O-glycosylated cell wall proteins are essential in root hair growth

Root hairs are single cells that develop by tip growth and are specialized in the absorption of nutrients. Their cell walls are composed of polysaccharides and hydroxyproline-rich glycoproteins (HRGPs) that include extensins (EXTs) and arabinogalactan-proteins (AGPs). Proline hydroxylation, an early posttranslational modification of HRGPs that is catalyzed by prolyl 4-hydroxylases (P4Hs), defines the subsequent O-glycosylation sites in EXTs (which are mainly arabinosylated) and AGPs (which are mainly arabinogalactosylated). We explored the biological function of P4Hs, arabinosyltransferases, and EXTs in root hair cell growth. Biochemical inhibition or genetic disruption resulted in the blockage of polarized growth in root hairs and reduced arabinosylation of EXTs. Our results demonstrate that correct O-glycosylation on EXTs is essential for cell-wall self-assembly and, hence, root hair elongation in Arabidopsis thaliana.     

Interlinked fast and slow positive feedback loops drive reliable cell decisions

Positive feedback is a ubiquitous signal transduction motif that allows systems to convert graded inputs into decisive, all-or-none outputs. Here we investigate why the positive feedback switches that regulate polarization of budding yeast, calcium signaling, Xenopus oocyte maturation, and various other processes use multiple interlinked loops rather than single positive feedback loops. Mathematical simulations revealed that linking fast and slow positive feedback loops creates a "dual-time" switch that is both rapidly inducible and resistant to noise in the upstream signaling system.     

拟南芥N末端乙酰转移酶Naa50参与调控根细胞有丝分裂

N末端乙酰转移酶Naa50在调控拟南芥生长发育方面至关重要。为探究Naa50是否通过调控细胞有丝分裂来影响植物生长发育,将naa50-1突变体与H2B-YFP株系杂交,利用H2B-YFP标示细胞分裂过程中染色体的分离行为。结果表明,与野生型相比,拟南芥naa50-1突变体根尖分生区细胞有丝分裂指数下降,染色体畸变率明显增加,微核率上升,且处在分裂中期的细胞比例增加。将naa50-1突变体与CYCB1-CUS株系杂交,检测根细胞中CYCB1蛋白表达,发现naa50-1突变体中CYCB1蛋白表达量明显增加。PI染色分析表明,naa50-1突变体根部存在部分死细胞。以上结果表明,在细胞有丝分裂过程中,Naa50促进遗传物质平均分配到两个子细胞中,促进细胞从分裂中期向后期转换;Naa50通过调控细胞分裂进而参与植物生长发育的调控。     

护发素对蚕豆根尖细胞有丝分裂不同时期染色体的影响

以蚕豆根尖为试验材料,探讨护发素对蚕豆根尖细胞有丝分裂过程染色体的影响.结果表明.本试验所使用的3个品牌护发素均对蚕豆根尖产生毒性效应.     

Retinoid signaling determines germ cell fate in mice

Germ cells in the mouse embryo can develop as oocytes or spermatogonia, depending on molecular cues that have not been identified. We found that retinoic acid, produced by mesonephroi of both sexes, causes germ cells in the ovary to enter meiosis and initiate oogenesis. Meiosis is retarded in the fetal testis by the action of the retinoid-degrading enzyme CYP26B1, ultimately leading to spermatogenesis. In testes of Cyp26b1-knockout mouse embryos, germ cells enter meiosis precociously, as if in a normal ovary. Thus, precise regulation of retinoid levels during fetal gonad development provides the molecular control mechanism that specifies germ cell fate.     

油菜素内酯对拟南芥和水稻根毛发育的影响

油菜素内酯(Brassinosteroids,BRs)是植物生长发育所必需的一类植物激素。本研究以拟南芥和水稻为研究对象,借助BR相关突变体,通过外源施加BR及BR合成抑制剂研究BR和根毛生长发育的关系。结果表明:在双子叶模式植物拟南芥中,BR抑制根毛的生长发育;在单子叶模式植物水稻中,BR对的根毛的发育存在剂量反应,在一定的浓度范围内促进根毛的生长发育。     

水分亏缺对玉米根毛区皮层解剖结构的影响

【目的】从解剖学角度探讨水分亏缺对苗期和拔节期玉米根毛区皮层结构的影响,为玉米抗旱机理研究提供参考。【方法】以有一定抗旱能力的高农901玉米种子为试材,采用盆栽试验,在苗期和拔节期分别设正常供水(CK)、轻度水分亏缺(LS)、中度水分亏缺(MS)和重度水分亏缺(SS)4个处理,使其土壤含水量分别达到田间最大持水量的75%～85%,65%～75%,55%～65%,45%～55%,处理结束后,分别取玉米根毛区(距离根尖端10cm)的根系样品制作半薄切片,常规甲苯胺蓝染色、番红-固绿和苯胺蓝荧光染色后,用Olympus BX51显微镜观察皮层细胞以及内皮层马蹄形加厚的变化情况,用image pro plus 6.0计算皮层厚度及内皮层细胞的长度和高度。【结果】随着水分亏缺程度的加剧,苗期和拔节期根毛区皮层厚度逐渐减少,皮层薄壁细胞减小变形,排列不规则;苗期和拔节期各水分亏缺处理的内皮层细胞长度和高度均逐渐减小,且在苗期时表现较为明显。荧光观察结果显示,苗期只有轻度水分亏缺处理的内皮层细胞两侧的马蹄形加厚程度较明显;拔节期轻度、中度水分亏缺处理的内皮层细胞出现了不同程度的马蹄形加厚,且在内切向壁上也出现了明显加厚,拔节期重度水分亏缺处理玉米内皮层细胞的加厚程度较小,且细胞发生了明显变形。【结论】苗期玉米对水分亏缺的反应比拔节期敏感;适度水分亏缺条件下,玉米通过缩减皮层厚度,增加内皮层木质化、栓质化程度,降低水分在根部径向运输的阻力,阻止根中水分的流失,使细胞保持基本形状,从而在一定程度上增强了对干旱环境的适应性。     

Differential lysosomal proteolysis in antigen-presenting cells determines antigen fate

Antigen-presenting cells (APCs) internalize antigens and present antigen-derived peptides to T cells. Although APCs have been thought to exhibit a well-developed capacity for lysosomal proteolysis, here we found that they can exhibit two distinct strategies upon antigen encounter. Whereas macrophages contained high levels of lysosomal proteases and rapidly degraded internalized proteins, dendritic cells (DCs) and B lymphocytes were protease-poor, resulting in a limited capacity for lysosomal degradation. Consistent with these findings, DCs in vivo degraded internalized antigens slowly and thus retained antigen in lymphoid organs for extended periods. Limited lysosomal proteolysis also favored antigen presentation. These results help explain why DCs are able to efficiently accumulate, process, and disseminate antigens and microbes systemically for purposes of tolerance and immunity.     

红松插条生根过程中形态和生根机理的观察

红松是扦插生根极难的树种之一，插条扦插１５０天才能形成根系．为给红松无性繁殖提供科学依据，我们进行了生根过程的形态和生理代谢的观察研究．结果表明：红松生根过程可分为五个时期，即形成环状愈伤组织期、愈伤组织木质部期、根源基期、初生根期、次生根期。红松生根过程形态的变化与生理代谢变化是一致的，其相关系数为ｒ＝０．９３。     

The role of the T cell receptor in positive and negative selection of developing T cells

Although many combinations of alpha beta T cell receptors are available to the T cells in any given organism, far fewer are actually used by mature T cells. The combinations used are limited by two selective processes, positive selection of T cells bearing receptors that will be useful to the host, and clonal elimination or inactivation of T cells bearing receptors that will be damaging to the host. The ways in which these two apparently contradictory processes occur, and the hypotheses that have been suggested to reconcile them, are discussed.     

Negative feedback regulation ensures the one receptor-one olfactory neuron rule in mouse

In the mouse olfactory system, each olfactory sensory neuron (OSN) expresses only one odorant receptor (OR) gene in a monoallelic and mutually exclusive manner. Such expression forms the genetic basis for OR-instructed axonal projection of OSNs to the olfactory bulb of the brain during development. Here, we identify an upstream cis-acting DNA region that activates the OR gene cluster in mouse and allows the expression of only one OR gene within the cluster. Deletion of the coding region of the expressed OR gene or a naturally occurring frame-shift mutation allows a second OR gene to be expressed. We propose that stochastic activation of only one OR gene within the cluster and negative feedback regulation by that OR gene product are necessary to ensure the one receptor-one neuron rule.     

Generation of calcium oscillations in fibroblasts by positive feedback between calcium and IP3

A wide variety of nonexcitable cells generate repetitive transient increases in cytosolic calcium ion concentration ([Ca2+]i) when stimulated with agonists that engage the phosphoinositide signalling pathway. Current theories regarding the mechanisms of oscillation disagree on whether Ca2+ inhibits or stimulates its own release from internal stores and whether inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DG) also undergo oscillations linked to the Ca2+ spikes. In this study, Ca2+ was found to stimulate its own release in REF52 fibroblasts primed by mitogens plus depolarization. However, unlike Ca2+ release in muscle and nerve cells, this amplification was insensitive to caffeine or ryanodine and required hormone receptor occupancy and functional IP3 receptors. Oscillations in [Ca2+]i were accompanied by oscillations in IP3 concentration but did not require functional protein kinase C. Therefore, the dominant feedback mechanism in this cell type appears to be Ca2+ stimulation of phospholipase C once this enzyme has been activated by hormone receptors.     

Interrelated striated elements in vestibular hair cells of the rat

Unusual fixation procedures revealed a series of interrelated striated organelles in type I and type II vestibular hair cells of the rat; these organelles seemed to be less well developed in cochlear hair cells. The findings suggest that contractile elements may play a role in sensory transduction in the inner ear, particularly in the vestibular system.     

Voltage- and tension-dependent lipid mobility in the outer hair cell plasma membrane

The mechanism responsible for electromotility of outer hair cells in the ear is unknown but is thought to reside within the plasma membrane. Lipid lateral diffusion in the outer hair cell plasma membrane is a sigmoidal function of transmembrane potential and bathing media osmolality. Cell depolarization or hyposmotic challenge shorten the cell and reduce membrane fluidity by half. Changing the membrane tension with amphipathic drugs results in similar reductions. These dynamic changes in membrane fluidity represent the modulation of membrane tension by lipid-protein interactions. The voltage dependence may be associated with the force-generating motors that contribute to the exquisite sensitivity of mammalian hearing.     

Cholinergic synaptic inhibition of inner hair cells in the neonatal mammalian cochlea

Efferent feedback onto sensory organs provides a means to modulate input to the central nervous system. In the developing mammalian cochlea, inner hair cells are transiently innervated by efferent fibers, even before sensory function begins. Here, we show that neonatal inner hair cells are inhibited by cholinergic synaptic input before the onset of hearing. The synaptic currents, as well as the inner hair cell's response to acetylcholine, are mediated by a nicotinic (alpha9-containing) receptor and result in the activation of small-conductance calcium-dependent potassium channels.     

Ethylene modulates stem cell division in the Arabidopsis thaliana root

The construction of multicellular organisms depends on stem cells-cells that can both regenerate and produce daughter cells that undergo differentiation. Here, we show that the gaseous messenger ethylene modulates cell division in the cells of the quiescent center, which act as a source of stem cells in the seedling root. The cells formed through these ethylene-induced divisions express quiescent center-specific genes and can repress differentiation of surrounding initial cells, showing that quiescence is not required for these cells to signal to adjacent stem cells. We propose that ethylene is part of a signaling pathway that modulates cell division in the quiescent center in the stem cell niche during the postembryonic development of the root system.