An Arabidopsis Homolog of YeastATG6/VPS30Is Essential for Pollen Germination

Abstract

Yeast (Saccharomyces cerevisiae) Atg6/Vps30 is required for autophagy and the sorting of vacuolar hydrolases, such as carboxypeptidase Y. In higher eukaryotes, however, roles for ATG6/VPS30 homologs in vesicle sorting have remained obscure. Here, we show that AtATG6, an Arabidopsis (Arabidopsis thaliana) homolog of yeast ATG6/VPS30, restored both autophagy and vacuolar sorting of carboxypeptidase Y in a yeast atg6/vps30 mutant. In Arabidopsis cells, green fluorescent protein-AtAtg6 protein localized to punctate structures and colocalized with AtAtg8, a marker protein of the preautophagosomal structure. Disruption of AtATG6 by T-DNA insertion resulted in male sterility that was confirmed by reciprocal crossing experiments. Microscopic analyses of AtATG6 heterozygous plants (AtATG6/atatg6) crossed with the quartet mutant revealed that AtATG6-deficient pollen developed normally, but did not germinate. Because other atatg mutants are fertile, AtAtg6 likely mediates pollen germination in a manner independent of autophagy. We propose that Arabidopsis Atg6/Vps30 functions not only in autophagy, but also plays a pivotal role in pollen germination.

A BAR-Domain Protein SH3P2, Which Binds to Phosphatidylinositol 3-Phosphate and ATG8, Regulates Autophagosome Formation in Arabidopsis

“…A conserved PI3K activity is essential for autophagy in plants, as silencing of ATG6 and VPS34 has been shown to limit cell death and reduce autophagosome formation in Arabidopsis and Nicotiana benthamiana plants (Liu et al, 2005;Fujiki et al, 2007;Patel and Dinesh-Kumar, 2008). When SH3P2-CFP (for cyan fluorescent protein) was coexpressed with VPS34-GFP and ATG6-RFP, we observed an enhanced production of VPS34-and ATG6-positive punctae that colocalized with SH3P2-CFP ( Figure 9C).…”

“…These included the PI3K complex (ATG6-YFP), ATG9 complex (ATG9-GFP), and ATG8 conjugate system (YFP-ATG8e and YFP-ATG8f) (Hanaoka et al, 2002;Yoshimoto et al, 2004;Fujiki et al, 2007). As shown in Figure 2A, ATG6-YFP and ATG9-GFP punctae largely colocalized with SH3P2-RFP, whereas the YFP-ATG8e and YFP-ATG8f dots only partially overlapped with SH3P2-RFP punctae.…”

The Arabidopsis Phosphatidylinositol 3-Kinase Is Important for Pollen Development

“…causes cell death at the tricellular stage in Arabidopsis pollen (Gupta et al, 2002). BECLIN1, an Arabidopsis homolog of yeast Beclin1/Atg6/Vps30, which associates with VPS34 and plays a role in autophagy and vacuolar protein sorting, is expressed predominantly in mature pollen, and disruption of this gene by T-DNA insertion results in male sterility due to severely reduced pollen germination (Fujiki et al, 2007;Qin et al, 2007). VACUOLESS1 (VPS16) and POKY POLLEN TUBE (VPS52), Arabidopsis VPS homologs, also play roles in pollen tube elongation (Hicks et al, 2004;Lobstein et al, 2004).…”

Role of autophagy in disease resistance and hypersensitive response-associated cell death

“…Beclin1 also forms complexes with the anti-apoptotic protein Bcl-2 in mammalian cells 49 and loss of Beclin1 in C. elegans triggers apoptotic cell death. 50 In Arabidopsis, Beclin1 is essential for pollen germination, a feature not associated with other ATG-null mutants, 51 but knockdown of Beclin1 through antisense or viral-induced gene silencing leads to premature chlorosis and cell death in both Arabidopsis and Nicotiana benthamiana. 52,53 It is therefore tempting to speculate that, like Beclin1 in metazoans, plant Beclin1 could also represent a molecular link between autophagy and another cell death route.…”