Heteroblasty in Arabidopsis thaliana (L.) Heynh

 Heteroblasty in Arabidopsis thaliana was analyzed in a variety of plants with mutations in leaf morphology using a tissue-specific β-glucuronidase gene marker. Some mutants exhibited their mutant phenotypes specifically in foliage leaves. The phenotypes associated with the foliage-leaf-specific mutations were also found to be induced ectopically in cotyledons in the presence of the lec1 mutation. Moreover, the features of an emf1lec1 double mutant showed that cotyledons can be partially converted into carpelloids. When heteroblastic traits were examined in foliage leaves in the presence of certain mutations or natural deviations by histochemical analysis of the expression of the tissue-specific marker gene, it was found that ectopic expression of the developmental program for the first foliage leaves in lec1 cotyledons seemed to affect the heteroblastic features of the first set of foliage leaves, while foliage leaves beyond the third position appeared normal. Similarly, in wild-type plants, discrepancies in heteroblastic features, relative to standard features, of foliage leaves at early positions seemed to be eliminated in foliage leaves at later positions. These results suggest that heteroblasty in foliage leaves might be affected in part by the heteroblastic stage of the preceding foliage leaves but is finally controlled autonomously at each leaf position. Received: 9 July 1999 / Accepted: 17 August 1999     

Gravitropism in leaves of Arabidopsis thaliana (L.) Heynh

In higher plants, stems and roots show negative and positive gravitropism, respectively. However, current knowledge on the graviresponse of leaves is lacking. In this study, we analyzed the positioning and movement of rosette leaves of Arabidopsis thaliana under light and dark conditions. We found that the radial positioning of rosette leaves was not affected by the direction of gravity under continuous white light. In contrast, when plants were shifted to darkness, the leaves moved upwards, suggesting negative gravitropism. Analysis of the phosphoglucomutase and shoot gravitropism 2-1 mutants revealed that the sedimenting amyloplasts in the leaf petiole are important for gravity perception, as is the case in stems and roots. In addition, our detailed physiological analyses revealed a unique feature of leaf movement after the shift to darkness, i.e. movement could be divided into negative gravitropism and nastic movement. The orientation of rosette leaves is ascribed to a combination of these movements.     

Effects of mechanical vibration on seed germination of Arabidopsis thaliana (L.) Heynh

The effects of sinusoidal vibration (40-120 Hz, amplitude equal to or smaller than 0.42 mm) on seed germination of Arabidopsis thaliana were examined. When the amplitude of vibration was fixed at 0.42 mm, vibration with frequencies higher than 70 Hz increased the rate of seed germination. When the frequency of vibration was fixed at 100 Hz, vibration with amplitudes larger than 0.33 mm also increased the rate of germination. The increase in the rate of germination appeared dependent on acceleration calculated from the frequency and amplitude of vibration. Vibration with a maximum acceleration of 70 m s(-2) increased the rate of germination, but the promotive effects leveled off at higher accelerations. Vibration had little effect on seed germination in a starch-deficient mutant, pgm. Thus, the amyloplasts appeared to act as a susceptor that senses mechanical vibrations. No vibration-induced promotion of germination was seen in an ethylene-insensitive mutant, etr1, or in the wild type in the presence of aminoethoxyvinylglycine, an inhibitor of ethylene synthesis, suggesting that vibration increased the rate of seed germination through the action of ethylene.     

Manipulation of ethylene biosynthesis

The plant hormone ethylene is involved in many plant processes ranging from seed germination to leaf and flower senescence and fruit ripening. Ethylene is synthesized from methionine, via S-adenosyl-L-methionine (SAM) and 1-amino-cyclopropane-1-carboxylic acid (ACC). The key ethylene biosynthetic enzymes are ACC synthase (ACS) and ACC oxidase (ACO). Manipulation of ethylene biosynthesis by chemicals and gene technology is discussed. Biotechnological modification of ethylene synthesis is a promising method to prevent spoilage of agricultural and horticultural products.     

Regulation of proline accumulation in Arabidopsis thaliana (L.) Heynh during development and in response to desiccation

Significant differences were observed in the amount and proportion of free amino acids in different organs of Arabidopsis thaliana (L.) Heynh, ecotype Columbia. The most notable were found for proline, which formed 17–26% of the total free amino acid concentration in reproductive tissues (floret and seed), but only 1–3% of the total free amino acid concentration in vegetative tissues (rosette leaf and root). Proline accumulation was associated with tissues that had relatively low water contents. Tissues which displayed high water contents, such as rosette leaves, contained low levels of proline. A significant increase in the levels of proline accumulation occurred in plants subjected to experimentally induced low water potentials as compared to unstressed plants. For instance, an 8–10-fold increase in proline was observed in the presence of 120 mmol kg^?1 NaCl or KCl, and a 20-fold increase was stimulated by 60 mmol kg^?1 PEG. However, in addition to the accumulation of proline, massive accumulation of Na⁺, K⁺ and Cl^? ions occurred in tissues of plants stressed with salt. No significant differences were observed in mineral ions in plants stressed with PEG. Isotope tracer experiments with ¹⁴C compounds established that glutamate, ornithine and arginine are precursors of the proline biosynthesis induced by PEG in response to low water potentials in Arabidopsis thaliana. We conclude that the accumulation of proline in response to PEG occurs through increased biosynthesis.     

Genphysiologie quantitativer Merkmale bei Arabidopsis thaliana (L.) Heynh.

Zusammenfassung In biometrischen Untersuchungen zur Genetik quantitativer Merkmale wird bisher weitgehend das Problem der Genregulation übersehen. Chlorophyll b-Defektmutanten der Crucifere Arabidopsis thaliana (L.) Heynh. erlauben eine direkte biochemische und physiologische Prüfung ihrer quantitativen Genwirkungen in der Biosynthesekette der Plastidenpigmente. Auf Grund bestimmter Testkreuzungen zwischen zwei Mutanten und der Normalform sowie eines kombinierten Verfahrens zur phänotypischen Klassifizierung bestehend aus präparativer Dünnschichtchromatographie aller Plastidenpigmente, visueller Blattfarbenbonitur, papierchromatographischem Einzelpflanzentest und quantitativer Pigmentmessung der Linien werden in Übereinstimmung mit anderen Autoren die nachweisbaren Genotypen einer Allelenreihe ch ⁺, ch ¹ und ch ² des ch-Locus zugeordnet. ch ¹/ch ¹ blockiert die Chlorophyll b-Synthese vollständig, ch ²/ch ² nur etwa zu 90% gegenüber dem Wildtyp und in der Heterozygote ch ²/ch ¹ wird offenbar die Blockade durch komplementative Effekte noch stärker aufgehoben (Superdominanz des ch ²- über das ch ¹-Allel). Die in den Spaltungsgenerationen erhöhten Chlorophyll- und Carotinoid-Varianzen aller Genotypen gegenüber den Eltern weisen auf einen evtl. polygenen Hintergrund der Gesamtpigmentbildung hin, dessen physiologischer Wirkungsbereich aber relativ zu dem des Hauptgens ch begrenzt ist. Orientierende In-vitro-Versuche ergeben, daß im plastidenfreien Blattgewebe-Extrakt des Wildtyps ch ⁺/ch ⁺ ein Wirkstoffprinzip vorhanden ist, das belichtete Chloroplasten der in vivo pigmentdefekten Genotypen ch ¹/ch ¹ und ch ²/ch ² zur Chlorophyll b-Synthese befähigt. Weiterführende Aussagen über Art, Umfang und Zeitpunkt der hierbei angenommenen Enzymaktivitäts- bzw. Substratsänderungen werden von Versuchen mit variierten Strahlungsbedingungen und verbesserten Nachweismethoden der Pigmentvorstufen erwartet.

---

Physiological genetics of quantitative characters in Arabidopsis thaliana (L.) Heynh.Part 1: segregation and biosynthesis of pigments in chlorophyll-b defect mutants

Summary In biometric studies on the genetics of quantitative characters the problem of regulation of the activity of genes is rarely considered. Mutants of Arabidopsis thaliana (L.) Heynh. (Cruciferae), defective for chlorophyll b, permit a direct biochemical and physiological determination of their quantitative gene effects in the biosynthetic pathway of the plastid pigments. In agreement with other authors, the detectable genotypes are found to be based on multiple alleles (ch ⁺, ch ¹ and ch ²) at the ch locus. This evidence was obtained by test crosses of two mutants with the wild type and by a phaenotypic classification of the progeny on the basis of preparatory thin layer chromatography, paper chromatography tests of single plants, and quantitative spectrophotometry of the lines. In ch ¹/ch ¹ the synthesis of chlorophyll b is completely blocked, in ch ²/ch ² only about 10 percent of the wild-type pigment is present, and in the heterozygote ch ²/ch ¹ a complementation effect is observed (i. e. superdominance of ch ² over ch ¹) resulting in still more pigment production. In the segregating generations the variances for chlorophyll and carotinoids of all the genotypes are higher than in the parents and therefore suggest a possible polygenic background for pigment development. However, the physiological effect of the genetic background is slight as compared with the effect of the major gene ch. Preliminary experiments show that in tissue extracts of wild type leaves (ch ⁺/ch ⁺) there is an effective agent which enables irradiated chloroplasts of the defective ch ¹/ch ¹ and ch ²/ch ² genotypes to synthesize chlorophyll b in vitro. Further information on the mechanism, extent and the time of action of the active agent should be obtained by varying the conditions of irradiation and by refining the tests for pigment precursors.

---

---

Die pigmentphysiologischen Untersuchungen wurden durch eine. Sachbeihilfe der Deutschen Forschungsgemeinschaft unterstützt.     

Population-level performance of Arabidopsis thaliana (L.) Heynh in dense monocultures

尽管几乎所有的农业物种在育种过程中都经历了从多物种环境向密集单种农田的过渡,但对非农业植物物种在单一栽培条件下的表现知之甚少。鉴于最近的研究成果突出杂草物种拟南芥为作物育种提供新视角的可能性,本研究的目的是了解播种密度对拟南芥在不同光周期环境下群体和个体表现的影响。我们研究了拟南芥Cvi-0生态型的表现。Cvi-0的选择基于一项初步实验,发现Cvi-0生态型植物表现出强烈的竞争性能。播种密度处理设置为每株17.6、8.8、4.4、2.2和1.1 cm²,光周期环境为日照12小时或16小时。结果表明,植物在所有密度处理下均获得了恒定的总种子产量。播种密度和日照时间处理会有一些相互作用：在高播种密度和长日照时间的处理下,种子重量更大,而日照时间短的处理下种子重量与密度呈负相关。这些结果阐明了一年生植物在面对激烈的种内竞争时能够采取不同的策略,并有助于为提高群体表现的作物育种提供新的前景。     

Indole glucosinolate and auxin biosynthesis in Arabidopsis thaliana (L.) Heynh. glucosinolate mutants and the development of clubroot disease

Mutants and wild type plants of Arabidopsis thaliana were analysed for differences in glucosinolate accumulation patterns, indole-3-acetic acid (IAA) biosynthesis and phenotype. A previously identified series of mutants, termed TU, with altered glucosinolate patterns was used in this study. Only the line TU8 was affected in shoot phenotype (shorter stems, altered branching pattern). Synthesis of IAA and metabolism were not much affected in the TU8 mutant during seedling development, although the content of free IAA peaked earlier in TU8 during plant development than in the wild type. Indole glucosinolates and IAA may, however, be involved in the development of clubroot disease caused by the obligate biotrophic fungus Plasmodiophora brassicae since the TU3 line had a lower infection rate than the wild type, and lines TU3 and TU8 showed decreased symptom development. The decline in clubroot formation was accompanied by a reduced number of fungal structures within the root cortex and slower development of the fungus. Indole glucosinolates were lower in infected roots of TU3 and TU8 than in control roots of these lines, whereas in wild-type plants the differences were not as prominent. Free IAA and indole-3-acetonitrile (IAN) were increased in infected roots of the wild type and mutants with normal clubroot symptoms, whereas they were reduced in infected roots of mutants TU3 and TU8. These results indicate a role for indole glucosinolates and IAN/IAA in relation to symptom development in clubroot disease. Received: 23 July 1998 / Accepted: 12 January 1999     

Abscisic acid deficiency prevents development of freezing tolerance in Arabidopsis thaliana (L.) Heynh.

Summary Abscisic acid (ABA) has been implicated as a regulatory factor in plant cold acclimation. In the present work, the cold-acclimation properties of an ABA-deficient mutant (aba) of Arabidopsis thaliana (L.) Heynh. were analyzed. The mutant had apparently lost its capability to cold acclimate: the freezing tolerance of the mutant was not increased by low temperature treatment but stayed at the level of the nonacclimated wild type. The mutational defect could be complemented by the addition of exogenous ABA to the growth medium, restoring freezing tolerance close to the wild-type level. This suggests that ABA might have a central regulatory function in the development of freezing tolerance in plants. Cold acclimation has been previously correlated to the induction of a specific set of proteins that have been suggested to have a role in freezing tolerance. However, these proteins were also induced in the aba mutant by low temperature treatment.     

The ethylene biosynthetic and perception machinery is differentially expressed during endosperm and embryo development in maize

The maize endosperm undergoes programmed cell death late in its development so that, with the exception of the aleurone layer, the tissue is dead by the time the kernel matures. Although ethylene is known to regulate the onset of endosperm cell death, the temporal and spatial control of the ethylene biosynthetic and perception machinery during maize endosperm development has not been examined. In this study, we report the isolation of the maize gene families for ACC synthase, ACC oxidase, the ethylene receptor, and EIN2 and EIL, which act downstream of the receptor. We show that ACC oxidase is expressed primarily in the endosperm, and only at low levels in the developing embryo late in its development. ACC synthase is expressed throughout endosperm development but, in contrast to ACC oxidase, it is transiently expressed to a significantly higher level in the developing embryo at a time that corresponds with the onset of endosperm cell death. Only two ethylene receptor gene families were identified in maize, in contrast to the five types previously identified in Arabidopsis. Members of both ethylene receptor families were expressed to substantially higher levels in the developing embryo than in the endosperm, as were members of the EIN2 and EIL gene families. These results suggest that the endosperm and embryo both contribute to the synthesis of ethylene, and they provide a basis for understanding why the developing endosperm is especially sensitive to ethylene-induced cell death while the embryo is protected.Electronic Supplementary Material Supplementary material is available in the online version of this article at Communicated by G. Jürgens     

Histological analysis of adventitious rooting in Arabidopsis thaliana (L.) Heynh seedlings

ABSTRACT

Adventititous rooting is essential for the post-embryonic growth of the root apparatus in various species. In Arabidopsis thaliana, adventitious rooting has been reported in some mutants, and auxin seems to be the inducer of the process. The objective of the study was to identify the tissues involved in adventitious rooting in the most commonly used ecotypes for molecular and genetic studies (i.e. Columbia, Wassilewskija and Landsberg erecta) both in the presence and absence of exogenous auxin. Seedlings of the three ecotypes were grown under various conditions. When grown under 16 hours light/day for 11 days, all seedlings showed adventitious roots, both with and without auxin, however, both adventitious and lateral rooting were enhanced by exogenous auxin (2 µM naphthaleneacetic acid). Independently of the presence of auxin and of the ecotype, the hypocotyl pericycle produced adventitious roots directly (i.e., according to the same pattern of lateral root formation by the pericycle cells in the primary root). However, in the presence of auxin, roots of indirect origin also, and mainly, formed and their formation was preceded by the exfoliation of the tissues external to the stele. Exfoliation was caused by cell hypertrophy, separation, and disintegration, which mainly involved the endodermis. At the exfoliation site, the pericycle, with a minor contribution of a few endodermal cells, produced the callus from which indirect roots arose. The finding that adventitious rooting occurs in the absence of auxin (all ecotypes) indicates that this process is part of the normal root apparatus in Arabidopsis, with the hypocotyl pericycle as the target tissue of the process. Exogenous auxin alters adventitious rhizogenesis mainly affecting the endodermis response.     

Brassinosteroids affect ethylene production in the primary roots of maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

To better understand the physiological roles of brassinosteroids (BRs) in the primary roots of maize, we examined their effect on ethylene production. Exogenously applied brassinolide (BL; 10^-9 to 10^-7 M) incrementally increased the level ethylene in a dose-dependent manner. This BL-induced production was enhanced in the presence of IAA, thereby implying a synergistic effect between BR and IAA. At 10^-7 M BL, the level of free ACC was increased, but that of conjugated ACC was diminished. Moreover, greater concentrations of BL proportionally increased ACC oxidase activity. In contrast, higher levels of IAA increased the endogenous content of conjugated ACC as well as ACC synthase activity. Based on these results, we conclude that BR activates ethylene production mainly via ACC oxidase, and interacts with IAA to produce ethylene. However, the functional site for ethylene production is different for each hormone.     

Ion channels in guard cells of Arabidopsis thaliana (L.) Heynh.

Despite the availability of many mutants for signal transduction, Arabidopsis thaliana guard cells have so far not been used in electrophysiological research. Problems with the isolation of epidermal strips and the small size of A. thaliana guard cells were often prohibiting. In the present study these difficulties were overcome and guard cells were impaled with double-barreled microelectrodes. Membrane-potential recordings were often stable for over half an hour and voltage-clamp measurements could be conducted. The guard cells were found to exhibit two states. The majority of the guard cells had depolarized membrane potentials, which were largely dependent on external K⁺ concentrations. Other cells displayed spontaneous transitions to a more hyperpolarized state, at which the free-running membrane potential (E_m) was not sensitive to the external K⁺ concentration. Two outward-rectifying conductances were identified in cells in the depolarized state. A slow outward-rectifying channel (s-ORC) had properties resembling the K⁺-selective ORC of Vicia faba guard cells (Blatt, 1988, J Membr Biol 102: 235–246). The activation and inactivation times and the activation potential, all depended on the reversal potential (E_rev) of the s-ORC conductance. The s-ORC was blocked by Ba²⁺ (K_1/2 = 0.3–1.3mM) and verapamil (K_1/2 = 15–20 μM). A second rapid outward-rectifying conductance (r-ORC) activated instantaneously upon stepping the voltage to positive values and was stimulated by Ba²⁺. Inward-rectifying channels (IRC) were only observed in cells in the hyperpolarized state. The activation time and activation potential of this channel were not sensitive to the external K⁺ concentration. The slow activation of the IRC (t_1/2 ≈ 0.5 s) and its negative activation potential (V_threshold = −155 mV) resemble the values found for the KAT1 channel expressed in Saccharomyces cerevisiae (Bertl et al., 1995, Proc Natl Acad Sci USA 92: 2701–2705). The results indicate that A. thaliana guard cells provide an excellent system for the study of signal transduction processes. Received: 28 March 1996 / Accepted: 11 November 1996