Gravity control of growth form in Brassica rapa and Arabidopsis thaliana (Brassicaceae): Consequences for secondary metabolism

Abstract

How gravity influences the growth form and flavor components of plants is of interest to the space program because plants could be used for food and life support during prolonged missions away from the planet, where that constant feature of Earth's environment does not prevail. We used plant growth hardware from prior experiments on the space shuttle to grow Brassica rapa and Arabidopsis thaliana plants during 16-d or 11-d hypergravity treatments on large-diameter centrifuge rotors. Both species showed radical changes in growth form, becoming more prostrate with increasing g-loads (2-g and 4-g). In Brassica, height decreased and stems thickened in a linear relationship with increasing g-load. Glucosinolates, secondary compounds that contribute flavor to Brassica, decreased by 140% over the range of micro to 4-g, while the structural secondary compound, lignin, remained constant at ∼15% (w/w) cell wall dry mass. Stem thickening at 4-g was associated with substantial increases in cell size (47%, 226%, and 33% for pith, cortex, and vascular tissue), rather than any change in cell number. The results, which demonstrate the profound effect of gravity on plant growth form and secondary metabolism, are discussed in the context of similar thigmostresses such as touch and wind.

Role of the plant cell wall in gravity resistance

“…Therefore, basipetal hypergravity produced by centrifugation has mainly been used for the analysis of gravity resistance Hoson et al, 2005), the same as for gravitropism (Hodick and Sievers, 1998;Fitzelle and Kiss, 2001). Hypergravity generally suppresses elongation growth but promotes the lateral expansion of plant organs (Soga et al, 2006;Allen et al, 2009). The effects of hypergravity on the mechanical properties of the cell wall have been analyzed in the stem organs and roots of various plant materials.…”

Rachis Lignification of Couroupita guianensis Aubl.

“…It may have caused the deactivation of the enzyme complex activity that regulates the rachis lignification. This was observed in plants which grow in microgravity systems, showing changes in the lignification process (Allen et al, 2009).…”

“…The influence of gravity in the lignification process was reported from experiments conducted under microgravity conditions (Stutter et al, 2006). Some laboratory experiments portraying conditions that simulate the gravity effect demonstrated the consequent accumulation of lignin in cell walls (Allen et al, 2009).…”

Effects of Altered Gravity Conditions on Lignin and Secondary Wall Formation in Herbaceous Dicots and Woody Plants

“…They also examined mechanical properties of the isolated secondary cell wall preparation and showed that its extensibility decreased in response to hypergravity. Morphometric analysis of Brassica rapa stem tissue confirmed that the areas of pith, cortex, and vascular tissue all increased at 4-G over the 16-d treatment (Allen et al, 2009). Vessel walls of cotyledons were significantly thinner in soybean seedlings grown in space than in 1-G conditions (P< 0.05) and the orientation of cellulose microfibrils, which are composed of 1-4-linked β-D-glucan chains and provide structural integrity to the cell walls, and their assembly in developing vessels were perturbed in space at the beginning of wall deposition (de Micco et al, 2008).…”

“…No differences were observed in lignin content of wheat leaves grown in µ G or 1 G (Stutte et al, 2006). Allen et al (2009) recently demonstrated that lignin remained constant over the micro-to 4-G range in stems of Brasica rapa. These recent flight experiments may tend to show no significant differences in lignin content, which might be due to well-ventilated systems employed in microgravity experiments, as suggested by Stutte et al (2006).…”