Stem Cell Fate versus Differentiation: the Missing Link.

The shoot apical meristem provides a microenvironment that ensures stem cell fate and proliferation via homeostasis between WUSCHEL (WUS) activity and CLAVATA signalling. New data from maize and arabidopsis reveal that an evolutionarily conserved signal deriving from primordium cells links WUS transcription to the morphogenetic programme.

Stem Cell Regulation by Arabidopsis WOX Genes.

Gene amplification followed by functional diversification is a major force in evolution. A typical example of this is seen in the WUSCHEL-RELATED HOMEOBOX (WOX) gene family, named after the Arabidopsis stem cell regulator WUSCHEL. Here we analyze functional divergence in the WOX gene family.

Symplesiomorphies in the WUSCHEL clade suggest that the last common ancestor of seed plants contained at least four independent stem cell niches.

Evolutionary studies addressing plant architecture have uncovered several significant dichotomies between lower and higher land plant radiations, which are based on differences in meristem histology and function. Here, we assess the establishment of different stem cell niches during land plant evolution based on genes of the stem cell-promoting WUSCHEL (WUS) clade of the WOX (WUSCHEL-related homeobox) gene family. WOX gene orthology was addressed by phylogenetic analyses of full-length WOX protein sequences and cellular expression pattern studies indicate process homology.

The invention of WUS-like stem cell-promoting functions in plants predates leptosporangiate ferns.

The growth of land plants depends on stem cell-containing meristems which show major differences in their architecture from basal to higher plant species. In Arabidopsis, the stem cell niches in the shoot and root meristems are promoted by WUSCHEL (WUS) and WOX5, respectively. Both genes are members of a non-ancestral clade of the WUS-related homeobox (WOX) gene family, which is absent in extant bryophytes and lycophytes.

Discrete shoot and root stem cell-promoting WUS/WOX5 functions are an evolutionary innovation of angiosperms.

The morphologically diverse bodies of seed plants comprising gymnosperms and angiosperms, which separated some 350 Ma, grow by the activity of meristems containing stem cell niches. In the dicot model Arabidopsis thaliana, these are maintained by the stem cell-promoting functions of WUS and WUSCHEL-related homeobox 5 (WOX5) in the shoot and the root, respectively. Both genes are members of the WOX gene family, which has a monophyletic origin in green algae.

Plant development revolves around axes.

Arabidopsis thaliana has become a paradigm for dicot embryo development, despite its embryology being non-representative of dicots in general. The recent cloning of heterologous genes involved in embryonic development from maize and construction of robust phylogenies has shed light on the conservation of transcription factor function and now facilitates a comparison of maize and Arabidopsis embryogenesis orthology. In this review, we focus on a comparison of expression domains of WUSCHEL HOMEOBOX LIKE (WOX), SHOOTMERISTEMLESS (STM), DORNROESCHEN (DRN) and CUP-SHAPED COTYLEDON (CUC) genes and their role in axialization in both species, showing that despite significantly divergent modes of embryogenesis, most notably in terms of axes and planes of symmetry, there is considerable conservation of function as well as notable differences between maize and Arabidopsis.

The evolution of plant regulatory networks: what Arabidopsis cannot say for itself.

Genetic and molecular analyses in the dicot model plant Arabidopsis thaliana have begun to shed some light on regulatory networks in plants. However, comparisons with other species are necessary to validate networks identified in model species on the evolutionary scale. Many key regulatory proteins are encoded by members of transcription factor gene families.

The shoot stem cell niche in angiosperms: expression patterns of WUS orthologues in rice and maize imply major modifications in the course of mono- and dicot evolution.

In Arabidopsis, stem cell homeostasis in the shoot apical meristem (SAM) is controlled by a feedback loop between WUS and CLV functions. We have identified WUS orthologues in maize and rice by a detailed phylogenetic analysis of the WOX gene family and subsequent cloning. A single WUS orthologue is present in the rice genome (OsWUS), whereas the allotetraploid maize genome contains 2 WUS paralogues (ZmWUS1 and ZmWUS2).

thick tassel dwarf1 encodes a putative maize ortholog of the Arabidopsis CLAVATA1 leucine-rich repeat receptor-like kinase.

Development in higher plants depends on the activity of meristems, formative regions that continuously initiate new organs at their flanks. Meristems must maintain a balance between stem cell renewal and organ initiation. In fasciated mutants, organ initiation fails to keep pace with meristem proliferation.

The maize duplicate genes narrow sheath1 and narrow sheath2 encode a conserved homeobox gene function in a lateral domain of shoot apical meristems.

The narrow sheath (ns) phenotype of maize is a duplicate factor trait conferred by mutations at the unlinked loci ns1 and ns2. Recessive mutations at each locus together confer the phenotypic deletion of a lateral compartment in maize leaves and leaf homologs. Previous analyses revealed that the mediolateral axis of maize leaves is comprised of at least two distinct compartments, and suggest a model whereby NS function is required to recruit leaf founder cells from a lateral compartment of maize meristems.