Minisymposium 23: Protein targeting

Abs # 44003: Vacuolar biogenesis in germinating pollen differs from that of sporophytes but nevertheless requires VACUOLESS1 for proper function

Presenter:	Hicks, Glenn R., ghicks@citrus.ucr.edu
Authors	Hicks, Glenn R. (A)   Rojo, Enrique  (A)   Hong, Seho  (A)   Carter, David  (A)   Raikhel, Natasha V (A)
Affiliations:	(A): Center for Plant Cell Biology and Dept of Botany and Plant Sciences, University of California, Riverside

Vacuoles perform multiple functions and VACUOLESS1 (VCL1) is essential for vacuole biogenesis with loss of expression in the vcl1-1 mutant leading to embryo lethality (Rojo et al, 2001 Dev Cell 1:303). VCL1 is a member of a protein complex, the AtC-VPS complex, found at the tonoplast and pre-vacuolar compartment and is likely involved in regulation of membrane fusion (Rojo et al, 2003 Mol Biol Cell 1:361). Vacuole biogenesis is prominent in the development of gametophytes yet is poorly understood. Given the importance of VCL1 in the development of sporophyte vacuoles, we asked if it contributes to vacuolar biogenesis during pollen germination. However, it was essential to first understand the dynamics of vacuoles in pollen. A tonoplast marker, δ-TIP::GFP, under a pollen-specific promoter permitted the examination of vacuole morphology in germinating Arabidopsis pollen. During the progression of biogenesis, pollen vacuoles are extremely dynamic with remarkable features such as elongated vacuoles and highly mobile cytoplasmic invaginations. Surprisingly, although there are no homologs of VCL1 in Arabidopsis, vcl1-1 does not appear to impact vacuolar morphology in pollen germinated in vitro. Vacuole morphology not withstanding, there is a defect in pollen fertility due to the presence of vcl1-1 as shown by genetics which demonstrated reduced transmission of vcl1-1 through pollen. Interestingly, vcl1-1 also affected the fertility of female gametophytes which possess prominent vacuoles. This result is consistent with the complex development of vacuoles that also exists in ovules. Our results point to the complexity of pollen vacuoles and suggest that the mechanism of vacuolar biogenesis in pollen may differ from that of sporophytes (Hicks et al, 2004, Plant Physiol 134:1227). It is interesting to note that Vsp16, the yeast homolog of VCL1, is critical for proper vacuole sorting, yet it is not an essential gene. In plants, VCL1 is essential for embryo development, yet our results indicate that this is not the case in pollen. Thus, pollen may permit the dissection of vacuole biogenesis in a system without the lethality associated with vacuole defects in the sporophyte. We will present our recent approaches for understanding the nature of the vcl1-1 defect in pollen and the role of VCL1 in the mechanism of vacuolar biogenesis.