Shoot meristem function and leaf polarity: the role of class III HD-ZIP genes

Abstract

The shoot apical meristem comprises an organized cluster of cells with a central region population of self-maintaining stem cells providing peripheral region cells that are recruited to form differentiated lateral organs. Leaves, the principal lateral organ of the shoot, develop as polar structures typically with distinct dorsoventrality. Interdependent interactions between the meristem and developing leaf provide essential cues that serve both to maintain the meristem and to pattern dorsoventrality in the initiating leaf. A key component of both processes are the class III HD-ZIP genes. Current findings are defining the developmental role of members of this family and are identifying multiple mechanisms controlling expression of these genes.

Figure 1. Leaves Arise on the Flanks of the Shoot Meristem

(A) Vegetative apex of Arabidopsis. The adaxial side of the leaf is adjacent to the central shoot apical meristem, whereas the opposite, abaxial, side of the leaf is farther from the shoot meristem. Dorsoventrality is established early in development and is clearly evident in the leaf on the left, which arches over the meristem due to differential growth on each side of the leaf. Scale bar is 50 μm. (B) Developing vegetative leaf of Arabidopsis. Adaxial trichomes on the larger leaf are one marker distinguishing dorsoventrality. Scale bar is 250 μm. (C) Diagrammatic representation of a leaf cross section with adaxial and abaxial outer epidermal and inner mesophyll marked. (D) Cross section of leaf midvein with adaxial xylem and abaxial phloem cells marked.

Figure 2. Class III HD–ZIP Genes in Arabidopsis

(A) Class III HD–ZIP genes encode 833–852 amino acid proteins with main domains indicated; an N-terminal HD-ZIP domain, and a 213–218 amino acid START domain. (B) Relationship between five Arabidopsis Class III HD–ZIP genes. (C) Representation of expression pattern of PHB in longitudinal section of embryo (left) and transverse section of shoot apex (right). In the embryo, expression is adaxial in cotyledons and in central provasculature. In the shoot apex, expression is adaxial in developing leaves and in the meristem. ad, adaxial; ab, abaxial; p, peripheral; c, central; m, meristem (D) Phenotype of rev mutants. In wild-type, axillary meristems in axils of leaves give rise to lateral branches (arrows). In rev mutants, axillary meristems are frequently absent, and fewer or no lateral shoots are produced.

Figure 3. MicroRNAs and Class III HD–ZIP Target Sequences

Diagrammatic representation of the PHB gene. Exons are represented as boxes and introns as lines. The three structural domains in the coding sequence are the HD-ZIP (yellow-orange) and the START (red) domains. Below them is the sequence of the microRNA binding site in the five Class III HD–ZIP genes and miR165 and miR166 sequences.