Scl binds to primed enhancers in mesoderm to regulate hematopoietic and cardiac fate divergence

Scl/Tal1 confers hemogenic competence and prevents ectopic cardiomyogenesis in embryonic endothelium by unknown mechanisms. We discovered that Scl binds to hematopoietic and cardiac enhancers that become epigenetically primed in multipotent cardiovascular mesoderm, to regulate the divergence of hematopoietic and cardiac lineages. Scl does not act as a pioneer factor but rather exploits a pre‐established epigenetic landscape. As the blood lineage emerges, Scl binding and active epigenetic modifications are sustained in hematopoietic enhancers, whereas cardiac enhancers are decommissioned by removal of active epigenetic marks. Our data suggest that, rather than recruiting corepressors to enhancers, Scl prevents ectopic cardiogenesis by occupying enhancers that cardiac factors, such as Gata4 and Hand1, use for gene activation. Although hematopoietic Gata factors bind with Scl to both activated and repressed genes, they are dispensable for cardiac repression, but necessary for activating genes that enable hematopoietic stem/progenitor cell development. These results suggest that a unique subset of enhancers in lineage‐specific genes that are accessible for regulators of opposing fates during the time of the fate decision provide a platform where the divergence of mutually exclusive fates is orchestrated.     

Tissue- and development-specific distributions of cytoskeletal elements in growing cells of the maize root apex

ABSTRACT

Indirect immunofluorescence performed using sections of actively growing maize root apices fixed and then embedded in low-melting-point Steedman's wax has proved efficient in revealing the arrangements and reorganizations of motility-related cytoskeletal elements which are associated with root cell development and tissue differentiation. This powerful, yet relatively simple, technique shows that specific rearrangements of both microtubular (MT) and actin microfilament (MF) arrays occur in cells as they leave the meristem and traverse the transitional region interpolated between meristem and elongation region. Cytoskeletal and growth analyses have identified the transition zone as critical for both cell and root development; it is in this zone that cell growth is channelled, by the cytoskeleton, into a strictly polarized mode which enables root tips to extend rapidly through the soil in search of water and nutrients. An integrated cytoskeletal network is crucial for both the cytomorphogenesis of individual cells and the overall morphogenesis of the plant body. The latter process can be viewed as a reflection of the tight control which cytoskeletal networks exert not only over cell division planes in the cells within meristematic apices but also over the orientation of cell growth in the meristem and elsewhere. Endoplasmic MTs interconnecting the plasma membrane with the nucleus are suggested to be involved in cell division control; they may also act as a two-way cytoskeletal communication channel for signals passing to and fro between the extracellular environment and the genome. Moreover, the dynamism of endoplasmic MTs exerts direct effects on chromatin structure and the accompanying nuclear architecture and hence can help exert a cellular level of control over cell growth and cell cycle progression. Because the inherent dynamic instability of MTs depends on the concentration of tubulin dimers within the cytoplasm, we propose that when asymmetric cell division occurs, it will result in two daughter cells which differ in the turnover rates of their MTs. This phenomenon could be responsible for different cell fates of daughter plant cells produced by such cell divisions.     

A new late Westphalian fossil marattialean fern from Nova Scotia

Sydneia manleyi gen. et sp. nov. is based on part of a fertile frond from the upper Westphalian D of the Sydney Coalfield, Nova Scotia, Canada. It has small synangia composed of laterally fused sporangia that are elongate and with a circular cross-section. The sporangia yielded variably sized monolete and trilete spores with laevigate and microspinate ornamentation; intermediate forms were also observed. The spores can be correlated with the sporae dispersae species Latosporites minutus , Punctatosporites oculus and Laevigatosporites minimus . Size distribution of the spores is variable and highly skewed, suggesting heterogeneity of the spores within the sporangium. Spore ultrastructure indicates that the fossil is part of a fern, and the morphology of the spores and synangia indicate marattialean affinities.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 142 , 199–212.     

Expression of the Auxin-Inducible GH3 Promoter/GUS Fusion Gene as a Useful Molecular Marker for Auxin Physiology

Expression of the GH3 promoter/GUS reporter (GH3/GUS) gene intransgenic tobacco is silent in most vegetative tissues andorgans, but can be specifically induced by auxin in all celltypes throughout the plant, indicating that auxin is a limitingfactor for activation of the GH3/GUS gene. In the absence ofexogenous auxin, expression of the GH3/GUS gene either correlateswith physiological events which are presumably mediated by auxinor occurs in cells where relatively high levels of endogenousauxin are expected. For example, the GH3/GUS gene is expressedin dormant lateral buds, tips of adventitious roots, and stemparenchyma cells where adventitious roots are initiating. TheGH3/GUS gene is activated at the cut surface nearest the tipof a leaf from which a strip of tissue has been excised, andin the basal end of an excised shoot segment. Also, the GH3/GUSgene is expressed predominantly in the lower side of a gravistimulatedshoot and in the dark side of a unilateral light-stimulatedshoot. Furthermore, the expression pattern of the GH3/GUS geneis similar to the distribution pattern of ¹⁴C-IAA that is appliedat the apical end of the gravistimulated shoot. Thus, the resultsreported here provide additional support for a crucial roleof auxin in shoot gravi- and photo-tropisms and adventitiousroot formation. Furthermore, the data presented in this reportalso indicate that expression of the GH3/GUS gene is an excellentmolecular marker for studying auxin transport and for monitoringchanges in either auxin concentration or cellular sensitivityto auxin in planta. ¹ This work was supported by NASA grant NAG10-0189 and a subgrantof NASA NAGW-1197 to YL, and NSF grant IBN9003956 to TG andGH.     

The amounts,subunit structures,and template-engaged activities of RNA polymerases in germinating soybean axes

During the first 24 hr of soybean axis imbibition and growth, there is about a 25-fold increase in RNA polymerase activity associated with isolated nuclei. Within this same period, there is no increase in the amount of RNA polymerase I or II protein in soybean axes. There is no alteration in subunit structure of RNA polymerase II during germination and growth, with the possible exception of conversion of the 215,000 subunit to a 180,000 polypeptide, and no alteration in phosphorylation pattern of RNA polymerase II subunits. These results suggest that the rates of RNA synthesis during imbibition, germination, and growth of soybean axes are not regulated by altering the amount or subunit structure or by posttranslational modification of RNA polymerase II subunits.