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Various Arabidopsis thaliana mutants with defects in phytohormone signal transduction or the reception of light were analysed with regard to their stomatal response in a red, red/blue light irradiation programme. Stomatal response to light was detected with a customized gas exchange measurement device, optimized for the small model plant. Small transpiration-kinetic variations of the two wild-type lines Columbia (Col) and Landsberg erecta (Ler) were observed. A comparison of the mutant lines to the respective wild type revealed significant differences for the phytochrome A (phyA-103), the abscisic acid insensitive (aba3-2) and the auxin resistant (axr1-3) mutant. Furthermore, the zeaxanthin-less mutant line npq1-2 showed no alterations in stomatal response to light.

meability of these channel proteins for water (Preston et al., 1992). Recent findings concerning the sensitivity of aqua- For a long time, plant physiologists, on the other hand, porins to heavy metal ions and their possible con- have not accepted the requirement of these molecules for sequences to plant physiology are discussed. the simple reason that the majority of plants do not Furthermore, results obtained from heterologous possess organs or sites where comparable tremendous expression of plant aquaporin cRNAs in Xenopus amounts of water are transported through living cells. It oocytes are related to the situation in the plant. An was assumed that the water permeability of a biomemassessment of the significance of water flux regulation brane itself is suYcient for the requirements of plant by modification of the aquaporin protein or change cellular water movement (Fettiplace and Haydon, 1980; of gene activity is given and new data concerning Cullis et al., 1996). Since the counterparts of AQP1 were the selectivity of plasma membrane aquaporins are identified in the plant plasma membrane and tonoplast, presented. this opinion is questioned. To date the existence of aquaporins in virtually all living organisms, including

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.