ACAULIS5 controls Arabidopsis xylem specification through the prevention of premature cell death

Cell size and secondary cell wall patterning are crucial for the proper functioning of xylem vessel elements in the vascular tissues of plants. Through detailed anatomical characterization of Arabidopsis thaliana hypocotyls, we observed that mutations in the putative spermine biosynthetic gene ACL5 severely affected xylem specification: the xylem vessel elements of the acl5 mutant were small and mainly of the spiral type, and the normally predominant pitted vessels as well as the xylem fibers were completely missing. The cell-specific expression of ACL5 in the early developing vessel elements, as detected by in situ hybridization and reporter gene analyses, suggested that the observed xylem vessel defects were caused directly by the acl5 mutation. Exogenous spermine prolonged xylem element differentiation and stimulated cell expansion and cell wall elaboration in xylogenic cell cultures of Zinnia elegans, suggesting that ACL5 prevents premature death of the developing vessel elements to allow complete expansion and secondary cell wall patterning. This was further supported by our observations that the vessel elements of acl5 seemed to initiate the cell death program too early and that the xylem defects associated with acl5 could be largely phenocopied by induction of premature, diphtheria toxin-mediated cell death in the ACL5-expressing vessel elements. We therefore provide, for the first time, mechanistic evidence for the function of ACL5 in xylem specification through its action on the duration of xylem element differentiation.     

The molecular basis and prospects in pancreatic development

Studies on the signaling mechanism that control the specification of endoderm-derived organs have only recently begun. While many studies revealed genes involved in the differentiation, growth and morphogenesis of the pancreas through studies of mutant mice, still little is known about how endoderm give rise to specific domains. Although many genes are known to have a role in pancreatic differentiation, growth and morphogenesis, few genes are known to take part in the specification of the pancreas so far. Hallmarks as well as gene markers for early development of the pancreas, which are however still very limited, will be useful for dissecting early events in pancreatic specification. Here, I give a summary on the origin of the dorsal and ventral pancreatic progenitors, signals for inductions, and genes so far known to function in pancreatic differentiation. I also give a future prospect in the use of ES cells and other experimental models, towards a comprehensive understanding of gene networks in the progenitor cells or intermediate cell types which arise during various stages of differentiation.     

NF-kappa B controls cell fate specification, survival, and molecular differentiation of immunoregulatory natural T lymphocytes

Ontogenetic, homeostatic, and functional deficiencies within immunoregulatory natural T (iNKT) lymphocytes underlie various inflammatory immune disorders including autoimmunity. Signaling events that control cell fate specification and molecular differentiation of iNKT cells are only partly understood. Here we demonstrate that these processes within iNKT cells require classical NF-kappaB signaling. Inhibition of NF-kappaB signaling blocks iNKT cell ontogeny at an immature stage and reveals an apparent, novel precursor in which negative selection occurs. Most importantly, this block occurs due to a lack of survival signals, as Bcl-x(L) overexpression rescues iNKT cell ontogeny. Maturation of immature iNKT cell precursors induces Bcl-2 expression, which is defective in the absence of NF-kappaB signaling. Bcl-x(L) overexpression also rescues this maturation-induced Bcl-2 expression. Thus, antiapoptotic signals relayed by NF-kappaB critically control cell fate specification and molecular differentiation of iNKT cells and, hence, reveal a novel role for such signals within the immune system.     

OCTOPUS, a polarly localised membrane-associated protein, regulates phloem differentiation entry in Arabidopsis thaliana

Vascular development is embedded into the developmental context of plant organ differentiation and can be divided into the consecutive phases of vascular patterning and differentiation of specific vascular cell types (phloem and xylem). To date, only very few genetic determinants of phloem development are known. Here, we identify OCTOPUS (OPS) as a potentiator of phloem differentiation. OPS is a polarly localised membrane-associated protein that is initially expressed in provascular cells, and upon vascular cell type specification becomes restricted to the phloem cell lineage. OPS mutants display a reduction of cotyledon vascular pattern complexity and discontinuous phloem differentiation, whereas OPS overexpressers show accelerated progress of cotyledon vascular patterning and phloem differentiation. We propose that OPS participates in vascular differentiation by interpreting longitudinal signals that lead to the transformation of vascular initials into differentiating protophloem cells.     

A chemical biology approach reveals an opposite action between thermospermine and auxin in xylem development in Arabidopsis thaliana

Thermospermine, a structural isomer of spermine, is produced through the action of ACAULIS5 (ACL5) and suppresses xylem differentiation in Arabidopsis thaliana. To elucidate the molecular basis of the function of thermospermine, we screened chemical libraries for compounds that can modulate xylem differentiation in the acl5 mutant, which is deficient in thermospermine and shows a severe dwarf phenotype associated with excessive proliferation of xylem vessels. We found that the isooctyl ester of a synthetic auxin, 2,4-D, remarkably enhanced xylem vessel differentiation in acl5 seedlings. 2,4-D, 2,4-D analogs and IAA analogs, including 4-chloro IAA (4-Cl-IAA) and IAA ethyl ester, also enhanced xylem vessel formation, while IAA alone had little or no obvious effect on xylem differentiation. These effects of auxin analogs were observed only in the acl5 mutant but not in the wild type, and were suppressed by the anti-auxin, p-chlorophenoxyisobutyric acid (PCIB) and α-(phenyl ethyl-2-one)-IAA (PEO-IAA), and also by thermospermine. Furthermore, the suppressor of acaulis51-d (sac51-d) mutation, which causes SAC51 overexpression in the absence of thermospermine and suppresses the dwarf phenotype of acl5, also suppressed the effect of auxin analogs in acl5. These results suggest that the auxin signaling that promotes xylem differentiation is normally limited by SAC51-mediated thermospermine signaling but can be continually stimulated by exogenous auxin analogs in the absence of thermospermine. The opposite action between thermospermine and auxin may fine-tune the timing and spatial pattern of xylem differentiation.     

Regulation of Vascular Development by CLE Peptide-receptor Systems

Cell division and differentiation of stem cells are controlled by non-cell-autonomous signals in higher organisms. The plant vascular meristem is a stem-cell tissue comprising procambial cells that produce xylem cells on one side and phloem cells on the other side. Recent studies have revealed that TDIF (tracheary element differentiation inhibitory factor)/CLE41/CLE44 peptide signal controls the procambial cell fate in a non-cell-autonomous manner. TDIF produced in and secreted from phloem cells is perceived by TDR/PXY, a leucine-rich repeat receptor kinase located in the plasma membrane of procambial cells. This signal suppresses xylem cell differentiation of procambial cells and promotes their proliferation. In addition to TDIF, some other CLE peptides play roles in vascular development. Here, we summarize recent advances in CLE signaling governing vascular development.     

Developmental Regulation of Nucleolus Size during Drosophila Eye Differentiation

When cell cycle withdrawal accompanies terminal differentiation, biosynthesis and cellular growth are likely to change also. In this study, nucleolus size was monitored during cell fate specification in the Drosophila eye imaginal disc using fibrillarin antibody labeling. Nucleolus size is an indicator of ribosome biogenesis and can correlate with cellular growth rate. Nucleolar size was reduced significantly during cell fate specification and differentiation, predominantly as eye disc cells entered a cell cycle arrest that preceded cell fate specification. This reduction in nucleolus size required Dpp and Hh signaling. A transient enlargement of the nucleolus accompanied cell division in the Second Mitotic Wave. Nucleoli continued to diminish in postmitotic cells following fate specification. These results suggest that cellular growth is regulated early in the transition from proliferating progenitor cells to terminal cell fate specification, contemporary with regulation of the cell cycle, and requiring the same extracellular signals.     

Hedgehog signaling is required for differentiation of endocardial progenitors in zebrafish

Endocardial cells form the inner endothelial layer of the heart tube, surrounded by the myocardium. Signaling pathways that regulate endocardial cell specification and differentiation are largely unknown and the origin of endocardial progenitors is still being debated. To study pathways that regulate endocardial differentiation in a zebrafish model system, we isolated zebrafish NFATc1 homolog which is expressed in endocardial but not vascular endothelial cells. We further demonstrate that Hedgehog (Hh) but not VegfA or Notch signaling is required for early endocardial morphogenesis. Pharmacological inhibition of Hh signaling with cyclopamine treatment resulted in nearly complete loss of the endocardial marker expression. Simultaneous knockdown of the two zebrafish sonic hedgehog homologs, shh and twhh or Hh co-receptor smoothened (smo) resulted in similar defects in endocardial morphogenesis. Inhibition of Hh signaling resulted in the loss of fibronectin (fn1) expression in the presumptive endocardial progenitors as early as the 10-somite stage which suggests that Hh signaling is required for the earliest stages of endocardial specification. We further show that the endoderm plays a critical role in migration but not specification or differentiation of the endocardial progenitors while notochord-derived Hh is a likely source for the specification and differentiation signal. Mosaic analysis using cell transplantation shows that Smo function is required cell-autonomously within endocardial progenitor cells. Our results argue that Hh provides a critical signal to induce the specification and differentiation of endocardial progenitors.     

T-brain homologue (HpTb) is involved in the archenteron induction signals of micromere descendant cells in the sea urchin embryo

Signals from micromere descendants play a crucial role in sea urchin development. In this study, we demonstrate that these micromere descendants express HpTb, a T-brain homolog of Hemicentrotus pulcherrimus. HpTb is expressed transiently from the hatched blastula stage through the mesenchyme blastula stage to the gastrula stage. By a combination of embryo microsurgery and antisense morpholino experiments, we show that HpTb is involved in the production of archenteron induction signals. However, HpTb is not involved in the production of signals responsible for the specification of secondary mesenchyme cells, the initial specification of primary mesenchyme cells, or the specification of endoderm. HpTb expression is controlled by nuclear localization of beta-catenin, suggesting that HpTb is in a downstream component of the Wnt signaling cascade. We also propose the possibility that HpTb is involved in the cascade responsible for the production of signals required for the spicule formation as well as signals from the vegetal hemisphere required for the differentiation of aboral ectoderm.     

Vascular instruction of pancreas development

Blood vessels course through organs, providing them with essential nutrient and gaseous exchange. However, the vasculature has also been shown to provide non-nutritional signals that play key roles in the control of organ growth, morphogenesis and homeostasis. Here, we examine a decade of work on the contribution of vascular paracrine signals to developing tissues, with a focus on pancreatic β-cells. During the early stages of embryonic development, blood vessels are required for pancreas specification. Later, the vasculature constrains pancreas branching, differentiation and growth. During adult life, capillaries provide a vascular niche for the maintenance of β-cell function and survival. We explore the possibility that the vasculature constitutes a dynamic and regionalized signaling system that carries out multiple and changing functions as it coordinately grows with the pancreatic epithelial tree.     

Chemical root to shoot signaling under drought

Chemical signals are important for plant adaptation to water stress. As soils become dry, root-sourced signals are transported via the xylem to leaves and result in reduced water loss and decreased leaf growth. The presence of chemical signals in xylem sap is accepted, but the identity of these signals is controversial. Abscisic acid (ABA), pH, cytokinins, a precursor of ethylene, malate and other unidentified factors have all been implicated in root to shoot signaling under drought. This review describes current knowledge of, and advances in, research on chemical signals that are sent from roots under drought. The contribution of these different potential signals is discussed within the context of their role in stress signaling.     

The what, where, when and how of Wnt/β-catenin signaling in pancreas development

The Wnt/β-catenin signaling pathway, conserved across the animal kingdom, is critical for the development of numerous tissues. Several recent studies have focused on the roles that this pathway plays at different stages of pancreatic organogenesis, including specification, proliferation, differentiation and function. Whereas, during early endoderm development, inhibition of the pathway is required for pancreatic specification, subsequent growth and differentiation of the fetal organ depends on the pathway being active. This appears especially true for exocrine acinar cells, the specification and differentiation of which also depend on β-catenin function. Whether the pathway plays an important role in development or function of endocrine islet cells, including insulin-producing β-cells, remains controversial. This question is particularly important in light of recent studies that implicate a downstream component of the pathway, TCF7L2, in human β-cell function. This review will cover recent work on Wnt/β-catenin signaling in pancreas development, emphasizing those points of controversy that most urgently require further investigation.