Hydrogen peroxide is required for abscisic acid-induced NH4+ accumulation in rice leaves

The role of H₂O₂ in abscisic acid (ABA)-induced NH₄⁺ accumulation in rice leaves was investigated. ABA treatment resulted in an accumulation of NH₄⁺ in rice leaves, which was preceded by a decrease in the activity of glutamine synthetase (GS) and an increase in the specific activities of protease and phenylalanine ammonia-lyase (PAL). GS, PAL, and protease seem to be the enzymes responsible for the accumulation of NH₄⁺ in ABA-treated rice leaves. Dimethylthiourea (DMTU), a chemical trap for H₂O₂, was observed to be effective in inhibiting ABA-induced accumulation of NH₄⁺ in rice leaves. Inhibitors of NADPH oxidase, diphenyleneiodonium chloride (DPI) and imidazole (IMD), and nitric oxide donor (N-tert-butyl-α-phenylnitrone, PBN), which have previously been shown to prevent ABA-induced increase in H₂O₂ contents in rice leaves, inhibited ABA-induced increase in the content of NH₄⁺. Similarly, the changes of enzymes responsible for NH₄⁺ accumulation induced by ABA were observed to be inhibited by DMTU, DPI, IMD, and PBN. Exogenous application of H₂O₂ was found to increase NH₄⁺ content, decrease GS activity, and increase protease and PAL-specific activities in rice leaves. Our results suggest that H₂O₂ is involved in ABA-induced NH₄⁺ accumulation in rice leaves.     

Combined activity of LACS1 and LACS4 is required for proper pollen coat formation in Arabidopsis

Very long chain lipids are important components of the plant cuticle that establishes the boundary surface of aerial organs. In addition, these lipids were detected in the extracellular pollen coat (tryphine), where they play a crucial role in appropriate pollen‐stigma communication. As such they are involved in the early interaction of pollen with the stigma. A substantial reduction in tryphine lipids was shown to compromise pollen germination and, consequently, resulted in male sterility. We investigated the role of two long‐chain acyl‐CoA synthetases (LACSs) in Arabidopsis with respect to their contribution to the production of tryphine lipids. LACS was shown to provide CoA‐activated very long chain fatty acids (VLCFA‐CoAs) to the pathways of wax biosynthesis. The allocation of sufficient quantities of VLCFA‐CoA precursors should therefore be relevant to the generation of tryphine lipids. Here, we report on the identification of lacs1 lacs4 double knock‐out mutant lines that were conditionally sterile and showed significant reductions in pollen coat lipids. Whereas the contributions of both LACS proteins to surface wax levels were roughly additive, their co‐operation in tryphine lipid biosynthesis was clearly more complex. The inactivation of LACS4 resulted in increased levels of tryphine lipids accompanied by morphological anomalies of the pollen grains. The additional inactivation of LACS1 neutralized the morphological defects, decreased the tryphine lipids far below wild‐type levels and resulted in conditionally sterile pollen.     

An NH2-terminal multi-basic RKR motif is required for the ATP-dependent regulation of hIK1

We previously demonstrated that the ATP/PKA-dependent activation of the human intermediate conductance, Ca2+-activated K+ channel, hIK1, is dependent upon a C-terminal motif. The NH2-terminus of hIK1 contains a multi-basic 13RRRKR17 motif, known to be important in the trafficking and function of ion channels. While individual mutations within this domain have no effect on channel function, the triple mutation (15RKR17/AAA), as well as additional double mutations, result in a near complete loss of functional channels, as assessed by whole-cell patch-clamp. However, cell-surface immunoprecipitation studies confirmed expression of these mutated channels at the plasma membrane. To elucidate the functional consequences of the (15)RKR(17)/AAA mutation we performed inside-out patch clamp recordings where we observed no difference in Ca2+ affinity between the wild-type and mutated channels. However, in contrast to wild-type hIK1, channels expressing the 15RKR17/AAA mutation exhibited rundown, which could not be reversed by the addition of ATP. Wild-type hIK1 channel activity was reduced by alkaline phosphatase both in the presence and absence of ATP, indicative of a phosphorylation event, whereas the 15RKR17/AAA mutation eliminated this effect of alkaline phosphatase. Further, single channel analysis demonstrated that the 15RKR17/AAA mutation resulted in a four-fold lower channel open probability (P(o)), in the presence of saturating Ca2+ and ATP, compared to wild-type hIK1. In conclusion, these results represent the first demonstration for a role of the NH2-terminus in the second messenger-dependent regulation of hIK1 and, in combination with our previous findings, suggest that this regulation is dependent upon a close NH2/C-terminal association.     

Regulated SUMOylation and ubiquitination of DdMEK1 is required for proper chemotaxis

MEK1, which is required for aggregation and chemotaxis in Dictyostelium, is rapidly and transiently SUMOylated in response to chemoattractant stimulation. SUMOylation is required for MEK1's function and its translocation from the nucleus to the cytosol and cortex, including the leading edge of chemotaxing cells. MEK1 in which the site of SUMOylation is mutated is retained in the nucleus and does not complement the mek1 null phenotype. Constitutively active MEK1 is cytosolic and is constitutively SUMOylated, whereas the corresponding nonactivatable MEK1 is not SUMOylated and nuclear. MEK1 is also ubiquitinated in response to signaling. A MEK1-interacting, ubiquitin E3 ligase RING domain-containing protein controls nuclear localization and MEK1 ubiquitination. These studies provide a pathway regulating the localization and function of MEK1.     

Growth inhibition by NH 4 + ions in arginine-requiring mutants of Chlamydomonas reinhardi

Summary In Chlamydomonas reinhardi, two types of arginine-requiring mutants can be distinguished: one type grows on media containing the usual concentration of NH₄Cl (400 mg/l); the other type can only grow on media poor in NH₄ ⁺ ions (NH₄ ⁺-sensitive mutants). The causes of NH₄ ⁺-sensitivity have been investigated. The results strongly support the view according to which NH₄ ⁺-sensitivity is a potential property of the wild-type strain, which becomes effective when a block is introduced in the biosynthetic pathway of arginine. Accordingly, two events are always necessary for obtaining an NH₄ ⁺-insensitive argininerequiring mutant, the first one being responsible for the nutritional requirement, the second one leading to reduced sensitivity to ammonium. This explains why arginine-requiring mutants preferentially arise on selective media deprived of ammonium ions.Chargé de Recherches du Fonds National de la Recherche Scientifique     

Negative regulation ofUltrabithorax expression byengrailed is required for proper specification of wing development inDrosophila melanogaster

In both vertebrates and invertebrates, homeotic selector genes confer morphological differences along the antero-posterior axis. However, insect wing development is independent of all homeotic gene functions, reflecting the ground plan of an ancestral pterygote, which bore wings on all segments. Dipteran insects such asDrosophila are characterized by a pair of wings in the mesothoracic segment. In all other segments, wing development is essentially repressed by different homeotic genes, although in the metathorax they are modified into a pair of halteres. This necessitates that during development all homeotic genes are to be maintained in a repressed state in wing imaginal discs. In this report we show that (i) the function of the segment polarity geneengrailed (en) is critical to keep the homeotic selector geneUltrabithorax (Ubx) repressed in wing imaginal discs, (ii) normal levels of En in the posterior compartment of haltere discs, however, are not enough to completely repressUbx, and (iii) the repression ofUbx byen is independent of Hedgehog signalling through which the long-range signalling ofen is mediated during wing development. Finally we provide evidence for a possible mechanism by whichen repressesUbx. On the basis of these results we propose thaten has acquired two independent functions during the evolution of dorsal appendages. In addition to its well-known function of conferring posterior fate and inducing long-range signalling to pattern the developing appendages, it maintains wing fate by keepingUbx repressed.     

Wnt4 is required for proper male as well as female sexual development

Genes previously implicated in mammalian sexual development have either a male- or female-specific role. The signaling molecule WNT4 has been shown to be important in female sexual development. Lack of Wnt4 gives rise to masculinization of the XX gonad and we showed previously that the role of WNT4 was to inhibit endothelial and steroidogenic cell migration into the developing ovary. Here we show that Wnt4 also has a function in the male gonad. We find that Sertoli cell differentiation is compromised in Wnt4 mutant testes and that this defect occurs downstream of the testis-determining gene Sry but upstream of Sox9 and Dhh, two early Sertoli cell markers. Genetic analysis shows that this phenotype is primarily due to the action of WNT4 within the early genital ridge. Analysis of different markers identifies the most striking difference in the genital ridge at early stages of its development between wild-type and Wnt4 mutant embryos to be a significant increase of steroidogenic cells in the Wnt4 -/- gonad. These results identify WNT4 as a new factor involved in the mammalian testis determination pathway and show that genes can have a specific but distinct role in both male and female gonad development.     

CDK4 regulation by TNFR1 and JNK is required for NF-kappaB-mediated epidermal growth control

Nuclear factor kappaB (NF-kappaB) mediates homeostatic growth inhibition in the epidermis, and a loss of NF-kappaB function promotes proliferation and oncogenesis. To identify mechanisms responsible for these effects, we impaired NF-kappaB action in the epidermis by three different genetic approaches, including conditional NF-kappaB blockade. In each case, epidermal hyperplasia was accompanied by an increase in both protein levels and tissue distribution of the G1 cell cycle kinase, CDK4. CDK4 up-regulation required intact TNFR1 and c-Jun NH2-terminal kinase (JNK) function. Cdk4 gene deletion concomitant with conditional NF-kappaB blockade demonstrated that CDK4 is required for growth deregulation. Therefore, epidermal homeostasis depends on antagonist regulation of CDK4 expression by NF-kappaB and TNFR1/JNK.