The Putative Arabidopsis Homolog of Yeast Vps52p Is Required for Pollen Tube Elongation, Localizes to Golgi, and Might Be Involved in Vesicle Trafficking

Abstract

The screening of the Versailles collection of Arabidopsis T-DNA transformants allowed us to identify several male gametophytic mutants, including poky pollen tube (pok). The pok mutant, which could only be isolated as a hemizygous line, exhibits very short pollen tubes, explaining the male-specific transmission defect observed in this line. We show that the POK gene is duplicated in the Arabidopsis genome and that the predicted POK protein sequence is highly conserved from lower to higher eukaryotes. The putative POK homolog in yeast (Saccharomyces cerevisiae), referred to as Vps52p/SAC2, has been shown to be located at the late Golgi and to function in a complex with other proteins, Vps53p, Vps54p, and Vps51p. This complex is involved in retrograde trafficking of vesicles between the early endosomal compartment and the trans-Golgi network. We present the expression patterns of the POK gene and its duplicate P2 in Arabidopsis, and of the putative Arabidopsis homologs of VPS53 and VPS54 of yeast. We show that a POK::GFP fusion protein localizes to Golgi in plant cells, supporting the possibility of a conserved function for Vps52p and POK proteins. These results, together with the expression pattern of the POK::GUS fusion and the lack of plants homozygous for the pok mutation, suggest a more general role for POK in polar growth beyond the pollen tube elongation process.Pollen tube growth is a vital process during male gametophyte development, since it allows male gametes to reach the ovules and achieve fertilization (Preuss, 1995). The elongation of the pollen tube, as well as that of animal axons, plant root hairs, fungal hyphae, and moss protonema, is achieved by a polarized mode of growth, termed tip growth (FranklinTong, 1999;Palanivelu and Preuss, 2000;Hepler et al., 2001), which involves the tip-localized exocytosis of Golgi-derived vesicles containing cell wall precursors (Franklin-Tong, 1999). The tip of the pollen tube is devoid of organelles, but contains almost exclusively Golgi-derived vesicles (Geitmann and Emons, 2000). In the shank of the tubes, an inverse fountain pattern of cytoplasmic streaming is observed; organelles and vesicles are transported toward the apex in the cortical region and basipetally in the central cytoplasm (Pierson et al., 1990). Actin filaments and microtubules, which are organized in longitudinal arrays more or less parallel to the elongation axis, act as tracks for cytoplasmic streaming and allow delivery of vesicles to the tip (Pierson et al., 1990;Vidali and Hepler, 2001). The use of membrane structure dyes, such as FM1-43 or FM4-64 (Parton et al., 2001;Camacho and Malhó , 2003), has revealed dynamic endo/ exocytosis processes at the tip of the pollen tube, but the molecular events underlying these processes are still poorly understood. The recent identification of the Golgi-associated tobacco (Nicotiana tabacum), Rab GTPase NtRab2, predominantly expressed in pollen, suggests that tip growth and vesicle trafficking could be tightly linked (Cheung et al., 2002). Moreover, a...

An Arabidopsis Homolog of YeastATG6/VPS30Is Essential for Pollen Germination

“…We also show that Arabidopsis Atg6/Vps30 partially complemented VPS function in atg6/vps30 mutant yeast. Several other Arabidopsis VPS homologs are required in pollen tubes (Hicks et al, 2004;Lobstein et al, 2004) because the polarized growth of pollen tubes requires membrane trafficking (Hepler et al, 2001;Monteiro et al, 2005). We found that disruption of AtATG6/VPS30 resulted in male sterility because the mutant pollen did not germinate.…”

“…5B). Likewise, POK/AtVPS52, another Arabidopsis VPS homolog implicated in pollen tube elongation (Lobstein et al, 2004), was expressed in different tissues and continuously expressed throughout pollen development (Lobstein et al, 2004;Fig. 5B).…”

The Maize aberrant pollen transmission 1 Gene Is a SABRE/KIP Homolog Required for Pollen Tube Growth

“…Recent genetic data confirm the importance of the Golgi for pollen tube growth. Mutations in genes encoding Golgi-localized proteins known to regulate vesicle trafficking in animal and yeast cells, such as a tobacco Rab2 GTPase (Cheung et al 2002) and an Arabidopsis homolog of yeast Vps52p (Lobstein et al 2004), are impaired in pollen tube growth. However, the mechanisms by which these proteins regulate pollen tube elongation are not yet understood.…”

The Arabidopsis Phosphatidylinositol 3-Kinase Is Important for Pollen Development

“…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). Together, these findings strongly support a central role for the PI3K-associated pathway in pollen development and/ or tube elongation.…”