The gravitropic set‐point angle (GSA): the identification of an important developmentally controlled variable governing plant architecture^*

Abstract

The angle at which an organ is maintained by gravitropism is characteristic of the organ, its developmental state and the prevailing environmental conditions. We propose that this angle be called the gravitropic set-point angle (GSA), defined as the angle with respect to the gravity vector (with a vertically downward orientation being 0 degrees) at which an organ is maintained as a consequence of gravitropism. Studies or the gravitropic behaviour of organs from trailing plants show that the GSA is subject to developmental regulation. Depending on the developmental age and prevailing environmental conditions, the GSA of an organ can be set at any value between 0 degrees and 180 degrees. The previously reported reversal of the sign of the gravitropic response in such organs, whether this is brought about developmentally or induced by light, represents the change from one common extreme (GSA = 180 degrees, conventionally referred to as negative orthogravitropism) to another (GSA = 0 degrees, or positive orthogravitropism). The concept of a variable gravitropic set-point offers a more unified view of all forms of gravitropic behaviour than has been advanced previously, and places a new constraint on models of gravitropism. Current models of gravitropism appear to be unable to explain either the ability of organs to change their orientation with respect to gravity as they develop, or the re-orientation that can be observed when some organs are exposed to new environmental conditions.

Gravity, light and plant form

“…For a plant to achieve maximum interception of light and gas exchange for photosynthesis, the lateral organs grow out from the main vertical stem at a variety of angles. The angle of growth of the different organs is controlled, at least in part, by the GSA that is a result of gravitropism (Digby & Firn 1995) and subject to light modulation (Myers et al 1994), probably by the action of phytochrome. Light-induced changes in gravitropism of primary shoots may also contribute to the effects of phytochrome on enhancement of phototropic curvature.…”

Shoot and root branch growth angle control—the wonderfulness of lateralness

“…Organs being maintained at angles to the gravity vector will have GSAs between these two values (for example, an organ growing perpendicular to the gravity vector would have a GSA of 90¡) ( Figure 1, Box 1). The GSA concept, introduced by Digby and Firn [1], is a wonderful way of thinking about gravitydependent growth because it distinguishes sharply between growth angle, which is a physical quantity, and gravitropism, which is a growth process. This distinction ensures that GSA and gravitropism are not conflated, bringing an analytical clarity to the task of determining how a particular growth angle might be generated.…”

Effects of mechanostimulation on gravitropism and signal persistence in flax roots

“…24 Memory, although even mathematically modeled, 25 is difficult to characterize especially for gravistimulation, because an organ (e.g., a horizontally reoriented root) perceives the stimulus for an extended time, during which the stimulus diminishes with the sine of the achieved curvature. The complexity of determining the longevity of a stimulus is further compromised by the variability of the gravitropic set point angle 26 and its modulation by other factors such as light. 27 Gravity compensation by clinorotation can be used to distinguish stimulus and response processes because it is possible to separate a pulse of unilateral gravistimulation (angular reorientation) from clinorotation, i.e., gravity-compensated, but omnilateral mechanostimulation.…”