plants Article Analysis of Graviresponse and Biological Effects of Vertical and Horizontal Clinorotation in Arabidopsis thaliana Root Tip Alicia Villacampa 1 , Ludovico Sora 1,2 , Raúl Herranz 1 , Francisco-Javier Medina 1 and Malgorzata Ciska 1,* 1 Centro de Investigaciones Biológicas Margarita Salas-CSIC, Ramiro de Maeztu 9, 28040 Madrid, Spain;
[email protected] (A.V.);
[email protected] (L.S.);
[email protected] (R.H.);
[email protected] (F.-J.M.) 2 Department of Aerospace Science and Technology, Politecnico di Milano, Via La Masa 34, 20156 Milano, Italy * Correspondence:
[email protected]; Tel.: +34-91-837-3112 (ext. 4260); Fax: +34-91-536-0432 Abstract: Clinorotation was the first method designed to simulate microgravity on ground and it remains the most common and accessible simulation procedure. However, different experimental set- tings, namely angular velocity, sample orientation, and distance to the rotation center produce different responses in seedlings. Here, we compare A. thaliana root responses to the two most commonly used velocities, as examples of slow and fast clinorotation, and to vertical and horizontal clinorotation. We investigate their impact on the three stages of gravitropism: statolith sedimentation, asymmetrical auxin distribution, and differential elongation. We also investigate the statocyte ultrastructure by electron microscopy. Horizontal slow clinorotation induces changes in the statocyte ultrastructure related to a stress response and internalization of the PIN-FORMED 2 (PIN2) auxin transporter in the lower endodermis, probably due to enhanced mechano-stimulation. Additionally, fast clinorotation, Citation: Villacampa, A.; Sora, L.; as predicted, is only suitable within a very limited radius from the clinorotation center and triggers Herranz, R.; Medina, F.-J.; Ciska, M.