Often used as a proxy for the transition to reproduction, flowering time (FT) is an integrative trait of two successive biological processes, bolting time (BT) and the interval between bolting and flowering time (INT). In this study, we aimed to identify candidate genes associated with these composite traits in using a field experiment. Genome-wide association (GWA) mapping was performed on BT, INT and FT based on a sample of 179 worldwide natural accessions genotyped for 216,509 SNPs. The high resolution conferred by GWA mapping indicates that FT is an integrative trait at the genetic level, with distinct genetics for BT and INT. BT is shaped largely by genes involved in the circadian clock whereas INT is shaped by genes involved in both the hormone pathways and cold acclimation. Finally, the florigen appears to be the main integrator of environmental and internal signals in ecologically realistic conditions. Based on FT scored in a previous field experiment, we also studied the genetics underlying reaction norms across two years. Only four genes were common to both years, emphasizing the need to repeat field experiments. The gene regulation model appeared as the main genetic model for genotype × year interactions.

Space experiments permit to understand better some phases of the gravitropic reaction which occurs when the orientation of the root changes in the gravitational field. In gravisensing cells (statocytes in the root cap), the nucleus is attached to the cell periphery, close to the plasma membrane, by actin filaments. The location of the amyloplasts (statoliths) depends also greatly on these elements of the cytoskeleton. A short period in microgravity (5 min.) modifies the location of the nucleus and of the amyloplasts in the statocytes. The tensions exerted by these very dense organelles on the actin network disappear and this network undergoes a relaxation. The kinetics of gravitropic curvature is also better understood. In fact, gravitropic reaction is regulated by a mechanism depending on gravity. In roots grown in space, then stimulated for 1 h on a 1 g centrifuge, and replaced in microgravity, the regulation limiting the curvature does not occur. It is hypothesized that the sedimentation of the amyloplasts on the endoplasmic reticulum placed at the basal pole of the statocytes could be responsible for this regulation. The contacts between these two organelles should have also a role in root growth. This hypothesis will be tested in our next space experiment (July 94). The experiments in near weightlessness also permit to determine the presentation time which is the duration of stimulation necessary to evoke a slight but significant curvature. Presentation time is 27 s. This short period allows a slight movement of the amyloplasts only (around 0.45 micrometer). The sequence of events leading to the curvature of the root is now well established: the first signal is the separation of the endoplasmic reticulum and the amyloplasts, when the root is subjected to a change in orientation. It is followed by the pressure of these organelles on the actin network which transmits this mechanical effect to the plasma membrane. The transduction of the effect occurs then by the activation of the ions channels (Ca++) and the carrier of a growth inhibitor (auxin), both located in the plasma membrane. This growth inhibitor provokes an asymmetrical growth in the distal part of the meristem and the proximal part of the cell elongation zone. At last, when the root tip reaches the direction of gravity, the amyoloplasts sediment on the endoplasmic reticulum and induce a signal of termination of the curavature.