Plant gravitropic response.

The gravitropic response of plants to a change in the gravity vector may be divided. in the phase of induction and expression. During the induction phase the amyloplasts, due to their greater density than the cytoplasmic density, shift their position in less than a minute. During this shift there is an interaction with the endoplasmic reticulum, although a role of actin-like proteins of the cytoskeleton may also be involved in this process. The endoplasmatic reticulum maintains a store of sequestered calcium through the action of an ATP-dependent calcium uptake mediated by the Ca2+, Mg(2+)-ATPase system present in the membrane of this organelle. The interaction of the amyloplast with the endoplasmic reticulum leads to the release of free calcium ions from the endoplasmic store. The increased free Ca2+ level in the cytoplasm may modify the activities of certain enzymes and receptor proteins. The gravitropic induction phase is completed when the lateral polarization of the tissues has taken place. These tissues contain information about changes in direction of the IAA transport system and in competition of the IAA-receptor system for the phytohormone. This information is fixed in "memory" and its expression is achieved when the lateral gradient of IAA concentration and of the activity of the IAA-receptor protein complexes is formed in the horizontally oriented plant organ. Flows of IAA and calcium ions in opposite directions may lead to the expression of laterally differentiated growth.

Evaluation of experiments involving the study of plant orientation and growth under different gravitational conditions.

The manifestation of gravitropic reaction in plants has been considered from the phylogenetic point of view. A chart has been suggested according to which it is supposed that the first indications of the ability to identify the direction of the gravitational vector were inherent in the most ancient eukaryotes, which gave rise to green, brown, yellow-green, golden and diatomaceous algae as well as fungi. The experiments on the role of gravity in plant ontogenesis are being continued. The sum total of the data obtained in a number of experiments in space shows that under these conditions a structurally modified but normally functioning gravireceptive apparatus is formed. The data confirming the modification, under changed gravity, of the processes of integral and cellullar growth of the axial organs of seedlings as well as of the anatomo-morphological structure and developmental rates of plants during their prolonged growth in space are presented. It is assumed that this fact testifies to the presence of systems interacting with gravity during plant ontogenesis. At the same time the necessity for further experiments in order to differentiate an immediate biological effect of gravity from the ones conditioned by it indirectly due to the changes in the behavior of liquids and gases is pointed out. The methodological aspects of biological experiments in space as the main source of reliable information on the biological role of gravity are discussed.

Formation and growth of callus tissue of Arabidopsis under changed gravity.

The changed gravity conditions do not prevent the process of cell dedifferentiation and formation of callus. Yet, callus grown on the clinostat and in space differs from the control one: its consistence is denser, occurence of meristematic centres is less frequent and it shows a reduced proliferative activity of cells. Average size of cell and nuclei area in the test variants is lower than in the control.

Interaction of growth-determining systems with gravity.

The experiments have been carried out with lettuce shoots on board the Salyut-7 orbital station the Kosmos-1667 biological satellite and under ground conditions at 180 degrees plant inversion. By means of the centrifuge Biogravistat-1M the threshold value of gravitational sensitivity of lettuce shoots has been determined on board the Salyut-7 station. It was found to be equal to 2.9 x 10(-3)g for hypocotyls and 1.5 x 10(-4)g for roots. The following results have been received in the experiment performed on board the Kosmos-1667 satellite: a) under microgravity the proliferation of the meristem cells and the growth of roots did not differ from the control; b) the growth of hypocotyls in length was significantly enhanced in microgravity; c) under microgravity transverse growth of hypocotyls (increase in cross sectional area) was significantly increased due to enhancement of cortical parenchyma cell growth. At 180 degrees inversion in Earth's gravity root extension growth and rate of cell division in the root apical meristem were decreased. The determination of DNA-fuchsin value in the nuclei of the cell root apexes showed that inversion affected processes of the cell cycle preceding cytokinesis.

Plant growth, development and embryogenesis during Salyut-7 flight.

The growth and geotropic movements of roots and hypocotyls of lettuce have been studied on board the Salyut 7 station in a stationary position and on the centrifuge at 0.01, 0.1 and 1 g. On the centrifuge at 0.1 and 0.01 g as well as under weightlessness, the final length of hypocotyls was by 8-16% greater than in control plants on the centrifuge at 1 g. The length of roots, however, was reduced by 17% at 0.01 g and under weightlessness; at 0.1 g their growth is much the same as at 1 g. On the Earth, while growing in a vertical position, and in space at 0 < or = g, the roots and hypocotyls deviate from the longitudinal axis of the seed. Average values of deviation eagles on the Earth are always equal to zero, while this is not always the case in space, which indicates the biological effect of microgravity conditions on board a spacecraft. The threshold of geotropic sensitiveness of lettuce hypocotyls, calculated from the linear regression parameters of the dependence of the response geotropic reaction upon the value of the centrifugal force, comprised 2.9 x 10(-3) g. In the Fiton 3 micro-greenhouse under spaceflight conditions, the plants of Arabidopsis thaliana (L) Heynh have, for the first time, undergone a full cycle of individual development. The seeds sown during the flight germinated, performed growth processes, formed vegetative and generative organs and, judging by the final result, they succeeded in fecundation, embryogenesis and ripening. Despite the noted modification of growth and development of plants in space, 42% of formed seeds appeared to be valuable biologically.

The state of gravity sensors and peculiarities of plant growth during different gravitational loads.

In the primary roots of lettuce shoots grown under altered gravitational conditions--180 degrees inversion on the centrifuged clinostat, horizontal clinostat and in dynamic weightlessness--localization of the cellular organelles, cell morphology and peculiarities of growth have been studied. Significant changes took place in the localization of amyloplasts on the horizontal clinostat. The changes of amyloplast position in the cap cells on the horizontal clinostat and under weightlessness are similar. A change of the normal shoot position (180 degrees inversion and horizontal clinostat) causes an inhibition of growth. Weightlessness increases the length of axial organs and cells in the zone of elongation, but decreases the nitotic index in comparison to the centrifuged control. The anlysis of the formation of generative organs has been carried out for Arabidopsis plants grown on board the orbital station Salyut-6. The ability of plants to undergo vegetative growth and to pass through early phases of generative development under weightlessness was confirmed.

Gravity as an obligatory factor in normal higher plant growth and development.

Gravity may influence different aspects of plant activity. The present report deals with two questions: gravity as an ecological factor determining spatial orientation of plant growth; and second, a possible requirement for gravity in the process of normal growth, morphogenesis and generative development of plants.

The development of seedling shoots under space flight conditions.

The assumption that gravity is the major factor in the process of formation of plant polar axis was used as a working basis for the experiment. It was hypothesized by Merkys in 1973 that the effect of gravity related to axial polarity is similar to the process which determines the lateral polarity of shoots under the influence of gravity. There are two possibilities: (i) the development and morphogenesis of shoots takes place directly under the influence of gravity, or (ii) this process, at least during the first growth phases, is determined in the course of the germ development in the seed. In accordance with that assumption, the experiment was carried out in 1973. A special system was used for germinating and cultivating "Pioneer" and "Grybovsky rannyj" peas. The duration of the experiment under flight conditions was 48 hours in darkness, at 20 degrees C. The experimental conditions were the following: 1, the experiment in flight; 2, imitation of flight conditions using the horizontal clinostat; 3, vertical clinostat; 4, control (vertical plants). When the system was brought down to earth, the material was fixed and subjected to morphological and biochemical analysis. On the basis of the analysis, the following conclusion was drawn: during the first growth phases, the morphogenesis of shoots and roots apparently does not change under flight conditions. This conclusion was confirmed by planting those seedlings under earth conditions; normal plants were obtained whose growth and development were similar to the control seedlings. The problem of the influence of changed gravity, or the lack of it, on the growth and development of plants is discussed. The possible role of gravity on the formation of the polarity axes is also discussed from the point of view of generative development and the determination of some peculiarities of morphogenesis.