Initial Infection and Colonization of Leaves and Stems of Cling Peach by Tranzschelia discolor.

ABSTRACT Initial infection processes and the subsequent colonization of leaves and young stems of peach by Tranzschelia discolor were studied. On leaves where multiple disease cycles of peach rust occur during the growing season, urediniospores germinated after 4 h of wetness. Germ tubes became septate and formed appressoria only over leaf stomata beginning 18 h after inoculation. No appressoria, however, formed over stomata of positive replicas of leaf surfaces indicating nonthigmotropic responses of germ tubes. On young, primary-growth stems (ca. 8 weeks old), stomata were mostly closed, less frequent than on leaves, and recessed from the surface of the cuticle of the epidermis. Although appressoria formation was not observed on inoculated stems, germ tube growth of urediniospores was directional toward stomata. Penetration of stem tissue is apparently a less common event that was reflected by a lower occurrence of stem lesions compared with that of leaf lesions in our potted plant inoculation studies and previous field observations. Still, stem lesions are important as sources of primary inoculum each spring and were reproduced in this study for the first time. Fungal colonization of leaves and stems was subepidermal-intercellular and haustoria were commonly found within mesophyll or cortical cells, respectively. No fungal colonization was observed in cambial stem tissue. Vascular tissue was also not colonized and delimited lesions in leaves and stems. Morphological host responses were not observed in infections on either leaves or young stems. In older stems (>32 weeks old), however, the infection was delimited by a wound periderm after uredinial formation. Furthermore, with continued secondary growth, stems recovered and fungal lesions became part of the bark tissue of woody branches. Thus, the fungus must infect primary-growth branches each year to establish stem lesions.

Ethylene-Mediated Programmed Cell Death during Maize Endosperm Development of Wild-Type and shrunken2 Genotypes.

We characterized the progression of programmed cell death during maize (Zea mays L.) endosperm development of starchy (Su; wild-type) and shrunken2 (sh2) genotypes and tested the involve ment of ethylene in mediating this process. Histological and viability staining demonstrated that endosperm cell death was initiated earlier and progressed more rapidly in sh2 endosperm compared with Su endosperm. Internucleosomal DNA fragmentation accompanied endosperm cell death and occurred more extensively in sh2 endosperm. 1-Aminocyclopropane-1-carboxylic acid levels peaked approximately 16 d after pollination (dap) in Su endosperm and gradually decreased during subsequent development, whereas two large 1-aminocyclopropane-1-carboxylic acid peaks were observed in sh2 endosperm, the first between 16 and 20 dap and the second at 36 dap. Ethylene levels were elevated in sh2 kernels compared with Su kernels, with an initial peak 20 dap approximately 3-fold higher than in Su kernels and a second peak 36 dap approximately 5-fold higher than that in Su kernels. Ethylene treatment of Su kernels resulted in earlier and more extensive endosperm cell death and DNA fragmentation. Aminoethoxyvinylglycine treatment of sh2 kernels reduced the extent of DNA fragmentation. We conclude that ethylene is involved in triggering programmed cell death in developing maize endosperm and is responsible for the aberrant phenotype of sh2 kernels.

Operationally defined apoplastic and symplastic aluminum fractions in root tips of aluminum-intoxicated wheat.

Knowledge of the mechanistic basis of differential aluminum (Al) tolerance depends, in part, on an improved ability to quantify Al located in the apoplastic and symplastic compartments of the root apex. Using root tips excised from seedlings of an Al-tolerant wheat cultivar (Triticum aestivum L. cv Yecora Rojo) grown in Al solutions for 2 d, we established an operationally defined apoplastic Al fraction determined with six sequential 30-min washes using 5 mm CaCl(2) (pH 4.3). Soluble symplastic Al was eluted by freezing root tips to rupture cell membranes and performing four additional 30-min CaCl(2) washes, and a residual fraction was determined via digestion of root tips with HNO(3). The three fractions were then determined in Yecora Rojo and a sensitive wheat cultivar (Tyler) grown at 18, 55, or 140 mum total solution Al (Al(T)). When grown at equal Al(T), Tyler contained more Al than Yecora Rojo in all fractions, but both total Al and fractional distribution were similar in the two cultivars grown at Al(T) levels effecting a 50% reduction in root growth. Residual Al was consistently 50 to 70% of the total, and its location was elucidated by staining root tips with the fluorophore morin and examining them using fluorescence and confocal laser scanning microscopy. Wall-associated Al was only observed in tips prior to any washing, and the residual fraction was manifested as distinct staining of the cytoplasm and nucleus but not of the apoplastic space. Accordingly, the residual fraction was allocated to the symplastic compartment for both cultivars, and recalculated apoplastic Al was consistently approximately 30 to 40% of the total. Distributions of Al in the two cultivars did not support a symplastic detoxification hypothesis, but the role of cytoplasmic exclusion remains unsettled.

Characteristics of alpha-Amylase during Germination of Two High-Sugar Sweet Corn Cultivars of Zea mays L.

The role of the scutellum and the aleurone in alpha-amylase production in the high-sugar sweet corn cultivars Illini X-tra Sweet (shrunken-2, sh2) and Illinois 677a (sugary, sugary enhancer; su se) was compared to that in the starchy (Su) hybrid Funks G4646 with the use of alpha-amylase enzyme assays, isoelectric focusing, electron microscopy, and laser scanning confocal microscopy. The scutellum of Illinois 677a had low levels of alpha-amylase activity compared to that of Funks G4646 through 10 days after imbibition, and the aleurone of Illini X-tra Sweet had negligible activity. On the isoelectric focusing gels, the Illinois 677a scutellum had fewer alpha-amylase isozymes at 7 days compared to the Funks G4646 scutellum. The Illini X-tra Sweet aleurone had no alpha-amylase isozymes. Funks G4646 scutellar epithelial and aleurone cells contained abundant rough endoplasmic reticulum, polysomes, and dictyosomes at 5 and 7 days, respectively. The scutellar epithelial cells of Illinois 677a contained fewer of these structures by 5 days, and the Illini X-tra Sweet aleurone contained mostly lipid bodies through 7 days. Few cytoplasmic membranes and little RNA were detected with laser scanning confocal microscopy in the Illini X-tra Sweet aleurone compared to Funks G4646 at 7 days. These data suggest that the scutellum of Illinois 677a and the aleurone of Illini X-tra Sweet have impaired abilities to produce alpha-amylase.

Identification and immunocytochemical localization of α-galactosidase in resting and germinated date palm (Phoenix dactylifera L.) seeds.

α-Galactosidases (EC 3.2.1.22) from resting and germinated date (Phoenix dactylifera L.) seeds were compared and localized using immunocytochemical methods. The enzyme was present in both the endosperm and embryo of resting seeds, in the endosperm undergoing digestion where the greatest specific activity was present, and in the haustorium of seedlings. The enzyme had a molecular mass of 140000 as determined by gel filtration and a pH optimum of 4.5. At least seven forms of the enzyme with isoelectric points ranging from 3.85 to 5.2 were detected in the haustorium whereas only four of these forms were present in the endosperm. The relative activity levels of the various forms also differed between the two tissues. On Western blots all enzyme forms were recognized by antibodies raised against mung-bean (Vigna radiata) α-galactosidase. Using immunogold techniques, label was shown to be present in the protein bodies of the resting embryo cells but to decrease in this organelle as the reserve protein was mobilized and to appear diffusely in the cytoplasm in subsequent stages. In resting endosperm cells, label occurred in the protein bodies and in a thin region of inner wall. In endosperm undergoing digestion, where different stages of protoplast and wall breakdown occurred, immunogold staining was localized in the flocculent contents of vacuoles which resulted from storageprotein breakdown, then dense staining occurred in the inner wall of cell cavities formed by the complete dissolution of the cytoplasm, and finally, staining was uniformly diffuse throughout the remaining endosperm wall adjacent to the haustorium surface. These observations indicate that the α-galactosidase present in cell walls of the date palm endosperm during mannan mobilization is not secreted by the haustorium but instead is probably a pregermination product stored mainly in the protein bodies of resting endosperm and is released to the wall following loss of membrane integrity.

Freeze-fracture observations on membranes of dry and hydrated pollen from Collomia, Phoenix and Zea.

Pollen from Collomia grandiflora Dougl. ex Lindl., Phoenix dactylifera L. and Zea mays L. was examined by freeze-fracture electron microscopy. Particular attention was paid to the organization of the cell membranes in the naturally dehydrated, as compared to the fully hydrated, state. All membranes examined had a normal bilayer organization similar to that seen in the hydrated cells of these and other plants. This organization of dry pollen membranes is discussed as it relates to physiological studies (e.g., leakage of ions during hydration), and to biophysical properties of biological and model membranes under various conditions of hydration and dehydration.

Identification of phosphate granules occurring in seedling tissue of two palm species (Phoenix dactylifera and Washingtonia filifera).

Haustoria of two palm species, Phoenix dactylifera L. (date) and Washingtonia filifera (Lindl.) Wendl were carefully dissected from seeds, and the ultrastructural characteristics of the large, electron-opaque granules present in the cells of this tissue were compared using standard aldehyde and OsO4 fixations. In Washingtonia, the granules were smaller than those in date and were more variable in size and presence of non-opaque inclusions. All granules appeared to be membrane bounded although they often filled the bounded space. No protein, lipid, carbohydrate or tannins were found in the granules by standard staining procedures. The granules stained positively with two different metallic-phosphate stains which contained either bismuth or lead. Energy dispersive X-ray microprobe analysis, done on aldehyde-fixed granules and those stained with both phosphate stains, confirmed the fact that phosphorus and calcium were present in the granules. The granules also bound the metallic stains as expected. All procedures consistently confirmed the presence of phosphate in the granules. The data are most consistent with the hypothesis that the granules are composed of polyphosphate.