Since the first ultrastructural investigations of sieve tubes in the early 1960s, their structure has been a matter of debate. Because sieve tube structure defines frictional interactions in the tube system, the presence of P protein obstructions shown in many transmission electron micrographs led to a discussion about the mode of phloem transport. At present, it is generally agreed that P protein agglomerations are preparation artifacts due to injury, the lumen of sieve tubes is free of obstructions, and phloem flow is driven by an osmotically generated pressure differential according to Mü nch's classical hypothesis. Here, we show that the phloem contains a distinctive network of protein filaments. Stable transgenic lines expressing Arabidopsis thaliana Sieve-Element-Occlusion-Related1 (SEOR1)-yellow fluorescent protein fusions show that At SEOR1 meshworks at the margins and clots in the lumen are a general feature of living sieve tubes. Live imaging of phloem flow and flow velocity measurements in individual tubes indicate that At SEOR1 agglomerations do not markedly affect or alter flow. A transmission electron microscopy preparation protocol has been generated showing sieve tube ultrastructure of unprecedented quality. A reconstruction of sieve tube ultrastructure served as basis for tube resistance calculations. The impact of agglomerations on phloem flow is discussed.

Matairesinol is a central precursor in planta in the biosynthesis of numerous lignans, including that of the important antiviral and anticancer agent, podophyllotoxin. In this study, the ϳ32-kDa NAD-dependent secoisolariciresinol dehydrogenase, which catalyzes the enantiospecific conversion of (؊)-secoisolariciresinol into (؊)-matairesinol in Forsythia intermedia, was purified >6,000-fold to apparent homogeneity. The 831-base pair cDNA clone encoding this 277-amino acid protein was next obtained from a library constructed from F. intermedia stem tissue, whose fully functional recombinant protein, produced by expression of this cDNA in Escherichia coli, catalyzed the same enantiospecific conversion via the corresponding lactol intermediate. A homologous secoisolariciresinol dehydrogenase gene was also isolated from a Podophyllum peltatum rhizome cDNA library, whose 834-base pair cDNA clone encoded a 278-amino acid protein with a calculated molecular mass of ϳ32 kDa. Expression of this protein in E. coli produced a fully functional recombinant protein that also catalyzed the enantiospecific conversion of (؊)-secoisolariciresinol into (؊)-matairesinol via the intermediary lactol. Various kinetic parameters were defined and established conversion of the intermediary lactol as being rate-limiting. With this overall enzymatic conversion now unambiguously defined, the entire biochemical pathway to the lignans, secoisolariciresinol and matairesinol, has been elucidated. Last, both secoisolariciresinol and matairesinol are metabolized in the gut of mammals, following digestion of high fiber dietary grains, seeds, and berries, into the so-called "mammalian" lignans, enterodiol and enterolactone, respectively; these in turn confer significant protection against the onset of breast and prostate cancers.

Nodulated legumes receive their nitrogen via nitrogen-fixing rhizobia, which exist in a symbiotic relationship with the root system. In tropical legumes like French bean (Phaseolus vulgaris) or soybean (Glycine max), most of the fixed nitrogen is used for synthesis of the ureides allantoin and allantoic acid, the major long-distance transport forms of organic nitrogen in these species. The purpose of this investigation was to identify a ureide transporter that would allow us to further characterize the mechanisms regulating ureide partitioning in legume roots. A putative allantoin transporter (PvUPS1) was isolated from nodulated roots of French bean and was functionally characterized in an allantoin transport-deficient yeast mutant showing that PvUPS1 transports allantoin but also binds its precursors xanthine and uric acid. In beans, PvUPS1 was expressed throughout the plant body, with strongest expression in nodulated roots, source leaves, pods, and seed coats. In roots, PvUPS1 expression was dependent on the status of nodulation, with highest expression in nodules and roots of nodulated plants compared with non-nodulated roots supplied with ammonium nitrate or allantoin. In situ RNA hybridization localized PvUPS1 to the nodule endodermis and the endodermis and phloem of the nodule vasculature. These results strengthen our prediction that in bean nodules, PvUPS1 is involved in delivery of allantoin to the vascular bundle and loading into the nodule phloem.Availability of reduced nitrogen is an important determinant in the growth and development of plants. Although in most vascular plant species the major transport form of reduced/organic nitrogen is as amino acids (including amides), tropical and subtropical legumes like cowpea (Vigna unguiculata), soybean (Glycine max), and French bean (Phaseolus vulgaris) transport large amounts of the nitrogenous compounds called ureides. The dominant forms of ureides in these species are allantoin and allantoic acid (Pate et al., 1980). In legumes that are adapted to temperate climates (e.g. pea [Pisum sativum] and faba bean [Vicia faba]), the amides Gln and Asn take on the major transport function (Herridge et al., 1978;Schubert, 1986). Ureides can comprise up to 90% of the total nitrogen transported in the xylem of nitrogenfixing tropical legumes (Herridge et al., 1978;Pate et al., 1980) and can be stored in high amounts in the different plant organs (Matsumoto et al., 1977a;Streeter, 1979; Layzell and LaRue, 1982). In soybean, ureides have been found to be in concentrations of 20 or 10 mm in the stem tissue or xylem (Layzell and LaRue, 1982;Rainbird et al., 1984) and 94 mm in nodule exudate (Streeter, 1979). In leaves, total ureide concentrations varied from approximately 1 to 3 mm, but analyses of the various leaf cells have shown that ureides can reach concentration levels up to 59 mm in the paraveinal mesophyll (Matsumoto et al., 1977b;Thomas and Schrader, 1981; Costigan et al., 1987). Due to the high concentrations in the vascular system and in certain plant tiss...