Auxin at the shoot apical meristem

Abstract

Plants continuously generate new tissues and organs through the activity of populations of undifferentiated stem cells, called meristems. Here, we discuss the so-called shoot apical meristem (SAM), which generates all the aerial parts of the plant. It has been known for many years that auxin plays a central role in the functioning of this meristem. Auxin is not homogeneously distributed at the SAM and it is thought that this distribution is interpreted in terms of differential gene expression and patterned growth. In this context, auxin transporters of the PIN and AUX families, creating auxin maxima and minima, are crucial regulators. However, auxin transport is not the only factor involved. Auxin biosynthesis genes also show specific, patterned activities, and local auxin synthesis appears to be essential for meristem function as well. In addition, auxin perception and signal transduction defining the competence of cells to react to auxin, add further complexity to the issue. To unravel this intricate signaling network at the SAM, systems biology approaches, involving not only molecular genetics but also live imaging and computational modeling, have become increasingly important.

Figure 1.

The shoot apical meristem of Arabidopsis thaliana. (A) Aerial part of a wild-type plant of the Columbia ecotype (Col-0). The SAM is responsible for the production of rosette leaves and, after floral transition, for the production of the stem, cauline leaves, lateral meristems, and flowers of the inflorescence. (B) Details of the tip of the inflorescence, showing the highly organized positioning of flowers around the main axis (a spiral). (C) A dissected inflorescence meristem. Older flowers have been removed to expose the meristem surrounded by young floral buds. (D) Longitudinal section of an inflorescence meristem showing the layered organization (L1, L2, and L3 cell layers). L1 and L2 are also called the tunica and L3 to the corpus. The functional zones are also represented. At the meristem summit the central zone (CZ) contains the stem cells, whereas primordia are initiated in the peripheral zone (PZ). The rib zone (RZ) produces the internal part of the stem.

Figure 2.

From dynamic transport to patterning: Auxin and organogenesis at the shoot apical meristem of Arabidopsis thaliana. (A) Immunodetection of PIN1 efflux carrier in the L1 (top view). The image was obtained by confocal microscopy. Note the subcellular polarized localization of the auxin transporter in most cells. The localization suggests that auxin accumulates in these young organ primordia (named P1, P2, and P3 from the oldest to the youngest organ). Adapted from de Reuille et al. (2006). (B) Expression of the synthetic DR5rev::GFP reporter in the inflorescence meristem (top view). Projection of serial optical sections obtained by confocal microscopy. The green corresponds to the GFP and the red to the autofluorescence of meristematic cells. DR5 expression in young emerging primordia indicates activation of auxin induced-genes. (C) Schematic representation of the role of auxin during organogenesis in the inflorescence meristem of Arabidopsis thaliana. See text for details.