双子叶植物幼苗顶端弯钩发育的分子机制

幼苗顶端弯钩的形成是双子叶植物暗形态建成过程中的一个重要事件。它保护双子叶植物幼嫩的子叶和脆弱的顶端分生组织在幼苗出土时免受机械损伤,进而保证幼苗的出苗率和成活率。幼苗顶端弯钩的形成是由于下胚轴顶端的两个对立面之间细胞分裂和延伸的不对称所引起的。目前,关于双子叶植物幼苗顶端弯钩发育分子机制的研究已有较大进展：发现生长素在顶端弯钩内外侧组织的梯度分布是顶端弯钩两侧细胞差异生长的重要驱动力;乙烯、赤霉素和油菜素内酯促进顶端弯钩的形成和维持;茉莉酸抑制顶端弯钩的形成;而光照则促进弯钩的打开;无弯钩1 (hookless1, HLS1)、乙烯不敏感3以及EIN3 相似蛋白1 (ethylene insensitive 3 /EIN3 like 1, EIN3/EIL1)、DELLA、组成型光形态发生1(constitutive photomorphogenic 1, COP1) 和光敏色素相互作用因子(phytochrome interacting factors, PIFs) 等多种因子已被发现参与顶端弯钩的发育过程,并介导了多种激素之间的互作。本文综述了双子叶植物幼苗顶端弯钩发育过程中的重要作用因子及调控网络,并对该领域的研究前景进行了展望。     

Expression of two HOOKLESS genes in peas (Pisum sativum L.)

The apical hook of dark-grown dicotyledonous plants results from asymmetric growth of its inner and outer sides. It is a protective structure that prevents damage to the shoot apical meristem and the young leaves as the seedling pushes through the soil. Two phytohormones, ethylene and auxin, are thought to be involved in regulating apical hook formation. HOOKLESS1 (HLS1) of Arabidopsis was recognized as an ethylene-response gene whose product is required for hook formation. We cloned two cDNAs from peas, Ps-HLS1 and Ps-HLS2, whose products are functional homologs of HLS1. Both Ps-HLS1 and Ps-HLS2 complement the hls1 mutation in Arabidopsis. Expression of Ps-HLS1 is enhanced by ethylene and by IAA. Because the effect of ethylene is counteracted by 2,5-norbornadiene, an inhibitor of ethylene action, it appears that the primary factor in apical hook formation in peas is ethylene.     

Ethylene-mediated enhancement of apical hook formation in etiolated Arabidopsis thaliana seedlings is gibberellin dependent

Dark-grown Arabidopsis seedlings develop an apical hook by differential elongation and division of hypocotyl cells. This allows the curved hypocotyl to gently drag the apex, which is protected by the cotyledons, upwards through the soil. Several plant hormones are known to be involved in hook development, including ethylene, which causes exaggeration of the hook. We show that gibberellins (GAs) are also involved in this process. Inhibition of GA biosynthesis with paclobutrazol (PAC) prevented hook formation in wild-type (WT) seedlings and in constitutive ethylene response (ctr)1-1, a mutant that exhibits a constitutive ethylene response. In addition, a GA-deficient mutant (ga1-3) did not form an apical hook in the presence of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC). Analysis of transgenic Arabidopsis seedlings expressing a green fluorescent protein (GFP)-repressor of ga1-3 (RGA) fusion protein suggested that ACC inhibits cell elongation in the apical hook by inhibition of GA signaling. A decreased feedback of GA possibly causes an induction of GA biosynthesis based upon the expression of genes encoding copalyl diphosphate synthase (CPS; GA1) and GA 2-oxidase (AtGA2ox1). Furthermore, expression of GASA1, a GA-response gene, suggests that differential cell elongation in the apical hook might be a result of differential GA-sensitivity.     

Auxin, ethylene and brassinosteroids: tripartite control of growth in the Arabidopsis hypocotyl

Dark-grown Arabidopsis seedlings develop an apical hook by differential cell elongation and division, a process driven by cross-talk between multiple hormones. Auxins, ethylene and gibberellins interact in the formation of the apical hook. In the light, a similar complexity of hormonal regulation has been revealed at the level of hypocotyl elongation. Here, we describe the involvement of brassinosteroids (BRs) in auxin- and ethylene-controlled processes in the hypocotyls of both light- and dark-grown seedlings. We show that BR biosynthesis is necessary for the formation of an exaggerated apical hook and that either application of BRs or disruption of BR synthesis alters auxin response, presumably by affecting auxin transport, eventually resulting in the disappearance of the apical hook. Furthermore, we demonstrate that ethylene-stimulated hypocotyl elongation in the light is largely controlled by the same mechanisms as those governing formation of the apical hook in darkness. However, in the light, BRs appear to compensate for the insensitivity to ethylene in hls mutants, supporting a downstream action of BRs. Hence, our results indicate that HLS1, SUR1/HLS3/RTY1/ALF1 and AMP1/HPT/COP2/HLS2/PT act on the auxin-ethylene interaction, rather than at the level of BRs. A model for the tripartite hormone interactions is presented.     

Interactions of light and ethylene in hypocotyl hook maintenance in Arabidopsis thaliana seedlings

Etiolated seedlings frequently display a hypocotyl or epicotyl hook which opens on exposure to light. Ethylene has been shown to be necessary for maintenance of the hook in a number of plants in darkness. We investigated the interaction of ethylene and light in the regulation of hypocotyl hook opening in Arabidopsis thaliana . We found that hooks of Arabidopsis open in response to continuous red, far-red or blue light in the presence of up to 100 μl l⁻¹ ethylene. Thus a change in sensitivity to ethylene is likely to be responsible for hook opening in Arabidopsis, rather than a decrease in ethylene production in hook tissues. We used photomorphogenic mutants of Arabidopsis to demonstrate the involvement of both blue light and phytochrome photosensory systems in light-induced hook opening in the presence of ethylene. In addition we used ethylene mutants and inhibitors of ethylene action to investigate the role of ethylene in hook maintenance in seedlings grown in light and darkness.     

Exploiting the triple response of Arabidopsis to identify ethylene-related mutants.

Alterations in the response of dark-grown seedlings to ethylene (the "triple response") were used to isolate a collection of ethylene-related mutants in Arabidopsis thaliana. Mutants displaying a constitutive response (eto1) were found to produce at least 40 times more ethylene than the wild type. The morphological defects in etiolated eto1-1 seedlings reverted to wild type under conditions in which ethylene biosynthesis or ethylene action were inhibited. Mutants that failed to display the apical hook in the absence of ethylene (his1) exhibited reduced ethylene production. In the presence of exogenous ethylene, hypocotyl and root of etiolated his1-1 seedlings were inhibited in elongation but no apical hook was observed. Mutants that were insensitive to ethylene (ein1 and ein2) produced increased amounts of ethylene, displayed hormone insensitivity in both hypocotyl and root responses, and showed an apical hook. Each of the "triple response" mutants has an effect on the shape of the seedling and on the production of the hormone. These mutants should prove to be useful tools for dissecting the mode of ethylene action in plants.     

To curve for survival: Apical hook development

Apical hook is a simple curved structure formed at the upper part of hypocotyls when dicot seeds germinate in darkness.The hook structure is transient but essential for seedlings’ survival during soil emergence due to its efficient protection of the delicate shoot apex from mechanical injury.As a superb model system for studying plant differential growth,apical hook has fascinated botanists as early as the Darwin age,and significant advances have been achieved at both the morphological and molec...     

Seedling emergence patterns and dormancy/germination physiology of <Emphasis Type="Italic">Primula modesta</Emphasis> in a subalpine region

On the basis of the germination/dormancy responses of seeds to temperature and light, and local seed rain, we attempted to interpret the seedling emergence patterns of Primula modesta Bisset et Moore (Primulaceae) in two different types of habitats in a subalpine zone of Mt. Asama: an oligotrophic flat moor and a grassland with relatively dense herbaceous vegetation cover. The seasonal pattern of seedling emergence was well explained by the dormancy/germination physiology revealed in laboratory germination tests. The seeds were demonstrated to have a strict light requirement even after experiencing moist chilling, which might facilitate the incorporation of the seeds into the soil seed bank. Despite sufficient seed production, the seedlings emerging were far less at the grassy site than the moor site, but the number of seedlings was significantly dependent on the seed rain within previous season and on the litter cover of the microsite in both sites. Therefore, the spatiotemporal patterns of seedling emergence in the habitats could be well explained by the spatiotemporal patterns of seed rain and safe-sites for germination.     

Ethylene and carbon dioxide as mediators in the response of the bean hypocotyl hook to light and auxins

Summary Ethylene inhibits hook opening in the bean hypocotyl and at high concentrations induces closure of the hook. Indoleacetic acid and 2,4-dichlorophenoxyacetic acid, whose inhibitory effect on hook opening resembles that of ethylene, stimulate ethylene production from the hook tissue, and this ethylene production is physiologically active in inhibiting hook opening. It is concluded that the inhibition of opening by auxin is due at least in a major part to auxin-induced ethylene production by the hook tissue.Carbon dioxide promotes hook opening, apparently by antagonizing the action of endogenous ethylene. The concentration of respiratory CO₂ in the internal gas space of the hook tissue is high enough to play a role in the regulation of hook opening.Red light causes a decrease in ethylene production and an increase in CO₂ evolution from the hook tissue. These effects are partially reversible by far-red light. It is concluded that both ethylene and CO₂ serve as natural growth regulators which mediate the hypocotyl hook-opening response to light in bean seedlings.     

Rodent-favored cache sites do not favor seedling establishment of shade-intolerant wild apricot (Prunus armeniaca Linn.) in northern China

Seed dispersal by scatter hoarders is widely assumed to involve directed dispersal, in which microhabitats selected for caching also benefit seedling establishment and growth. However, in many systems, this may not be true if animal-favored cache sites do not match the safe sites for plants, or if cache sites benefit the plant in one life stage and not another. Here, we investigated whether cache sites selected by rodents are favorable for seedling establishment and growth of shade-intolerant wild apricot (Prunus armeniaca Linn.) in northern China. We tracked tagged seeds and compared the germination and growth of seedlings from rodent-cached seeds with that of naturally established seedlings in a secondary forest and shrubland stand. Rodents preferred to cache seeds under shrubs with medium canopy cover (31–60 %) in litter substrate in the secondary forest, and under shrubs with high canopy cover (>60 %) in soil or litter substrate in the shrubland stand, neither of which conveyed an advantage for seedling establishment. Although fewer caches were made along shrub edges, or under low canopy cover (≤30 %) in the secondary forest, or along shrub edges, open areas of grass, or under low canopy cover in the shrubland stand, these cache sites consistently contributed to higher survival rates. The microhabitats of grass, soil, or low canopy cover significantly promoted the emergence, survival, and growth of naturally established seedlings. Our results are best explained by the conflicting demands of rodents for caching seeds in more secure sites and P. armeniaca’s high-light requirements for seedling recruitment. We argue that the relationship between favored cache sites for seed hoarders and safe sites for plants will often not match but may still allow a reasonable rate of establishment and regeneration.     

Effects of red light and ethylene on growth of etiolated lettuce seedlings

Low concentrations of ethylene inhibit hypocotyl elongation of etiolated lettuce seedlings (Lactuca sativa cv. Grand Rapids), whereas red light does not inhibit it. The plumular hook tightens in response to either ethylene or red light. A combination of these two factors gives an additive response. Red light has no effect on ethylene production and red light will cause hook closure even under hypobaric pressure which removes endogenous ethylene. This suggests that ethylene and red light act independently in causing hook closure.     

Effect of light environment during soil disturbance on germination and emergence pattern of weeds

This paper describes results from experiments which investigated the effects of light intensity during soil disturbance on germination and emergence pattern of weeds. Different emergence patterns were demonstrated for seeds which are instantly flash induced compared to seeds which are induced to germinate by integrating a weak light signal over a period of time. A reduced and delayed emergence is achieved after a disturbance in darkness compared to a soil disturbance in daylight. The increased emergence after soil disturbance in daylight is due to additional plants originating from seeds placed at a soil depth in the pots where daylight cannot penetrate and induce seeds to germinate, but which are induced during the short exposure period. A close relationship between soil disturbance intensity and number of weed plants emerging was found in field experiments with shallow harrowings. It was also shown that a portion of the increased number of seedlings arising when soil disturbance is carried out in daylight, compared to soil cultivation in darkness, originates from seeds germinating from deeper soil layers, resulting in a deeper average germination depth.     

Effects of soil disturbance,fire and litter accumulation on the establishment of Cistus clusii seedlings

Cistus clusii is an early successional shrub, the most drought-resistant species of Cistus which colonises perturbed areas in the southern Iberian Peninsula and regenerates entirely from seeds after a fire. Some of the factors controlling germination and seedling establishment were examined in a field experiment in which we tested the effect of soil disturbance, fire, and litter accumulation on seedling emergence. In a first experiment, soil disturbance and fire were applied to plots in the field with a factorial design in April 1992. In a second experiment, we measured the effect of adding C. clusii litter. The number of seedlings, annual plants and grasses, and the coverage of mosses were recorded in 1993 and 1994.Soil disturbance increased the number of C. clusii seedlings almost seven-fold but did not change the number of grasses, annual plants nor moss cover. Fire significantly increased the number of C. clusii seedlings only in combination with soil disturbance, and it did not affect the presence of annuals, grasses nor moss cover. Litter increased the number of C. clusii seedlings in the following two years.Overall, soil disturbance as a single factor had the most significant effect on seedling emergence, though plots disturbed and treated with fire had the highest number of seedlings, implying that germination of C. clusii seeds was enhanced by processes that alter the hard seed coat and break the physical dormancy imposed by the testa. In addition, recruitment of C. clusii was dependent on rainfall, as drought significantly reduced the number of seedlings appearing in winter.     

Regulation of differential growth in the apical hook of Arabidopsis.

Arabidopsis seedlings develop a hook-like structure at the apical part of the hypocotyl when grown in darkness. Differential cell growth processes result in the curved hypocotyl hook. Time-dependent analyses of the hypocotyl showed that the apical hook is formed during an early phase of seedling growth and is maintained in a sequential phase by a distinct process. Based on developmental genetic analyses of hook-affected mutants, we show that the hookless mutants (hls1, cop2) are involved in an early aspect of hook development. From time-dependent analyses of ethylene-insensitive mutants, later steps in hook maintenance were found to be ethylene sensitive. Regulation of differential growth was further studied through examination of the spatial pattern of expression of two hormone-regulated genes: an ethylene biosynthetic enzyme and the ethylene receptor ETR1. Accumulation of mRNA for AtACO2, a novel ACC (1-aminocyclopropane-1-carboxylic acid) oxidase gene, occurred within cells predominantly located on the outer-side of the hook and was tightly correlated with ethylene-induced exaggeration in the curvature of the hook. ETR1 expression in the apical hook, however, was reduced by ethylene treatment. Based on the expression pattern of ETR1 and AtACO2 in the hook-affected mutants, a model for hook development and maintenance is proposed.     

Light exaggerates apical hook curvature through phytochrome actions in tomato seedlings

Contrary to the established notion that the apical hook of dark-grown dicotyledonous seedlings opens in response to light, we found in tomato (Solanum lycopersicum L.) that the apical hook curvature is exaggerated by light. Experiments with several tomato cultivars and phytochrome mutants, irradiated with red and far-red light either as a brief pulse (Rp, FRp) or continuously (Rc, FRc), revealed: the hook-exaggeration response is maximal at the emergence of the hypocotyl from the seed; the effect of Rp is FRp-reversible; fluence–response curves to a single Rp or FRp show an involvement of low and very low fluence responses (LFR, VLFR); the effect of Rc is fluence-rate dependent, but that of FRc is not; the phyA mutant (phyA hp-1) failed to respond to an Rp of less than 10⁻² μmol m⁻² and to an FRp of all fluences tested as well as to FRc, thus indicating that the hook-exaggeration response involves phyA-mediated VLFR. The Rp fluence–response curve with the same mutant also confirmed the presence of an LFR mediated by phytochrome(s) other than phyA, although the phyB1 mutant (phyB1 hp-1) still showed full response probably due to other redundant phytochrome species (e.g., phyB2). Simulation experiments led to the possible significance of hook exaggeration in the field that the photoresponse may facilitate the release of seed coat when seeds germinate at some range of depth in soil. It was also observed that seed coat and/or endosperm are essential to the hook exaggeration.     

Effects of bryophytes and grass litter on seedling emergence vary by vertical seed position and seed size

Establishment of plants through seeds is often constrained by the quality of microsites, which is in part controlled by the nature and amount of ground cover. The latter consists of living shoots of vascular plants or bryophytes and/or the dead remains of the dominant species. In the present article, we report the results of a controlled pot experiment with five species characteristic of floodplain grasslands. We manipulated the amounts of grass litter and/or mosses to study (1) differences between ground cover types with respect to their effects on microenvironment and seedling emergence and (2) how these effects interact with seed size and seed sowing position. Increasing amounts of both cover types led to increasing soil humidity, whereas temperature amplitude and illumination were decreased. However, since grass litter decomposed much faster than bryophytes, light conditions for germination under grass litter improved considerably with time. Although seedling emergence varied significantly between species, ground cover types and cover amounts, seed position alone explained about 50% of the variation in the data set. Additionally, we found an important interaction between seed size, seed position and cover type: large-seeded species showed a fitness advantage when seeds were situated beneath a cover, irrespective of cover type, which disappeared when seeds were shed on top of a cover layer. We suggest that this interaction may be ecologically and evolutionarily relevant because it may lead to changes in species composition and diversity of plant communities as a consequence of changes in the amount and type of ground cover.     

Convergence of signaling pathways in the control of differential cell growth in Arabidopsis

Seedling apical hook development involves a complex interplay of hormones and light in the regulation of differential cell growth. However, the underlying molecular mechanisms that integrate these diverse signals to control bending of the embryonic stem are poorly understood. The Arabidopsis ethylene-regulated HOOKLESS1 (HLS1) gene is essential for apical hook formation. Herein, we identify two auxin response regulators that act downstream of HLS1 to control cell elongation in the hypocotyl. Extragenic suppressors of hls1 were identified as mutations in AUXIN RESPONSE FACTOR 2 (ARF2). The level of ARF2 protein was decreased by ethylene, and this response required HLS1. Exposure to light decreased HLS1 protein levels and evoked a concomitant increase in ARF2 accumulation. These studies demonstrate that both ethylene and light signals affect differential cell growth by acting through HLS1 to modulate the auxin response factors, pinpointing HLS1 as a key integrator of the signaling pathways that control hypocotyl bending.     

Automorphosis of etiolated pea seedlings in space is simulated by a three-dimensional clinostat and the application of inhibitors of auxin polar transport

Etiolated pea (Pisum sativum L. cv. Alaska) seedlings grown under microgravity conditions in space show automorphosis: bending of epicotyls, inhibition of hook formation and changes in root growth direction. In order to determine the mechanisms of microgravity conditions that induce automorphosis, we used a three-dimensional clinostat and obtained the successful induction of automorphosis-like growth of etiolated pea seedlings. Kinetic studies revealed that epicotyls bent at their basal region towards the clockwise direction far from the cotyledons from the vertical line (0 degrees) at approximately 40 degrees in seedlings grown both at 1 g and in the clinostat within 48 h after watering. Thereafter, epicotyls retained this orientation during growth in the clinostat, whereas those at 1 g changed their growth direction against the gravity vector and exhibited a negative gravitropic response. On the other hand, the plumular hook that had already formed in the embryo axis tended to open continuously by growth at the inner basal portion of the elbow; thus, the plumular hook angle initially increased; this was followed by equal growth on the convex and concave sides at 1 g, resulting in normal hook formation; in contrast, hook formation was inhibited on the clinostat. The automorphosis-like growth and development of etiolated pea seedlings was induced by auxin polar transport inhibitors (9-hydroxyfluorene-9-carboxylic acid, N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid), but not by anti-auxin (p-chlorophenoxyisobutyric acid) at 1 g. An ethylene biosynthesis inhibitor, 1-aminooxyacetic acid, inhibited hook formation at 1 g, and ethylene production of etiolated seedlings was suppressed on the clinostat. Clinorotation on the clinostat strongly reduced the activity of auxin polar transport of epicotyls in etiolated pea seedlings, similar to that observed in space experiments (Ueda J, Miyamoto K, Yuda T, Hoshino T, Fujii S, Mukai C, Kamigaichi S, Aizawa S, Yoshizaki I, Shimazu T, Fukui K (1999) Growth and development, and auxin polar transport in higher plants under microgravity conditions in space: BRIC-AUX on STS-95 space experiment. J Plant Res 112: 487492). These results suggest that clinorotation on a three-dimensional clinostat is a valuable tool for simulating microgravity conditions, and that automorphosis of etiolated pea seedlings is induced by the inhibition of auxin polar transport and ethylene biosynthesis.