Auxin control of root development

Abstract

A plant's roots system determines both the capacity of a sessile organism to acquire nutrients and water, as well as providing a means to monitor the soil for a range of environmental conditions. Since auxins were first described, there has been a tight connection between this class of hormones and root development. Here we review some of the latest genetic, molecular, and cellular experiments that demonstrate the importance of generating and maintaining auxin gradients during root development. Refinements in the ability to monitor and measure auxin levels in root cells coupled with advances in our understanding of the sources of auxin that contribute to these pools represent important contributions to our understanding of how this class of hormones participates in the control of root development. In addition, we review the role of identified molecular components that convert auxin gradients into local differentiation events, which ultimately defines the root architecture.

Figure 1.

Cellular organization and the “inverted fountain” of auxin movement in the root tip of Arabidopsis. (A) Magnified view of the meristematic zone of the root tip that highlights the transport-mediated “reflux” of auxin from the lateral root cap through the epidermis to the basal meristem and then back towards the root tip. (B) The proximal–distal organization of the root arises from the iterative process of cell division and expansion of four types of stem cell initials, which are immediately adjacent to the mitotically inactive quiescent center cells: The epidermal/lateral root cap initials, which give rise to the epidermis and lateral root cap, the collumella initials, which contribute to the central portion of the root cap, the cortex/endodermal initials sustain the formation of the ground tissue, and the vascular initials give rise to the vascular tissues and the pericycle. Indicated on the left of the root are the root apical meristem (AM), elongation zone (EZ), differentiation zone (DZ), and the basal meristem (BM). (C) Cross section of root in the elongation zone highlighting the circumferential and radial organization of the root. The position of trichoblast and atrichoblast cell files are indicated. Also shown is the location of the protoxylem and protophloem that give rise to the diarch symmetry of the mature Arabidopsis root. (D) Cross section of an immature root showing the radial organization of cell files.

Figure 2.

Spatial and temporal factors govern the formation of lateral root primordia. The regular spacing of lateral root primordia arises from pulses of auxin signaling in the basal meristem. The basal meristem encompasses the set of cells that are transitioning from the meristematic region into the elongation zone and thus includes cells that are undergoing division as well as elongation. In seedlings that are grown in constant light, pulses of auxin signaling, highlighted in purple, occur with a periodicity of 15 h. The response to auxin signaling in the xylem cells primes the adjacent pericycle cells (shown in red) so that they are competent to become lateral root founder cells (shown in green) upon a second, auxin-dependent signal in the differentiation zone (De Smet et al. 2007). As such, the pulses of auxin in the basal meristem coupled with the continuous growth of the root leads to the observed regular spacing of lateral root primordia. HAG, hours after germination.