The Bcl-2 family of proteins: regulators of cell death and survival

The Bcl-2 protein inhibits apoptosis induced by a variety of signals, in a range of cell types and in diverse organisms, and it is implicated in both normal development and oncogenesis. Despite this central role, the mechanism of action of Bcl-2 is not yet clear. Recent studies have uncovered a number of Bcl-2-related gene products that regulate apoptosis either negatively or positively, and Bcl-2 forms heterodimers with at least one of these proteins, Bax. This article discusses the role of the Bcl-2 family of proteins in the light of these findings.     

Protein phosphatase 2A--a 'ménage à trois'

Protein phosphorylation is probably the major regulatory mechanism employed by eukaryotic cells. Much work has been devoted to the role of protein kinases and their modulation by hormones, growth factors and neurotransmitters. It is now appreciated that protein phosphatases are also key players in actively regulating many cellular processes. In this article we discuss recent advances in our understanding of the function of protein phosphatase 2A, one of the major serine/threonine-specific protein phosphatases.     

银杏、杜仲和栝楼体内的皮质醇含量

1993年8月,我们对本校园的银杏（o破办biAdxl）、杜仲（ffil‘xi1Tll7lm叨mm协的）、括楼op把h刀m尬h拐h对从w础）的叶片（4周左右,达到最大叶面积）、芽（05～Icm大小）、茎（当年生茎中部）及雌花（全花）,按Simons和Grin－w沁h（＊。njffe,1989,67：288）的方法加以修改进行了皮质醇含量的测定。即：取Zg新鲜的植物组织或器官,加10ml95％乙醇,匀浆后加等体积的二氯甲烷,提取48h,然后用0．lmol／L醋酸溶液洗3次,收集二氯甲烷相,抽干,残留物加lml无水乙醇溶解后,以2000Xg离心15min,取0．lml上清液于聚乙烯小管中（双管）…     

The Extraction and Assay of 1-Kestose:Sucrose Fructosyl Transferase from Leaves of Wheat

Isolating the enzymes responsible for fructan synthesis in plants has been hampered by unsuitable assays used during purification. It is believed that there are two enzymes necessary for fructan synthesis in higher plants, one initiating synthesis utilizing sucrose as donor and the other elaborating the polymer using fructan oligomers as donor. In this paper, a rapid quantitative assay is described to measure the latter fructosyl transfer. The activity was absent from leaves that were not synthesizing fructan. Activity in crude extracts showed a hyperbolic dependence upon sucrose concentration. Activity against 1-kestose showed a pronounced optimum, suggesting that self-transfer also occurred.     

Conditioning of Parsley (Petroselinum crispum L.) Suspension Cells Increases Elicitor-Induced Incorporation of Cell Wall Phenolics

The elicitor-induced incorporation of phenylpropanoid derivatives into the cell wall and the secretion of soluble coumarin derivatives (phytoalexins) by parsley (Petroselinum crispum L.) suspension cultures can be potentiated by pretreatment of the cultures with 2,6-dichloroisonicotinic acid or derivatives of salicylic acid. To investigate this phenomenon further, the cell walls and an extracellular soluble polymer were isolated from control cells or cells treated with an elicitor from Phytophthora megasperma f. sp. glycinea. After alkaline hydrolysis, both fractions from elicited cells showed a greatly increased content of 4-coumaric, ferulic, and 4-hydroxybenzoic acid, as well as 4-hydroxybenzaldehyde and vanillin. Two minor peaks were identified as tyrosol and methoxytyrosol. The pretreatment effect is most pronounced at a low elicitor concentration. Its specificity was elaborated for coumarin secretion. When the parsley suspension cultures were preincubated for 1 d with 2,6-dichloroisonicotinic, 4- or 5-chlorosalicylic, or 3,5- dichlorosalicylic acid, the cells exhibited a greatly increased elicitor response. Pretreatment with isonicotinic, salicylic, acetylsalicylic, or 2,6-dihydroxybenzoic acid was less efficient in enhancing the response, and some other isomers were inactive. This increase in elicitor response was also observed for the above-mentioned monomeric phenolics, which were liberated from cell walls upon alkaline hydrolysis and for "lignin-like" cell wall polymers determined by the thioglycolic acid method. It was shown for 5-chlorosalicylic acid that conditioning most likely improves the signal transduction leading to the activation of genes encoding phenylalanine ammonia lyase and 4-coumarate: coenzyme A ligase. The conditioning thus sensitizes the parsley suspension cells to respond to lower elicitor concentrations. If a similar mechanism were to apply to whole plants treated with 2,6-dichloroisonicotinic acid, a known inducer of systemic acquired resistance, one can hypothesize that fungal pathogens might be recognized more readily and effectively.     

Water Relation Alterations Observed during Hypersensitive Reaction Induced by Bacteria

Upon exposure to pathogenic bacteria, resistant and nonhost plants undergo a hypersensitive reaction (HR) that is expressed as rapid plant cell death. If sufficient concentrations of these bacteria are inoculated to such plant tissue, then that portion of the tissue rapidly collapses and becomes necrotic. As the tissue collapses the water relations of inoculated tissues become markedly disturbed. We measured a decline in the relative water content (RWC) in the leaf-like cotyledons of cotton (Gossypium hirsutum cv Immune 216) within the first 4 h (cut at 1 h) after inoculation with Pseudomonas syringae pv tabaci. However, the decrease in RWC was not caused by a decrease in initial fresh weight but by increased water uptake during incubation in water. By 8 h after inoculation, cotyledons still on the plant had lost turgidity, and their area decreased. K+ efflux was also observed concurrently with the decrease in RWC, providing a reason for the loss of turgidity in the tissue. These observations suggest that cells lose turgor and change shape from cylinders with large intercellular spaces to those of a more tabular shape. During this change cell walls come closer together, providing an avenue for increased water uptake through capillary action. The stomatal diffusive resistance of intact cotyledons increased; hence, water loss through stomata is not the cause of the observed wilting and RWC decline. An increase in K+ per dry weight suggests that phloem loading or movement may also be impaired during bacterially induced HR.