Vegetative and Sperm Cell-Specific Aquaporins of Arabidopsis Highlight the Vacuolar Equipment of Pollen and Contribute to Plant Reproduction

The water and nutrient status of pollen is crucial to plant reproduction. Pollen grains of Arabidopsis (Arabidopsis thaliana) contain a large vegetative cell and two smaller sperm cells. Pollen grains express AtTIP1;3 and AtTIP5;1, two members of the Tonoplast Intrinsic Protein subfamily of aquaporins. To address the spatial and temporal expression pattern of the two homologs, C-terminal fusions of AtTIP1;3 and AtTIP5;1 with green fluorescent protein and mCherry, respectively, were expressed in transgenic Arabidopsis under the control of their native promoter. Confocal laser scanning microscopy revealed that AtTIP1;3 and AtTIP5;1 are specific for the vacuoles of the vegetative and sperm cells, respectively. The tonoplast localization of AtTIP5;1 was established by reference to fluorescent protein markers for the mitochondria and vacuoles of sperm and vegetative cells and is at variance with the claim that AtTIP5;1 is localized in vegetative cell mitochondria. AtTIP1;3-green fluorescent protein and AtTIP5;1-mCherry showed concomitant expression, from first pollen mitosis up to pollen tube penetration in the ovule, thereby revealing the dynamics of vacuole morphology in maturating and germinating pollen. Transfer DNA insertion mutants for either AtTIP1;3 or AtTIP5;1 showed no apparent growth phenotype and had no significant defect in male transmission of the mutated alleles. By contrast, a double knockout displayed an abnormal rate of barren siliques, this phenotype being more pronounced under limited water or nutrient supply. The overall data indicate that vacuoles of vegetative and sperm cells functionally interact and contribute to male fertility in adverse environmental conditions.