The dose–response curve of the gravitropic reaction: a re‐analysis

Abstract

The dose-response curve of the gravitropic reaction is often used to evaluate the gravisensing of plant organs. It has been proposed (Larsen 1957) that the response (curvature) varies linearly as a function of the logarithm of the dose of gravistimulus. As this model fitted correctly most of the data obtained in the literature, the presentation time (tp, minimal duration of stimulation in the gravitational field to induce a response) or the presentation dose (dp, minimal quantity in g.s of stimulation to induce a response) were estimated by extrapolating down to zero curvature the straight line representing the response as a function of the logarithm of the stimulus. This method was preferred to a direct measurement of dp or tp with minute stimulations, since very slight gravitropic response cannot be distinguished from the background oscillations of the extremity of the organs. In the present review, it is shown that generally the logarithmic model (L) does not fit the experimental data published in the literature as well as the hyperbolic model (H). The H model in its simplest form is related to a response in which a ligand-receptor system is the limiting phase in the cascade of events leading to the response (Weyers et al. 1987). However, it is demonstrated that the differential growth, responsible for the curvature (and the angle of curvature), would vary as a hyperbolic function of the dose of stimulation, even if several steps involving ligand-receptor systems are responsible for the gravitropic curvature. In the H model, there is theoretically no presentation time (or presentation dose) since the curve passes through the origin. The value of the derivative of the H function equals a/b and represents the slope of the cune at the origin. It could be therefore used to estimate gravisensitivity. This provides a measurement of graviresponsiveness for threshold doses of stimulation. These results imply that the presentation time (or presentation dose) derived from the L model cannot be used anymore as an estimate of gravisensitivity. On the contrary, the perception time (minimal duration of a repeated stimulation which induces a response), which is less than 1 s, should be related to the perception of gravity. The consequences of these results on the mode of action and the nature of graviperception are discussed.

Effects of mechanostimulation on gravitropism and signal persistence in flax roots

“…Investigations with excess starch mutants indicate that gravisensing is not saturated 41 and studies of the presentation time show that much shorter reorientation times than signal saturation are sufficient to induce curvature and suggest that amyloplast sedimentation is not necessary for the initiation of gravitropism. 40 While earlier studies focused on the detection limit (presentation time) that is sufficient to induce curvature, 28,35 this study demonstrates saturation of the gravity stimulus likely coincides with the amyloplast sedimentation. Studies on gravity-induced gene expression, hormone redistribution and signal transduction should use this time as reference point for maximal effects of a (single) acceleration or reorientation stimulus.…”

“…28,29 Despite the usefulness of a clinostat as a gravity compensator, it introduces its own problems such as the destruction of columella cells in the root cap. 30,31 Clinostatinduced complications are likely to interfere with the original gravistimulus and sensed and probably processed as mechanostimulation that is superimposed to the gravity stimulus and response.…”

The transparent testa4 Mutation Prevents Flavonoid Synthesis and Alters Auxin Transport and the Response of Arabidopsis Roots to Gravity and Light[W]

“…Despite the early differences in tt4(2YY6) root gravity responses, gravity perception appears unhampered in this mutant. Examination of the presentation time and the gravity sensitivity, two estimates of the minimum time of gravity stimulation that will induce gravitropic bending, were obtained from two graphical methods (Larsen, 1957;Perbal et al, 2002). Both methods indicated that the tt4(2YY6) mutant is not impaired in gravity perception.…”

“…The clinostat randomizes the gravity vector by rotating the samples continuously at 1g, allowing additional gravity-independent elongation, to assess the response to the initial gravity stimulus. The angle of gravitropic curvature change was measured after clinorotation and plotted using the linear-log model (Larsen, 1957) and the hyperbolic method (Perbal et al, 2002) to estimate the minimum time likely to induce bending and determine gravisensitivity, respectively. These graphs are included as supplemental data online.…”

Analyses of a Gravistimulation-Specific Ca2+ Signature in Arabidopsis using Parabolic Flights

“…This constant 1g background poses significant challenges for defining the temporal sensitivity of the plant gravisensing machinery. In previous studies, the minimum duration of gravistimulation needed to induce a detectable response, "the presentation time" for gravitropic responses (Perbal and DrissEcole, 2003), has been estimated indirectly (Perbal et al, 2002) and is probably less than 10 s in Lepidium sativum roots (Hejnowicz et al, 1998).…”