Meristematic cell proliferation and ribosome biogenesis are decoupled in diamagnetically levitated Arabidopsis seedlings

Abstract

BackgroundCell growth and cell proliferation are intimately linked in the presence of Earth’s gravity, but are decoupled under the microgravity conditions present in orbiting spacecraft. New technologies to simulate microgravity conditions for long-duration experiments, with stable environmental conditions, in Earth-based laboratories are required to further our understanding of the effect of extraterrestrial conditions on the growth, development and health of living matter.ResultsWe studied the response of transgenic seedlings of Arabidopsis thaliana, containing either the CycB1-GUS proliferation marker or the DR5-GUS auxin-mediated growth marker, to diamagnetic levitation in the bore of a superconducting solenoid magnet. As a control, a second set of seedlings were exposed to a strong magnetic field, but not to levitation forces. A third set was exposed to a strong field and simulated hypergravity (2 g). Cell proliferation and cell growth cytological parameters were measured for each set of seedlings. Nucleolin immunodetection was used as a marker of cell growth. Collectively, the data indicate that these two fundamental cellular processes are decoupled in root meristems, as in microgravity: cell proliferation was enhanced whereas cell growth markers were depleted. These results also demonstrated delocalisation of auxin signalling in the root tip despite the fact that levitation of the seedling as a whole does not prevent the sedimentation of statoliths in the root cells.ConclusionsIn our model system, we found that diamagnetic levitation led to changes that are very similar to those caused by real- [e.g. on board the International Space Station (ISS)] or mechanically-simulated microgravity [e.g. using a Random Positioning Machine (RPM)]. These changes decoupled meristematic cell proliferation from ribosome biogenesis, and altered auxin polar transport.

Mechanisms of disruption of meristematic competence by microgravity inArabidopsisseedlings

“…In samples exposed to magnetic levitation, auxin distribution always appeared abnormal at all 3 levels of effective gravity, 12 with a pattern resembling the distribution observed after drug-induced inhibition of polar transport 14 (Fig. 1, first row).…”

“…However, as shown in Figure 1, the effect is maximized in the 0g* position, associated with other alterations in cell proliferation and cell growth parameters; in this position, samples are supporting a synergy between magnetic field and microgravity simulation. 12 Second, diamagnetic levitation acts at the molecular level and depends on diamagnetic properties and density of each material; since the setting of the 0g* position within the magnet corresponds to the diamagnetic levitation of water, there will be different materials in the cells which will not levitate in these conditions. Specifically, this is the case of starch granules which are the main component of statoliths, the starch-based organelles in the columella cells responsible of the gravitropism.…”

“…whole mount preparation of roots were stained and observed by light microscopy. in the upper row, microscopical images of DR5::GUS-stained root meristems from seedlings grown for 4 d in the magnetic levitation facilities; 12 From left to right: samples from the 1g external control, 0g*, 1g*, and 2g* positions in the magnet. in the lower row, the same approach was used in experiments using mechanical/inertial facilities for altered gravity simulation.…”

“…some more complexity, have recently been published. 12 The novel aspects were the use of the transgenic line CYCB1::GUS, allowing the in situ detection of the expression of the cyclin B1 gene, and the sequential character of the study, comprising 2 sampling points in the seedling development, namely at two and 4 days after seed hydration. Apart from this, the magnet architecture and the properties of the applied magnetic field allowed us the use of 3 different positions for placing the samples, each one characterized by a different level of effective gravity (g*).…”

Suppression subtractive hybridization library construction and identification of epidermal bladder cell related genes in the common ice plant, Mesembryanthemum crystallinum L.

“…Ribosomal biogenesis is necessary for the production of proteins required for cell proliferation and growth (Manzano et al, 2013). MW31 (McNSA2) transcript abundance was differentially expressed between the wild-type and the EBC-less mutant (Figure 1).…”