Elizabeth Kordyum

Biology of Plant Cells in Microgravity and under Clinostating

Experimental data on plant cell reproduction, growth, and differentiation in spaceflight and under clinostating that partially reproduce the biological effects of microgravity are elucidated. The rearrangements of organelle structural and functional organization in unicellular plant organisms as well as in meristematic, differentiating, and differentiated cells of multicellular organisms in these conditions are considered. The focus is on the changes in the interrelations of prokaryotic and eukaryotic organisms under altered gravity. Ideas on the acceleration of differentiation and aging of cells in microgravity and clinostating and the organisms adaptive possibilities for carrying out its own functions are discussed.

The stress protein level under clinorotation in context of the seedling developmental program and the stress response

Heat-shock proteins (HSP70 and HSP90) are present in plant cells under the normal growth conditions. At the same time, a variety of environmental disruptions results in their rapid synthesis as a substantial part of adaptation. HSP amounts can be indicative of a cellular stress level. Altered gravity (clinorotation) is unnatural for plants, so it may be a kind of stress. The aim of this study was to analyze the influence of horizontal clinorotation on the HSP70 and HSP90 level during seedling development. Pea (Pisum sativum L.) seedlings grown for 3 days from seed imbibitions in stationary control and under slow clinorotation (2 rpm) are used for this investigation. Western blot analysis indicated that HSP70 and HSP90 were abundant in the embryos of dry seeds and their amount decreased significantly during seed germination. But under horizontal clinorotation, their level in seedlings remained higher compared to the control. Furthermore, a comparison of the influence of horizontal and vertical clinorotation on the HSP level was carried out. On the ELISA data, HSP70 and HSP90 amounts in the 3-day old seedlings were higher after horizontal clinorotation than after vertical. The obtained data show an increased HSP70 and HSP90 level in pea seedlings under clinorotation. Both, rotation and change in the cell position relatively to a gravity vector affect the HSP level.

Molecular mechanisms of gravity perception and signal transduction in plants

Gravity is one of the environmental cues that direct plant growth and development. Recent investigations of different gravity signalling pathways have added complexity to how we think gravity is perceived. Particular cells within specific organs or tissues perceive gravity stimulus. Many downstream signalling events transmit the perceived information into subcellular, biochemical, and genomic responses. They are rapid, non-genomic, regulatory, and cell-specific. The chain of events may pass by signalling lipids, the cytoskeleton, intracellular calcium levels, protein phosphorylation-dependent pathways, proteome changes, membrane transport, vacuolar biogenesis mechanisms, or nuclear events. These events culminate in changes in gene expression and auxin lateral redistribution in gravity response sites. The possible integration of these signalling events with amyloplast movements or with other perception mechanisms is discussed. Further investigation is needed to understand how plants coordinate mechanisms and signals to sense this important physical factor.

A role for the cytoskeleton in plant cell gravisensitivity and Ca2+-signaling in microgravity

Space flight and clinostat experiments have been used to alter the influence of gravity. Lower and higher plants, both specialized and non-specialized to gravity perception, have shown through changes in ultrastructure and metabolism (including the intracellular calcium balance) that cells are gravisensitive (Claasen and Spooner, 1994; Halstead and Dutcher, 1987; Kiss, 2000; Kordyum, 1997). In the presented paper, an attempt was made to summarize some experimental data and concepts concerning certain cell gravity-sensing systems in the gravitational field and their interactions with the changed environment of microgravity basing on the cytoskeleton behavior and Ca signaling in altered gravity. It was proposed that a distinction be made between cell gravisensing and cell graviperception. Gravisensing is related to cell structure and metabolism stability in the gravitational field and their changes in microgravity. Graviperception is related to the active use of a gravitational stimulus by cells, which are specialized to gravity perception, for realizing normal plant orientation in space, for growth and vital activity (gravitropism, gravitaxis) (Kordyum and Guikema, 2001). The structure of graviperceptive cells is diverse, but gravisensors are well known. These are statoliths of different types that change their position in the direction of the gravity vector and thus initiate the next steps of the gravitational response. The structural and functional organization of graviperceptive cells is determined genetically. In most cell types not specialized for perception of gravity, the primary sensors are not clearly defined. The idea of positional homeostasis (Nace, 1983) was the first to focus attention on the role of the cytoskeleton in cell graviresponse; it also explained the fixed stable position and optimal cell orientation in the gravitational field as a state of mechanical stress of the cytoskeletal elements and those that maintain cell membrane integrity. The cytoskeleton is also considered as an integral unspecialized cell gravireceptor (Tairbekov, 1990). A significant role in stability of the cell’s spatio-temporal organization in the gravitational field and its gravisensing has been attributed to the cytoskeleton in some other concepts on cell gravisensing. These include static stimulation (Sievers et al., 1991), passive gravistimulation (Barlow, 1992), protoplast pressure (Wayne et al., 1990), putative tensegrity (Ingber, 1993), and restrained gravisensing (Baluska and Hasenstein, 1997). The cytoskeleton is known to participate in cytoplasmic streaming and cell organelle motion, mitosis, cytokinesis, endoand exocytosis, as well as in intracellular transport of substances—all activities that are potentially gravitysensitive through the cytoskeleton (Cipriano, 1993). The intracellular cytoskeleton, the extracellular matrix, and the cytoplasmic membrane are assumed to represent compartments of sufficient macromolecular organization to be sensitive to gravity-induced phenomena (Claasen and Spooner, 1994). The cytoskeleton and extracellular matrix are indispensable for cellular and developmental processes that are directly or indirectly linked through the cellmembrane acting as an intermediary. The most useful models for investigating cell gravisensitivity in space flight or on the clinostat are graviperceptive cells with statoliths, e.g. root cap statocytes, alga Chara rhizoids, apical cells of moss protonema. The apical and subapical zones of Chara rhizoids contain thin bundles of microfilaments and the basal zone contains thicker ones. The gravitropically responsive apical part contains statoliths—compartments filled with crystallites of barium sulfate (Sievers et al., 1991). It was concluded that although the arrangement of microtubules is essential for polar cytoplasmic zonation and the functional polar organization of the actin cytoskeleton, it is not involved in the primary events of * Tel: +38 044 212 3236; fax: +38 044 212 3236. E-mail address: cell@svitonline.com (E.L. Kordyum). Cell Biology International 27 (2003) 219–221 Cell Biology International

The Role Of The Cytoskeleton In Plant Cell Gravisensitivity

This chapter highlights current ideas on the role of the cytoskeleton in plant cell gravisensing. It is important to distinguish between cell graviperception and cell gravisensing. The first implies the active perception of a gravitational stimulus by cells which are specialized for gravity perception, and the second refers to cell structure and stability in the gravitational field and their changes in response to microgravity. Special attention is given to the rearrangements of actin microfilaments and tubulin microtubules in multicellular organs as well as in the tip-growing plant cells experiencing microgravity, under clinorotation and gravistimulation. It is assumed that the cytoskeleton takes part both in plant cell graviperception and gravisensing and helps to provide growth stability for plants. The perspectives of cytoskeleton research are outlined.

Clinorotation impacts root apex respiration and the ultrostructure of mitochondria.

Mitochondrial respiration in plants provides energy for biosynthesis, and its balance with photosynthesis determines the rate of plant biomass accumulation. However, there are very limited data on the influence of altered gravity on the functional status of plant mitochondria. In the given paper, we presented the results of our investigations of root respiration, the mitochondrion ultrastructure, and AOX expression of pea 1‐, 3‐ and 5‐day old seedlings grown under slow horizontal clinorotation by using an inhibitor analysis, electron microscopy, and quantitative real‐time RT‐PCR. It was in the first time shown that enhancement of the respiration rate in root apices of pea etiolated seedlings at the 5th day of clinorotation does not connected with increasing of both alternative oxidize capacity and AOX expression. We assumed this phenomenon is provided by more intensive oxidation of respiratory substrates. At the structural level, mitochondria in cells of the distal elongation zone were the most sensitive to clinorotation that confirms the special physiological status of this zone. The performed investigation revealed an enough resistance of plant mitochondria to the influence of altered gravity that, on our opinion, is one of components providing plant adaptation to microgravity in space flight.

Structure and biogenesis of ribonucleoprotein bodies in epidermal cells of Caragana arborescens L

Epidermal cells of leaf petioles, pedicles, and sepals in Caragana arborescens L. are characterized with a unique biogenesis of intracellular bodies, the presence of which continues during 10–12xa0days in spring, from budding till flowering and fruit inception. Initially, a nuclear body is formed as a derivative of the nucleolus at the beginning of elongation of the protodermal cells, whereas a cytoplasmic body is formed in the proximity of the nuclear envelope later. Nuclear bodies and cytoplasmic bodies do not contain DNA, lipids, and starch, and they consist of RNA tightly packaged with proteins mainly in the form of short thin fibrils with thickness of 6xa0nm. By the end of cell elongation and the beginning of differentiation, nuclear bodies disappear, while cytoplasmic bodies become surrounded by a homogenous zone (halo). Later, the bundles of parallel-oriented fibrils derived from the body radially pass through the homogenous zone and gradually disperse in the cytoplasm. In the differentiated epidermal cells, no traces of cytoplasmic bodies are observed; there is only one nucleolus in the nucleus. It is hypothesized that cytoplasmic bodies may function as an RNA depot, which is utilized later in cell metabolism during the formation of fruits and seeds.

Cytoskeleton During Aerenchyma Formation in Plants

Aerenchyma is a plant tissue characterized by prominent intercellular spaces facilitating gas diffusion between roots and the aerial environment. The classical formation of intercellular spaces is thought to be the result of schizogeny and lysogeny during development of wetland species and in some dry‐land species in response to different abiotic stress, including drought, high temperature, and nutrient deficiency. The plant cytoskeleton is known to play a major role in cellular organization and signaling pathways. It is emphasized a lot of ambiguity as to the cytoskeleton function in the constitutive and induced aerenchyma development, especially at the earliest stages of this process. In the present review, we focus on some aspects of the cytoskeleton behavior during the formation of schizogenous and lysigenous aerenchyma in wetland and terrestrial plants growing both in the nature and experimental conditions.