Analysis of polarity establishment during megagametogenesis and early zygotic embryogenesis in Zea mays

摘要

Asymmetric cell division is one of the mechanisms to generate cellular diversity in multicellular organisms including flowering plants. Two major pathways have been reported by which plants perform asymmetric cell division, namely, (i) the “one mother-two different daughters” and (ii) the “coenocytes-cellularization” pathway. In the “one mother-two different daughters” pathway, a mother cell divides to generate two daughter cells which are either different in size and fate or which are initially comparable in size but subsequently get different fates (Ranganath, 2007). This pathway is exemplified by the asymmetric division of the zygote in maize, giving rise to the small apical cell and the large basal cell. Microarray hybridizations were performed comparing the egg cell, the apical cell and the basal cell to identify candidate transcripts that are polarly localized in the egg cell and which segregated either to the apical or basal cell of the two-celled proembryo after zygotic division. Several interesting candidate genes were identified to be expressed in the maize egg cell and differentially expressed in the apical or basal cell. The majority of differentially expressed genes encode for proteins involved in gene regulation, metabolism and proteins with unknown function. Furthermore, other classes of transcripts encode for proteins involved in signaling, vesicle trafficking, cytoskeleton, RNA metabolism, protein/protein interaction, cell wall biogenesis/structure, transport, targeting proteins for degradation and protein folding. The “coenocytes-cellularization” pathway is based on the formation of a coenocyte, involving nuclear migration to specific locations and cellularization, which occurs during megagametogenesis in maize. The role of two proteins, namely, ZmEAL1 (Zea mays EA1 Like1) and ZmDSUL (Zea mays DiSUMO-Like), was analyzed during female gametophyte development. ZmEAL1 is a small secreted peptide expressed specifically in the egg cell in the embryo sac. After fertilization, ZmEAL1 is expressed in the embryo proper until seven days after pollination. ZmEAL1 is involved in cell fate maintenance during female gametophyte development. On the other hand, ZmDSUL transcripts were detected exclusively in the egg apparatus and zygote of maize. ZmDSUL appears to be essential for nuclei segregation and positioning, the prerequisite for cell specification during FG maturation.ud