ECHIDNA Protein Impacts on Male Fertility in Arabidopsis by Mediating trans-Golgi Network Secretory Trafficking during Anther and Pollen Development      

Abstract

The trans-Golgi network (TGN) plays a central role in cellular secretion and has been implicated in sorting cargo destined for the plasma membrane. Previously, the Arabidopsis (Arabidopsis thaliana) echidna (ech) mutant was shown to exhibit a dwarf phenotype due to impaired cell expansion. However, ech also has a previously uncharacterized phenotype of reduced male fertility. This semisterility is due to decreased anther size and reduced amounts of pollen but also to decreased pollen viability, impaired anther opening, and pollen tube growth. An ECH translational fusion (ECHPro:ECH-yellow fluorescent protein) revealed developmentally regulated tissue-specific expression, with expression in the tapetum during early anther development and microspore release and subsequent expression in the pollen, pollen tube, and stylar tissues. Pollen viability and production, along with germination and pollen tube growth, were all impaired. The ech anther endothecium secondary wall thickening also appeared reduced and disorganized, resulting in incomplete anther opening. This did not appear to be due to anther secondary thickening regulatory genes but perhaps to altered secretion of wall materials through the TGN as a consequence of the absence of the ECH protein. ECH expression is critical for a variety of aspects of male reproduction, including the production of functional pollen grains, their effective release, germination, and tube formation. These stages of pollen development are fundamentally influenced by TGN trafficking of hormones and wall components. Overall, this suggests that the fertility defect is multifaceted, with the TGN trafficking playing a significant role in the process of both pollen formation and subsequent fertilization.

A Golgi-Released Subpopulation of the Trans-Golgi Network Mediates Protein Secretion in Arabidopsis

“…1, A and B). Interestingly, a TGN-localized membrane protein ECHIDNA and its interacting partner YPT/RAB GTPASE INTERACTING PROTEIN 4a (YIP4a) and YIP4b (Gendre et al, 2011(Gendre et al, , 2013, which collectively facilitate the secretory pathway (Boutté et al, 2013;Gendre et al, 2013;Fan et al, 2014;McFarlane et al, 2014), only partly colocalize with TGN markers such as VHAa1 and SYP61 (Gendre et al, 2013). This finding further supports the potential subdiversification of GI-TGNs, which might be mediated by ECHIDNA and/or additional trafficking components such as SNAREs, small GTPases, and adaptor proteins.…”

Genetic and Biochemical Mechanisms of Pollen Wall Development

“…Mutation in rice Waxdeficient anther 1 (WDA1), an epidermis-specific expressed gene, causes defective exine [42], while mutation in Arabidopsis ECHIDNA shows partial male sterility owing to reduced and disorganized thickening of anther endothecium [109]. In kiwifruit (Actinidia deliciosa), delayed PCD in the middle layer induces male sterility [110].…”

Independent yet overlapping pathways ensure the robustness and responsiveness of trans-Golgi network functions in Arabidopsis

“…ECHIDNA was originally identified by virtue of its transcriptional upregulation in elongating cells in both Arabidopsis thaliana (Arabidopsis) and hybrid aspen; consistently, echidna mutants are impaired in cell elongation (Gendre et al, 2011). In addition, echidna mutants have defects in hook formation and in pollen formation, release and fertilization (Boutté et al, 2013;Fan et al, 2014;Gendre et al, 2013). Electron tomography of echidna mutants, together with a genetic analysis of protein and polysaccharide secretion, yielded striking phenotypes, which suggest that ECHIDNA is primarily required for the genesis of secretory vesicles (Boutté et al, 2013;Gendre et al, 2013;McFarlane et al, 2013).…”