Minor vein structure and sugar transport in Arabidopsis thaliana

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. Received: 5 October 1999 / Accepted: 20 November 1999     

PLASMODESMATAL FREQUENCY AND OTHER STRUCTURAL ASPECTS OF ASSIMILATE COLLECTION AND PHLOEM LOADING IN LEAVES OF SONCHUS OLERACEUS (ASTERACEAE), A SPECIES WITH MINOR VEIN TRANSFER CELLS

Leaves of Sonchus oleraceus (Asteraceae) were examined with the electron microscope to determine plasmodesmatal frequencies and other structural features relating to the collection of photoassimilate and its subsequent loading into minor veins. Few plasmodesmata occur between mesophyll cells, which contain chloroplasts that are sometimes connected to both the plasmalemma and the tonoplast by membranous tubules. The minor veins consist of tracheary elements, sieve-tube members, vascular parenchyma cells, and companion cells. The latter two cell types are transfer cells, with some of the fingerlike wall ingrowths in companion cells being traversed lengthwise by plasmodesmata. The frequencies of plasmodesmata at the mesophyllbundle sheath boundary and within are higher at some interfaces than at corresponding interfaces in nine other species, including some that previously had been characterized as loading assimilate via the symplast. It is thus premature to designate all species containing transfer cells in their minor veins as loading assimilate only via the apoplast.     

STUDIES ON THE LEAF OF POPULUS DELTOIDES (SALICACEAE): ULTRASTRUCTURE,PLASMODESMATAL FREQUENCY,AND SOLUTE CONCENTRATIONS

The minor veins and contiguous tissues of mature leaves of Populus deltoides Bartr. ex Marsh. were examined with the electron microscope to determine the ultrastructural characteristics of the component cells and to determine the structure, distribution, and frequency of plasmodesmata between the various cell types. In addition, plasmolytic studies were carried out to determine the solute concentrations of the various cell types of the minor veins and contiguous tissues. The cells comprising the mesophyll and bundle sheath contain all the components typical of photosynthetic cells. Paraveinal mesophyll cells and bundle-sheath cells have fewer microbodies and smaller chloroplasts than do palisade parenchyma cells. Vascular parenchyma and companion cells tend to intergrade with one another structurally but can be distinguished from one another by their characteristic plastids. The mature, enucleate sieve-tube member is lined by a parietal layer of cytoplasm consisting of plasmalemma, endoplasmic reticulum, mitochondria, plastids, and P-protein. Plasmodesmata occur along all possible routes from the palisade parenchyma cells to the sieve tubes of the minor veins, and their frequency increases with increasing proximity to the sieve-tube members. Plasmolytic studies revealed that the paraveinal mesophyll cells had a higher C₅₀ (estimated mannitol concentration plasmolyzing, on the average, 50% of a given cell type) than any other cell type of the leaf. Concentration gradients existed along the palisade cell/bundle-sheath cell/companion cell (or vascular parenchyma cell) route as well as along the paraveinal mesophyll cell/bundle-sheath cell/companion cell (or vascular parenchyma cell) route. Considering the frequency of plasmodesmata along these routes, it is conceivable that photosynthate diffuses from palisade cells to the companion cells along concentration gradients. Within the minor veins, the C₅₀ was higher for sieve-tube members than for either companion cells or vascular parenchyma cells, indicating that loading of the sieve tubes is an active, energy-dependent process.     

Plasmodesmatal frequency in relation to short-distance transport and phloem loading in leaves of barley (Hordeum vulgare). Phloem is not loaded directly from the symplast

We investigated the phloem loading pathway in barley, by determining plasmodesmatal frequencies at the electron microscope level for both intermediate and small blade bundles of mature barley leaves. Lucifer yellow was injected intercellularly into bundle sheath, vascular parenchyma, and thin-walled sieve tubes. Passage of this symplastically transported dye was monitored with an epifluorescence microscope under blue light. Low plasmodesmatal frequencies endarch to the bundle sheath cells are relatively low for most interfaces terminating at the thin- and thick-walled sieve tubes within this C₃ species. Lack of connections between vascular parenchyma and sieve tubes, and low frequencies (0.5% plasmodesmata per μm cell wall interface) of connections between vascular parenchyma and companion cells, as well as the very low frequency of pore-plasmodesmatal connections between companion cells and sieve tubes in small bundles (0.2% plasmodesmata per μm cell wall interface), suggest that the companion cell-sieve tube complex is symplastically isolated from other vascular parenchyma cells in small bundles. The degree of cellular connectivity and the potential isolation of the companion cell-sieve tube complex was determined electrophysiologically, using an electrometer coupled to microcapillary electrodes. The less negative cell potential (average –52 mV) from mesophyll to the vascular parenchyma cells contrasted sharply with the more negative potential (–122.5 mV) recorded for the companion cell-thin-walled sieve tube complex. Although intercellular injection of lucifer yellow clearly demonstrated rapid (0.75 μm s^-1) longitudinal and radial transport in the bundle sheath-vascular parenchyma complex, as well as from the bundle sheath through transverse veins to adjacent longitudinal veins, we were neither able to detect nor present unequivocal evidence in support of the symplastic connectivity of the sieve tubes to the vascular parenchyma. Injection of the companion cell-sieve tube complex, did not demonstrate backward connectivity to the bundle sheath. We conclude that the low plasmodesmatal frequencies, coupled with a two-domain electropotential zonation configuration, and the negative transport experiments using lucifer yellow, precludes symplastic phloem loading in barley leaves.     

Vascular invasion routes and systemic accumulation patterns of tobacco mosaic virus in Nicotiana benthamiana

Plant viruses must enter the host vascular system in order to invade the young growing parts of the plant rapidly. Functional entry sites into the leaf vascular system for rapid systemic infection have not been determined for any plant/virus system. Tobacco mosaic virus (TMV) entry into minor, major and transport veins from non-vascular cells of Nicotiana benthamiana in source tissue and its exit from veins in sink tissue was studied using a modified virus expressing green fluorescent protein (GFP). Using a surgical procedure that isolated specific leaf and stem tissues from complicating vascular tissues, we determined that TMV could enter minor, major or transport veins directly from non-vascular cells to produce a systemic infection. TMV first accumulated in abaxial or external phloem-associated cells in major veins and petioles of the inoculated leaf and stems below the inoculated leaf. It also initially accumulated exclusively in internal or adaxial phloem-associated cells in stems above the inoculated leaf and petioles or major veins of sink leaves. This work shows the functional equivalence of vein classes in source leaves for entry of TMV, and the lack of equivalence of vein classes in sink leaves for exit of TMV. Thus, the specialization of major veins for transport rather than loading of photoassimilates in source tissue does not preclude virus entry. During transport, the virus initially accumulates in specific vascular-associated cells, indicating that virus accumulation in this tissue is highly regulated. These findings have important implications for studies on the identification of symplasmic domains and host macromolecule vascular transport.     

Vascular Anatomy of Angiospermous Leaves,with Special Consideration of the Maize Leaf

In this brief review an attempt has been made to discuss some of the important features of the vascular anatomy of angiospermous leaves, especially those related to assimilate transport. Accordingly, emphasis has been placed on the small or minor veins, which are closely related spatially to the mesophyll, and which play a major role in the uptake and subsequent transport of photosynthates from the leaf. The small veins are enclosed by bundle sheaths that intervene between the mesophyll and vascular tissues and greatly increase the area for contact with mesophyll cells. In the minor veins of dicotyledonous leaves, parenchymatic cells having organelle-rich protoplasts and numerous cytoplasmic connections with sieve elements dominate quantitatively. It is these so-called intermediary cells that apparently are directly involved with the loading of assimilates into the sieve elements. In the maize leaf the small and intermediate bundles have two types of sieve tubes, relatively thin-walled ones that have numerous cytoplasmic connections with companion cells, and thick-walled ones that lack companion cells but have numerous connections with vascular parenchyma cells. The companion cell-sieve tube complexes are virtually isolated symplastically from other cells of the vascular bundle and from the bundle sheath. Thick-walled sieve tubes similar to those in the maize leaf have been recorded in the leaves of other grasses.     

Intercellular Symplastic Connection and Isolation of the Unloading Zone in Flesh of the Developing Grape Berry

The uhrastructure and intercellular connection of the sugar unloading zone (i. e. the phloem in the dorsal vascular bundle and the phloem-surrounding the assimilate sink-cells) of grape ( Vitis vinifera x V. labrusca cv. Jingchao) berry was observed via transmission electron microscopy. The results showed that during the early developmental stages of grape berry, numerous plasmodesmata were found in the phloem between sieve element (SE) and companion cell (CC), between SE/CC complexes, between SE/CC complex and phloem parenchyma cell and in between phloem parenchyma cells, which made the phloem a symplastic integration, facilitating sugar unloading from sieve elements into both companion cells and phloem parenchyma cells via a symplastic pathway. On the contrary, there was almost no plasmodesma between phloem and its surrounding flesh photoassimilate sink-cells, neither in between the flesh photoassimilate sink-cells giving rise to a symplastic isolation both between phloem and its surrounding flesh photoassimilate sink-cells, as well as among the flesh photoassimilate sink-cells. This indicated that both the sugar unloading from phloem and pestphloem transport of sugars should be mainly via an apoplastic pathway. Dining the ripening stage, most of the plasmodesmata between SE/CC complex and the surrounding phloem parenchyma cells were shown to be blocked by the electron-opaque globules, and a phenomenon of plasmolysis was found in a number of companion cells, indicating a symplastic isolation between SE/CC complex and its surrounding parenchynm cells during this phase. The symplastic isolation between the whole phloem and its surrounding photoassimilate sink-cells during the early developmental stages shifted to a symplastic isolation within the phloem during the ripening phase, and thus the symplastic pathway of sugar unloading from SE/CC complex during the early development stages should be replaced by a dominant apoplastic unloading pathway from SE/CC complex in concordance.     

Leaf structure in relation to solute transport and phloem loading in Zea mays L.

Small and intermediate (longitudinal) vascular bundles of the Zea mays leaf are surrounded by chlorenchymatous bundle sheaths and consist of one or two vessels, variable numbers of vascular parenchyma cells, and two or more sieve tubes some of which are associated with companion cells. Sieve tubes not associated with companion cells have relatively thick walls and commonly are in direct contact with the vessels. The thick-walled sieve tubes have abundant cytoplasmic connections with contiguous vascular parenchyma cells; in contrast, connections between vascular parenchyma cells and thin-walled sieve tubes are rare. Connections are abundant, however, between the thin-walled sieve tubes and their companion cells; the latter have few connections with the vascular parenchyma cells. Plasmolytic studies on leaves of plants taken directly from lighted growth chambers gave osmotic potential values of about-18 bars for the companion cells and thin-walled sieve tubes (the companion cell-sieve tube complexes) and about-11 bars for the vascular parenchyma cells. Judging from the distribution of connections between various cell types of the vascular bundles and from the osmotic potential values of those cell types, it appears that sugar is actively accumulated from the apoplast by the companion cell-sieve tube complex, probably across the plasmalemma of the companion cell. The thick-walled sieve tubes, with their close spatial association with the vessels and possession of plasmalemma tubules, may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream. The transverse veins have chlorenchymatous bundle sheaths and commonly contain a single vessel and sieve tube. Parenchymatic elements may or may not be present. Like the thick-walled sieve tubes of the longitudinal bundles, the sieve tubes of the transverse veins have plasmalemma tubules, indicating that they too may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream.     

LEAF OF SPINACIA OLERACEA (SPINACH): ULTRASTRUCTURE,AND PLASMODESMATAL DISTRIBUTION AND FREQUENCY,IN RELATION TO SIEVE-TUBE LOADING

Minor veins and contiguous tissues of the Spinacia oleracea leaf were analyzed by electron microscopy to determine the characteristics of the component cells and the structure, distribution, and frequency of plasmodesmata between the various cell types of the leaf. Mesophyll and bundle-sheath cells contain components typical of photosynthetic cells although the latter cell type contains smaller chloroplasts and fewer mitochondria and microbodies than the mesophyll cells. In addition, the mesophyll cells contain numerous invaginations of the plasmalemma bordering the chloroplasts and evaginations of the outer membrane of the opposing chloroplast envelope. In places, these membranes appear continuous with each other. The minor veins consist of tracheary elements, xylem parenchyma cells, sieve-tube members, companion and phloem parenchyma cells, and other cells simply designated vascular parenchyma cells. The companion and phloem parenchyma cells are typically larger than the sieve-tube members with the companion cells containing a much denser cytoplasm that the phloem parenchyma. Cytoplasmic connections occur along all possible routes from the mesophyll to the sieve-tube members and consist of either simple or branched plasmodesmata between parenchymatic elements or pore-plasmodesmata between the sieve-tube members and parenchyma cells. The highest frequency of plasmodesmata occurs between the sieve-tube members and companion cells, although the value is essentially the same as between the various parenchymatic elements of the phloem. Compared to several previously studied species, the frequency of plasmodesmata between cell types of the spinach leaf is low. These results are discussed in relation to apoplastic vs. symplastic solute transport and sieve-tube loading in this species.     

Relation of beet yellows virus to the phloem and to movement in the sieve tube

In minor veins of leaves of Beta vulgaris L. (sugar beet) yellows virus particles were found both in parenchyma cells and in mature sieve elements. In parenchyma cells the particles were usually confined to the cytoplasm, that is, they were absent from the vacuoles. In the sieve elements, which at maturity have no vacuoles, the particles were scattered throughout the cell. In dense aggregations the particles tended to assume an orderly arrangement in both parenchyma cells and sieve elements. Most of the sieve elements containing virus particles had mitochondria, plastids, endoplasmic reticulum, and plasma membrane normal for mature sieve elements. Some sieve elements, however, showed evidence of degeneration. Virus particles were present also in the pores of the sieve plates, the plasmodesmata connecting the sieve elements with parenchyma cells, and the plasmodesmata between parenchyma cells. The distribution of the virus particles in the phloem of Beta is compatible with the concept that plant viruses move through the phloem in the sieve tubes and that this movement is a passive transport by mass flow. The observations also indicate that the beet yellows virus moves from cell to cell and in the sieve tube in the form of complete particles, and that this movement may occur through sieve-plate pores in the sieve tube and through plasmodesmata elsewhere.     

Ethylene-induced microtubule reorientations: mediation by helical arrays

Pruned source-sink transport systems from predarkened plants of Amaranthus caudatus L. and Gomphrena globosa L. were used to study the localization of ¹⁴C-labeled photosynthate imported into experimentally induced sink leaves by microautoradiography. During a 6-h (Amaranthus) or a 4-h (Gomphrena) transport period, ¹⁴C-assimilates were translocated acropetally from a mature source leaf provided with ¹⁴CO₂, into a younger induced sink leaf (dark/-CO₂). In addition, a young still-expanding source leaf exposed to ¹⁴CO₂ exported ¹⁴C-assimilates basipetally into a mature induced sink leaf (dark/-CO₂). Microautoradiographs showed that imported ¹⁴C-photosynthate was strongly accumulated in the sieve element/companion cell complexes of midveins, secondary veins, and minor veins of both the mature and the expanding sink leaf. Some label was also present in the vascular parenchyma and bundlesheath cells. In petioles, ¹⁴C-label was concentrated in the sieve element/companion cell complexes of all bundles indicating that assimilates were imported and distributed via the phloem. Moreover, a considerable amount of radioactivity unloaded from the sieve element/companion cell complexes of petiolar bundles, was densely located at sites of secondary wall thickenings of differen-tiating metaxylem vessels, and at sites of chloroplasts of the vascular parenchyma and bundle-sheath cells. These observations were more striking in petioles of Gomphrena than Amaranthus.Abbreviation se/cc sieve element/companion cell     

Current perspectives on plasmodesmata: structure and function

Recent studies on plasmodesmata have shown that these important intercellular passages for communication and transport are much more sophisticated in both structure and regulatory abilities than previously imagined. A complex, but not well understood, substructure has been revealed by a variety of increasingly reliable ultrastructural techniques. Proteinaceous particles are seen within the cytoplasmic sleeve surrounding the desmotubule. Dye-coupling studies have provided experimental evidence for the physical pathway of solute movement, supporting conclusions about substructural dimensions within plasmodesmata drawn from the ultrastructural studies. Calcium has been identified as a major factor in the regulation of intercellular communication via plasmodesmata. Evidence from studies on virus movement through plasmodesmata suggests a direct interaction between virallycoded movement proteins and plasmodesmata in the systemic spread of many viruses. There is increasing evidence, albeit indirect, that in some plant species phloem loading may involve transport of photoassimilate entirely within the symplast from mesophyll cells to the sieve element-companion cell complexes of minor veins.     

Photoassimilate-transport characteristics of nonchlorophyllous and green tissue in variegated leaves of Coleus blumei Benth

The nonchlorophyllous (albino) tissue of mature C. blumei leaves is a sink for photoassimilate. Transport from the green to the albino region of the same leaf was inhibited by cold and anoxia. When the green tissue of mature leaves was removed, the remaining albino portion imported labeled translocate from other mature leaves in the phloem. Photoassimilate unloading in the albino region of mature leaves was studied by quantitative autoradiography. The unloading was inhibited by cold but not by anoxia. No labeled photoassimilate could be detected in the free space of mature albino tissue by compartmental efflux analysis as phloem unloading proceeded in a N₂ atmosphere, indicating that unloading, may occur by a symplastic pathway as it apparently does in sink leaves of other species. The minor veins of mature albino leaf tissue did not accumulate exogenous [¹⁴C]sucrose. Minor veins of green tissue in the same leaves accumulated [¹⁴C]sucrose but, in contrast to other species studied to date, this accumulation was insensitive to the inhibitor p-chloromercuribenzensulfonic acid (PCMBS).In its capacity to import and unload photoassimilate, and in the inability, of the minor veins to accumulate exogenous sucrose, the albino region of the mature C. blumei lamina differs from mature albino tobacco leaves and darkened mature leaves of other species. This, together with evidence indicating that phloem loading in C. blumei and other species may occur by different routes and with different sensitivity to PCMBS, indicates that the mechanism of transfer of photoassimilates between veins and surrounding tissues, and the mechanism of the sink-source transition, may not be the same in the leaves of all species. It is speculated that the unusual properties of the C. blumei leaf may be a consequence of the presence, in the minor veins, of intermediary cells, large companion cells connected to the bundle sheath by abundant plasmodesmata.Abbreviation PCMBS p-chloromercuribenzenesulfonic acid     

Ultrastructure of ferritin in the leaves of <Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis> under stress conditions

The location and structure of ferritin in the parenchyma of leaf minor veins of the common ice plant (Mesembryanthemum crystallinum L.) treated with exogenous putrescine under salinity conditions were investigated by electron microscopy. Considerable aggregates of ferritin were detected in the chloroplasts of bundle sheath cells, in companion phloem cells, and other parenchyma cells of leaf minor veins. The structure of ferritin in the vascular parenchyma chloroplasts suggests that it was partially degraded and converted to phytosiderin. This point of view is based on indistinct structure of Fe-containing cores of ferritin molecules, break of distance between the cores, and their pronounced ability to aggregate and produce larger structures. Aggregation of Fe-containing cores apparently pointed to the destruction of ferritin protein envelope or its partial degradation. In a certain stage of ferritin destruction, electron-dense material and the structures resembling small vesicles appeared between the Fe-containing cores. Electron-dense inclusions, whose structure was similar to that of phytosiderin, were also detected in the vacuoles. Examination of the cross sections done without additional staining showed that the same as ferritin, phytosiderin in the chloroplasts and vacuoles was dark-colored against weakly colored cellular structures. In the vascular parenchyma of control plant leaves, the level of ferritin and phytosiderin was greater than in the mesophyll and much lower than in the plants simultaneously treated with NaCl and putrescine. In control material, iron cores of ferritin and phytosiderin were more light-colored and 2–3 times smaller in size than in the experimental treatment. Destruction of ferritin essentially did not occur in the mesophyll but was observed in the chloroplasts of bundle sheath cells on the border between the mesophyll and vascular bundle. The presence of much ferritin and phytosiderin on the border between the mesophyll and the vessels is accounted for by the fact that the vascular parenchyma is a buffer area that maintains a specific concentration of iron in the mesophyll of leaves and other parts of the plant. Within the cell, the role of such a buffer is performed by ferritin and vacuoles. Transformation of ferritin to insoluble hydrophobic phytosiderin is supposed to be an efficient way of withdrawing the excess of active iron from the cellular metabolism and therefore of relaxing oxidative stress. Ferritin and phytosiderin were detected not only in parenchyma cells of leaf minor veins but in sieve tubes as well. This suggests that iron may be transported within the plant as a component of protein complex.     

Bromovirus movement protein conditions for the host specificity of virus movement through the vascular system and affects pathogenicity in cowpea

Previously, we reported that CCMV(B3a), a hybrid of bromovirus Cowpea chlorotic mottle virus (CCMV) with the 3a cell-to-cell movement protein (MP) gene replaced by that of cowpea-nonadapted bromovirus Brome mosaic virus (BMV), can form small infection foci in inoculated cowpea leaves, but that expansion of the foci stops between 1 and 2 days postinoculation. To determine whether the lack of systemic movement of CCMV(B3a) is due to restriction of local spread at specific leaf tissue interfaces, we conducted more detailed analyses of infection in inoculated leaves. Tissue-printing and leaf press-blotting analyses revealed that CCMV(B3a) was confined to the inoculated cowpea leaves and exhibited constrained movement into leaf veins. Immunocytochemical analyses to examine the infected cell types in inoculated leaves indicated that CCMV(B3a) was able to reach the bundle sheath cells through the mesophyll cells and successfully infected the phloem cells of 50% of the examined veins. Thus, these data demonstrate that the lack of long-distance movement of CCMV(B3a) is not due to an inability to reach the vasculature, but results from failure of the virus to move through the vascular system of cowpea plants. Further, a previously identified 3a coding change (A776C), which is required for CCMV(B3a) systemic infection of cowpea plants, suppressed formation of reddish spots, mediated faster spread of infection, and enabled the virus to move into the veins of inoculated cowpea leaves. From these data, and the fact that CCMV(B3a) directs systemic infection in Nicotiana benthamiana, a permissive systemic host for both BMV and CCMV, we conclude that the bromovirus 3a MP engages in multiple activities that contribute substantially to host-specific long-distance movement through the phloem.     

Movement of lucifer yellow in leaves of Coleus blumei Benth

Abstract Individual spongy mesophyll cells in green areas of variegated Coleus blumei leaves were injected with the symplast-mobile dye lucifer yellow and its movement to other cell types was followed with fluorescence microscopy. In 13 trials, the dye remained in the injected cell twice, moved only to other mesophyll cells five times, and moved up to and along minor veins six times. Where extensive movement of the dye occurred, the tissue was fixed with 4% glutaraldehyde, dehydrated, embedded in plastic, sectioned at 3 μm, and examined again with fluorescence microscopy. The dye was found in abaxial bundle-sheath cells for up to 200 μm or more distant from the site of injection near the minor vein, but no convincing evidence was found for its presence in the vascular tissue itself. It thus appears that superficial whole-mount views of lucifer yellow movement along leaf minor veins cannot be taken as certain evidence for symplastic transport of the dye into and along the vascular tissues.     

西瓜小叶脉超微结构研究

用透射电子显微技术研究了西瓜叶片小叶脉,结果表明,小叶脉是由大型维管束鞘细胞包围的维管束,维管束呈现大的头部和线形的柄部,柄部是单列细胞的木质部,由维管薄壁细胞和导管分子组成;头部是韧皮部,由维管薄壁细胞、伴胞和筛管分子组成。同一小叶脉内常见有超微结构特征显著不同的两种伴胞:一种伴胞体积小,与维管束鞘细胞接触面较小或不接触,细胞内有大液泡,细胞壁上没有胞间连丝或只有少数不分枝的胞间连丝,这种伴胞为2a型;另一种伴胞体积大,通常位于韧皮部两翼,不含大液泡而含大量小泡,与维管束鞘细胞接触面较大,接触面上有大量具分枝的胞间连丝,分枝部分比未分枝部分直径小,这种伴胞为中间细胞类型。显然,西瓜是小叶脉内兼具两种类型伴胞的植物。     

西瓜小叶脉超微结构研究

用透射电子显微技术研究了西瓜叶片小叶脉,结果表明,小叶脉是由大型维管束鞘细胞包围的维管束,维管束呈现大的头部和线形的柄部,柄部是单列细胞的木质部,由维管薄壁细胞和导管分子组成;头部是韧皮部,由维管薄壁细胞、伴胞和筛管分子组成。同一小叶脉内常见有超微结构特征显著不同的两种伴胞：一种伴胞体积小,与维管束鞘细胞接触面较小或不接触,细胞内有大液泡,细胞壁上没有胞间连丝或只有少数不分枝的胞间连丝,这种伴胞为2a型;另一种伴胞体积大,通常位于韧皮部两翼,不含大液泡而含大量小泡,与维管束鞘细胞接触面较大,接触面上有大量具分枝的胞间连丝,分枝部分比未分枝部分直径小,这种伴胞为中间细胞类型。显然,西瓜是小叶脉内兼具两种类型伴胞的植物。     

A morphometric analysis of the phloem-unloading pathway in developing tobacco leaves

A morphometric analysis of developing leaves of Nicotiana tabacum L. was conducted to determine whether imported photoassimilates could be unloaded by symplastic transport and whether interruption of symplastic transport could account for termination of import. Five classes of veins were recognized, based on numbers of cells in transverse section. Photoassimilate is unloaded primarily from Class III veins in tissue nearing the end of the sink phase of development. Smaller veins (Class IV and V) do not transport or unload photoassimilate in sink tissue because the sieve elements of these veins are immature until after the tissue stops importing. In Class III veins the sieve element-companion cell (SE-CC) complexes are surrounded by phloem parenchyma which abuts the bundle sheath. Along the most obvious unloading route, from SE-CC complex to phloem parenchyma to bundle sheath to mesophyll cells, the frequency of plasmodesmata at each interface increases. To determine whether this pattern of plasmodesmatal contact is consistent with symplastic unloading we first demonstrated, by derivation from Fick's law that the rate of diffusion from a compartment is proportional to a number N which is equal to the ratio of surface area to volume of the compartment multiplied by the frequency of pores (plasmodesmata) which connect it to the next compartment. N was calculated for each compartment within the vein which has the SE-CC complex as its center, and was shown to be statistically the same in all cases except one. These observations are consistent with a symplastic unloading route. As the leaf tissue matures and stops importing, plasmodesmatal frequency along the unloading route decreases and contact area between cells also decreases as intercellular spaces enlarge. As a result, the number of plasmodesmata between the SE-CC complex and the first layer of mesophyll cells declines in nonimporting tissue to 34% of the number found in importing tissue, indicating that loss of symplastic continuity between the phloem and surrounding cells plays a role in termination of photoassimilate unloading.Abbreviation SE-CC sieve element-companion cell     

桑叶细脉中伴胞的超微结构研究

为了解桑叶细脉中伴胞的超微结构,采用透射电子显微技术对桑叶细脉中伴胞进行观察,着重伴胞与相邻细胞界面上胞间连丝发生频率.结果表明,(1)伴胞含丰富细胞器,细胞壁光滑,无壁内突;(2)伴胞细胞壁上具有大量胞间连丝,胞间连丝通常聚集,并常发生分枝;(3)伴胞与不同类型细胞界面上的胞间连丝发生频率有差异,伴胞-维管束鞘细胞界面上发生频率为25.12±1.83个/μm²,伴胞-伴胞界面上20.18±1.7个2/μm²,伴胞-维管薄壁细胞界面上5.42±0.6个/μm².基于上述观察,认为桑叶细脉中的伴胞属于1-2a型,韧皮部装载途径属于共质体类型.