Auxin-mediated Gravitropism of Arabidopsis thaliana

Gravitropism is an important tropic response in plants that allows the ability to respond to changes in orientation and to modify growth to maintain specific patterns of growth with respect to gravity. Gravitropism is controlled by a group of hormones called auxins. There are three steps that contribute to the response: the perception of the gravity signal, the differential flow of auxin, and the transcriptional control of auxin related genes. The so-called sine law of gravitropism states that the magnitude of a plants’ gravity response is proportional to the sine of the angle between the organ axis and the vertical. This model has since been described in many species, but the molecular basis of the mechanism remains unclear. Using kinetics experiments, auxin-mediated gravity response in Arabidopsis roots was confirmed to be angle-dependent. The auxin reporter R2D2 was used to quantify previously unreported angle-dependent auxin asymmetries that likely govern gravity response in primary and lateral roots. The subcellular localisation of auxin efflux carriers, known as PINs, were quantified in the gravity-sensing cells of primary roots. It was found that as the angle of stimulation increases, PINs are targeted more to the basal plasma membrane. Evidence for angle-specific contributions of PINs was also found. Other components of gravity response were also considered. The role of auxin signalling in the gravity-perception and response in Arabidopsis roots and shoots was investigated using gravity-sensing cell- and epidermal cell-specific promoters expressing mutated versions of Aux/IAA and ARF auxin signalling regulators. Gravitropism assays with these transgenic lines indicate that auxin signalling is necessary in the gravity-sensing cells of the primary root, however, its exact role is still unclear.