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Plant Growth Strategies Are Remodeled by Spaceflight

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Plant Growth Strategies Are Remodeled by Spaceflight

Anna-Lisa Paul et al. BMC Plant Biol.

Abstract

Background: Arabidopsis plants were grown on the International Space Station within specialized hardware that combined a plant growth habitat with a camera system that can capture images at regular intervals of growth. The Imaging hardware delivers telemetric data from the ISS, specifically images received in real-time from experiments on orbit, providing science without sample return. Comparable Ground Controls were grown in a sister unit that is maintained in the Orbital Environment Simulator at Kennedy Space Center. One of many types of biological data that can be analyzed in this fashion is root morphology. Arabidopsis seeds were geminated on orbit on nutrient gel Petri plates in a configuration that encouraged growth along the surface of the gel. Photos were taken every six hours for the 15 days of the experiment.

Results: In the absence of gravity, but the presence of directional light, spaceflight roots remained strongly negatively phototropic and grew in the opposite direction of the shoot growth; however, cultivars WS and Col-0 displayed two distinct, marked differences in their growth patterns. First, cultivar WS skewed strongly to the right on orbit, while cultivar Col-0 grew with little deviation away from the light source. Second, the Spaceflight environment also impacted the rate of growth in Arabidopsis. The size of the Flight plants (as measured by primary root and hypocotyl length) was uniformly smaller than comparably aged Ground Control plants in both cultivars.

Conclusions: Skewing and waving, thought to be gravity dependent phenomena, occur in spaceflight plants. In the presence of an orienting light source, phenotypic trends in skewing are gravity independent, and the general patterns of directional root growth typified by a given genotype in unit gravity are recapitulated on orbit, although overall growth patterns on orbit are less uniform. Skewing appears independent of axial orientation on the ISS - suggesting that other tropisms (such as for oxygen and temperature) do not influence skewing. An aspect of the spaceflight environment also retards the rate of early Arabidopsis growth.

Figures

Figure 1
Figure 1
Novel imaging hardware records distinctive patterns of growth for spaceflight and ground control plants. The GFP Imaging System (GIS) is unique spaceflight hardware that is designed to be housed within the Advanced Biological Research System (ABRS) orbital growth chamber (A). The GIS contains six slots that accommodate 10 cm2 petri plates, three in an upper tier and three in a lower tier (B). The middle lower tier plate is positioned directly in front of the GIS camera and collects a set of images every 6 hours and includes an image-unique time-stamp; examples shown are from the initiation of the experiment (C) and at the end (D). Time stamps for these images are in the lower left corner. Time stamps are in the convention of year_month_day_hour_minute, thus, 1C, 2010_02_11_03_37, corresponds to 2010, February 11, 03:37 and 2010_02_26_09_39 to 2010, February 26, 09:39. The proximity to the LED light source influences the patterns of root growth between lower tier plates (E) and upper tier plates (F).
Figure 2
Figure 2
Quantification of growth patterns with overlays provides information on the growth rates and habit. Root growth patterns of plants 8.5 days old from the ground control (A) and flight experiment (B) were quantified with mapping options in Adobe Illustrator CS3. Numerical values were assigned to both the absolute distance grown and the degree of deviation from the vertical (C) which creates an overlay of data containing information on the growth rates and habit (detail in D).
Figure 3
Figure 3
Plants on orbit grew more slowly than comparable ground controls. Root growth patterns of plants 8.5 days old from the ground control (A) and flight experiment (B) are shown with mapping overlays. Dotted line marks division between cultivar WS and Col-0. A colored trace was made at each 6 hour increment of growth as each plant grew, out to 8.5 days (A, B). Traces alternated red and blue for clarity. Sections rendered in orange and light blue represent extrapolations for missing individual photographs. Numerical values were calculated for each root and hypocotyl length (grid=13mm), and then the average values for each set of cultivars plotted with respect to treatment (C). Standard deviation was calculated using "n-1" method: Ground control (GC-green bars) - WS: n=11, StDev roots=0.91, StDev hypocotyls=0.70, Col-0: n=7, StDev roots=1.00, StDev hypocotyls=0.60; Flight (FLT-blue bars) - WS: n=13, StDev roots=0.67, StDev hypocotyls=0.80, Col-0: n=7, StDev roots=0.67, StDev hypocotyls=0.40. (D) Traced sections enlarged and the region representing the first 48hours of discernible growth highlighted with a gold bracket. (E) The meristematic zone in roots of ground control (GC) and flight (FLT) plants. Epidermis, cortex, endodermis and quiescent center (QC) are indicated in blue arrows. The gold line along the epidermis cell file from the quiescent center measures 225μm. (F) The elongation zone in roots of ground control and flight plants. The blue frame defines a region of about 225μm along the length of each root from the transition zone into the elongation zone. The cross bars in the rectangle define cell boarders in this area for each root.
Figure 4
Figure 4
Plants on orbit grew skew and wave, but in a novel way. Enlargements of 5 day old WS ground control and flight plants to show near vertical root growth (A, B) compared to the waving behavior of 5 day old WS plants grown on 45 degree inclined agar plates in unit gravity (C). Enlargements of 8.5 day old WS and Col-0 ground control and flight plants to show skewing and waving patterns of root growth (D, E) compared to the waving behavior of 8.5 day old WS and Col-0 plants grown on 45 degree inclined agar plates in unit gravity (F).
Figure 5
Figure 5
Quantification and mapping of growth patterns. The direction of root growth over time was quantified by taking measurements of the angle of a root segment in relation to a vertical line, with right being positive degrees and left as negative degrees. Values for each plant cultivar in ground control (A) and flight (B) in each image, spanning growth through 8.5 days, were averaged and then plotted with Microsoft Excel (see Methods for operational details). The two different cultivars represented on the imaging plate are WS (blue line) and Col-0 (green line). The y-axis shows the degree of deviation from the vertical the root presents at each 6 hour time point and the x-axis shows the corresponding dates the images were taken. Each cultivar plot line is an average taken from measurements of several roots: GC WS: 8 roots; GC Col-0: 3 roots; FLT WS: 7 roots; FLT Col-0: 4 roots. The error bars included with each time point reflect the Standard Error of the Mean.
Figure 6
Figure 6
The influence of micro-habitat on plant growth patterns. The GIS is configured to hold six 10 cm2 petri plates in two tiers, numbered sequentially: bottom tier plates are numbered 1 (imaging plate opposite the camera), 3 and 5; upper tier plates are numbered 2 (directly over plate 1), 4 and 6. Air is circulated by entering the base just below plate 3 and exiting just below plate 5 (A). Photographs taken of the flight plants when the plates were removed from the GIS for harvest on orbit are shown in from the upper tier (Plates 4, 2 and 6) and the lower tier (plate 1 and plate 5) (B). Note that not all plates were harvested on orbit, and that lower tier harvest plate images are limited.

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