Abstract
Changes in the cell growth rate of an in vitro cellular system in Arabidopsis thaliana induced by short exposure to an altered gravity environment have been estimated by a novel approach. The method consisted of defining three structural nucleolar types which are easy and reliable indicators of the ribosome biogenesis activity and, consequently, of protein biosynthesis, a parameter strictly correlated to cell growth in this cellular system. The relative abundance of each nucleolar type was statistically assessed in different conditions of gravity. Samples exposed to simulated microgravity for 200 min showed a significant decrease in nucleolar activity compared to 1g controls, whereas samples exposed to hypergravity (2g) for the same period showed nucleolar activity slightly increased,. These effects could be considered as an early cellular response to the environmental alteration, given the short duration of the treatment. The functional significance of the structural data was validated by a combination of several different well-known parameters, using microscopical, flow cytometry, qPCR and proteomic approaches, which showed that the decreased cell growth rate was decoupled from an increased cell proliferation rate under simulated microgravity, and the opposite trend was observed under hypergravity. Actually, not all parameters tested showed the same quantitative changes, indicating that the response to the environmental alteration is time-dependent. These results are in agreement with previous observations in root meristematic cells and they show the ability of plant cells to produce a response to gravity changes, independently of their integration into plant organs.
Highlights
Plant development is modulated by environmental factors
The structural study was done on samples obtained from experiments performed in two mechanical devices for generation of altered gravity, namely Random Positioning Machine (RPM) and Large Diameter Centrifuge (LDC) and their respective controls
Altered gravity conditions produce a decoupling of the cell growth and cell proliferation processes in root meristematic cells of seedlings (Matía et al, 2010; Manzano et al, 2012b, 2013)
Summary
Plant development is modulated by environmental factors. Among them, gravity is the unique parameter that has remained constant on Earth throughout the entire history of life. The modification of this parameter would lead to significant physiological changes and would eventually trigger adaptive mechanisms Understanding these changes is important for increasing our basic knowledge on plant physiology, but it is essential for plant space biology, if we consider that gravity is one of the most important factors differentiating the Earth environment from the outer space or exoplanets. Plants will be essential for the coming enterprises of manned space exploration as part of the life support systems required to feed humans during long spaceflights and for survival on exoplanets For this purpose, plants need to be adapted to grow in near-zero gravity (space) and in fractional gravity, e.g., on the Moon, where gravitational acceleration is 1/6g, or on Mars, where gravity is 3/8g
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