Embryonic development in Arabidopsis thaliana: from the zygote division to the shoot meristem

Abstract

Postembryonic organ formation of plants is fueled with cells from the stem cell niches in the shoot and root meristems. During the last two decades many players that regulate stem cell maintenance have been identified. With these factors in hand, the mechanisms establishing stem cell niches during embryo development can be addressed. Here we discuss current models of how the shoot meristem stem cell niche arises during Arabidopsis embryo pattern formation.

Keywords: asymmetry; body plan; cell fate; stem cell niche.

Figure 1

Arabidopsis shoot apical meristem organization. The shoot meristem with the central zone (CZ), the peripheral zone (PZ), and the rib zone (RZ) is shown. The three layers (L1–L3) are indicated. Stem cells (shown in blue, expressing CLV3) and the organizing center (OC; shown in red, expressing WUS) interact in a negative feedback loop. In the leaf primordium, adaxial and abaxial cell fates are marked by expression of HD-ZIP III and KANADI family genes, respectively. Modified from Gross-Hardt and Laux (2003).

Figure 2

Apical pole establishment during embryogenesis. (A) Gene interactions during the asymmetric division of the Arabidopsis zygote. SSP mRNA acts as a signal delivered by the pollen to promote elongation and asymmetric division of the zygote via the YDA pathway. The WRKY/WOX pathway promotes polar organelle localization in and asymmetric division of the zygote. Vacuoles (white), nuclei (circles) are indicated. Expression domains and PIN7 localization are color-coded as shown in (B). Modified from Zhang and Laux (2011). (B) Apical and basal lineage development. The zygote co-expresses WOX2, WOX8, and WOX9, the expression patterns of which subsequently mark the gradual establishment of the body plan. RAM, root meristem primordium; SAM, shoot meristem primordium. (C) Partitioning of the apical embryo domain. Upper row: longitudinal views of embryos, displaying WUS, CLV3, and DRN, DRNL expression patterns. Bottom row: top views of embryos. Expression domains of marker genes showing the gradual establishment of bilateral symmetry are indicated. Figure modified from Laux et al. (2004). For references and gene symbols see text.

Figure 3

Function of AGO1, ZLL, and miR165/166 during embryogenesis. In wild type, ZLL protein is thought to sequester miR165/166 away from AGO1, thus allowing accumulation of HD-ZIPIII mRNA in the shoot meristem region and maintenance of stem cells (expressing CLV3). In the zll embryo, higher levels of miR165/166 can bind to AGO1, resulting in a reduction of HD-ZIPIII transcript levels, termination of the shoot meristem and of CLV3 expression. Gene expression domains are indicated.