We have the following objectives:
MORE INFORMATION ON PHYSICAL PRINCIPLES UNDERLYING OUR EXPERIMENT.
We are exploiting heterogeneity of magnetic susceptibility (æ) of plant gravity receptor cells (statocytes) to displace statoliths (dense starch-filled organelles, a.k.a. amyloplasts) inside the cells. Intracellular sedimentation of statoliths is presumed to be the primary step of gravity perception in plants. A possibility to displace them without affecting the rest of the plant by centrifugation or re-orienting in the gravity field is a potent research tool for studying plant graviperception and signal transduction. Statoliths are stronger diamagnetics, than the cytoplasm [1-6]. In a non-uniform magnetic field they are repulsed from stronger field zones by the ponderomotive magnetic force: Fm= (æp-æcp)Vgrad(H2/2), where æp is the magnetic susceptibility of plastids, æcp - that of cytoplasm, grad(H2/2) is the dynamic factor of the magnetic field. If magnetic field is very strong and very non-uniform (so-called high gradient magnetic field [HGMF]), with grad(H2/2)= 109 to 1010 Oe2/cm, this force can be equivalent to the gravity force. In our experiments with several types of magnetic systems [1-10] we were able to displace amyloplasts inside receptor cells in roots [2-6] and shoots [7,8,10] of higher plants, and inside protonemata of the moss Ceratodon purpureus [9]. This displacement (intracellular magnetophoresis) caused positively gravitropic organs (roots of flax and Arabidopsis, wwr-mutant of Ceratodon) to curve in the direction of amyloplast displacement (i.e., away from the stronger field), while negatively gravitropic organs (shoots of tomato and barley, WT of Ceratodon) curved in the direction opposite to the amyloplast displacement. This pattern of the physiological response of the plants is consistent with gravitropism, and the kinetics of the curvature is similar to the gravitropic curvature [1-10]. Starchless mutant of Arabidopsis did not curve in HGMF, indicating, that starch-filled bulk organelles are necessary for the effect, and that other cell components are not significantly affected by the field [4]. This shows, that magnetic ponderomotive forces acting on amyloplasts can simulate gravity for plants.
This approach not only can provide information on physical heterogeneity of the cells and organelles, but also can serve as a research tool for signal transduction in plants. Such manipulation of amyloplasts is likely to answer some of the basic questions of the sequence of events in gravisensing and a possible involvement of the cytoskeleton in graviperception. We can also expect answers as to whether amyloplast displacement or the force that amyloplasts exert on the ER system leads to the cascade of events that results in curvature. Small size of the area of HGMF with a significant dynamic factor allows to stimulate only a small portion of receptor cells of a bigger plant organ, allowing to study relative sensitivity and importance of different regions of the organ for gravity perception and response [7,8,10], which is impossible to do by any other method.
|
Figure 1. High Gradient Magnetic Field (HGMF) in the vicinity of an edge of a ferromagnetic wedge magnetized by a (uniform) external magnetic field. The density of field lines is proportional to the field intensity, red arrows indicate the direction of force acting on diamagnetic substances. Diamagnetic amyloplasts would move away from the wedge edge and positively gravitropic roots are expected to curve as shown. |
Our publications on the topic.