Dicotyledonous Seedlings Research Articles

Effects of Sheep Grazing and Nitrogen Addition on Dicotyledonous Seedling Abundance and Diversity in Alpine Meadows

Seedling is a crucial stage in the growth and development of plants, and the expansion and persistence of plant populations can be achieved through seed regeneration. Sheep grazing, fertilization, light, soil moisture, vegetation diversity and biomass, and litter all have potential impacts on species regeneration. We measured vegetation diversity, annual net primary productivity (ANPP), litter, ground photosynthetically active radiation (PAR), and soil moisture of alpine meadows under sheep grazing and nitrogen addition treatments, and studied their effects on the dicotyledonous seedling abundance and diversity using linear regression models (LMs) and structural equation models (SEMs). We found that sheep grazing reduced ANPP, increased vegetation diversity and PAR, and decreased soil moisture. Fertilization increased ANPP and litter, decreased vegetation diversity and PAR, but had no effect on soil moisture. Sheep grazing and fertilization both reduced the abundance of dicotyledonous seedlings, and simultaneously fertilization can reduce the diversity of dicotyledonous seedlings, while sheep grazing had no effect on the diversity of dicotyledonous seedlings. LMs showed that vegetation diversity, ANPP, and litter, rather than light and soil moisture, affected dicotyledonous seedling abundance and diversity. SEMs revealed that sheep grazing and fertilization indirectly influenced seedling regeneration through vegetation diversity rather than ANPP and litter. Our research will increase our understanding of the dicotyledonous plant regeneration process in alpine grasslands and facilitate the development of strategies for management and protection of alpine grassland.

Antagonistic effects of grazers and shrubs on the emergence and establishment of seedlings in a sub-humid grassland of South America.

Biotic interactions are key processes that strongly affect the performance of seedlings in plant communities. In this work, we evaluated the effect of grazing and shrubs on the emergence and establishment of seedlings with different life-forms (dicotyledonous and monocotyledonous) in a sub-humid grassland community. We delimited five areas of 25m2 in which we established four permanent plots of 1.0m2. Two plots were centered at shrubs (beneath the canopy) and two in open spaces (without shrubs). Half of the shrubs and open plots were fenced to avoid grazing. During two consecutive years we marked all emerged seedlings and followed their fate. Grazing promoted dicotyledonous seedling emergence but reduced establishment. On the contrary, shrubs reduced dicotyledonous emergence but enhanced establishment. We did not detect any effect of shrubs or grazing on the emergence of monocotyledonous seedlings. Most seedlings emerged during fall and winter and died during spring and summer. Recruitment from seeds in this grassland is rare and complex, involving biotic and abiotic factors, with different responses to grazing and shrubs depending on seedling life-form and ontogeny.

Salicylic Acid Suppresses Apical Hook Formation via NPR1-Mediated Repression of EIN3 and EIL1 in Arabidopsis.

Salicylic acid (SA) and ethylene (ET) are important phytohormones that regulate numerous plant growth, development, and stress response processes. Previous studies have suggested functional interplay of SA and ET in defense responses, but precisely how these two hormones coregulate plant growth and development processes remains unclear. Our present work reveals antagonism between SA and ET in apical hook formation, which ensures successful soil emergence of etiolated dicotyledonous seedlings. Exogenous SA inhibited ET-induced expression of HOOKLESS1 (HLS1) in Arabidopsis (Arabidopsis thaliana) in a manner dependent on ETHYLENE INSENSITIVE3 (EIN3) and EIN3-LIKE1 (EIL1), the core transcription factors in the ET signaling pathway. SA-activated NONEXPRESSER OF PR GENES1 (NPR1) physically interacted with EIN3 and interfered with the binding of EIN3 to target gene promoters, including the HLS1 promoter. Transcriptomic analysis revealed that NPR1 and EIN3/EIL1 coordinately regulated subsets of genes that mediate plant growth and stress responses, suggesting that the interaction between NPR1 and EIN3/EIL1 is an important mechanism for integrating the SA and ET signaling pathways in multiple physiological processes. Taken together, our findings illuminate the molecular mechanism underlying SA regulation of apical hook formation as well as the antagonism between SA and ET in early seedling establishment and possibly other physiological processes.

Activation of HLS1 by Mechanical Stress via Ethylene-Stabilized EIN3 Is Crucial for Seedling Soil Emergence.

The seeds of terrestrial flowering plants often start their life cycle in subterranean darkness. To protect the fragile apical meristematic tissues and cotyledons from mechanical injuries during soil penetration, dicotyledonous seedlings form an elegant apical hook at the top of the hypocotyl. The apical hook has been considered as an adaption structure to the subterranean environment. However, the role of the apical hook in seedling emergence and the molecular mechanism of apical hook formation under real-life conditions remain highly speculative. Here, we find that HOOKLESS 1 (HLS1), a critical gene in apical hook formation in Arabidopsis thaliana, is required for seedling emergence from the soil. When grown under soil, hls1 mutant exhibits severe emergence defects. By contrast, HLS1 overexpression in the hls1 background fully restores emergence defects and displays better emergence capacity than that of WT. Our results indicate that HLS1 transcription is stimulated in response to the mechanical stress of soil cover, which is dependent on the function of the transcription factors ETHYLENE INSENSITIVE 3 (EIN3) and EIN3-LIKE 1 (EIL1). Soil-conferred mechanical stress activates the ethylene signaling pathway to stabilize EIN3 by repressing the activity of the F-box proteins EBF1 and EBF2. These combined results reveal a signaling pathway in which plant seedlings transduce the mechanical pressure of soil cover to correctly modulate apical hook formation during soil emergence.

Strategies of seedlings to overcome their sessile nature: auxin in mobility control.

Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. A prominent demonstration of this developmental plasticity is their ability to bend organs in order to reach the position most optimal for growth and utilization of light, nutrients, and other resources. Shortly after germination, dicotyledonous seedlings form a bended structure, the so-called apical hook, to protect the delicate shoot meristem and cotyledons from damage when penetrating through the soil. Upon perception of a light stimulus, the apical hook rapidly opens and the photomorphogenic developmental program is activated. After germination, plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. On the other hand, when roots and shoots are diverted from their upright orientation, they immediately detect a change in the gravity vector and bend to maintain a vertical growth direction. Noteworthy, despite the diversity of external stimuli perceived by different plant organs, all plant tropic movements share a common mechanistic basis: differential cell growth. In our review, we will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and consequently, organ bending. In particular, we highlight common and specific features of regulatory pathways in control of the bending of organs and a role for the plant hormone auxin as a key regulatory component.

Analysis of apical hook formation in Alaska pea with a 3-D clinostat and agravitropic mutant ageotropum.

The formation of the apical hook in dicotyledonous seedlings is believed to be effected by gravity in the dark. However, this notion is mostly based on experiments with the hook formed on the hypocotyl, and no detailed studies are available with the developmental manners of the hook, particularly of the epicotyl hook. The present study aims at clarifying the dynamics of hook formation including the possible involvement of gravity. Time-course studies with normal Alaska pea (Pisum sativum L., cv. Alaska) and an agravitropic pea mutant, ageotropum, under the 1-g conditions and on a 3-D clinostat revealed that (1) the apical hook of the epicotyl forms by the development of the arc-shaped plumule of the embryo existing in the non-germinated seed. The process of formation consists of two stages: development and partial opening, which are controlled by some intrinsic property of the plumule, but not gravity. Approximately when the epicotyl emerges from the seed coat, the hook is established in both pea varieties. In Alaska the established hook is sustained or enhanced by gravity, resulting in a delay of hook opening compared with on a clinostat, which might give an incorrect idea that gravity causes hook formation. (2) During the hook development and opening processes the original plumular arc holds its orientation unchanged to be an established hook, which, therefore, is at the same side of the epicotyl axis as the cotyledons. This is true for both Alaska and ageotropum under 1-g conditions as well as on the clinostat, supporting finding (1). (3) Application of auxin polar transport inhibitors, hydroxyfluorenecarboxylic acid, naphthylphthalamic acid, and triiodobenzoic acid, suppressed the curvature of hook by equal extents in Alaska as well as ageotropum, suggesting that the hook development involves auxin polar transport probably asymmetrically distributed across the plumular axis by some intrinsic property of the plumule not directly related with gravity action.

Reduced phototropism in pks mutants may be due to altered auxin‐regulated gene expression or reduced lateral auxin transport

Phototropism allows plants to orient their photosynthetic organs towards the light. In Arabidopsis, phototropins 1 and 2 sense directional blue light such that phot1 triggers phototropism in response to low fluence rates, while both phot1 and phot2 mediate this response under higher light conditions. Phototropism results from asymmetric growth in the hypocotyl elongation zone that depends on an auxin gradient across the embryonic stem. How phototropin activation leads to this growth response is still poorly understood. Members of the phytochrome kinase substrate (PKS) family may act early in this pathway, because PKS1, PKS2 and PKS4 are needed for a normal phototropic response and they associate with phot1 in vivo. Here we show that PKS proteins are needed both for phot1- and phot2-mediated phototropism. The phototropic response is conditioned by the developmental asymmetry of dicotyledonous seedlings, such that there is a faster growth reorientation when cotyledons face away from the light compared with seedlings whose cotyledons face the light. The molecular basis for this developmental effect on phototropism is unknown; here we show that PKS proteins play a role at the interface between development and phototropism. Moreover, we present evidence for a role of PKS genes in hypocotyl gravi-reorientation that is independent of photoreceptors. pks mutants have normal levels of auxin and normal polar auxin transport, however they show altered expression patterns of auxin marker genes. This situation suggests that PKS proteins are involved in auxin signaling and/or lateral auxin redistribution.

Initiation of phototropic growth: The where, the how and the now

For centuries, phototropism has given researchers a convenient experimental system to study how light directs plant growth. Yet, despite over a century of research, deciphering how plants reorientate their growth to maximize photosynthetic light capture and promote seedling survival has proved remarkably challenging. In this article, we present a brief historical overview of our current understanding of the molecular mechanisms underlying shoot phototropism in higher plants and discuss recent progress that now provides new insights on where this adaptive growth response is initiated in dicotyledonous seedlings.

Differential growth at the apical hook: all roads lead to auxin

The apical hook is a developmentally regulated structure that appears in dicotyledonous seedlings when seeds germinate buried in the soil. It protects the shoot apical meristem and cotyledons from damage while the seedling is pushing upwards seeking for light, and it is formed by differential cell expansion between both sides of the upper part of the hypocotyl. Its apparent simplicity and the fact that it is dispensable when seedlings are grown in vitro have converted the apical hook in one of the favorite experimental models to study the regulation of differential growth. The involvement of hormones –especially auxin—in this process was manifested already in the early studies. Remarkably, a gradient of this hormone across the hook curvature is instrumental to complete its development, similar to what has been proposed for other processes involving the bending of an organ, such as tropic responses. In agreement with this, other hormones—mainly gibberellins and ethylene—and the light, regulate in a timely and interconnected manner the auxin gradient to promote hook development and its opening, respectively. Here, we review the latest findings obtained mainly with the apical hook of Arabidopsis thaliana, paying special attention to the molecular mechanisms for the cross-regulation between the different hormone signaling pathways that underlie this developmental process.

The auxin influx carriers AUX1 and LAX3 are involved in auxin-ethylene interactions during apical hook development inArabidopsis thalianaseedlings

Dark-grown dicotyledonous seedlings form a hook-like structure at the top of the hypocotyl, which is controlled by the hormones auxin and ethylene. Hook formation is dependent on an auxin signal gradient, whereas hook exaggeration is part of the triple response provoked by ethylene in dark-grown Arabidopsis seedlings. Several other hormones and light are also known to be involved in hook development, but the molecular mechanisms that lead to the initial installation of an auxin gradient are still poorly understood. In this study, we aimed to unravel the cross-talk between auxin and ethylene in the apical hook. Auxin measurements, the expression pattern of the auxin reporter DR5::GUS and the localization of auxin biosynthesis enzymes and influx carriers collectively indicate the necessity for auxin biosynthesis and efficient auxin translocation from the cotyledons and meristem into the hypocotyl in order to support proper hook development. Auxin accumulation in the meristem and cotyledons and in the hypocotyl is increased approximately 2-fold upon treatment with ethylene. In addition, a strong ethylene signal leads to enhanced auxin biosynthesis at the inner side of the hook. Finally, mutant analysis demonstrates that the auxin influx carrier LAX3 is indispensable for proper hook formation, whereas the auxin influx carrier AUX1 is involved in the hook exaggeration phenotype induced by ethylene.

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) micromol 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.

Adventitious shoot formation in decapitated dicotyledonous seedlings starts with regeneration of abnormal leaves from cells not located in a shoot apical meristem

Regeneration of new shoots in plant tissue culture is often associated with appearance of abnormally shaped leaves. We used the adventitious shoot regeneration response induced by decapitation (removal of all preformed shoot apical meristems, leaving a single cotyledon) of greenhouse-grown cotyledon-stage seedlings to test the hypothesis that such abnormal leaf formation is a normal regeneration progression following wounding and is not conditioned by tissue culture. To understand why shoot regeneration starts with defective organogenesis, the regeneration response was characterized by morphology and scanning electron and light microscopy in decapitated cotyledon-stage Cucurbita pepo seedlings. Several leaf primordia were observed to regenerate prior to differentiation of a de novo shoot apical meristem from dividing cells on the wound surface. Early regenerating primordia have a greatly distorted structure with dramatically altered dorsoventrality. Aberrant leaf morphogenesis in C. pepo gradually disappears as leaves eventually originate from a de novo adventitious shoot apical meristem, recovering normal phyllotaxis. Similarly, following comparable decapitation of seedlings from a number of families (Chenopodiaceae, Compositae, Convolvulaceae, Cucurbitaceae, Cruciferae, Fabaceae, Malvaceae, Papaveraceae, and Solanaceae) of several dicotyledonous clades (Ranunculales, Caryophyllales, Asterids, and Rosids), stems are regenerated bearing abnormal leaves; the normal leaf shape is gradually recovered. Some of the transient leaf developmental defects observed are similar to responses to mutations in leaf shape or shoot apical meristem function. Many species temporarily express this leaf development pathway, which is manifest in exceptional circumstances such as during recovery from excision of all preformed shoot meristems of a seedling.

Anatomy of axis contraction in seedlings from a fire prone habitat

The hypocotyls or roots of many seed plants contract during seedling growth. Anatomical evidence is here reported for the first time that G-fibers (gelatinous or tension wood fibers) may cause contraction of roots and hypocotyls in dicotyledonous seedlings long after germination. To document repositioning of seedling buds, selected perennials (20 dicotyledons and one cycad) native to the fire-prone pine rocklands of subtropical South Florida were germinated and measured for 4-5 mo. The height of cotyledonary nodes above the soil decreased because of axis contraction or bending in eight species. Anatomy suggested that two mechanisms operate: (1) previously well-documented collapse of parenchyma cells in two species (Convolvulaceae and Zamiaceae) and (2) newly documented production of G-fibers in six species (all Fabaceae). Contraction or bending of the hypocotyl and/or taproot moved the cotyledonary and later buds of the seedling closer to the soil surface or buried them. Bud repositioning by these mechanisms may protect the lateral buds from injury by fire or other environmental stresses and allow resprouting.

Dwarf Forest Recovery after Disturbances in the Luquillo Mountains of Puerto Rico

Dwarf forest in Puerto Rico's Luquillo Mountains varies according to substrate and topography with very short, dense forest growing on exposed, rocky sites. High elevation level sites suffered considerable damage during past hurricanes whereas the trees on certain lower slopes were protected by ridges or spurs. Post-disturbance recovery of dwarf forest on two types of sample plots near East Peak was slow. Nearly 37 years after a 1968 airplane crash, Cyathea bryophila (R. Tryon) Proctor and Eugenia borinquensis Britton accounted for 71% of the 25.3 t ha-1 total aboveground dry weight biomass (hereafter biomass) on 780 m2 of the crash site. This is only 30% of the 80 t ha-1 average biomass of the surrounding dwarf forest prior to Hurricane Hugo of 1989. Also, six 250 m2 permanent plots (stratified by topography with sites on ridges, slopes, and ravines) showed delayed post-hurricane mortality, declining in mean stem numbers from 2,956 stems ha-1 in 1990 to 2,268 stems ha-1 in 2005. Average plot biomass decreased from 72.8 t ha-1 in 1990 to 61.6 t ha-1 in 2000, increasing slightly to 62.9 t ha-1 in 2005. Recovery on all sites was characterized by an immediate invasion of grass cover along with an influx of ferns, followed by dicotyledonous seedlings and saplings, and finally small trees. More than one-half of the arborescent species growing in dwarf forest are endemic to Puerto Rico where they play a prominent role in post disturbance recovery; moreover, 85% of the trees do not exceed 15 m in height anywhere in the Luquillo Mountains.

Growth and morphological responses to different UV wavebands in cucumber (Cucumis sativum) and other dicotyledonous seedlings

We examined the influence of short-term exposure of different UV wavebands on the fine-scale kinetics of hypocotyl growth of dim red light-grown cucumbers (Cucumis sativus L.) and other selected dicotyledonous seedlings to evaluate: (1) whether responses induced by UV-B radiation (280-320 nm) are qualitatively different from those induced by UV-A (320-400 nm) radiation, and (2) whether different wavebands within the UV-B elicit different responses. Responses to brief (30 min) irradiations with 3 different UV wavebands all included transient inhibition of elongation during irradiation followed by wavelength specific responses. Irradiations with proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm) induced inhibition of hypocotyl elongation within 20 min of onset of irradiation, while UV-B including only wavelengths longer than 290 nm (and only 8% of UV-B between 290 and 300 nm) induced inhibition of hypocotyl elongation with a lag of 1-2 h. The response to short wavelength UV-B was persistent for at least 24 h, while the response to long wavelength UV-B lasted only 2-3 h. The UV-A treatment induced reductions in elongation rates of approximately 6-9 h following exposure followed by a continued decline in rates for the following 15-18 h. Short wavelength UV-B also induced positive phototropic curvature in both cucumber and Arabidopsis seedlings, and this response was present in nph-1 mutant Arabidopsis seedlings defective in normal blue light phototropism. Reciprocity was not found for the response to short wavelength UV-B. The short wavelength and long wavelength UV-B responses differed in dose-response relationships and both short wavelength responses (phototropic curvature and elongation inhibition) increased sharply at wavelengths below 300 nm. These results indicate that different photosensory processes are involved in mediating growth and morphological responses to short wavelength UV-B (280-300 nm), long wavelength UV-B (essentially 300-320 nm) and UV-A. The existence of two separate types of hypocotyl inhibition responses to UV-B, with one that depends on the intensity of the light source, provides alternate interpretations to findings in other studies of UV-B induced photomorphogenesis and may explain inconsistencies between action spectra for inhibition of stem growth.

Nuclear and cytosolic events of light-induced, phytochrome-regulated signaling in higher plants.

Plants as sessile organisms cannot walk away but have to adapt to changes in their natural environments. Light is obviously the most important external factor, being the energy source. To regulate their fitness, the photosynthetic capacity of plants must be adapted to changes in the ambient light environment. Therefore, plants have evolved a large set of photoreceptors to monitor light quality and quantity, ranging from the UV to the infrared part of the spectrum. At least three different photoreceptor classes have been identified and analyzed: (i) UV‐B receptors, primarily characterized by action spectroscopy (Wellmann, 1983); (ii) the blue UV‐A photoreceptors, cry1 and cry2 and phototropine (Ahmad and Cashmore, 1993; Christie et al ., 1998; Lin et al ., 1998); and (iii) the red/far‐red reversible phytochromes. These photoreceptors control different aspects of plant growth and development, and in this review we focus on the physiological and molecular events regulated by the most characterized plant photoreceptors, the phytochromes. In higher plants, phytochromes are encoded by a small multigene family; five genes ( PHYA–PHYE , Sharrock and Quail, 1989; Clack et al ., 1994) have been identified in Arabidopsis . Phytochromes exist as dimers composed of two 125 kDa polypeptides, each carrying a covalently linked tetrapyrrol chromophore in the N‐terminal domain and dimerization domains in the C‐terminal domain. The photosensory function of the molecule is based on its capacity for reversible interconversion between the red light‐absorbing Pr form and the far‐red light absorbing Pfr form following sequential absorption of red and far‐red light. Photosignal perception by the receptor is followed by conformational changes, and activates, through an as yet poorly understood mechanism, signaling pathways leading to changes in the expression of genes that underlie developmental responses to light (Quail et al ., 1995). In order to finely tune their …

The Arabidopsis det3 mutant reveals a central role for the vacuolar H(+)-ATPase in plant growth and development.

In all multicellular organisms growth and morphogenesis must be coordinated, but for higher plants, this is of particular importance because the timing of organogenesis is not fixed but occurs in response to environmental constraints. One particularly dramatic developmental juncture is the response of dicotyledonous seedlings to light. The det3 mutant of Arabidopsis develops morphologically as a light-grown plant even when it is grown in the dark. In addition, it shows organ-specific defects in cell elongation and has a reduced response to brassinosteroids (BRs). We have isolated the DET3 gene by positional cloning and provide functional and biochemical evidence that it encodes subunit C of the vacuolar H(+)-ATPase (V-ATPase). We show that the hypocotyl elongation defect in the det3 mutant is conditional and provide evidence that this is due to an alternative mechanism of V-ATPase assembly. Together with the expression pattern of the DET3 gene revealed by GFP fluorescence, our data provide in vivo evidence for a role for the V-ATPase in the control of cell elongation and in the regulation of meristem activity.

Asymmetric Responsiveness to Ethylene Mediates Cell Elongation in the Apical Hook of Peas

The apical hook of dark-grown dicotyledonous seedlings is a protective structure resulting from an inhibition of cell elongation on the inner portion of the hook. This differential growth response is mediated by ethylene. Expression of the gene encoding 1-aminocyclopropane-1-carboxylate oxidase (ACO), the terminal enzyme in ethylene biosynthesis, is induced by ethylene via a positive feedback loop. Therefore, the ACO transcript can serve as a molecular marker for both ethylene formation and ethylene responsiveness. We examined the distribution of ACO mRNA of pea, Ps-ACO1 , and of ACO enzyme activity in the apical hook of etiolated pea seedlings. In situ hybridization showed that cells on the inner, concave side of pea hooks accumulated more Ps-ACO1 mRNA than did cells on the outer, convex side. The distribution of ACO enzyme activity followed the same pattern. A direct correlation was observed between the cellular distribution of Ps-ACO1 mRNA, ACO enzyme activity, and the inhibition of cell elongation. Pea seedlings treated with a saturating concentration of ethylene still accumulated higher levels of the Ps-ACO1 transcript on the inner side of the apical hook, demonstrating an increased responsiveness to ethylene in this tissue. These results indicate that an asymmetrically distributed component of the ethylene signal transduction pathway mediates hook formation. Based on existing genetic evidence, we propose that this component is downstream from the serine/threonine protein kinase CTR1.

Hook formation in sunflower seedlings is directed by both positive gravitropism and a form of circumnutation

ABSTRACT The apical hook that forms at the apex of most dicotyledonous seedlings during germination has in the past been variously described either as the consequence of positive gravitropism or as an autonomous process. Recently, however, emphasis has shifted more on the role of positive gravitropism as the inducer of this structure. To address the question on the nature and origin of the hook, experiments were performed on sunflower seedlings, not only under gravitational conditions but also by means of a clinostat, i.e. in the presence of multidirectionally compensated gravitational stimulation. The results of this work show that at least two processes are involved in the formation of a hook: positive gravitropism and an autonomous movement. Consequently, both Darwin, with his conviction that an innate process such as circumnutation was involved, and the supporters of positive gravitropism, were partly correct. Moreover, it was found that, in contrast to the hook formed under normal unidirectional gravitational conditions, the one formed on the clinostat showed a preferential plane for its development: the plane that passes parallel to the closed cotyledons. This characteristic also suggests that a form of circumnutation may be part of the basic mechanism for hook formation.

Light signal transduction in plants.

Plants, as sessile organisms, have developed intricate signal transduction networks used to adapt to the changing environment surrounding them. One of the most important dynamic environmental factors is light, and the light control of dicotyledonous seedling development is one of the best studied examples. Before the developing seedling emerges from the soil it has the classic skotomorphogenic or etiolated morphology, containing a long hypocotyl with closed cotyledons and an apical hook. Plastids remain as etioplasts that lack developed membrane structures and have no photosynthetic capacity. Emergence of the seedling into the light triggers a profound developmental switch to photomorphogenesis, which results in short hypocotyls, open and expanded cotyledons and the lack of an apical hook. Chloroplasts develop and photosynthetic gene expression is derepressed. This changing morphological pattern is the result of vastly different modes of gene expression in the dark and the light. The sunlight that reaches the earth's surface ranges from UV to visible to infrared. Although the visible light (400700 nm) is most effective in driving photosynthesis, higher plants have adapted a sophisticated sensory system that is also able to sense the UV and far-red light. Moreover, higher plants not only sense the quality of the light surrounding them but also the direction and quantity of light. Extensive research in the past decades, particularly the last few years, has significantly improved our understanding of the photosensory system and the downstream signaling processes. Many components involved in this regulatory network have been identified and some of them have been characterized molecularly. This review attempts to provide a brief summary of recent progress, with particular emphasis on the effect of light on Arubidopsis seedling development. There are several excellent reviews on this and related subjects in the last 2 years (1-6) that are recommended for further information.