Establishment of Embryonic Stem Cell Niches in Maize

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Specification of stem cell precursor cells leading to the establishment of embryonic meristems occurs already at the radial 16-celled dermatogen stage in the model plant Arabidopsis. Gene regulatory networks required for cell specification include the auxin-induced MONOPTEROS (MP)-BODENLOS (BDL) module consisting of the ARF transcription factor MP and its repressor BDL as well as WUSCHEL (WUS) and WUS-related homeobox (WOX) transcription factors. Compared with Arabidopsis, almost nothing is known about how meristems are established and organized during embryogenesis in cereals. Cell fate decisions during embryogenesis in Arabidopsis are strictly coupled to stereotypic cell division patterns, while cell division planes in cereal embryogenesis appear chaotic and random. Thus, how are meristem precursor and stem cells specified in cereals? The few descriptive reports studying hormone transport and responses as well as gene expression pattern of WUS/WOX homologs by in situ hybridization suggest that these processes are not conserved between maize and Arabidopsis. Meristem initiation occurs much later in maize, after symmetry breaking at the transition stage when the embryo consists already of hundreds of cells. Moreover, our preliminary data predict that auxin generated in the endosperm can trigger symmetry breaking, that this cell specification might initially be prevented by the embryo surrounding region (ESR), a specialized endosperm structure lacking in Arabidopsis, that ARF-IAA modules different from MP-BDL regulate embryonic meristem establishment, and that this involves also cereal-specific molecular players. Thus, to understand meristem establishment in cereals, this ambitious project aims (i) to identify and study the role of auxin-regulated ARF-IAA modules expressed at the transition stage of maize embryos, (ii) to elucidate the role of the ESR in early embryo patterning, (iii) to understand the role of the ZmWUS1/ZmWOX5 gene regulatory network for the establishment of embryonic stem cell niches and (iv) to identify the function and targets of the cereal-specific EAL1 signal peptide that is secreted asymmetrically at the apical embryo domain at the transition stage.