Abstract
BackgroundPreviously, we demonstrated that pollen chamber formation (PCF) in G. biloba ovules was a process of programmed cell death (PCD) within the nucellar cells at the micropylar end. However, the signal triggering the cascades of the programmed events in these nucellar cells remains unexplored.ResultsA transcriptomic strategy was employed to unravel the mechanism underlying the nucellar PCD via the comparative profiles of RNA-seq between pre-PCF and post-PCF ovules. A total of 5599 differentially expressed genes (DEGs) with significance was identified from G. biloba ovules and classified into three main categories of GO annotation, including 17 biological processes, 15 cellular components and 17 molecular functions. KEGG analysis showed that 72 DEGs were enriched in “Plant hormone signal transduction”. Furthermore, 99 DEGs were found to be associated with the PCD process, including the genes involved in ethylene signaling pathway, PCD initiation, and PCD execution. Moreover, calcium-cytochemical localization indicated that calcium could play a role in regulating PCD events within the nucellar cells during pollen chamber formation in G. biloba ovules.ConclusionsA putative working model, consisting of three overlapping processes, is proposed for the nucellar PCD: at the stage of PCD preparation, ethylene signaling pathway is activated for transcriptional regulation of the downstream targets; subsequently, at the stage of PCD initiation, the upregulated expression of several transcription factors, i.e., NAC, bHLH, MADS-box, and MYB, further promotes the corresponding transcript levels of CYTOCHROME C and CALMODULINs, thereby, leads to the PCD initiation via the calcium-dependent signaling cascade; finally, at the stage of PCD execution, some proteases like metacaspases and vacuolar processing enzyme for hydrolysis, together with the process of autophagy, play roles in the clearance of cellular components. Afterwards, a pollen chamber is generated from the removal of specific nucellar cells in the developing ovule.
Highlights
We demonstrated that pollen chamber formation (PCF) in G. biloba ovules was a process of programmed cell death (PCD) within the nucellar cells at the micropylar end
At the early stage of ovule development, nucellar cells at the micropylar end are morphologically similar in their size and shape with those at other parts of nucellus (Fig. 1a)
The upregulated expression of ethyleneinsensitive protein 3 (EIN3) in the pre-PCF ovules were likely to promote the nucellar PCD in this study. These results indicated that ethylene signaling pathway was activated for transcriptional regulation of the downstream targets at the stage of nucellar PCD preparation in G. biloba ovules
Summary
We demonstrated that pollen chamber formation (PCF) in G. biloba ovules was a process of programmed cell death (PCD) within the nucellar cells at the micropylar end. Pollen chamber formation (PCF) is resulted from the degeneration of 5 ~ 7 layers of nucellar cells at the micropylar end of ovule [2]. Developmental PCD (dPCD) has occurred concomitantly with reproductive and vegetative developments, for instance, cell death of the nucellar. Similar to many plant developmental processes, dPCD is frequently coordinated by hormone signaling through transcriptional control, most commonly ethylene [8, 15,16,17]. Ethylene and its signaling pathways promotes several types of dPCD, such as root aerenchyma formation and leaf senescence [17,18,19]. GA could promote tapetum PCD [21], and auxin, cytokinin and brassinosteroid promote the PCD during tracheary element (TE) differentiation [22]
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