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The quiescent center is viewed as an architectural template in the root apical meristem of all angiosperm and gymnosperm root tips. In roots of Arabidopsis thaliana (L.) Heynh., the quiescent center inhibits differentiation of contacting initial cells and maintains the surrounding initial cells as stem cells. Here, the role of the quiescent center in the development of the maize (Zeca mays L.) root cap has been further explored. Three maize root-specific genes were identified. Two of these were exclusively expressed in the root cap and one of them encoded a GDP-mannose-4,6-dehydratase. Most likely these two genes are structural, tissue-specific markers of the cap. The third gene, a putative glycine-rich cell wall protein, was expressed in the cap and in the root epidermis and, conceivably is a positional marker of the cap. Microsurgical and molecular data indicate that the quiescent center and cap initials may regulate the positional and structural expression of these genes in the cap and thereby control root cap development.

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B-deficient bean (Phaseolus vulgaris L.) nodules examined by light microscopy showed dramatic anatomical changes, mainly in the parenchyma region. Western analysis of total nodule extracts examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that one 116-kD polypeptide was recognized by antibodies raised against hydroxyproline-rich glycoproteins (HRGPs) from the soybean (Glycine max) seed coat. A protein with a comparable molecular mass of 116 kD was purified from the cell walls of soybean root nodules. The amino acid composition of this protein is similar to the early nodulin (ENOD2) gene. Immunoprecipitation of the soybean ENOD2 in vitro translation product showed that the soybean seed coat anti-HRGP antibodies recognized this early nodulin. Furthermore, we used these antibodies to localize the ENOD2 homolog in bean nodules. Immunocytochemistry revealed that in B-deficient nodules ENOD2 was absent in the walls of the nodule parenchyma. The absence of ENOD2 in B-deficient nodules was corroborated by performing hydroxyproline assays. Northern analysis showed that ENOD2 mRNA is present in B-deficient nodules; therefore, the accumulation of ENOD2 is not affected by B deficiency, but its assembly into the cell wall is. B-deficient nodules fix much less N2 than control nodules, probably because the nodule parenchyma is no longer an effective O2 barrier.

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In dark grown pea (Pisum sativum) seedlings ethylene causes the triple response in which elongation growth is inhibited, radial growth is promoted, and orientation of shoots to gravity is altered. The distribution of extensin and peroxidase activity in pea epicotyls upon ethylene treatment was studied by tissue printing on nitrocellulose paper. It was found that the localization of extensin and peroxidase activity changes after 72 and 96 hours of ethylene treatment. In untreated plants, peroxidase activity is detected only in the vascular bundles. Nonetheless, after 72 and 96 hours of ethylene treatment peroxidase activity is hardly detected in the vascular system but present in the epidermal and cortical cells. Extensin increases in the epidermal and cortical cells upon ethylene treatment but it also appears in the vascular system when peroxidase activity is no longer detected.

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In soybean seed coats the accumulation of the hydroxyproline-rich glycoprotein extensin is regulated in a developmental and tissue-specific manner. The time course of appearance of extensin during seed development was studied by Western blot analysis and by immunogold-silver localization. Using these techniques extensin was first detected at 16-18 d after anthesis, increasing during development to high levels at 24 d after anthesis. Immunogold-silver localization of extensin in the seed coat showed marked deposition of the glycoprotein in the walls of palisade epidermal cells and hourglass cells. The immunolocalization of extensin in developing soybean seeds was also made by a new technique--tissue printing on nitrocellulose paper. It was found that extensin is primarily localized in the seed coat, hilum, and vascular elements of the seed.

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In soybean (Glycine max (L.) Merr.) root nodules the level of hydroxyproline-containing molecules is developmentally regulated. Hydroxyproline accumulates in both nodule cortex and medulla. In the cortex, the hydroxyproline is mainly localized in the cell wall, presumably as extensin, but in the medulla it is mainly in the soluble fraction as an arabinogalactan protein (AGP). Nodule-specific AGPs are present at early nodulation. The highest concentration of AGP is in the nodule medulla, followed by nodule cortex, uninfected roots, leaves, flowers, pods and seeds. Root nodules and all organs of the soybean plant that were tested were found to express a tissue-specific set of arabinogalactan proteins.

RESUMEN

In soybean seeds the level of hydroxyproline is regulated in a developmental and tissue-specific manner. The seed coat contains approximately 77% of the total hydroxyproline in the seed at all stages of development. We determined the ratio of hydroxyproline to dry weight in a number of tissues within the seed; however, only the seed coat shows an increase in this ratio during development. Within the many cell layers of the seed coat, hydroxyproline is most abundant in the external layer. The hydroxyproline is present as an hydroxyproline-rich cell wall glycoprotein. The protein is rich in hydroxyproline (36%), lysine (11%), proline (10%), histidine (9%), tyrosine (9%), and serine (8%). The carbohydrate portion is 90 mole% arabinose and 10 mole% galactose. The arabinose residues are attached to hydroxyproline mostly in the form of trisaccharides. The apparent molecular weight of this glycoprotein is 100,000 daltons.