Abstract: Leaf and minor vein structure were studied in Arabidopsis thaliana (L) Heynh to gain insight into the mechanism(s) of phloem loading Vein density (length of veins per unit leaf area) is extremely low Almost all veins are intimately associated with the mesophyll and are probably involved in loading In transverse sections of veins there are, on average, two companion cells for each sieve element Phloem parenchyma cells appear to be specialized for delivery of photoassimilate from the bundle sheath to sieve element-companion cell complexes: they make numerous contacts with the bundle sheath and with companion cells and they have transfer cell wall ingrowths where they are in contact with sieve elements Plasmodesmatal frequencies are high at interfaces involving phloem parenchyma cells The plasmodesmata between phloem parenchyma cells and companion cells are structurally distinct in that there are several branches on the phloem parenchyma cell side of the wall and only one branch on the companion cell side Most of the translocated sugar in A thaliana is sucrose, but raffinose is also transported Based on structural evidence, the most likely route of sucrose transport is from bundle sheath to phloem parenchyma cells through plasmodesmata, followed by efflux into the apoplasm across wall ingrowths and carrier-mediated uptake into the sieve element-companion cell complex

Abstract: In maize leaves growth at low temperatures causes decreases in maximum catalytic activities of photosynthetic enzymes and reduced amounts of proteins, rather than effects on regulation or co-ordination of the photosynthetic processes. To test the hypothesis that differential localization of antioxidants between the different types of photosynthetic cell in maize leaves is a major determinant of the extreme sensitivity of maize leaves to chilling damage, oxidative damage to proteins, induced by incubation of maize leaves with paraquat, has been measured and compared with the effects incurred by growth at low temperatures. While the increase in protein carbonyl groups caused by paraquat treatment was much greater than that caused by low temperature growth conditions, most carbonyl groups were detected on bundle sheath proteins in both stress conditions. With one or two exceptions proteins located in the mesophyll tissues were free of protein carbonyl groups in both situations. Paraquat treatment caused a complete loss of the psaA gene products, modified the photosystem II reaction centre polypeptide, D1, and increased the number of peptides arising from breakdown of ribulose 1,5-bisphosphate carboxylase oxygenase (Rubisco). In contrast, growth at 15 °C increased the abundance (but not number) of Rubisco breakdown products and decreased that of the psaB gene product while the psaA gene product and PEP carboxylase were largely unaffected. Since bundle sheath proteins are more susceptible to oxidative damage than those located in the mesophyll cells, strategies for achieving a more balanced system of antioxidant defence may be effective in improving chilling tolerance in maize.

Abstract: Light and electron microscopy were used to relate histological and ultrastructural differences of barley leaves treated with different concentrations of salicylic acid (SA, 100 µM-1 mM). Light microscopy revealed that the thickness of all leaf tissue components decreased in SA-treated plants. The effect was most pronounced on the width of the adaxial epidermis and on the size of the bulliform cells. The chloroplast ultrastructure was also affected by SA treatment. Swelling of grana thylakoids in various degrees, coagulation of the stroma, and increase in chloroplast volume were observed. 1 mM SA caused a vast destruction of the whole plastid structure.

Abstract: The distribution of antioxidants between bundle sheath and mesophyll cells of maize leaves was analysed in plants grown at 20°C, 18°C and 15°C. The purity of the isolated bundle sheath and mesophyll fractions was determined using compartment-specific marker enzymes. In plants grown at 15°C, ascorbate peroxidase, CuZn-superoxide dismutase (CuZn-SOD) and monodehydroascorbate reductase activities were increased in the bundle sheath cells, and glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase activities were enhanced in the mesophyll cells. SOD was absent from the mesophyll of plants grown at 20°C but an Fe-SOD activity was found in the mesophyll of plants grown at 15°C. Foliar Mn-SOD activities were decreased at 15°C compared to 20°C. Catalase was undetectable in the mesophyll extracts of plants grown at 15°C. Ascorbate and glutathione contents were considerably higher in the mesophyll than the bundle sheath fractions of plants grown at 20 °C. The ratios of reduced to oxidized forms of these antioxidants were significantly decreased in the bundle sheath, but increased in the mesophyll of leaves grown at 15°C. Foliar H 2 O 2 accumulated at 15 °C compared to 20°C. Most of the foliar H 2 O 2 was localized in the mesophyll tissues at all growth temperatures. The differential distribution of antioxidants between leaf bundle sheath and mesophyll tissues, observed at 20°C, is even more pronounced when plants are grown at 15°C and may contribute to the extreme sensitivity of maize to low temperatures.

Abstract: Phloem loading of sucrose is decisive for the speed of mass flow, because sucrose is the dominant solutein the sieve tube sap of nearly all plant species. The export rate of carbon is linearly correlated to the concentration of sucrose in green leaves. Saturation of export was not observed, because surplus of assimilates is converted to starch, a process which is regulated by the sucrose level in the cytosol. Consequently, an increase of sucrose synthesis by overexpression of SPS did not enhance carbon export (at least under normal ambient conditions). Saturation of sucrose export could be observed only in experimental systems, where sucrose was fed directly to the phloem (e.g. in Ricinus seedling) or where constraints on transport activity were imposed by genetic manipulation either on the transporters (e.g. in sucrose transporter antisense plants) or on the path of sucrose (e.g. in plants trans ormed with TMV movement protein, or by incubation in salts). The balance between carbon storage and carbon export is subject to adaptation to meet growth requirements under special circumstances. For example, in a starch-deficient mutant, the day time export rate is nearly doubled compared to wild type plants. Furthermore, plants under short day illumination greatly accelerated starch storage compared to plants under long day illumination (a modulation which persists even a few days after a shift to long day conditions). Plants with a higher assimilation rate due to elevated ambient CO2 increase the nightly carbon export rate, whereas the export rate in day time rate appeared to work at its upper limit. The overall efficiency of sucrose export and incorporation into biomass is ca 0.65, which is close to the theoretical value of 0.75. Sucrose transport along the phloem strands is modulated according to the input at the source, but the individual phloem strands show also partial coordination with respect to sucrose concentrations (as revealed by NMR-imaging), especially obvious after physical interruption of some vascular bundles.

Abstract: In this paper four classes of co-operative root-shoot interations are addressed. (I) Nitrogen concentrations in the xylem sap originating from the root and in the phloem sap as exported from source leaves are much lower than those required for growth by apices and developing organs. Enrichment of xylem sap N is achieved by xylem to xylem (X-X) transfer, by which reduced N, but not nitrate, is abstracted from the xylem of leaf traces and loaded into xylem vessels serving the shoot apex. Nitrogen enrichment of phloem sap from source leaves is enacted by transfer of reduced N from xylem to phloem (X-P transfer). Quantitative data for the extent of the contribution of X-X and X-P transfer to the nutrition of young organs of Ricinus communis L. and for their change with time are presented. (II) Shoot and root cooperate in nitrate reduction and assimilation. The partitioning of this process between shoot and root is shifted towards the root under conditions of nitrate-and K-deficiency and under salt stress, while P deficiency shifts nitrate reduction almost totally to the shoot. All four changes in partitioning can be attributed to the need for cation-anion balance during xylem transport and the change in electrical charge occurring with nitrate reduction. (III) Even maintenance of the specificity of ion uptake by the root may — in addition to its need for energy — require a shoot-root interaction. This is shown to be needed in the case of the maintenance of K/Na selectivity under the highly adverse condition of salt stress and absence of K supply from the soil. (IV) Hormonal root to shoot interactions are required in the whole plant for sensing mineral imbalances in the soil. This is shown and addressed for conditions of salt stress and of Pdeficiency, both of which lead to a strong ABA signalling from root to shoot but result in different patterns of response in the shoot.

Abstract: Using Lucifer Yellow we have demonstrated that the phloem-loading pathway from the mesophyll to the bundle sheath—vascular parenchyma interface inZea mays source leaves follows a symplasmic route in small and intermediate vascular bundles in control as well as in the green sections of mutant sucrose-export-defective (SXD-1) plants. In the anthocyanin-rich mutant leaf sections, Lucifer Yellow transport was prohibited along the same path, at the bundle sheath—vascular parenchyma interface in particular. Plasmodesmata at the latter interface in SXD-1 anthocyanin-rich leaf sections appear to be structurally altered through callose deposition at the plasmodesmal orifices. We suggest that a transport bottleneck at the bundle sheath—vascular parenchyma interface is thus orchestrated and regulated through callose formation, preventing symplasmic transport across this important loading interface.

Abstract: Glutathione reductase (GR; EC 1.6.4.2) activity was assayed in bundle sheath and mesophyll cells of maize (Zea mays L. var H99) from plants grown at 20 degrees C, 18 degrees C, and 15 degrees C. The purity of each fraction was determined by measuring the associated activity of the compartment-specific marker enzymes, Rubisco and phosphoenolpyruvate carboxylase, respectively. GR activity and the abundance of GR protein and mRNA increased in plants grown at 15 degrees C and 18 degrees C compared with those grown at 20 degrees C. In all cases GR activity was found only in mesophyll fractions of the leaves, with no GR activity being detectable in bundle sheath extracts. Immunogold labeling with GR-specific antibodies showed that the GR protein was exclusively localized in the mesophyll cells of leaves at all growth temperatures, whereas GR transcripts (as determined by in situ hybridization techniques) were observed in both cell types. These results indicate that post-transcriptional regulation prevents GR accumulation in the bundle sheath cells of maize leaves. The resulting limitation on the capacity for regeneration of reduced glutathione in this compartment may contribute to the extreme chilling sensitivity of maize leaves.

Abstract: Heterobaric leaves are characterized by transparent regions in their lamina, due to the occurrence of bundle sheath extensions. Fused silica fibre-optic microprobes were used to monitor light gradients and part of the spectral regime along the bundle sheath extensions, as well as along the mesophyll in the heterobaric leaves of two representative plants, one mesomorphic (Vitis vinifera L.) and one xeromorphic (Quercus coccifera L.). It was found that the attenuation of collimated visible light by the bundle sheath extensions of both plants was weaker than the attenuation by the photosynthetic parenchyma layers. However, only a small portion of the amount of light that strikes the leaf surface is transmitted through these structures. The adaxial epidermis covering the bundle sheath extensions, as well as the mesophyll, afforded similar effective protection against UV radiation in both tissues. The relative amount of the forward-scattered visible light inside the bundle sheath extensions approached that detected by the microprobe at the adaxial illuminated leaf surface. Moreover, light transmitted through the bundle sheath extensions was enriched mainly in the blue and red regions, compared to light transmitted through the photosynthetic tissue. The time course of photosynthetic starch formation in the leaves of V. vinifera detected by iodine staining showed that the accumulation of starch during the first minutes of illumination was high within photosynthetic parenchyma cells adjacent to the bundle sheath extensions. The data showed that bundle sheath extensions act as transparent ‘windows’ which enrich the neighbouring mesophyll areas with high levels of photosynthetically active radiation (400–700 nm). The phenomenon was more pronounced in the thick and compact sclerophyllous leaves of Q. coccifera by virtue of the greater abundance of bundle sheath extensions as compared to that in V. vinifera. The enhancement of the light micro-environment within the deep internal layers of the mesophyll may affect the photosynthetic performance of such leaves, giving adaptive advantages.

Abstract: Stems of plane tree seedlings were cytologically investigated regarding the involvement of tyloses and gels in defense reactions to Ceratocystis fimbriata f. sp platani, the canker stain disease agent. In the lumen of infected xylem vessels, cytochemical tests revealed the occurrence of compounds labeled for cellulose and which seem to be involved in gel formation. Immunogold methods indicated that pectin-rich material accumulated in the paramural area of vessel-associated parenchyma cells and around tylosis walls. Tylosis formation and deposition of pectin-containing gels were associated with metabolic changes of vessel-associated parenchyma cells. It is suggested that parenchyma cells play a key role in the defense of plane tree seedlings through plugging of xylem vessels leading to compartmentalization of the vascular fungus.

Abstract: DNA gel blot analysis suggested that asparagine synthetase (AS; EC 6.3.5.4) occurred as a single gene in rice. A fusion protein consisting of 17 kDa tagged-region from pET32a(+) expression plasmid and 42 kDa N-terminal region of rice AS was first expressed in Escherichia coli. The resulting polypeptide was purified and a mono-specific antibody for rice AS was prepared after affinity-purification with the antigen. Immunoblotting revealed a high content of AS protein in the leaf sheath at the second position from the fully expanded top leaf and in grains at the middle stage of ripening. Accumulation of mRNA for AS was also observed in these organs. During the ripening of the spikelets, the AS protein contents increased during the first 21 days after flowering, then declined rapidly. Immunolocalization analysis revealed signals for AS protein in the companion cells of vascular bundles of leaf sheath and phloem-parenchyma cells, nucellar projection, and nucellar epidermis of dorsal vascular bundles of grains.

Abstract: The small subunit of ribulose-bisphosphate carboxylase (Rubisco), encoded by rbcS, is essential for photosynthesis in both C3 and C4 plants, even though the cell specificity of rbcS expression is different between C3 and C4 plants. The C3 rbcS is specifically expressed in mesophyll cells, while the C4 rbcS is expressed in bundle sheath cells, and not mesophyll cells. Two chimeric genes were constructed consisting of the structural gene encoding β-glucuronidase (GUS) controlled by the two promoters from maize (C4) and rice (C3) rbcS genes. These constructs were introduced into a C4 plant, maize. Both chimeric genes were specifically expressed in photosynthetic organs, such as leaf blade, but not in non-photosynthetic organs. The expressions of the genes were also regulated by light. However, the rice promoter drove the GUS activity mainly in mesophyll cells and relatively low in bundle sheath cells, while the maize rbcS promoter induced the activity specifically in bundle sheath cells. These results suggest that the rice promoter contains some cis-acting elements responding in an organ-pecific and light-inducible regulation manner in maize but does not contain element(s) for bundle sheath cell-specific expression, while the maize promoter does contain such element(s). Based on this result, we discuss the similarities and differences between the rice (C3) and maize (C4) rbcS promoter in terms of the evolution of the C4 photosynthetic gene.

Abstract: A beta-glucosidase that cleaves the biologically inactive hormone conjugates cytokinin-O- and kinetin-N3-glucosides is encoded by the maize Zm-p60.1 gene. The expression of the Zm-p60.1 gene was analyzed by Northern blot analysis and in-situ hybridization. It was found that the expression levels of the Zm-p60.1-specific mRNA changed after pollination of carpellate inflorescences. The Zm-p60.1 cDNA was expressed in E. coli and antibodies were raised against this protein. An antibody was used to determine the tissue-specific localization of this protein. By in situ immunolocalization experiments, this protein was found to be located in cell layers below the epidermis and around the vascular bundles of the coleoptile. In the primary leaf, the Zm-p60.1 protein was detected in cells of the outermost cell layer and around the vascular tissue. In floral tissue, Zm-p60.1 was present in the glumes, the carpels and in the outer cell layer of the style. In coleoptiles, as determined by immuno-electronmicroscopy, the Zmp60.1 protein was located exclusively in the plastids.

Abstract: Penetration, development and reproduction of a Harmony pathotype of Meloidogyne arenaria was studied on two resistant grape rootstocks, 10-23B and RS-3. Associated resistance mechanisms were examined microscopically. Cabernet Sauvignon was used as a susceptible control. The plants were inoculated with freshly hatched second-stage juveniles (J2). Significantly more J2 penetrated roots of Cabernet than 10-23B by 4 days after inoculation. Penetration was delayed by one week in roots of RS-3. Cabernet roots contained a greater number of nematodes coupled with faster development than that found in resistant rootstocks. Hypersensitive responses were expressed in the presence of penetrating J2 in resistant rootstocks only. This occurred along the root epidermis, among the cortical cells and along the differentiating vascular bundle. Cortical necrosis halted or delayed migration of J2 to vascular tissues and vascular necrosis prevented establishment of successful feeding sites which arrest

Abstract: Analyses of vascular saps supplying source and sink organs havedemonstrated the presence of major endogenous hormones and/or theirprecursors. Indol-3yl-acetic acid, a number of gibberellins, cytokininsand abscisic acid, as well as the precursor for ethylene production havebeen found in these vascular saps, allowing the sites of hormonalsynthesis and putative target tissues to be deduced. Exogenously appliedhormones are also readily loaded into these vascular pathways and may betranslocated over considerable distances from a point of application.Observations such as these indicate a possible co-ordination systembetween source and sink regulated by the synthesis and transport ofendogenous hormones. It is widely accepted that the partitioning ofassimilates between photosynthetic source organs and utilising sinkorgans is regulated by endogenous plant hormones. The key intermediatesteps involved in assimilate transport, such as phloem loading andunloading, have been shown to be responsive to applied hormones,although the role of endogenous hormones in these processes remainsessentially unresolved. Results of the analyses of vascular saps fromRicinus communis, which have been obtained using a range ofphysicochemical methods, are compared and contrasted with those obtainedby the application of exogenous hormones or their precursors. Theseresults are evaluated critically and interpreted in the light of currentmodels of source:sink regulatory processes and the long-distancetransport of auxins and cytokinins in higher plants.

Abstract: This short review concentrates on the transport and metabolism of sucrose and hexoses in the storage parenchyma of the sugarcane stalk. Unloading of sucrose from the phloem in the stalk has to proceed symplastically, because barriers around the bundle sheath of ripening stalks prevent apoplastic solute flow. Consequently, unloaded sucrose first appears in the cytosol of storage parenchyma. Sucrose then is subject to several cyclic processes in parallel: a metabolic cycle of sucrose hydrolysis and synthesis, a cycle of sucrose efflux and hexose uptake through plasmalemma-located transport systems, and a cycle of sucrose and hexose transfer into and out of the vacuole. The rate of these cyclic processes changes during the ripening of the internodes. In stalk parenchyma, where the transport rate through the tonoplast seems relatively low, intravacuolar acid invertase exerts the major control over the sucrose content of the cell. In case of rapid sucrose transfer over the tonoplast, as it happens in suspension cells, that control is lifted.

Abstract: To characterize novel genes functioning specifically in mesophyll cells (MCs) or bundle sheath cells (BSCs) of C4 plants, differential screening of a maize cDNA library was conducted using 32P-labeled single-strand cDNAs prepared from MCs and bundle sheath strands (BSS) as probes. Ten genes encoding thylakoid membrane proteins in chloroplasts were identified as MC-abundant genes. These included genes for chlorophyll a/b binding proteins, plastocyanin, PsaD, PsbT, PsbR, PsbO, PsaK, PsaG, PsaN and ferredoxin. Seven genes identified as BSS-abundant genes encoded PEP carboxykinase, salt-inducible SalT homolog, heavy metal-inducible metallothionein-like protein, ABA- and drought-inducible glycine-rich protein, and three proteins of unknown function (one of which was named Bss1). In situ hybridization analyses for several selected genes revealed that mRNAs for the metallothionein-like protein and Bss1 were accumulated specifically in BSCs, and that mRNA for the SalT homolog was accumulated in vascular cells around phloem cells. Results suggest that the functional differentiation of MC chloroplasts accompany preferential expression of these small proteins in photosystem complexes and that BSCs are the major site of stress responses.

Abstract: The alb1 mutant of Lotus japonicus (Ljsym74) forms empty nodules in which most of the bacteria remain in abnormally enlarged infection threads and fail to enter the host plant cells The alb1 mutant was also found to be defective in differentiation of ramified nodule vascular bundles; only a single vascular bundle differentiates at the proximal end of the alb1 nodules and it fails to differentiate further Histochemical analysis using fluorescein-conjugated wheat-germ agglutinin (F-WGA) indicated that the mutation in the ALB1 gene specifically affects the differentiation of vascular bundles in nodules Analysis of nodulin gene expression revealed that the expression of an early nodulin gene, ENOD40, was very low in alb1 nodules At early developmental stages of alb1 nodules, the pattern of ENOD40 transcription was essentially the same as that in wild-type nodules; transcripts were localized in dividing cortical cells and in the pericycle of the root stele opposite nodule primordia, as in wild-type nodules However, mature alb1 nodules exhibited very weak or no expression of ENOD40 in the peripheral cells of the undeveloped nodule vascular bundle The ENOD40 expression pattern in alb1 nodules is distinct from that in another ineffective mutant, fen1 (Ljsym76), in which ENOD40 expression persists prior to premature senescence These findings lead us to speculate that ENOD40 may play a role in the differentiation of nodule vascular bundles

Abstract: Evidence suggests that C 4 plants produce greater amounts of biomass per unit of intercepted photosynthetically active radiation. This is due in large part to two factors. First, C 4 plants have a greater quantum yield than C 3 plants at 30 °C (the C 4 advantage diminishes at lower temperatures and as atmospheric CO 2 partial pressures rise). Second, C 4 plants have greater rates of CO 2 assimilation per unit leaf nitrogen (this benefit diminishes as leaf area index and/or canopy nitrogen content increases). The protein cost of C 4 enzymes per unit chlorophyll is calculated and found to be similar to that of C 3 photosynthesis. However, the rate of CO 2 assimilation per unit nitrogen in C 4 plants is greater than that of C 3 plants because high CO 2 partial pressure in the bundle sheath cells enables Rubisco to operate near its maximum catalytic rate and suppresses photorespiration. Rice leaf anatomy is examined with respect to locating the C 4 metabolism. Chloroplasts in bundle sheath cells represent only a minute fraction of those present in the rice leaf. In addition, whereas mesophyll cells are immediately adjacent to bundle sheath cells in terrestrial C 4 leaves, there are numerous mesophyll cells between adjacent veins in rice, which would diminish the efficiency of the C 4 cycle. To engineer the C 4 pathway into rice is therefore a formidable challenge.

Abstract: In order to investigate the roles of different cell types, metabolite compartmentation in barley (Hordeum vulgare L.) leaf tissue was mapped at the single-cell level, using single-cell sampling and analysis (SiCSA) tech- niques. The partitioning of recently fixed photoassimilate was investigated for the first time at single-cell resolution, using BAMS (biological accelerator mass spectroscopy) for precise measurement of 14 C in femtomole quantities. The data obtained by BAMS qualitatively reflect concentrations of sugars in different cell types measured by SiCSA. Calculation of 14 C-specific activities showed that the radioactive label saturated the mesophyll and parenchymatous bundle sheath (PBS) pools within the 45-min labelling period. During the photoperiod, sucrose concentration increased to 200 mM in mesophyll cells. The concentration of malate also increased during the photoperiod in meso- phyll and PBS cells. Epidermal cells contained very low concentrations of sugar but high concentrations of malate (120-180 mM) and did not show significant diurnal changes. Accumulation of sugars and fructan synthesis could be induced in mesophyll and PBS cells by reduced export of sugars from leaves or, alternatively, when sugars were sup- plied from excised leaf blade bases immersed in a sucrose solution in the dark. The epidermis accumulated addi- tional malate in step with the accumulation of sugar by the mesophyll/PBS cells during the long-term reduction of export. Immunolocalisation of Rubisco and cytochrome oxidase proteins was used to analyse the distribution of enzymes of photoassimilation and respiration between functionally different cells in mature leaves of barley.

Abstract: The distance between sites of synthesis of assimilates and the site of phloem loading can be large, and specialized leaf cell layers such as the paraveinal mesophyll (PVM) might act to enhance the efficiency of transport. A number of techniques were used to analyse PVM of legume leaves with respect to a hypothesized function in transfer of assimilates between tissues. Of 39 legume species examined, PVM was found in 22. Leaves of all PVM- containing species had multiple palisade parenchyma layers, while non-PVM species generally had only one distinct palisade layer. Morphometric analysis identified a significant correlation between PVM presence and greater numbers of palisade cells per unit leaf surface area. Comparison of photosynthetic rates of four PVM and four non- PVM species showed the PVM species had higher rates on a leaf area basis than all but one of the non-PVM species. Microautoradiography of 14 CO2 pulse-chase studies in soybean demonstrated PVM is an intermediary tissue in transfer of assimilates to vascular bundles. In addition, PVM cells but not mesophyll cells, were enriched in a sucrose binding protein previously found to be associated with sucrose-transporting tissues. The structural, positional and transport data support the hypothesis that the PVM acts as a transport pathway between the vascular system and photoassimilatory cells of the leaf, and has probably evolved to overcome diffusion limitations imposed by multiple palisade layers.

Abstract: New morphometric data gathered from cross-sections of two Lower Devonian land plants (Rhynia gwynne-vaughanii and Asteroxylon mackiei) are interpreted in terms of the evolution of the function of vascular bundles in early land plants. The following conclusions can be drawn from these new data: (1) The ratio of the cross-sectional area of the xylem (representing the conducting volume supplying the axis with water) to the xylem perimeter (representing the “contact area” between xylem and parenchyma through which water leaves the xylem and enters the parenchyma) is not constant for Rhynia axes, almost constant for Asteroxylon axes, and different between Rhynia and Asteroxylon. Thus, Bower´s hypothesis that the ratio of cross-sectional area of the xylem to xylem perimeter is constant during ontogenetic development is true for Asteroxylon. That this ratio is constant during phylogeny, however, is not supported by our data. (2) The ratio between cross-sectional area of xylem to parenchyma is higher in ...

Abstract: The cloning of promoter sequences of two invertase genes from potato (Solanum tuberosum L.) is described. Histochemical analysis of series of reporter transgenic lines reveals phloem-specific expression from both promoters, with one expressed preferentially in internal phloem and the other in external phloem of stem vascular bundles.

Abstract: Viral movement in the leaf tissues of a resistant host, Cucumis figarei, inoculated with the pepo strain of Cucumber mosaic virus (CMV) and incubated at 24°C or 36°C was investigated by fluorescence in situ hybridization (FISH), leaf-press blotting, tissue printing and immunogold-silver staining techniques. Observation by FISH revealed that at 24°C most infection sites with CMV at 0.01 mg/ml or 0.1 mg/ml were limited to a single cell during the incubation period, that the number of infection sites increased from 24hpi (hours post inoculation) to 80 hpi in the leaves inoculated with CMV at 0.5 mg/ml, and that the size as well as the number of infection sites rapidly increased with time in the leaves inoculated with CMV at 2.0 mg/ml. These results suggested that one factor for the resistance of C. figarei at 24°C might be an inhibition of viral movement in and out of the infection sites. Leaf-press blotting and tissue blotting indicated that CMV remained in the infection sites at 24°C, whereas it spread from the inoculated leaves to other parts of the plants through vascular systems at 36°C. Immunogold-silver staining demonstrated that at 24°C CMV infected bundle sheath (BS) cells in minor veins, whereas at 36°C it invaded not only BS cells, but also phloem parenchyma (PP)/ companion cell (CC) or PP/intermediary cell (IC) complexes in minor veins in the regions with chlorotic symptoms. These results indicated that at 24°C CMV had difficulty in passing through the interface between BS and PP/CC or PP/ IC complexes and that viral entry from mesophyll to the phloem pathway was inhibited in the inoculated leaves.

Abstract: The isolation of bundle sheath protoplasts from leaves of Zea mays L. for patch clamp whole-cell experiments presents special problems caused by the suberin layer surrounding these cells. These problems were overcome by the isolation technique described here. Two different types of whole-cell response were found: a small response caused by MB-1 (maize bundle sheath conductance type 1) which was instantaneously activated, and another caused by MB-2 (maize bundle sheath conductance type 2) consisting of an instantaneous response (maize bundle sheath K+ instantaneous current type 2; MB-KI2) similar to but stronger than the current through MB-1 plus a small time-dependent outward rectifying component (maize bundle sheath activated outward rectifying current; MB-AOR) with voltage-dependent delayed activation. The occurrence of MB-AOR was often accompanied by a smaller contribution from an inward rectifying channel at negative potentials. Activation of MB-2 required ATP. It is suggested that MB-1 and MB-2 are related to bundle sheath cells with and without direct contact with the xylem vessels. In mesophyll cells, only one type of response caused by MM-2 (maize mesophyll conductance type 2) was found with an instantaneous (maize mesophyll K+ instantaneous current type 2, MM-KI2) and a voltage-dependent delayed component (maize mesophyll activated outward rectifying current, MM-AOR). The most striking difference between bundle sheath and mesophyll cells was the pH dependence of K+ uptake. At pH 7.2, uptake of K+ by MB-2 was identical to that by MM-2 over the whole voltage range. However, acidification stimulated K+ conductance in bundle sheath cells, whereas a decrease was found for MM-2. At pH 6.15, the bundle sheath channel MB-2 had more than a 10-fold higher K+ uptake at positive and negative potentials than MM-2. The channel MB-1, too, was stimulated by low pH. This seems to indicate a putative role for MB-1 and MB-2 in charge balance during uptake of nutrients via cotransport from the xylem into the symplasm.

Abstract: The distribution and the ontogenesis of lateral roots have been investigated in the Mediterranean woody species Quercus suber L. (cork oak). Lateral roots arose in protoxylem-based ranks and a tendency to clumping was observed. Three stages are distinguished in lateral root primordium development. Lateral root primordia are derived mainly from pericycle cells. The endodermis contributed to the initial lateral root development, forming an endodermal cover that sloughs off with lateral root emergence. The unemerged lateral roots show an incipient layered root meristem; this meristem can be classified as a closed type meristem. Primary vascular connection takes place with the xylem strand opposite the lateral root primordium and the two adjacent phloem strands. Primary vascular connector elements are derived from pericyclic derivative cells. Vascular parenchyma cells contribute mainly in the development of the cambium and the subsequent secondary xylem and phloem connector elements. The secondary vascular el...

Abstract: The relation between the vascular system and the photosynthate translocation pathway was investigated in main stems, sympodiums, and inflorescences of tomato plants. The plants had the 1st inflorescence on the main shoot with 9 leaves, and the 1st, 2nd and 3rd lateral shoots with 3 leaves from each terminal node. The shoot alternated right- and left-handed phyllotaxis. There were 4 vascular bundles in the hypocotyl and 8 vascular bundles in the main stem and sympodiums. Four vascular bundles were located at the base of the 1st inflorescence. Two were connected to the 7th leaf and to the 2nd or 4th flowers ; the other two were connected to the 8th leaf and the 1st or 3rd flowers. When 14CO2 was fed to 7th or 8th leaf, the highest radioactivity was recovered in the internode just below the treated leaf among the internodes on the main stem and sympodiums. In each internode, particularly strong radioactivity was recovered in 3 of 8 vascular bundles. Among the pedicels in the 1st inflorescence, the highest radioactivity was recovered in the 1st and 3rd ones when the 8th leaf was treated, and in the 2nd and 4th pedicels when the 7th leaf was fed with 14CO2.