoint in the underlying tissues when epidermal cell division is restricted. Finally, the L1 layer-specific expression of both KRP1/ICK1 and KRP4 showed no effects on the structure and function of the SAM, suggesting that the effects of these cyclin-dependent kinase inhibitors are context dependent.     

An Arabidopsis homolog of RAPTOR/KOG1 is essential for early embryo development

The RAPTOR/KOG1 proteins are binding partners of the target of rapamycin (TOR) kinase that is present in all eucaryotes and plays a central role in the stimulation of cell growth and metabolism in response to nutrients. We show in this report that two genes are coding for RAPTOR/KOG1 homologs in the Arabidopsis and rice genomes. Disruption of the Arabidopsis AtRaptor1 gene leads to a very early arrest of embryo development whereas disruption of the AtRaptor2 gene, which is expressed at a lower level than AtRaptor1, has no visible effects on embryo and plant development. We also observed that mutations in the AtRaptor1 gene result in an earlier halt of embryo development than disruption of the AtTor gene.     

Investigation of cytokinin-deficient phenotypes in Arabidopsis by ectopic expression of orchid DSCKX1

The plant hormone cytokinin plays a major role in regulating plant growth and development. Here we generated cytokinin-reduction Arabidopsis plants by overexpressing a heterologous cytokinin oxidase gene DSCKX1 from Dendrobium orchid. These transgenic plants exhibited reduced biomass, rapid root growth, decreased ability to form roots in vitro, and reduced response to cytokinin in growing calli and roots. Furthermore, the expression of KNAT1, STM, and CycD3 genes was significantly reduced in the transgenic plants, suggesting that cytokinin may function to control the cell cycles and shoot/root development via regulation of these genes.     

Dissecting Drosophila embryonic brain development using photoactivated gene expression

The Drosophila brain is generated by a complex series of morphogenetic movements. To better understand brain development and to provide a guide for experimental manipulation of brain progenitors, we created a fate map using photoactivated gene expression to mark cells originating within specific mitotic domains and time-lapse microscopy to dynamically monitor their progeny. We show that mitotic domains 1, 5, and 9 give rise to discrete cell populations within specific regions of the brain. Two novel observations were that the antennal sensory system, composed of four disparate cell clusters, arose from mitotic domain 5 and that mitotic domain B produced glial cells, while neurons were produced from mitotic domains 1, 5, and 9. Time-lapse analysis of marked cells showed complex mitotic and migratory patterns for cells derived from these mitotic domains. Photoactivated gene expression was also used either to kill, to induce ectopic divisions, or to alter cell fate. This revealed that deficits were not repopulated, while ectopic cells were removed and extra glia were tolerated.     

The regulation of Dkk1 expression during embryonic development

During embryogenesis, the Dkk1 mediated Wnt inhibition controls the spatiotemporal dynamics of cell fate determination, cell differentiation and cell death. Furthermore, the Dkk1 dose is critical for the normal Wnt homeostasis, as alteration of the Dkk1 activity is associated with various diseases. We investigated the regulation of Dkk1 expression during embryonic development. We identified nine conserved non-coding elements (CNEs), located 3′ to the Dkk1 locus. Analyses of the regulatory potential revealed that four of these CNEs in combination drive reporter expression very similar to Dkk1 expression in several organs of transgenic embryos. We extended the knowledge of Dkk1 expression during hypophysis, external genitalia and kidney development, suggesting so far to unexplored functions of Dkk1 during the development of these organs. Characterization of the regulatory potential of four individual CNEs revealed that each of these promotes Dkk1 expression in brain and kidney. In combination, two enhancers are responsible for expression in the pituitary and the genital tubercle. Furthermore, individual CNEs mediates craniofacial, optic cup and limb specific Dkk1 regulation. Our study substantially improves the knowledge of Dkk1 regulation during embryonic development and thus might be of high relevance for therapeutic approaches.     

Ndrg4 is required for normal myocyte proliferation during early cardiac development in zebrafish

NDRG4 is a novel member of the NDRG family (N-myc downstream-regulated gene). The roles of NDRG4 in development have not previously been evaluated. We show that, during zebrafish embryonic development, ndrg4 is expressed exclusively in the embryonic heart, the central nervous system (CNS) and the sensory system. Ndrg4 knockdown in zebrafish embryos causes a marked reduction in proliferative myocytes and results in hypoplastic hearts. This growth defect is associated with cardiac phenotypes in morphogenesis and function, including abnormal heart looping, inefficient circulation and weak contractility. We reveal that ndrg4 is required for restricting the expression of versican and bmp4 to the developing atrioventricular canal. This constellation of ndrg4 cardiac defects phenocopies those seen in mutant hearts of heartstrings (hst), the tbx5 loss-of-function mutants in zebrafish. We further show that ndrg4 expression is significantly decreased in hearts with reduced tbx5 activities. Conversely, increased expression of tbx5 that is due to tbx20 knockdown leads to an increase in ndrg4 expression. Together, our studies reveal an essential role of ndrg4 in regulating proliferation and growth of cardiomyocytes, suggesting that ndrg4 may function downstream of tbx5 during heart development and growth.     

Structural analysis of Arabidopsis thaliana nucleoside diphosphate kinase-2 for phytochrome-mediated light signaling

In plants, nucleoside diphosphate kinases (NDPKs) play a key role in the signaling of both stress and light. However, little is known about the structural elements involved in their function. Of the three NDPKs (NDPK1-NDPK3) expressed in Arabidopsis thaliana, NDPK2 is involved in phytochrome-mediated signal transduction. In this study, we found that the binding of dNDP or NTP to NDPK2 strengthens the interaction significantly between activated phytochrome and NDPK2. To better understand the structural basis of the phytochrome-NDPK2 interaction, we determined the X-ray structures of NDPK1, NDPK2, and dGTP-bound NDPK2 from A.thaliana at 1.8A, 2.6A, and 2.4A, respectively. The structures showed that nucleotide binding caused a slight conformational change in NDPK2 that was confined to helices alphaA and alpha2. This suggests that the presence of nucleotide in the active site and/or the evoked conformational change contributes to the recognition of NDPK2 by activated phytochrome. In vitro binding assays showed that only NDPK2 interacted specifically with the phytochrome and the C-terminal regulatory domain of phytochrome is involved in the interaction. A domain swap experiment between NDPK1 and NDPK2 showed that the variable C-terminal region of NDPK2 is important for the activation by phytochrome. The structure of Arabidopsis NDPK1 and NDPK2 showed that the isoforms share common electrostatic surfaces at the nucleotide-binding site, but the variable C-terminal regions have distinct electrostatic charge distributions. These findings suggest that the binding of nucleotide to NDPK2 plays a regulatory role in phytochrome signaling and that the C-terminal extension of NDPK2 provides a potential binding surface for the specific interaction with phytochromes.