Three-dimensional visualization of plant tissues and organs by X-ray micro-computed tomography

Studies visualizing plant tissues and organs in 3D by micro-computed tomography (CT) published since approximately 2015 are reviewed. In this period, the number of publications in the field of plant sciences dealing with micro-CT have increased along with the development of high-performance lab-based micro-CT system as well as the continuous development of cutting-edge technologies at synchrotron radiation facilities. A widespread use of commercially-available lab-based micro-CT systems enabling phase-contrast imaging technique, which is suitable for visualization of biological specimens composed of light elements, appears to have facilitated these studies. Unique features of the plant body, which are particularly utilized for imaging of plant organs and tissues by micro-CT, are having functional air spaces and specialized cell walls, such as lignified ones. In this review we briefly describe the basis of micro-CT technology first and then get down into details of its application to 3D visualization in plant sciences, which are categorized as follows: imaging of various organs, caryopses, seeds, other organs (reproductive organs, leaves, stems and petioles), various tissues (leaf venations, xylems, air-filled tissues, cell boundaries, cell walls), embolisms, root systems, hoping that wide users of microscopes and other imaging technologies will be interested also in micro-CT and obtain some hints for deeper understanding of structure of plant tissues and organs in 3D. Majority of current morphological studies using micro-CT still appear to be at qualitative level. Development of methodology for accurate 3D segmentation is needed for transition of the studies at from qualitative level to quantitative level in the future.

Three-dimensionally visualized rhizoid system of moss, Physcomitrium patens, by refraction-contrast X-ray micro-computed tomography

Land plants have two types of shoot-supporting systems, root system and rhizoid system, in vascular plants and bryophytes. However, since the evolutionary origin of the systems is different, how much they exploit common systems or distinct systems to architect their structures is largely unknown. To understand the regulatory mechanism of how bryophytes architect the rhizoid system responding to environmental factors, we have developed the methodology to visualize and quantitatively analyze the rhizoid system of the moss, Physcomitrium patens, in 3D. The rhizoids having a diameter of 21.3 µm on the average were visualized by refraction-contrast X-ray micro-computed tomography using coherent X-ray optics available at synchrotron radiation facility SPring-8. Three types of shape (ring-shape, line and black circle) observed in tomographic slices of specimens embedded in paraffin were confirmed to be the rhizoids by optical and electron microscopy. Comprehensive automatic segmentation of the rhizoids, which appeared in three different form types in tomograms, was tested by a method using a Canny edge detector or machine learning. The accuracy of output images was evaluated by comparing with the manually segmented ground truth images using measures such as F1 score and Intersection over Union, revealing that the automatic segmentation using machine learning was more effective than that using the Canny edge detector. Thus, machine learning-based skeletonized 3D model revealed quite dense distribution of rhizoids. We successfully visualized the moss rhizoid system in 3D for the first time.

How plants grow under gravity conditions besides 1 g: perspectives from hypergravity and space experiments that employ bryophytes as a model organism

Plants have evolved and grown under the selection pressure of gravitational force at 1 g on Earth. In response to this selection pressure, plants have acquired gravitropism to sense gravity and change their growth direction. In addition, plants also adjust their morphogenesis in response to different gravitational forces in a phenomenon known as gravity resistance. However, the gravity resistance phenomenon in plants is poorly understood due to the prevalence of 1 g gravitational force on Earth: not only it is difficult to culture plants at gravity > 1 g(hypergravity) for a long period of time but it is also impossible to create a < 1 genvironment (μg, micro g) on Earth without specialized facilities. Despite these technical challenges, it is important to understand how plants grow in different gravity conditions in order to understand land plant adaptation to the 1 g environment or for outer space exploration. To address this, we have developed a centrifugal device for a prolonged duration of plant culture in hypergravity conditions, and a project to grow plants under the μg environment in the International Space Station is also underway. Our plant material of choice is Physcomitrium (Physcomitrella) patens, one of the pioneer plants on land and a model bryophyte often used in plant biology. In this review, we summarize our latest findings regarding P. patens growth response to hypergravity, with reference to our on-going "Space moss" project. In our ground-based hypergravity experiments, we analyzed the morphological and physiological changes and found unexpected increments of chloroplast size and photosynthesis rate, which might underlie the enhancement of growth and increase in the number of gametophores and rhizoids. We further discussed our approaches at the cellular level and compare the gravity resistance in mosses and that in angiosperms. Finally, we highlight the advantages and perspectives from the space experiments and conclude that research with bryophytes is beneficial to comprehensively and precisely understand gravitational responses in plants.

Visualization of Arabidopsis Root System Architecture in 3D by Refraction-Contrast X-Ray Micro-Computed Tomography

Plant roots change their morphological traits in order to adapt themselves to different environmental conditions, resulting in alteration of the root system architecture. To understand this mechanism, it is essential to visualize morphology of the entire root system. To reveal effects of long-term alteration of gravity environment on root system development, we have performed an experiment in the International Space Station using Arabidopsis plants and obtained dried root systems grown in rockwool slabs. X-ray computed tomography (CT) technique using industrial X-ray scanners has been introduced to visualize root system architecture of crop species grown in soil in 3D non-invasively. In the case of the present study, however, root system of Arabidopsis is composed of finer roots compared with typical crop plants and rockwool is also composed of fibers having similar dimension to that of the roots. A higher spatial resolution imaging method is required for distinguishing roots from rockwool. Therefore, in the present study, we tested refraction-contrast X-ray micro-CT using coherent X-ray optics available at the beamline of the synchrotron radiation facility SPring-8 for bio-imaging. We have found that wide field of view but with low resolution obtained at the experimental Hutch 3 of this beamline provided an overview map of the root systems, while narrow field of view but with high resolution obtained at the experimental Hutch 1 provided extended architecture of the secondary roots, by clear distinction between roots and individual rockwool fibers, resulting in successful tracing of these roots from their basal regions.

Functions and structure of roots and their contributions to salinity tolerance in plants

Soil salinity is an increasing threat to the productivity of glycophytic crops worldwide. The root plays vital roles under various stress conditions, including salinity, as well as has diverse functions in non-stress soil environments. In this review, we focus on the essential functions of roots such as in ion homeostasis mediated by several different membrane transporters and signaling molecules under salinity stress and describe recent advances in the impacts of quantitative trait loci (QTLs) or genetic loci (and their causal genes, if applicable) on salinity tolerance. Furthermore, we introduce important literature for the development of barriers against the apoplastic flow of ions, including Na⁺, as well as for understanding the functions and components of the barrier structure under salinity stress.

Vegetative and reproductive growth of Arabidopsis under microgravity conditions in space

We have performed a seed-to-seed experiment in the cell biology experiment facility (CBEF) installed in the Kibo (Japanese Experiment Module) in the International Space Station. The CBEF has a 1 × g compartment on a centrifuge and a microgravity compartment, to investigate the effects of microgravity on the vegetative and reproductive growth of Arabidopsis thaliana (L.) Heynh. Seeds germinated irrespective of gravitational conditions after water supply on board. Thereafter, seedlings developed rosette leaves. The time of bolting was slightly earlier under microgravity than under space 1 × g. Microgravity enhanced the growth rate of peduncles as compared with space 1 × g or ground control. Plants developed flowers, siliques and seeds, completing their entire life cycle during 62-days cultivation. Although the flowering time was not significantly affected under microgravity, the number of flowers in a bolted plant significantly increased under microgravity as compared with space 1 × g or ground control. Microscopic analysis of reproductive organs revealed that the longitudinal length of anthers was significantly shorter under microgravity when compared with space 1 × g, while the length of pistils and filaments was not influenced by the gravitational conditions. Seed mass significantly increased under microgravity when compared with space 1 × g. In addition, seeds produced in space were found not to germinate on the ground. These results indicate that microgravity significantly influenced the reproductive development of Arabidopsis plants even though Earth's gravitational environment is not absolutely necessary for them to complete their life cycle.

Correction to: A hypergravity environment increases chloroplast size, photosynthesis, and plant growth in the moss Physcomitrella patens

The original article can be found online.

Plasma membrane-anchored chloroplasts are necessary for the gravisensing system of Ceratopteris richardii prothalli

The prothalli of the fern Ceratopteris richardii exhibit negative gravitropism when grown in darkness. However, no sedimentable organelles or substances have been detected in the prothallial cells, suggesting that a non-sedimentable gravisensor exists. We investigated whether chloroplasts are involved in the gravisensing system of C. richardii prothalli. We used a clumped-chloroplast mutant, clumped chloroplast 1 (cp1), in which the chloroplasts are detached from the plasma membrane and clustered around the nucleus likely because of a partial deletion in the KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT 1 gene. The cp1 mutation resulted in prothalli that had a significantly diminished gravitropic response, while the phototropic response occurred normally. These results suggest that plasma membrane-anchored chloroplasts in prothallial cells function as one of the gravisensors in C. richardii prothalli.

A hypergravity environment increases chloroplast size, photosynthesis, and plant growth in the moss Physcomitrella patens

The physiological and anatomical responses of bryophytes to altered gravity conditions will provide crucial information for estimating how plant physiological traits have evolved to adapt to significant increases in the effects of gravity in land plant history. We quantified changes in plant growth and photosynthesis in the model plant of mosses, Physcomitrella patens, grown under a hypergravity environment for 25 days or 8 weeks using a custom-built centrifuge equipped with a lighting system. This is the first study to examine the response of bryophytes to hypergravity conditions. Canopy-based plant growth was significantly increased at 10×g, and was strongly affected by increases in plant numbers. Rhizoid lengths for individual gametophores were significantly increased at 10×g. Chloroplast diameters (major axis) and thicknesses (minor axis) in the leaves of P. patens were also increased at 10×g. The area-based photosynthesis rate of P. patens was also enhanced at 10×g. Increases in shoot numbers and chloroplast sizes may elevate the area-based photosynthesis rate under hypergravity conditions. We observed a decrease in leaf cell wall thickness under hypergravity conditions, which is in contrast to previous findings obtained using angiosperms. Since mosses including P. patens live in dense populations, an increase in canopy-based plant numbers may be effective to enhance the toughness of the population, and, thus, represents an effective adaptation strategy to a hypergravity environment for P. patens.

Single microfilaments mediate the early steps of microtubule bundling during preprophase band formation in onion cotyledon epidermal cells

The preprophase band (PPB) is a cytokinetic apparatus that determines the site of cell division in plants. It originates as a broad band of microtubules (MTs) in G2 and narrows to demarcate the future division site during late prophase. Studies with fluorescent probes have shown that PPBs contain F-actin during early stages of their development but become actin depleted in late prophase. Although this suggests that actins contribute to the early stages of PPB formation, how actins contribute to PPB-MT organization remains unsolved. To address this question, we used electron tomography to investigate the spatial relationship between microfilaments (MFs) and MTs at different stages of PPB assembly in onion cotyledon epidermal cells. We demonstrate that the PPB actins observed by fluorescence microscopy correspond to short, single MFs. A majority of the MFs are bound to MTs, with a subset forming MT-MF-MT bridging structures. During the later stages of PPB assembly, the MF-mediated links between MTs are displaced by MT-MT linkers as the PPB MT arrays mature into tightly packed MT bundles. On the basis of these observations, we propose that the primary function of actins during PPB formation is to mediate the initial bundling of the PPB MTs.

Role of ethylene signalling in the formation of constitutive aerenchyma in primary roots of rice

Although the extent of aerenchyma (interconnected gas-filled space) formed in the cortex of the roots of some species can be promoted by submergence and ethylene, such roots also form a somewhat less extensive aerenchyma under well-aerated conditions. It has been unclear whether or not ethylene is also involved in promoting this constitutive aerenchyma. To confirm the potential of ethylene to stimulate aerenchyma development and test the possibility that gas regulates constitutive aerenchyma, a novel sandwich method was employed in rice roots. This involved germinating japonica rice (Oryza sativa) caryopses sandwiched between two agar slabs with or without 1-aminocyclopropane-1-carboxylic acid (ACC) at 1 µM. The roots were then grown for 4 days in the dark in the presence or absence of gaseous 1-methylcyclopropene (1-MCP), an inhibitor of ethylene action. Examination of aerenchyma development by light microscopy demonstrated a more extensive aerenchyma in cross-section on the ACC-treated side that also commenced closer to the root tip. In the presence of 1-MCP at 0.1 or 1 ppm, aerenchyma formation was inhibited in the presence or absence of ACC. 1-Methylcyclopropene also overcame ACC-inhibited root elongation. The results indicate that ethylene signalling is involved in aerenchyma development in primary roots of rice and that this may include the regulation of constitutive aerenchyma. In addition, root elongation was slowed in control roots in the presence of 1 ppm 1-MCP, supporting previous studies demonstrating that endogenous levels of ethylene stimulate root elongation.

Growth stimulation in inflorescences of an Arabidopsis tubulin mutant under microgravity conditions in space

Cortical microtubules are involved in plant resistance to hypergravity, but their roles in resistance to 1 g gravity are still uncertain. To clarify this point, we cultivated an Arabidopsis α-tubulin 6 mutant (tua6) in the Cell Biology Experiment Facility on the Kibo Module of the International Space Station, and analyzed growth and cell wall mechanical properties of inflorescences. Growth of inflorescence stems was stimulated under microgravity conditions, as compared with ground and on-orbit 1 g conditions. The stems were 10-45% longer and their growth rate 15-55% higher under microgravity conditions than those under both 1 g conditions. The degree of growth stimulation tended to be higher in the tua6 mutant than the wild-type Columbia. Under microgravity conditions, the cell wall extensibility in elongating regions of inflorescences was significantly higher than the controls, suggesting that growth stimulation was caused by cell wall modifications. No clear differences were detected in any growth or cell wall property between ground and on-orbit 1 g controls. These results support the hypothesis that cortical microtubules generally play an important role in plant resistance to the gravitational force.

High-pressure freezing and low-temperature processing of plant tissue samples for electron microscopy

Use of electron tomography methods improves image resolution of transmission electron microscopy especially in the z-direction, enabling determination of complicated 3D structures of organelles and cytoskeleton arrays. The increase in resolution necessitates preservation of cellular structures close to the native states with minimum artifacts. High-pressure freezing (HPF) that immobilizes molecules in the cell instantaneously has been used to avoid damages caused by convention chemical fixation. Despite the advantages of HPF, cells could still be damaged during dissection prior to HPF. Therefore, it is critical to isolate cells/tissues of interest quickly and carefully. The samples frozen by HPF are often processed by freeze substitution (FS), and FS should be carried out under appropriate conditions. Here we describe dissection, HPF, and FS methods that we have utilized to prepare plant samples for electron tomography/immuno-electron microscopy.

Effects of hypergravity stimulus on global gene expression during reproductive growth in Arabidopsis

The life cycle of higher plants consists of successive vegetative and reproductive growth phases. Understanding effects of altered gravity conditions on the reproductive growth is essential, not only to elucidate how higher plants evolved under gravitational condition on Earth but also to approach toward realization of agriculture in space. In the present study, a comprehensive analysis of global gene expression of floral buds under hypergravity was carried out to understand effects of altered gravity on reproductive growth at molecular level. Arabidopsis plants grown for 20-26 days were exposed to hypergravity of 300 g for 24 h. Total RNA was extracted from flower buds and microarray (44 K) analysis performed. As a result, hypergravity up-regulated expression of a gene related to β-1,3-glucanase involved in pectin modification, and down-regulated β-galactosidase and amino acid transport, which supports a previous study reporting inhibition of pollen development and germination under hypergravity. With regard to genes related to seed storage accumulation, hypergravity up-regulated expression of genes of aspartate aminotransferase, and down-regulated those related to cell wall invertase and sugar transporter, supporting a previous study reporting promotion of protein body development and inhibition of starch accumulation under hypergravity, respectively. In addition, hypergravity up-regulated expression of G6PDH and GPGDH, which supports a previous study reporting promotion of lipid deposition under hypergravity. In addition, analysis of the metabolic pathway revealed that hypergravity substantially changed expression of genes involved in the biosynthesis of phytohormones such as abscisic acid and auxin.

Micro-CT observations of the 3D distribution of calcium oxalate crystals in cotyledons during maturation and germination in Lotus miyakojimae seeds

The cotyledon of legume seeds is a storage organ that provides nutrients for seed germination and seedling growth. The spatial and temporal control of the degradation processes within cotyledons has not been elucidated. Calcium oxalate (CaOx) crystals, a common calcium deposit in plants, have often been reported to be present in legume seeds. In this study, micro-computed tomography (micro-CT) was employed at the SPring-8 facility to examine the three-dimensional distribution of crystals inside cotyledons during seed maturation and germination of Lotus miyakojimae (previously Lotus japonicus accession Miyakojima MG-20). Using this technique, we could detect the outline of the embryo, void spaces in seeds and the cotyledon venation pattern. We found several sites that strongly inhibited X-ray transmission within the cotyledons. Light and polarizing microscopy confirmed that these areas corresponded to CaOx crystals. Three-dimensional observations of dry seeds indicated that the CaOx crystals in the L. miyakojimae cotyledons were distributed along lateral veins; however, their distribution was limited to the abaxial side of the procambium. The CaOx crystals appeared at stage II (seed-filling stage) of seed development, and their number increased in dry seeds. The number of crystals in cotyledons was high during germination, suggesting that CaOx crystals are not degraded for their calcium supply. Evidence for the conservation of CaOx crystals in cotyledons during the L. miyakojimae germination process was also supported by the biochemical measurement of oxalic acid levels.

Salinity tolerance mechanisms in glycophytes: An overview with the central focus on rice plants

Elevated Na(+) levels in agricultural lands are increasingly becoming a serious threat to the world agriculture. Plants suffer osmotic and ionic stress under high salinity due to the salts accumulated at the outside of roots and those accumulated at the inside of the plant cells, respectively. Mechanisms of salinity tolerance in plants have been extensively studied and in the recent years these studies focus on the function of key enzymes and plant morphological traits. Here, we provide an updated overview of salt tolerant mechanisms in glycophytes with a particular interest in rice (Oryza sativa) plants. Protective mechanisms that prevent water loss due to the increased osmotic pressure, the development of Na(+) toxicity on essential cellular metabolisms, and the movement of ions via the apoplastic pathway (i.e. apoplastic barriers) are described here in detail.

The pea stem: a unique experimental system to study the development of the Casparian strip

The Casparian strip is commonly observed in the endodermis of roots of vascular plants and, in some cases, also in the stems. Pea stems develop the Casparian strip, and its development has been reported to be regulated by blue light. In addition, for the purpose of photobiological studies, pea stems provide a unique experimental system for other physiological studies of the development of the Casparian strip. In this article, I have briefly summarized (1) the effects of environmental factors on the development of the Casparian strip, (2) the advantage of using pea stems for physiological studies of the development of the Casparian strip, and (3) cellular events indicated to be involved in the development of the Casparian strip, focusing on the studies using pea stems as well as other recent studies.

Demonstration of osmotically dependent promotion of aerenchyma formation at different levels in the primary roots of rice using a 'sandwich' method and X-ray computed tomography

BACKGROUND AND AIMS

The effect of environmental factors on the regulation of aerenchyma formation in rice roots has been discussed for a long time, because aerenchyma is constitutively formed under aerated conditions. To elucidate this problem, a unique method has been developed that enables sensitive detection of differences in the development of aerenchyma under two different environmental conditions. The method is tested to determine whether aerenchyma development in rice roots is affected by osmotic stress.

METHODS

To examine aerenchyma formation both with and without mannitol treatment in the same root, germinating rice (Oryza sativa) caryopses were sandwiched between two agar slabs, one of which contained 270 mm of mannitol. The roots were grown touching both slabs and were thereby exposed unilaterally to osmotic stress. As a non-invasive approach, refraction contrast X-ray computed tomography (CT) using a third-generation synchrotron facility, SPring-8 (Super photon ring 8 GeV, Japan Synchrotron Radiation Research Institute), was used to visualize the three-dimensional (3-D) intact structure of aerenchyma and its formation in situ in rice roots. The effects of unilateral mannitol treatment on the development of aerenchyma were quantitatively examined using conventional light microscopy.

KEY RESULTS

Structural continuity of aerenchyma was clearly visualized in 3-D in the primary root of rice and in situ using X-ray CT. Light microscopy and X-ray CT showed that the development of aerenchyma was promoted on the mannitol-treated side of the root. Detailed light microscopic analysis of cross-sections cut along the root axis from the tip to the basal region demonstrated that aerenchyma developed significantly closer to the root tip on the mannitol-treated side of the root.

CONCLUSIONS

Continuity of the aerenchyma along the rice root axis was morphologically demonstrated using X-ray CT. By using this 'sandwich' method it was shown that mannitol promoted aerenchyma formation in the primary roots of rice.

Development of the Casparian strip is delayed by blue light in pea stems

To understand the regulatory mechanisms involved in tissue development by light, the kinetics of regulation of Casparian strip (CS) development in garden pea stems was studied. We found that short-term irradiation with white light delayed the development of the CS and used this delay to assess the quantitative effect of light on CS development. We examined the effect of the duration and fluence rates of white light treatment on CS development and observed a significant relationship between fluence and the delay in CS development indicating that the Bunsen-Roscoe law of reciprocity holds for this response. The effect of white light irradiation was not inhibited in the presence of a photosynthetic inhibitor, DCMU, or a carotenoid biosynthesis inhibitor, Norflurazon, indicating that the delay in CS development by light is a photomorphogenetic response rather than a subsidiary effect mediated by photosynthetic activity. An action spectrum for the response displayed a major peak in the blue-light region, suggesting a dominant role for blue-light receptors. A minor peak in the red-light region also suggested the possible involvement of phytochromes. Although phytochromes are known to contribute to blue-light responses, phytochrome-deficient mutants showed a normal delay of CS development in response to blue light, indicating that the response is not mediated by phytochrome and suggesting a role for one or more specific blue-light receptors.

Involvement of auxin dynamics in hypergravity-induced promotion of lignin-related gene expression in Arabidopsis inflorescence stems

Recent studies have shown that hypergravity enhances lignification through up-regulation of the expression of lignin biosynthesis-related genes, although its hormonal signalling mechanism is unknown. The effects of hypergravity on auxin dynamics were examined using Arabidopsis plants that were transformed with the auxin reporter gene construct DR5::GUS. Hypergravity treatment at 300 g significantly increased β-glucuronidase activity in inflorescence stems of DR5::GUS plants, indicating that endogenous auxin accumulation was enhanced by hypergravity treatment. The hypergravity-related increased expression levels of both DR5::GUS and lignin biosynthesis-related genes in inflorescence stems were suppressed after disbudding, indicating that the increased expression of lignin biosynthesis-related genes is dependent on an increase in auxin influx from the shoot apex.

Casparian strip development and its potential function in salt tolerance

The root system is particularly affected by unfavourable conditions because it is in direct contact with the soil environment. Casparian strips, a specialised structure deposited in anticlinal walls, are characterised by the impregnation of the primary wall pores with lignin and suberin. The Casparian strips in the endo- and exodermis of vascular plant roots appear to play an important role in preventing the non-selective apoplastic bypass of salts into the stele along the apoplast under salt stress. However, only a few investigations have examined the deposition and function of these apoplastic barriers in response to salt stress in higher plants.

A role of endocytosis in plant cytokinesis

The preprophase band (PPB) of microtubules (MTs) marks the site of the future division plane irrespective of the orientation of the equatorial plane. Because the PPB MTs disappear during prometaphase, some positional information is thought to remain in the cortical cytoplasm after the disappearance of the PPB MTs. Cytoskeletal proteins are known to be excluded from the PPB site during mitosis. These depleted zones of cytoskeletal proteins are potential candidates for a "negative memory" system. However, how these depleted zones of the cytoskeletal proteins are produced remains unknown. In a recent paper, we have quantified the distribution of clathrin-coated pits and vesicles as well as of secretory structures during PPB formation using a combination of high-pressure freezing and electron tomography techniques. Our results demonstrated that the rate of endocytosis is enhanced in PPB regions. We postulate that the removal of membrane proteins by endocytosis plays a role in the creation of PPB "memory" structures.

The preprophase band is a localized center of clathrin-mediated endocytosis in late prophase cells of the onion cotyledon epidermis

The preprophase band (PPB) marks the site on the plant cell cortex where the cell plate will fuse during the final stage of cytokinesis. Recent studies have shown that several cytoskeletal proteins are depleted at the PPB site, but the processes that bring about these changes are still unknown. We have investigated the membrane systems associated with the PPB regions of epidermal cells of onion cotyledons by means of serial thin sections and electron tomograms. In contrast with specimens preserved by chemical fixatives, our high-pressure frozen cells demonstrated the presence of large numbers of clathrin-coated pits and vesicles in the PPB regions. The vesicles were of two types: clathrin-coated and structurally related, non-coated vesicles. Quantitative analysis of the data revealed that the number of clathrin-coated pits and vesicles is higher in the PPB regions than outside of these regions. Immunofluorescent microscopy using anti-plant clathrin-antibody confirmed this result. In contrast, no differences in secretory activities were observed. We postulate that the removal of membrane proteins by endocytosis plays a role in the formation of PPB 'memory' structures.

Effects of hypergravity conditions on elongation growth and lignin formation in the inflorescence stem of Arabidopsis thaliana

The effects of hypergravity on elongation growth and lignin deposition in secondary cell walls of the Arabidopsis thaliana (L.) Heynh. inflorescence stem were examined in plants grown for 3 days after exposure to hypergravity in the direction from shoot to root at 300 g for 24 h. The content of acetylbromide-extractable lignins in a secondary cell wall fraction prepared by enzyme digestion of inflorescence stem segments removing primary cell wall components was significantly increased by the hypergravity stimulus. Xylem vessels, particularly in a region closer to the base of the inflorescence stem, increased in number. Gadolinium chloride at 0.1 mM, a blocker of mechanoreceptors, partially suppressed the effect of hypergravity on lignin deposition in the secondary cell wall fraction. These results suggest that mechanoreceptors are responsible for hypergravity-induced lignin deposition in secondary cell walls in A. thaliana inflorescence stems.

Optical microscopy of Arabidopsis seedlings fixed in non-fresh FAA using Kennedy Fixation Tubes

Samples will be kept in non-fresh fixative for longer period than usual in Kennedy Space Center Fixation Tubes (KFT) when they will be fixed for microscopy in the International Space Station (ISS). It is necessary to examine characteristics of samples prepared under such conditions and to improve preservation of the samples. In this study, morphology of Arabidopsis tissues prepared under such conditions was examined under a light microscope. Deformation of cell shape was observed in tissues with well-developed intercellular spaces when they were fixed in non-fresh fixative for longer period in KFTs.

Lead tolerance and accumulation in the gametophytes of the fern Athyrium yokoscense

The fern Athyrium yokoscense is known to be highly tolerant to lead toxicity, and is a lead hyperaccumulator that can accumulate over 1,000 microg g(-1) of lead in its dry matter. In this work, we examined whether the gametophytic generation of A. yokoscense also resists lead toxicity like the sporophytic generation. Spore germination in A. yokoscense was more tolerant to Pb2+, compared to that in other fern species, such as Pteridium aquilinum, Lygodium japonicum and Pteris vittata. In addition, the early gametophyte development of A. yokoscense was not much affected by 10 microM Pb2+, as evaluated from the prothallial growth and rhizoid development. We also showed that Athyrium gametophytes could accumulate more than 10,000 microg g(-1) of lead, and that the lead was localized in the cytosol and vacuole of rhizoidal cells, as determined by a transmission electron micrograph. These results indicate that Athyrium gametophytes have the ability to accumulate lead in the rhizoids. Furthermore, the gametophytes were found to include a large amount of proanthocyanidins (condensed tannins). Because proanthocyanidins have a latent ability to complex with lead ions, the possible roles of proanthocyanidins in the lead tolerance and accumulation of Athyrium gametophytes are discussed.

Effects of hypergravity environment on lignin formation in Arabidopsis

Lignin and the secondary wall formation are essential for evolution of land plants. In this study, effects of hypergravity environment on the morphology of the secondary wall and the lignin content were examined in Arabidopsis thaliana. Xylem vessels showed intense staining with phloroglucinol-HCl and autofluorescence under UV light at the basal region of the flower stalk when seedlings grown for 3 days after hypergravity treatment for 24 hours. And, the flower stalk exposed to hypergravity showed slight increase in the lignin content. These results suggest that the lignin formation is positively regulated under hypergravity.

Development of the Casparian strip in primary roots of maize under salt stress

The Casparian strip in the endodermis of vascular plant roots appears to play an important role in preventing the influx of salts into the stele through the apoplast under salt stress. The effects of salinity on the development and morphology of the Casparian strip in primary roots of maize ( Zea mays L.) were studied. Compared to the controls, the strip matured closer to the root tip with increase in the ambient concentration of NaCl. During growth in 200 mM NaCl, the number and the length of the endodermal cells in the region between the root tip and the lowest position of the endodermal strip decreased, as did the apparent rate of production of cells in single files of endodermal cells (the rate of cell formation being equal to the rate at which cells are lost from the meristem). The estimated time required for an individual cell to complete the formation of the strip after generation of the cell in the presence of 200 mM NaCl was not very different from that required in controls. Thus, salinity did not substantially affect the actual process of formation of the strip in individual cells. The radial width of the Casparian strip, a morphological parameter that should be related to the effectiveness of the strip as a barrier, increased in the presence of 200 mM NaCl. The mean width of the lignified region was 0.92 microm in distilled water and 1.33 microm in 200 mM NaCl at the lowest position of the strip. The mean width of the strip relative to that of the radial wall at this position was significantly greater after growth in the presence of 200 mM NaCl than in the controls, namely, 20.5% in distilled water and 33.9% in 200 mM NaCl. These observations suggest that the function of the strip is enhanced under salt stress.

[Effects of clinorotation on the structure of xylem in Arabidopsis roots]

Lignin, which is the most important component of vascular tissue, is essential material for the structural support of higher plants and considered to play a critical role in evolution of land plants. It has been postulated that development of secondary wall is mediated by gravity. However, effects of gravity on the development and the morphology of secondary cell wall have not been well investigated. In this study, effects of averaging of gravity vector using 2-D clinostat rotation on the structure of xylem was examined. The results indicated that the morphology of xylem is regulated by the orientation of gravity vector.

Improved method for visualizing coated pits, microfilaments, and microtubules in cryofixed and freeze-substituted plant cells

We have optimized the conditions for visualizing microfilaments, microtubules, and coated pits in the cortical cytoplasm of high-pressure frozen and freeze-substituted plant cells, in both tobacco root tips and onion cotyledons, individual microfilaments and the supramolecular structure of coated pits can be seen clearly in freeze-substituted samples treated with OsO4 at 40 degrees C followed by 5% uranyl acetate. Treatment with uranyl acetate alone resulted in poorly stained cytoplasmic organelles, whereas microfilaments were difficult to discern in specimen treated with OsO4 alone. The combination of a 40 degrees C OsO4 staining step followed by staining with uranyl acetate at 4 degrees C should prove useful for more detailed plant cytoskeletal/membrane studies in the future.

Radial widening of the Casparian strip follows induced radial expansion of endodermal cells

The Casparian strip, the barrier to apoplastic transport that is located at the endodermis in roots and stems, is formed by individual endodermal cells and is constructed as a highly organized mesh within the primary wall. Since little is known about the mechanism of formation of the strip, we tried to obtain morphological evidence for the existence, prior to suberization and lignification, of some regulatory system at the expected site of the strip. Endodermal cells in etiolated pea stems were induced to expand in the radial direction by piercing the stems through the cortex before formation of the strip. The radial width of the strip increased significantly with the expansion of the radial walls of these endodermal cells. The expansion of the cells occurred before the formation of the strip. However, strips that had already been formed when the stems were pierced did not increase in width despite an induced expansion of the radial walls. These observations suggest that some positional information exists in the radial wall of endodermal cells that defines the future site of formation of the strip and its width.

An agravitropic mutant of Arabidopsis, endodermal-amyloplast less 1, that lacks amyloplasts in hypocotyl endodermal cell layer

We have isolated a new recessive mutant of Arabidopsis thaliana for gravitropism, endodermal-amyloplast less 1 (eal1). eal1 shows reduced gravitropism in hypocotyl, and completely lacks gravitropism in inflorescence stems; root gravitropism is not affected. Starch staining with I-KI solution reveals almost no amyloplasts in eal1 hypocotyls when grown on a sucrose-free medium, though the root columella cells contain as many amyloplasts as wild type. On a medium containing 1% sucrose, eal1 hypocotyls contain as many starch granules as those of wild type, suggesting that starch synthesis is not affected in eal1. The endodermal cell layer which is thought to function as statocytes in hypocotyls is present in eal1. These results suggest that differentiation or development of gravity-responsive amyloplasts are affected in eal1 hypocotyls.