The first sign characteristic of giant cell induction is the cell cycle (re)activation in parenchyma vascular cells. Giant cells will then be subjected to synchronous repeated mitosis without complete cytokinesis. Nodule formation is initiated by dedifferentiation of root cortical cells followed by cell proliferation, establishing a cluster of meristematic cells that give rise to the nodule primordium (Patriarca et al. 2004; Crespi and Frugier 2008). To initiate organogenesis, both nematodes and symbiotic bacteria recruit cell cycle regulators, such as cyclins or cyclin-dependent kinases, at early stages of the interactions (Table 1) (de Almeida Engler et al. 1999; Barcala et al. 2010; Maunoury et al. 2010). In nodule, following division, cells undergo endoreduplication - repeated cycles of DNA replication without mitosis -allowing their differentiation into enlarged symbiotic cells (Vinardell et al. 2003). Recently, Maunoury et al. (2010) hypothesized that it is endoreduplication that may act as a transcriptome switch for the cell to evolve from progenitor primordium or meristematic cell to a symbiotic cell. High expression of CCS52 was observed in endoreduplicating nodule tissues of M. truncatula (Cebolla et al. 1999). CCS52 promotes endoreduplication by activation of the anaphase-promoting complex (APC) resulting in mitotic cyclin destruction and mitosis arrest. Upregulation of CCS52 was also observed in giant cells and their surrounding cells (Koltai et al. 2001; Favery et al. 2002) suggesting that endoreduplication process occurs in giant cells. Its function still remains unclear but should represent an important process in the development of functional giant cells to support the enhanced metabolic demands imposed by RKN. More generally host endoreduplication is an emerging theme in biotroph-plant interactions (Wildermuth 2010).
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