Abstract
WUSCHEL-related homeobox (WOX) gene is a plant-specific clade of homeobox transcription factors. Increasing evidences reveal that WOXs play critical roles in early embryogenesis, which involves zygote development, initiation of zygote division, and apical or basal cell lineage establishment. However, how WOXs regulate these developmental events remains largely unknown, and even detailed expression pattern in gametes and early proembryos is not yet available. Here, 13 WOX family genes were identified in Nicotiana tabacum genome. Comparative analysis of 13 WOX family genes with their homologs in Arabidopsis thaliana reveals relatively conserved expression pattern of WUS and WOX5 in shoot/root apical meristem. Whereas variations were also found, e.g., lacking homolog of WOX8 (a marker for suspensor cell) in tobacco genome and the expression of WOX2/WOX9 in both apical cell and basal cell. Transient transcriptional activity analysis revealed that WOXs in WUS clade have repressive activities for their target's transcription, whereas WOXs in ancient and intermediate clade have activation activities, giving a molecular basis for the phylogenetic classification of tobacco WOXs into three major clades. Expression pattern analysis revealed that some WOXs (e.g., WOX 13a) expressed in both male and female gametes and some WOXs (e.g., WOX 11 and WOX 13b) displayed the characteristics of parent-of-origin genes. Interestingly, some WOXs (e.g., WOX2 and WOX9), which are essential for early embryo patterning, were de novo transcribed in zygote, indicating relevant mechanism for embryo pattern formation is only established in zygote right after fertilization and not carried in by gametes. We also found that most WOXs displayed a stage-specific and cell type-specific expression pattern. Taken together, this work provides a detailed landscape of WOXs in tobacco during fertilization and early embryogenesis, which will facilitate the understanding of their specific roles in these critical developmental processes of embryogenesis.
Highlights
Homeobox (HB) protein is a larger superfamily of eukaryotic transcription factors, which was first discovered in Drosophila melanogaster, and subsequently in many other eukaryotic organisms, ranging from sponges to vertebrates and mammals (Gehring et al, 1994)
Despite the loss of several individual WUSCHEL-related homeobox (WOX), duplication of the tobacco genome is evident as that two tobacco WOX family genes (NtWOX3 and NtWOX13) have multiple paralogs derived from chromosomal duplication within the genome, which has been found in other plants such as WOX2 and WOX5 in maize (Figure S3 and Table S1)
Researches revealed that WOXs in A. thaliana have important roles in different developmental processes, especially in embryo development including cell fate determination of apical and basal cell lineages and the establishment of the apical-basal pattern (Haecker et al, 2004; Breuninger et al, 2008)
Summary
Homeobox (HB) protein is a larger superfamily of eukaryotic transcription factors, which was first discovered in Drosophila melanogaster, and subsequently in many other eukaryotic organisms, ranging from sponges to vertebrates and mammals (Gehring et al, 1994). WUSCHEL-related homeobox (WOX) genes are distinguished as a plant-specific clade of HB transcription factors by phylogenetic relatedness of its HD from other HB transcription factors (van der Graaff et al, 2009). WOX family genes were found to be ubiquitous present in the genomes of different plants, ranging from green algae to angiosperms (van der Graaff et al, 2009). There is only one WOX in the genome of green algae, whereas over 10 WOXs have been identified in the genome of angiosperms (Mukherjee et al, 2009). Phylogenetic differences of WOX family genes between monocots and eudicots or between angiosperms and gymnosperms have been revealed, respectively (van der Graaff et al, 2009)
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