A pump for the auxin fountain: AUX1 and root gravitropism

Abstract

Roots generally grow downwards. If a plant topples over, its roots will bend during their subsequent growth and reorient themselves vertically. This process is known as gravitropism, and is divided into three distinct phases. The first phase is the perception of gravity by the columella cells in the root cap. These cells are filled with heavy, starch-filled organelles called amyloplasts, which sediment in a gravitational field and define the direction of root growth. In the second phase, the amyloplast sedimentation is converted into a biochemical signal, which is transmitted to the root distal elongation zone (DEZ). Auxin arrives at the root apex from the shoot via the phloem. The asymmetrical redistribution of auxin back towards the DEZ through the outer cell-layers of the root is thought to constitute this signal. In the third phase, the auxin gradient generated laterally across the root in the DEZ results in differential cell elongation and root bending. The controlled acropetal (to the root apex) and basipetal (toward the DEZ) redistribution of auxin is described as the ‘auxin fountain’ hypothesis.Ranjan Swarup and his European colleagues [1xLocalization of the auxin permease AUX1 suggests two functionally distinct hormone transport pathways operate in the Arabidopsis root apex. Swarup, R. et al. Genes Dev. 2001; 15: 2648–2653Crossref | PubMed | Scopus (368)See all References][1] have investigated the role of the auxin influx transporter (AUX1) in the ‘auxin fountain’. As the aux1 mutant shows an agravitropic root phenotype, and root gravitropism is intimately linked to auxin redistribution within the root, their experiments also provide an insight into the processes involved in root gravitropism. They make three pertinent observations:First, they observe that the AUX1 protein and the auxin efflux carrier protein (AtPIN1) are located in the plasma membrane on opposite sides of protophloem cells. They speculate that the juxtaposition of AUX1 on the upper surface, and AtPIN1 on the lower surface of the protophloem cells is required for polar (acropetal) auxin transport to the root apex. This is consistent with the observation that roots lacking AUX1 do not deliver auxin to the root apex, and suggests that AUX1 is intimately involved in auxin transport to this region.Second, they observe that the AUX1 protein is present in the columella cells most responsive to gravity. In these cells, AUX1 most frequently has an intracellular location but, in some cells, it is present on the plasma membrane. It is tempting to speculate that those cells in which AUX1 is present in the plasma membrane are involved in the differential basipetal redistribution of auxin to the DEZ.Third, they observe that the AUX1 protein is present in the cells of the lateral root cap (LRC) overlying the DEZ where gravitropic root curvature is initiated. High (intracellular) auxin concentrations inhibit root elongation. The AUX1 protein is located evenly over the plasma membrane of the LRC cells, suggesting that it has a role in auxin uptake by these cells but not in polar transport. In the roots of wild-type plants, auxin is elevated throughout the DEZ, but in the aux1 mutant it is absent. This observation is consistent with the mutation disrupting auxin transport to this region. This could be a consequence of either impaired acropetal transport to the root apex or impaired basipetal redistribution to the DEZ.Thus, Swarup and colleagues identify the AUX1 protein as not only the pump in the protophloem required for the ‘auxin fountain’, but also as a likely contributor to both the basipetal redistribution of auxin and to the sensitization of the DEZ to auxin. By expressing AUX1 in specific cell-types within the root of the aux1 mutant, they hope to elucidate these three discrete roles further.