Control of Gravitropic Setpoint Angle in higher plant lateral branches

The overall architecture of higher plants is determined by the number and arrangement of lateral branches around the main root-shoot axis. The principal function of these shoot and root branches is to hold leaves and other organs to the sun, and below ground, to facilitate the uptake of nutrients and water, and provide secure anchorage for the plant. Lateral root and shoot branches are often maintained at specific angles with respect to gravity, a quantity known as the gravitropic set-point angle (GSA). While primary root and shoot organs are typically approximately vertical, GSA values of lateral shoots and roots are most often non-vertical, allowing the plant to optimise the capture of resources both above- and below-ground. Despite the importance of branch angle as a fundamental parameter of plant form, until now research has focused on the mechanisms controlling numbers of lateral roots and shoots and studies on gravitropism have been all but confined to the primary root-shoot axis. In this work, the central questions of how stable, straight growth of a branch at a nonvertical angle is maintained and of how the value of that angle is set have been addressed. It was found that non-vertical GSAs of lateral shoots and roots depend upon an angle offset that is the product of latent balancing gravitropic and anti-gravitropic asymmetries in auxin transport and response. The work also showed that auxin specifies GSA values dynamically through development by regulating the magnitude of the anti-gravitropic offset in lateral organs, and further that variation in auxin sensitivity in the gravity-sensing cells of the root and shoot is sufficient to alter GSA. Finally, environmental signals such as light, temperature and nutrient deficiency were found to be able to effect changes in lateral organ GSA through modulation of auxin signalling. The involvement of auxin in regulating GSA is another example of auxin’s spectacular capacity to self-organise in multiple developmental contexts. It also provides a mechanism for integration of environmental signals that alter plant architecture through the modulation of growth angle and a conceptual framework for understanding the specification of GSA throughout higher plants.