Symposium I: Tropisms
Abs #
10003: Shape shifting in plants: gravitropism and plant form
Over 100 years ago Darwin pointed out that “Under the influence of gravitation certain [plant] parts are excited to place themselves more or less transversely to the line of its action”. However, because of the ease of working with young seedlings and their pronounced gravity responses, most research on gravitropism has focused on primary roots and shoots, which typically grow vertically. That research has provided tremendous insight into the molecular machinery that controls tropistic responses in the primary root and shoot. Yet, the bulk of a plant consists of lateral organs that do not grow parallel to the gravity vector and which can change their growth orientation during development. Using Arabidopsis as a model, we have found that lateral organs are capable of both positive and negative gravitropism. For example, reorienting a lateral root upward or downward will result in positive or negative curvature, respectively, of the lateral root so that it grows back towards its original orientation. This behavior of lateral parts is consistent with the notion that the orientation is determined in some way by a gravitropic set-point angle (GSA) as described by Firn and Digby (1997 Planta 203: S159-S163). Moreover, the orientation of Arabidopsis lateral roots changes developmentally in a way that appears to be dependent on changes in the GSA. Overall, our studies are showing that Arabidopsis lateral roots, rosette leaves and inflorescence branches display GSA-dependent developmental changes in their orientation. The developmental control of the GSA in lateral organs of Arabidopsis provides a useful system for investigating the regulatory mechanism involved in determining the directionality of tropistic responses. We have isolated novel Arabidopsis mutants that have altered orientations of lateral organs while maintaining normal gravitropism-dependent vertical orientation of the primary axes. These mutants have been tentatively named mgsa mutants, for their modified gravitropic set-point angle. Our investigation of the mgsa mutants show that many aspects of plant architecture are determined to a large extent by developmentally regulated changes in the gravitropic responses of the lateral parts of a plant.
