Polysaccharide moiety of the boron-polysaccharide complex (T. shown to be rhamnogalacturonan II by glycosyl-linkage analysis and the presence of diagnostic monosaccharides, including apiose, aceric acid, 2-Omethylfucose, and 3-deoxy-~-mann~2-octulosonic acid. Remova1 of boron from the complex reduced the molecular weight by one-half without causing a significant increase in the number of reducing end groups, indicating that boron, as boric acid, links two rhamnogaladuronan II chains together to form the boron-polysaccharide complex.Boron (B) is an essential microelement for higher plants (reviewed by Loomis and Durst, 1992); however, its primary function is not known. A number of metabolic disorders that are consequences of B deficiency have been discussed in recent reviews (Dugger, 1983;Parr and Loughman, 1983; Loomis and Durst, 1992). Skok and McIlrath (1958) pointed out that the locus of B in the plant cell was little known, and the issue has never been resolved. We have shown that in cultured tobacco cells more than 98% of the B is present in the cell wall (Matoh et al., 1992), and that a BPC can be isolated from radish (Xapkanus sativus) cell walls (Matoh et al., 1993a). In this paper, the structure of the polysaccharide moiety of the radish BPC is reported and its physiological function is discussed. MATERIALS AND METHODS Preparation of the BPCThe BPC was prepared from radish (Xaphanus sativus) roots as described previously (Matoh et al., 1993a) with some modifications. Briefly, radish roots were grated and squeezed, and the macerated tissue was freezedried. The dried tissue (4 g) was treated with Pectinase-SS (0.lY0, w/v; Kyowa Chemical Products, NishiNakajima, Osaka, Japan) in 400 mL of 20 mM sodium acetate, pH 4.0, for 48 h at 25°C on a rotary shaker (130 rpm). The suspension was centrifuged min), and the supernatant was adjusted to pH 8.0 with 2 M Tris and then applied to a DEAE-Sepharose column (4.8 X 60 cm, C1-form, Pharmacia) equilibrated with 20 mM Tris-HC1, pH 8.0. The column was eluted with a 6-L linear gradient of O to 0.5 M NaCl in the column buffer, and the fractions containing B were pooled and dialyzed. The BPC was purified by rechromatography on the same DEAE-Sepharose column. Fractions containing B were subjected to gel filtration using a Superdex 75 column (2.6 X 60 cm, Pharmacia) equilibrated with 20 mM Tris-HC1, pH 8.0, containing 0.1 M NaC1. B-rich fractions were pooled, dialyzed against water, and lyophilized. Partia1 Hydrolysis of the BPC with AcidThe BPC (1 mg) was incubated in 0.1 M HCl(1 mL) for 15 min at 25OC. After neutralization with NaOH, an aliquot (100 pL) was analyzed by size-exclusion chromatography (YMC-pack Diol-120, 300 X 8 mm [YMC, Karasuma-Oike, Kyoto, Japan], Shimadzu 6A HPLC system). The column was equilibrated and eluted at a flow rate of 0.5 mL min-' with 50 mM sodium acetate, pH 5.2, containing 0.2 M NaC1. Saccharides were detected fluorometrically (Shimadzu RF 530 detector) after postcolumn labeling of their reducing termini with 2-cyanoacetamide (Honda et al., ...

(D.W.M.) Brefeldin A (BFA) inhibits exocytosis but allows endocytosis, making it a valuable agent to identify molecules that recycle at cell peripheries. In plants, formation of large intracellular compartments in response to BFA treatment is a unique feature of some, but not all, cells. Here, we have analyzed assembly and distribution of BFA compartments in development-and tissue-specific contexts of growing maize (Zea mays) root apices. Surprisingly, these unique compartments formed only in meristematic cells of the root body. On the other hand, BFA compartments were absent from secretory cells of root cap periphery, metaxylem cells, and most elongating cells, all of which are active in exocytosis. We report that cell wall pectin epitopes counting rhamnogalacturonan II dimers cross-linked by borate diol diester, partially esterified (up to 40%) homogalacturonan pectins, and (134)-␤-d-galactan side chains of rhamnogalacturonan I were internalized into BFA compartments. In contrast, Golgi-derived secretory (esterified up to 80%) homogalacturonan pectins localized to the cytoplasm in control cells and did not accumulate within characteristic BFA compartments. Latrunculin B-mediated depolymerization of F-actin inhibited internalization and accumulation of cell wall pectins within intracellular BFA compartments. Importantly, cold treatment and protoplasting prevented internalization of wall pectins into root cells upon BFA treatment. These observations suggest that cell wall pectins of meristematic maize root cells undergo rapid endocytosis in an F-actin-dependent manner.Eukaryotic cells perform endomembrane flow accomplished by vesicles shuttling among endoplasmic reticulum (ER), Golgi apparatus (GA), the plasma membrane (PM), and endosomes (for plants see Robinson et al., 1998; Hawes et al., 1999). These compartments and pathways of endomembrane flow are highly conserved in unicellular yeast, higher plants, and animals (for plant cells, see Robinson et al., 1998; Hawes et al., 1999). A major breakthrough in our current understanding of this complex endomembrane flow was provided by rediscovery of the fungal metabolite brefeldin A (BFA; Fujiwara et al., 1988). BFA action prevents vesicle formation in the exocytosis pathway by stabilizing abortive complexes between conserved ADP ribosylation factor 1 (ARF1) and the Sec7 domain of its guanine nucleotide exchange factor during the assembly of coat protein complexes of budding vesicles (for plants see, Pimpl et al., 2000;Robineau et al., 2000). Because of this action, BFA inhibits anterograde vesicular pathways while allowing endocytosis and some retrograde pathways to proceed further (Miller et al., 1992; Gaynor et al., 1998; Belanger and Quatrano, 2000). Moreover, BFA inhibits the endosome to vacuole transport in budding yeast (Gaynor et al., 1998).The introduction of BFA to investigate the cell biology of endomembrane flow in plant cells occurred some years later (Satiat-Jeunemaitre and Hawes, 1992), but most of the major findings concerning the effects of...

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Phytophthora stem and root rot, caused by Phytophthora sojae, is one of the most destructive diseases of soybean [Glycine max (L.) Merr.], and the incidence of this disease has been increasing in several soybean-producing areas around the world. This presents serious limitations for soybean production, with yield losses from 4 to 100%. The most effective method to reduce damage would be to grow Phytophthora-resistant soybean cultivars, and two types of host resistance have been described. Race-specific resistance conditioned by single dominant Rps (“resistance to Phytophthora sojae”) genes and quantitatively inherited partial resistance conferred by multiple genes could both provide protection from the pathogen. Molecular markers linked to Rps genes or quantitative trait loci (QTLs) underlying partial resistance have been identified on several molecular linkage groups corresponding to chromosomes. These markers can be used to screen for Phytophthora-resistant plants rapidly and efficiently, and to combine multiple resistance genes in the same background. This paper reviews what is currently known about pathogenic races of P. sojae in the USA and Japan, selection of sources of Rps genes or minor genes providing partial resistance, and the current state and future scope of breeding Phytophthora-resistant soybean cultivars.