Auxin Receptors and Auxin Binding Proteins

In the last few years, a large number of auxin-binding proteins (ABPs) have been reported. Implicitly or explicitly, interest in such proteins resides in their possible role as auxin receptors. Many of these proteins are characterized as ABPs solely by their susceptibility to covalent photolabeling by tritiated azido-indole-3-acetic acid. In most cases where the labeled polypeptides have been identified, they turn out to have roles unconnected with primary auxin perception. It seems likely that auxin is binding to sites of catholic specificity in these cases and the influence of experimental protocols on the data is discussed. Because the term ABP implies that auxin binding affects the function of that protein, the importance of establishing further criteria before photolabeled peptides can be termed ABPs is emphasized. Applying such criteria, only a very few ABPs are currently of interest and only one of these, maize ABP1, has been characterized in detail. This protein is located primarily within the lumen of the endoplasmic reticulum, although an important fraction appears to function on the outside of the plasma membrane. The protein has a wide species distribution and it now seems highly probable that it is a genuine auxin receptor, the only protein for which such a function has yet been established. This conclusion is based on three independent lines of electrophysiological evidence, together with confocal imaging of cytoplasmic pH changes.     

Auxin transport

Polar transport of auxin is essential for normal plant growth and development. On a cellular level, directional auxin transport is primarily controlled by an efflux carrier complex that is characterized by the PIN-FORMED (PIN) family of proteins. Detailed developmental studies of PIN distribution and subcellular localization have been combined with the analysis of changes in localized auxin levels to map PIN-mediated auxin movement throughout Arabidopsis tissues. Plant orthologs of mammalian multidrug-resistance/P-glycoproteins (MDR/PGPs) also function in auxin efflux. MDR/PGPs appear to stabilize efflux complexes on the plasma membrane and to function as ATP-dependent auxin transporters, with the specificity and directionality of transport being provided by interacting PIN proteins.     

Auxin metabolism

Auxin metabolism encompasses transport, conjugation, deconjugation, conversion, and catabolism. The balance between auxin metabolism and biosynthesis determines the actual level of the hormone in a given cell and consequently plays an important role in many developmental processes from seed germination to fruit ripening. Mass spectrometry used in conjunction with stable isotope labeling studies has enabled comprehensive examination of auxin biosynthesis and turnover along with the identification of many auxin conjugate. It appears that the conjugate moiety may signal the metabolic fate (e.g. storage and eventual hydrolysis to free hormone, or catabolism). Recently identified auxin-metabolizing enzymes are encoded by gene families which vary in specificity for auxin metabolites. The expression patterns of these genes will reveal a great deal about the mechanics of auxin metabolism.     

生长素反应因子

生长素反应因子(ARF)是1997年发现的新一类转录因子家族,它们与生长素早期反应基因启动子中的生长素反应元件(AuxRE)TGTCTC特异性地结合,调节这类基因的转录活性.文章阐述生长素反应因子和其结构的特点、作用模式的设想的研究进展.     

生长素受体与信号转导机制研究进展

对生长素受体ABP1和TIR1及调控泛素化蛋白降解的生长素信号转导途径研究进展进行综述。     

Auxin and phyllotaxis

Our understanding of phyllotaxis is still largely based on surgical and pharmacological experiments carried out before 1970. Recent experiments implicate the plant hormone auxin in the regulation of phyllotaxis. A recent paper shows how the polar auxin transport mutant, pin1-1, which fails to make flowers, affects the expression of well known meristem genes. This work opens the door for the genetic analysis of phyllotaxis.     

Auxin Content and Auxin Catabolism in Relation to the Growth Polarity

The auxin level in the root fragments of carrot cultivated in vitro is inversely related to the auxin-oxidase activity. In the morphological basal region, auxin catabolism is low and, in consequence, auxin content is high. This accumulation of the endogenous auxin leads to the induction of callus. In such new tissues, IAA-oxidase activity is also very low and, similarly, the auxin content is high: thus, cells can growth rapidly. Consequently the growth polarity is directly related to the auxin catabolism.     

Auxin Antagonistic Effects of Auxin Precursors of the Substituted Amide Type

Cucumber seeds were germinated in Petri-dishes for 6 days withdifferent doses of a number of N-monosubstituted amide derivativesof some chlorophenoxy-alkane-carboxylic acids. Doses of theamides studied were roughly equivalent to 0.1, 1, 10, 100, and1,000 p.p.m. in water. The gross morphological effects of theamide treatments were studied, both with and without the additionof 1 and 10 p.p.m., respectively, of the corresponding carboxylicacid. The amides studied were the N-methyl-, N-ethyl-, N-propyl-,N-iso-propyl-, N-butyl-, and N-benzyl-amides of 2,4-dichlorophenoxyaceticacid (2,4-D), -(2,4-dichlorophenoxy)-propionic acid (2,4-DP),2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and -(2,4,5-trichlorophenoxy)-propionicacid (2,4,s-TP). Thirteen of the twenty-one amide derivatives studied antagonizedthe auxintype plant-growth-regulating effects of their parentcarboxylic acid. The antagonisms observed involved not onlya decrease in the stimulatory effects of the externally appliedauxin acids (counteractions of the abnormal swellings of differentplant parts), but also reduced significantly their inhibitoryeffects, i.e. the inhibitions of growth in length of the hypocotyland of the roots. Since some auxin-type stimulatory effectswere observed with all of the amides studied, it is suggestedthat the thirteen amide derivatives play a double role as plantgrowth regulators in cucumber tissues: (1) to a certain extentthey are transformed by plant enzymes to the corresponding carboxylicacid, i.e. to auxin; and (2) the amides themselves exert anti-auxineffects.     

早期生长素响应蛋白在生长素信号转导中的作用

3种早期生长素响应蛋白--生长素/吲哚乙酸蛋白(Aux/IAAs)、生长素响应因子(ARFs)和泛素介导的蛋白降解途径组分在生长素的信号转导中起着关键性的作用.目前的研究结果支持负调控模型的说法,即Aux/IAAs蛋白以生长素依赖的方式通过泛素相关的蛋白降解机制为26S蛋白酶降解.当Aux/IAAs-Aux/IAAs以及Aux/IAAs-ARFs二聚体含量降低时,ARFs-ARFs水平升高,ARFs-ARFs结合在生长素调控基因启动子的生长素响应元件(AuxREs)上调节一系列基因的表达,进而引导植物的正常生长和发育.