Suppression of Cortical Microtubule Reorientation and Stimulation of Cell Elongation in Arabidopsis Hypocotyls under Microgravity Conditions in Space

How microgravity in space influences plant cell growth is an important issue for plant cell biology as well as space biology. We investigated the role of cortical microtubules in the stimulation of elongation growth in Arabidopsis (Arabidopsis thaliana) hypocotyls under microgravity conditions with the Resist Tubule space experiment. The epidermal cells in the lower half of the hypocotyls of wild-type Columbia were longer in microgravity than at on-orbit 1 g, which precipitated an increase in the entire hypocotyl length. In the apical region, cortical microtubules adjacent to the outer tangential wall were predominantly transverse to the long axis of the cell, whereas longitudinal microtubules were predominant in the basal region. In the 9th to 12th epidermal cells (1 to 3 mm) from the tip, where the modification of microtubule orientation from transverse to longitudinal directions (reorientation) occurred, cells with transverse microtubules increased, whereas those with longitudinal microtubules decreased in microgravity, and the average angle with respect to the transverse cell axis decreased, indicating that the reorientation was suppressed in microgravity. The expression of tubulin genes was suppressed in microgravity. These results suggest that under microgravity conditions, the expression of genes related to microtubule formation was downregulated, which may cause the suppression of microtubule reorientation from transverse to longitudinal directions, thereby stimulating cell elongation in Arabidopsis hypocotyls.