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Several processes during sexual reproduction in higher plants involve the movement of water between cells or tissues, such as occurs during dehiscence of the anther and hydration of the pollen grain after it is deposited on a stigma. To get more insight in these processes, a set of putative aquaporins was cloned and it was found that at least 15 are expressed in reproductive organs, which indicates that the control of water flow is important for reproduction. Functional studies in Xenopus laevis oocytes using two of the cDNAs showed that NtPIP2;1 is an efficient aquaporin, whereas NtPIP1;1 is not. Expression studies on RNA and protein levels showed that PIP1 and PIP2 genes are differently expressed in reproductive organs: PIP1 RNA accumulates in the stigma, and PIP1 and PIP2 RNA can be detected in most tissues of the anther.

This paper gives a summary of a project to characterize a tobacco aquaporin. The cDNA and gene, including the 5' upstream region, for the tobacco aquaporin NtAQP1 has been isolated and the encoded protein characterized. The significance of promoter regions for an abscisic acid- and gibberellic acid-induced gene expression could be restricted to a region between -1450 and -1112 upstream of the transcription start point by transient transformation of a bicistronic vector into tobacco protoplasts. NtAQP1 expression in tobacco plants was found to be elevated in flowers, stems and roots. In roots, the protein was detected close to xylem vessels in pitch-like structures. Studies with a NtAQP1-GFP fusion indicated a plasma membrane location. For a functional analysis, the cDNA was expressed in Xenopus oocytes. NtAQP1 was found to be a heavy metal-insensitive aquaporin with additional permeability for glycerol. Mutation of a threonine at position 233 to a cysteine transformed NtAQP1 into a heavy metal-sensitive aquaporin.