Heterologous expression in yeast cells revealed that NtAQP1, a member of the so-called PIP1 aquaporin subfamily, did not display increased water transport activity in comparison with controls. Instead, an increased CO 2 -triggered intracellular acidification was observed. NtPIP2;1, which belongs to the PIP2 subfamily of plant aquaporins, behaved as a true aquaporin but lacked a CO 2 -related function. Results from split YFP experiments, protein chromatography, and gel electrophoresis indicated that the proteins form heterotetramers when coexpressed in yeast. Tetramer composition had effects on transport activity as demonstrated by analysis of artificial heterotetramers with a defined proportion of NtAQP1 to NtPIP2;1. A single NtPIP2;1 aquaporin in a tetramer was sufficient to significantly increase the water permeability of the respective yeast cells. With regard to CO 2 -triggered intracellular acidification, a cooperative effect was observed, where maximum rates were measured when the tetramer consisted of NtAQP1 aquaporins only. The results confirm the model of an aquaporin monomer as a functional unit for water transport and suggest that, for CO 2 -related transport processes, a structure built up by the tetramer is the basis of this function.Water moves across biological membranes by diffusion. In most living organisms, the rate of water diffusion can be increased via pore-forming transmembrane proteins, the socalled aquaporins. These consist of six-membrane-spanning helices; N and C termini of the proteins face the cytosol. The helix-connecting loops B and E are themselves short helices that dip into the membrane from opposite sides and form the water-conducting channel (1, 2). Aquaporin monomers can assemble into tetramers (3, 4). Evidence for their function as water transport facilitators was observed by results from experiments using heterologous expression systems, such as Xenopus laevis oocytes (5). Despite this initially detected function, a facilitated membrane transport for glycerol or volatile substances like CO 2 or NH 3 was postulated (6 -9). Plant aquaporins were subdivided into protein groups according to the cellular location in which they have been initially detected and on the basis of sequence similarities (10). Accordingly, the PIPs (plasma membrane-intrinsic proteins) were split into two major groups, the PIP1 and PIP2 aquaporins. Compared with PIP1 proteins, PIP2 proteins have a shorter N-terminal extension and a longer C terminus (11)(12)(13)(14). In general, plant PIP2 proteins have been shown to facilitate membrane water transport in heterologous expression systems; however, PIP1 proteins display low or no activity in this respect (8,11,(15)(16)(17)(18). For the human AQP1 as well as for the tobacco PIP1 aquaporin NtAQP1, an increased cellular acidification rate under CO 2 -enriched buffer was obtained in oocytes expressing the respective aquaporin in addition to a carbonic anhydrase (9,19,20). The observations on NtAQP1, together with the fact that plants deficient in NtAQP1 express...

Summary• Here, nitrogen management within the plant was compared in an early-senescing maize hybrid and in a late-senescing maize hybrid, both grown under field conditions with a high fertilisation input involving large quantities of fertiliser.• We monitored, in representative leaf stages, the changes in metabolite content, enzyme activities and steady-state levels of transcripts for marker genes of N primary assimilation, N recycling and leaf senescence.• The hybrids differed in terms of persistence of leaf greenness, the expression of marker genes and the concentration of enzymes used to describe the transition from N assimilation to N recycling. The transcription of leaf-senescence marker genes did not differ. Agronomic studies confirmed the ability of the late-senescing hybrid to absorb and store more N in shoots.• Despite the differences in the mode of N management adopted by the two hybrids, we conclude that leaf senescence occurs independently of the source-to-sink transition at the high level of fertilisation used involving large quantities of fertiliser. The possibility of improving N metabolic efficiency in the latest maize hybrids is discussed.

SummaryDodder or Cuscutaceae are holoparasitic plants subsisting on other dicotyledonous plants. The infection process is initiated by adherence of Cuscuta prehaustoria to the host surface, followed by penetration attempts by hyphae. In the case of a successful infection, these organs connect the parasite's vascular tissue to that of the host. Here we show that contact of Cuscuta reflexa prehaustoria to tomato induces the expression of a new arabinogalactan protein (AGP), attAGP, in the tomato precisely at the site of dodder attack. We show that attAGP is a plasma membrane-bound cell wall-localized protein. Using the RNAi technique and attAGPtargeted virus-induced gene silencing, we observed a correlation between attAGP expression level and force of attachment of the parasite to host tomatoes. If the expression level of attAGP was reduced, the C. reflexa attachment capability was significantly reduced, too. We conclude that C. reflexa infection induced a signal in the host leading to expression of tomato attAGP, which promotes the parasite's adherence.

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