Stimulation of elongation growth and xyloglucan breakdown in Arabidopsis hypocotyls under microgravity conditions in space

Seedlings of Arabidopsis thaliana (L.) Heynh. (ecotype Columbia and an ethylene-resistant mutant etr1-1) were cultivated for 68.5, 91.5 and 136 h on board during the Space Shuttle STS-95 mission, and changes in the elongation growth and the cell wall properties of hypocotyls were analyzed. Elongation growth of dark-grown hypocotyls of both Columbia and etr1-1 was stimulated under microgravity conditions in space. There were no clear differences in the degree of growth stimulation between Columbia and etr1-1, indicating that the ethylene level was not abnormally high in the cultural environment of this space experiment. Microgravity also increased the mechanical extensibility of cell walls in both cultivars, and such an increase was attributed to the increase in the apparent irreversible extensibility. The levels of cell wall polysaccharides per unit length of hypocotyls decreased in space. Microgravity also reduced the weight-average molecular mass of xyloglucans in the hemicellulose-II fraction. Also, the activity of xyloglucan-degrading enzymes extracted from hypocotyl cell walls increased under microgravity conditions. These results suggest that microgravity reduces the molecular mass of xyloglucans by increasing xyloglucan-degrading activity. Modifications of xyloglucan metabolism as well as the thickness of cell wall polysaccharides seem to be involved in an increase in the cell wall extensibility, leading to growth stimulation of Arabidopsis hypocotyls in space.     

Plant growth processes in Arabidopsis under microgravity conditions simulated by a clinostat.

The life cycle of Arabidopsis plants was examined by growing them on a horizontal clinostat. Seeds on agar media were allowed to germinate and seedlings were grown under a simulated microgravity on a horizontal clinostat. Clinorotation (3 rpm) did not appear to interfere with germination of plant seeds and development of cotyledons and leaves. Stress relaxation parameters of the cell wall, the minimum relaxation time and the relaxation rate did not appear to be affected by clinostat rotation. On the other hand, the length of inflorescences was reduced to 61-62% by clinostat rotation. Rotation was found to inhibit the polar transport of auxin, although inflorescence growth and auxin transport were not completely inhibited. From these facts, it is possible that the life cycle in Arabidopsis plants could be accomplished in space, although growth phenomena involving auxin transport and its action may be disturbed. Plants may have a capacity to grow in space and we may be able to cultivate crops in space.     

Modification of growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls grown under microgravity conditions in space

We carried out a space experiment, denoted as Aniso Tubule, to examine the effects of microgravity on the growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls, using lines in which microtubules are visualized by labeling tubulin or microtubule‐associated proteins (MAPs) with green fluorescent protein (GFP). In all lines, GFP‐tubulin6 (TUB6)‐, basic proline‐rich protein1 (BPP1)‐GFP‐ and spira1‐like3 (SP1L3)‐GFP‐expressing using a constitutive promoter, and spiral2 (SPR2)‐GFP‐ and GFP‐65 kDa MAP‐1 (MAP65‐1)‐expressing using a native promoter, the length of hypocotyls grown under microgravity conditions in space was longer than that grown at 1 g conditions on the ground. In contrast, the diameter of hypocotyls grown under microgravity conditions was smaller than that of the hypocotyls grown at 1 g. The percentage of cells with transverse microtubules was increased under microgravity conditions, irrespective of the lines. Also, the average angle of the microtubules with respect to the transverse cell axis was decreased in hypocotyls grown under microgravity conditions. When GFP fluorescence was quantified in hypocotyls of GFP‐MAP65‐1 and SPR2‐GFP lines, microgravity increased the levels of MAP65‐1, which appears to be involved in the maintenance of transverse microtubule orientation. However, the levels of SPR2 under microgravity conditions were comparable to those at 1 g. These results suggest that the microgravity‐induced increase in the levels of MAP65‐1 is involved in increase in the transverse microtubules, which may lead to modification of growth anisotropy, thereby developing longer and thinner hypocotyls under microgravity conditions in space.     

Morphogenic effects of abiotic stress: reorientation of growth in Arabidopsis thaliana seedlings

Abiotic stress responses include changes in physiological and biochemical processes as well as morphological and developmental patterns. It has remained an enigma which mechanisms are responsible for stress-induced morphogenesis. In this paper we demonstrate that stress induced phenotypes comprise a re-orientation rather than a cessation of growth. Moreover, strong similarities between the phenotypes induced by excess copper, paraquat, salicylic acid and a hydrogen peroxide analogue, indicate that a common molecular-physiological response system mediates these morphogenic stress responses. It is proposed that reactive oxygen species play a key role in controlling the architectural changes in stressed Arabidopsis thaliana seedlings.We found that phenotypes of plants exposed to stress resemble, in terms of the redistribution of growth, plants altered in phytohormone metabolism. We also found that plants in which polar auxin transport is blocked with TIBA, strongly resemble, but are not identical to, plants exposed to abiotic stress. Based on the stress induced formation of lateral roots, we surmise that stress induces local auxin accumulation near the root pericycle.     

Effects of dichromate on growth and root system architecture of Arabidopsis thaliana seedlings

The hexavalent form of chromium [Cr(VI)] is toxic for most organisms; however, very little information is available regarding the effects of this metal on plant morphogenesis. In this work, we investigated the effects of Cr(VI) on the growth and development of Arabidopsis thaliana, a species widely used as a model for studying the diverse physiological and cellular processes in plants. Elongation of root hairs and biomass production were stimulated by relatively low concentrations (100 μM) of Cr(VI) as potassium dichromate. Concentrations of Cr(VI) greater than 200 μM were toxic to plants as revealed both by arrested growth of roots and shoots and the development of chlorosis in leaves. At 200 μM the primary root growth was totally inhibited but the plants continued their growth manifesting different alterations in root development. These alterations correlated with changes in mitotic activity and in cellular expansion. The analyses of A. thaliana transgenic plants that express the auxin-inducible marker DR5:uidA, and the response of the auxin-resistant mutants axr2 and aux1–7 to dichromate suggest that auxins do not participate as mediators in the cellular and physiological responses to this metal. The primary root growth inhibition by 200 μM dichromate was alleviated by more than 70% by increasing the sulfate, phosphate or nitrate concentration in the media, which suggests a relation of dichromate with these mineral nutrients.     

The response of growth rate to temperature in seedlings of several Allium crop species

Relative growth rates (R_w), leaf area ratios (LAR), net assimilation rates (NAR), and leaf weight ratios (LWR) were estimated for seedlings of onion, leek, Japanese bunching onion and chives grown at constant temperatures of 10, 12, 15, 19, 23, 27 and 31 C. R_w was greatest at 19 C in Japanese bunching onion and chives, and at 27 C in onions and leeks. R_w increased approximately linearly between 10 and 19 C in all species; between 19 and 27 C it either increased further, decreased or remained constant depending on species but at 31 C R_w was lower than the maximum in all cases. LAR's were greatest at 27 C in onions and leeks and at 23 C in the other two species. Between 10 and 19 C both LAR and NAR increased in all species. Between 19 and 27 C increases in LAR were compensated for by decreases in NAR with the result that R_w showed little change with temperature. Averaging over all temperatures the species were ranked in order of decreasing R_w, NAR and LWR as onion, leek, Japanese bunching onion and chives. For LAR this ordering was reversed with respect to onions and leeks. Plant to plant variability increased in the order onion, leek, Japanese bunching onion then chives. Although onions were the fastest growing of the Alliums tested their R_w was lower than that of several other vegetable species for which R_w was similarly measured.     

Dynamics of root growth in microgravity

An experiment to study the growth of garden cress roots in microgravity is described. The experiment, denoted RANDOM, was an ESA Biorack experiment in the IML-2 flight in July 1994. In the absence of gravity, it can be anticipated that the roots would show random growth, changing their direction randomly. The hypothesis that such random growth movements occur according to random walk theory, leads to predictions as to the detailed manner in which deviations increase with time. The experiment was designed to test this random walk hypothesis. The paper concentrates on the technological aspects of studying the roots in microgravity. The development of suitable plant chambers, fitting containers developed by ESA, is described as well as the techniques used to grow the seeds between agar slices. hardware was developed to record photographically root movements between the agar slices. Photos were taken once per hour. Some plant chambers were designed to allow fixation of plant material in space. The practical solutions found using glutaraldehyde for prefixation in the Spacelab, within the restrictions given, are described. The experimental results show that the growth pattern in fact followed the prediction from the random walk approach. The average changes in the growth direction stayed constant and equal to zero during the experiment while the squared angular deviations increased proportional to time. Furthermore, plant material prefixed in orbit was permanently fixed after the flight. Light microscopy and electron microscopy pictures are shown as examples of the results achieved. The long prefixation period meant a drawback for the quality of the fixation process. However, sections suitable for study were achieved. The main goals of the RANDOM experiment were therefore achieved.     

Interference of in vitro and in vivo growth of several intestinal bacteria by Lactococcus strains

The ability of Lactococcus strains to inhibit the growth of intestinal bacteria was examined. In in vitro cocultures, we observed that among eighteen Lactococcus strains tested, the ability to inhibit growth of Escherichia coli varied, with the L. lactis N7 showing the greatest growth inhibition. Strain N7 (8.94x1010 CFU/day for 7 days) was orally administered to mice, and the viable count of strain N7 in feces appeared at a level of 104-5 CFU/g. After administration, the proportion of Bacteroidaceae to total intestinal bacteria decreased. Lactococci may act as probiotic bacteria by inhibiting the growth of harmful bacteria.     

铜胁迫对拟南芥幼苗生长和基因组DNA甲基化的影响

通过Murashige and Skoog(MS)培养实验,利用甲基化敏感扩增多态性(methylation-sensitive amplified polymorphism,MSAP)技术研究Cu2+胁迫对拟南芥幼苗基因组DNA甲基化水平与甲基化模式的变化,同时比较其与幼苗鲜重、根系生长对Cu2+胁迫的敏感性。结果表明:0、0.25、1.0 mg獉L-1Cu2+处理15 d后,幼苗根长及鲜重变化差异不显著,而幼苗基因组MSAP率随Cu2+浓度的增加呈先增高后降低的趋势,分别为15.93%、16.28%和15.83%;高浓度Cu2+胁迫下(3.0 mg獉L-1),根长显著变短,鲜重显著降低,MSAP率为14.26%;Cu2+胁迫(0.25~3.0 mg獉L-1)下,拟南芥幼苗基因组超甲基化(M型)位点及去甲基化(D型)位点数均呈显著增加趋势,Msp I酶较Hpa II酶对胁迫反应更敏感。因此,拟南芥幼苗MSAP变化对低浓度Cu2+胁迫响应敏感,可作为Cu污染的早期诊断和生态风险评价。     

Arabidopsis thaliana sku mutant seedlings show exaggerated surface-dependent alteration in root growth vector.

Roots of wild-type Arabidopsis thaliana seedlings in the Wassilewskija (WS) and Landsberg erecta (Ler) ecotypes often grow aslant on vertical agar surfaces. Slanted root growth always occurs to the right of the gravity vector when the root is viewed through the agar surface, and is not observed in the Columbia ecotype. Right-slanted root growth is surface-dependent and does not result directly from directional environmental stimuli or gradients in the plane of skewing. We have isolated two partially dominant mutations in WS (sku1 and sku2) that show an exaggerated right-slanting root-growth phenotype on agar surfaces. The right-slanting root-growth phenotype of wild-type and mutant roots is not the result of diagravitropism or of an alteration in root gravitropism. It is accompanied by a left-handed rotation of the root about its axis within the elongation zone, the rate of which positively correlates with the degree of right-slanted curvature. Our data suggest that the right-slanting root growth phenotype results from an endogenous structural asymmetry that expresses itself by a directional root-tip rotation.     

Morphogenesis of rice and Arabidopsis seedlings in space.

Seedlings of rice (Oryza saliva L.) and Arabidopsis (A. thaliana L.) were cultivated for 68.5 hr in the RICE experiment on board during Space Shuttle STS 95 mission, and changes in their growth and morphology were analyzed. Microgravity in space stimulated elongation growth of both rice coleoptiles and Arabidopsis hypocotyls by making their cell walls extensible. In space, rice coleoptiles showed an inclination toward the caryopsis in the basal region and also a spontaneous curvature in the same direction in the elongating region. These inclinations and curvatures were more prominent in the Koshihikari cultivar compared to a dwarf cultivar, Tan-ginbozu. Rice roots elongated in various directions including into the air on orbit, but two thirds of the roots formed a constant angle with the axis of the caryopsis. In space, Arabidopsis hypocotyls also elongated in a variety of directions and about 10% of the hypocotyls grew into the agar medium. No clear curvatures were observed in the elongating region of Arabidopsis hypocotyls. Such a morphology of both types of seedlings was fundamentally similar to that observed on a 3 D clinostat. Thus, it was confirmed by the RICE experiment that rice and Arabidopsis seedlings perform an automorphogenesis under not only simulated but also true microgravity conditions.