Aquaporin tetramer composition modifies the function of tobacco aquaporins.

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.

Clover and ryegrass are tolerant species to ammonium nutrition.

The application of nitrification inhibitors (NIs) together with nitrogen fertilizers in grasslands is an effective alternative to reduce nitrate leaching and nitrogenous gases emissions to the atmosphere. Nevertheless, the use of NIs increases the amount of ammonium available for the plant that, due to its reported toxic effect in plants, can have a direct effect on crop production. Grassland species have traditionally suffered from intensive grazing and urea deposition and, therefore, a tolerance to ammonium nutrition could be expected in these species. Plants of Trifolium repens L. var. huia and Lolium perenne L. var. Herbus were grown under two nitrogen nutrition regimes (nitrate or ammonium) and three different nitrogen concentrations (0.5, 2.5 and 5 mmol/L). The effect of nitrogen form was determined on biomass production parameters, gas-exchange and water relations parameters as well as polyamine (PA) and ion tissue contents. Both grassland species showed tolerance to ammonium nutrition due to their capacity to adjust several metabolic processes in a species-specific way. Gas exchange measurements and biomass production (expressed as dry weight (DW)) were unaffected by the nitrogen form or dose in both species except for a decrease in root total DW in ryegrass plants grown under ammonium nutrition. Hydraulic conductance (L(0)) increased in ryegrass with increasing ammonium doses but no change due to the nitrogen source was observed in water potential (Psi(w)) values. Both species, and specially ryegrass, accumulated free ammonium mainly in roots when grown under ammonium nutrition and its translocation to the shoot via xylem was also observed. A clear difference in cations and PAs pattern was observed in each species when comparing both nitrogen nutrition regimes.

An attack of the plant parasite Cuscuta reflexa induces the expression of attAGP, an attachment protein of the host tomato.

Dodder 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 attAGP-targeted 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.

Nitrogen management and senescence in two maize hybrids differing in the persistence of leaf greenness: agronomic, physiological and molecular aspects.

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.