Báo cáo lâm nghiệp: "Phloem S. Delrot loading and unloading"

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Phloem and loading unloading S. Delrot J.L. Bonnemain Laboratoire de Physiologie et Biochimie Végétales, CNRS URA81, 25, Faubourg Saint- du rue Cyprien, 86000 Poifiers, France Introduction characterized by their osmotic pressure. The high osmotic pressure of the phloem Phloem transport of assimilates provides sap is due to the presence of many so- the materials needed for the build up of lutes: sugars, amino acids, ions (Ziegler, the herbaceous plant or the tree. Under- 1975). Concerning sugars, in many spe- standing this mechanism is therefore cies, sucrose is the predominant mobile important to control the edification of the sugar: This is the case for most herba- plant. Considerable work has been devot- ceous plants and for tree species be- ed to transport in the past (for recent longing to gymnosperms (Picea abies, reviews, see Giaquinta, 1983; Delrot and Pinus strobus) or angiosperms (monocoty- Bonnemain, 1985; Delrot, 1987, 1989; Van ledons, palm-tre!e; dicotyledons, willow). In plants in addition Bel, 1987), but much further work is need- to sucrose, the other ed, especially on woody species, because sap contains oligosaccharides phloem the information available on basic pro- belonging to the raffinose family and char- cesses, such as loading into and unload- acterized by the attachment of one or ing from the sieve tubes, mainly concerns more galactose residues to the sucrose herbaceous species. Therefore, this short molecule. Some members of Bignonia- overview will often refer to herbaceous ceae, Tiliaceae and Ulmaceae belong to species but the general principles which this group of plants. A third group is made will be given may be used to understand of species containing sugar alcohols in the assimilate transport in trees. Actually, the phloem sap, for example mannitol (Olea- scant information available shows wide ceae; Fraxinus, Syringa), sorbitol (Prunus variety in the anatomical, physiological, serotina, Malus domestica), or dulcitol and biochemical situations involved in (Celastraceae). As regards amino acids, assimilate transport. gluamine/glutarnate and asparagine/as- partate are the quantitatively predominant compounds (1--30 mM each), together General with serine, but there are exceptions. For background example, proline is the predominant amino Nature of translocated substances acid in the sieve tube sap, of Robinia. In some species, the phloem sap also distance transport of assimilates contains ureides, allantoin and allantoic Long in specialized cells (sieve tubes) acid (Acer, Platanus, Aesculus) or citrul- occurs
attention will be paid mainly to the events line (Betula, Carpinus, Alnus, Juglans). occurring in the source and in the sink. There is no evidence that any of these nitrogenous substances is excluded from the sieve tubes, in contrast to the loading of sugars, which is a highly selective pro- in Lateral transport and phloem loading cess. In all investigated cases, the predo- the leaf minant cation in sieve tube sap is potas- sium, while the predominant anion is generally phosphate and sometimes chlo- In the leaf, the assimilates which are not ride. Another striking feature of the phloem used for growth may be either stored in a sap is its alkaline pH (7.5-8.5). The con- storage compartment (vacuole or chloro- centration of the phloem sap exhibits nyc- plast) or exported via a mobile compart- themeral variations (Hocking, 1980) and ment (cytosol or endoplasmic reticulum). its content exhibits seasonal variations Lateral transport up to the conducting (Ziegler, 1975), as well as variations bundle may be apoplastic, in the cell wall, depending upon the location in the plant if assimilates are leaked into the apoplast, (Hocking, 1980; Vreugdenhil, 1985). or symplastic, via the plasmodesmata which connect the mesophyll cells to one another. The final step of lateral transport The different steps involved in long dis- is the active loading of assimilates into the tance transport conducting complex. Until recently, the only evidence available suggested that active loading occurred from the apoplast, Assimilate transport involves 3 steps but some authors now argue that loading which are lateral transport from the chloro- might also occur via the plasmodesmata in plast to the conducting bundle in the leaf species. some (source), translocation in the sieve tubes be markedly different examples will Two (path), and lateral transport from the sieve status of to illustrate the given present tubes to the receiving cells (sink). Lateral knowledge, the diversity of the situations transport in the source, which ends in the encountered, and the questions being active loading of the assimilates in the debated. sieve tube, provides the driving force for translocation, while the activity in the dif- ferent sinks controls the direction of trans- Apoplastic loading port. Although the presence of actin and myosin-like proteins in the phloem of Evidence detailed elsewhere (Delrot, some species may give support to the and references therein) hypothesis of active translocation powered 1989, 1987, shows that in Beta vulgaris and Vicia by contractile filaments (Kursanov et al., faba, loading of sugars is mediated by 1983; Turkina et al., 1987), translocation in the path is thought to be rather passive, a proton-sucrose cotransport process across the plasmalemma of the conduct- particularly in species whose phloem ing complex (companion cell-sieve tube). transport is not sensitive to temperature for a wide range of values (Faucher et aL, This evidence may be summarized as fol- lows. Plasmolytic studies show the exis- 1982). Yet, mechanisms must function in the stem to prevent excessive leakage of tence of a steep, uphill concentration gra- assimilates from the conducting tissue to dient at the boundary of the sieve the external parenchyma. In the following, tube-companion cell complex. Loading is
in the phloem specific for 1988) and their presence sucrose, since exogenous that of raffinose, must be well hexoses are not absorbed by the veins. It as as sap, is promoted by adenosine triphosphate, explained by a transport mediated by an- other carrier, by metabolism inside fusiccocin (an activator of the plasmalem- ma proton-pump), but inhibited by un- the conducting complex or by symplastic transport from the mesophyll. The use couplers and metabolic inhibitors. Sucrose is present in the apoplast and is the major of the non-permeant sulfhydryl rea- gent p-chloromercuribenzenesulfonic acid mobile sugar. Apoplastic sucrose concen- (PCMBS) has demonstrated the presence tration undergoes nycthemeral changes and is sensitive to treatments which block of a thiol protected by the substrate in the active site of the sucrose carrier of broad- export in various herbaceous species. The sieve tube is associated with specialized bean leaf tissue. This property has been used to label differentially the plasmalem- transfer cells possessing numerous wall ma proteins protected by sucrose. The ingrowths, which increase the volume of the apoplast and the surface area of plas- data obtained with purified plasmalemma malemma available for exchanges. The from sugar beet and from broadbean sieve tube and the transfer cell are con- leaves indicate that an intrinsic polypepti- nected by plasmodesmata, but in contrast, de of 42 kDa is differentially labeled by N- very few plasmodesmata are found at the ethylmaleimide, in the presence of sucro- se and not in the presence of the boundary between the conducting com- plex and the surrounding cells. In Vicia non-transported sucrose analogue palati- faba, the number of plasmodesmata de- nose (Pichelin-Poitevin et al., 1987; Gallet creases as the proximity of the cells con- et aL, 1989). A polyclonal antiserum raised sidered to the conducting complex in- against the 42 kDa polypeptide is able to The conducting complex is inhibit selectively uptake of sucrose by leaf creases. therefore an insulated unit, and all the pro- protoplasts, but has no effect on the upta- perties described above strongly suggest ke of amino acids and hexoses (Lemoine apoplastic loading. The existence of a pro- et al., 1989). These data suggest that the ton extruding activity more concentrated or intrinsic 42 kDa polypeptide of the plasma- more active in the veins than in the sur- lemma is (part of) the sucrose carrier. rounding tissues, and the demonstration of sucrose-induced alkalizations of the me- Symplastic loading dium indicate that uptake of sucrose in leaf tissues, and more particularly in the veins, occurs with proton cotransport. This Madore et al. (1986) and Van Bel (1987) is further substantiated by uptake exper- have argued that some observations make iments which show that the sucrose carrier feasible the possibility that loading into the obeys 2 substrate kinetics, with the proton sieve tubes may be symplastic i.e., via the and sucrose as the substrates. The su- plasmodesmata!. First, in some species, crose carrier is able to recognize sucrose, electron microscopy shows more or less maltose, raffinose and a-phenylglucoside numerous ptasmodesmata connecting the (M’Batchi et al., 1985). Yet, it is able to conducting complex with the surrounding transport sucrose, maltose and a-phenyl- cells (Van Bel, 1987). In addition, several glucoside, but not raffinose, probably authors have reported on particular cells because of steric hindrance. Sorbitol and (paraveinal mesophyll), which seem to be stachyose are not transported by the located in a strategic position which would sucrose carrier (M’Batchi and Delrot, allow them to act as cells collecting the
in broadbean, for assimilates from the mesophyll and giving opposite to that found them back to the conducting cells. The example. In soybean, these ’collecting’ cells seem to have a more acidic cell wall leaf of Populus deltoides, studied by Rus- sin and Evert (1984; 1985a, b) provides than the surrounding cells, suggesting that an excellent example of this situation (Fig. they possess strongly active proton extru- 1This species possesses a paraveinal ding systems (Canny, 1987). Plasmolytic mesophyll and there are numerous plas- studies with cottonwood also pointed to a modesmata between all cell types, in- situation completely different from that found in the case of apoplastic loading cluding the cells of the conducting com- plex. In the mesophyll, the highest (sugar beet). Indeed, in Populus del- frequency of plasmodesmata is found bet- toides, the highest osmotic pressure is not found in the sieve tube, but in the paravei- ween the cells of paraveinal mesophyll nal mesophyll; there is an osmotic gra- and the other cell types. The density of dient along the palisade cell-bundle shea- plasmodesmata increases from the meso- phyll to the sieve tube and this situation is th cell-companion cell (or vascular
parenchyma cell) route and along the paraveinal mesophyll-bundle sheath cell-companion cell path. Yet, within the conducting bundle, the osmotic pressure is higher in the sieve tube than in the other cells (companion cell, vascular parenchy- ma cells). The problem is to know whether these osmotic gradients are due to mobile sugars or to other solutes (ions). Several structural, ultrastructural and physiological observations therefore sug- gest that symplastic transport in the leaf may be followed by symplastic loading in some species. The next questions can then be summarized as follows: are the plasmodesmata around the conducting complex open, and if they are open, are they able to build up, or to maintain osmo- tic gratients? and may these gradients be selective for one mobile form of sugar (sucrose, raffinose, sorbitol, etc.)? Although this kind of experiment has not yet been conducted with woody species, to our knowledge, injection of fluorescent dyes into the mesophyll cells has shown in several herbaceous species that the dye actually entered the veins but gave no clear demonstration of dye entry into the companion cell-sieve tube complex itself. ter and the cytoplasmic annulus would The data presented above shows that function as a ’one-way’ valve or that the osmotic gradients may be found between desmotubule is open and that active load- cells connected by plasmodesmata. ing is mediated by an energized carrier Now, considering the structure of plas- located on the endoplasmic reticulum or modesmata (Fig. 2), how can we explain the tonoplast (which communicates with that they would accumulate sucrose in the the reticulum). Much additional work is conducting complex and not hexoses? needed to test these hypotheses. The diameter of the plasmodesmata is about 50 nm and the continuity of the Gamalei and Pakhomova (1980) and plasma membrane from cell to cell is quite Gamalei (1984) surveyed the structure evident. A central structure, the desmotu- and the repartition of plasmodesmata at bule passes axially along the cylinder. The the boundary of the conducting complex. desmotubule is seen as an extension of According the Gamalei (1984), the struc- the endoplasmic reticulum, but it is not ture of the minor veins may be classified known whether the desmotubule is open into 3 categories (Fig. 3). The type I-vein, or not. The only way to build up a selec- characterized by plasmodesmata fields, is tive concentration gradient across this typical for plants transporting oligosaccha- structure is to hypothesize that the sphinc- rides (mainly raffinose) and is an adapta-
effects of non-per- comparison of the and tion to symplastic transport (Fig. 38). and permeant osmotic buffers Types 11 (Fig. 3A) and III (Fig. 3C), typical meant shows that the important factor is cell tur- for sucrose transporting species, allow gor. The effects of cell turgor on loading apoplastic transport. Both typesI and III, may be due in part to the sensitivity of the found more frequently in the recent groups transmembrane potential difference to the of phanerogams, would be derived from osmotic conditions (Li and Delrot, 1987). type II, found in the older groups of phane- Yet the effects of turgor on the plasma rogams. TypeI includes gymnosperms membrane ATPase are not sufficient to and dicotyledon families containing tree explain the osmotic sensitivity of loading species, while types II and III include and other phenomena must be involved. mainly herbaceous dicotyledons (except Furthermore, due to the large osmotic Fagaceae, type!)). ). changes needed to affect loading in vitro, it is not known what part osmotic regula- tion of this process actually plays in vivo. Possible regulation of loading Various reports have concluded that phytohormones could directly control Apart from the numerous metabolic pro- phloem loading. Malek and Baker (1978) cesses which affect the availability of the found that auxin promoted phloem loading sugar export pool and which will not be in castor bean, while Vreugdenhil (1983) considered here, 2 main factors may affect reported inhibition of sucrose uptake by phloem loading: the cell turgor and hormo- abscisic acid in discs prepared from the nal status. Phloem loading is promoted by cotyledons of the same species. More hyperosmotic media in various species recently, Daie {1987) studied the effects of (sugar beet, bean, broadbean, celery), lt———————B r r’t
load- loading is apoplastic. Using broadbean gibberellic acid and auxin on phloem ing in isolated vascular bundles and stem segments, Aloni et aL (1986) showed tissue of celery. She found that that sucrose efflux from the phloem was phloem both hormones (1 pM) were able to stimu- mediated by 81 carrier sensitive to PCMBS. late sucrose uptake in these materials Indeed, the efflux of preloaded [!4C]- within 2 h of treatment. This effect was enhanced when unlabeled sucrose was present in the efflux medium, also apparent on the uptake of mannitol, sucrose was which is also translocated in celery, but compared to a control. This exchange could not be detected with 3-O-methyglu- mechanism is inhibited by PCMBS. This cose, which does not enter the veins. The efflux is not active because it is stimulated hormonal effects were therefore attributed by the addition of protonophores. After efflux from the phloem into the apoplast of to phloem loading. Again, the mechanism of this regulation and the actual part it the stem, sucrose is either hydrolyzed by plays in vivo remain to be elucidated. a cell wall invertase, as in sugar cane (Fig. 4B), or not hydrolyzed as in broadbean Phloem loading and carbon partitioning (Fig. 4C). The resulting sugars, either hex- be affected in the short-term by artifi- can oses or sucrose, are then actively taken cial manipulation of the source-sink rela- up by the receiving cells. tionships. For example, in broadbean, In the stems of trees (Populus), the den- heat-girdling of a petiole still attached to sity of plasmodesmata (8/,um in the ray ) 2 the plant leads to an apparent inhibition of cells is almost as high as in the paraveinal loading (Ntsika and Delrot, 1986), which cells of the leaf and allows radial transport seems to be due to the diversion of !4C of sugars via the symplastic pathway from the mobile pool to starch (Grusak, (Sauter and K;loth, 1986). Delrot and Ntsika, unpublished data). In fruits, the examples studied so far indicate that the first steps of unloading in the maternal tissues are symplastic but there is a symplastic discontinuity between Phloem unloading and accumulation by the 2 generations and uptake of assimi- the receiving cells lates by the embryo occurs necessarily from the apoplast. In this case, the limiting step for import is the rate of uptake across While the pathway for loading may depend the plasmalemma of the embryo cells, upon the species investigated, the path- which in turn depend upon the metabolism way for phloem unloading depends mainly and the compartmentation of assimilate in upon the receiving organs, not only on the the receiving cell. Two examples illustrate species. this configuration. The first one is the fruit In young importing leaves or in root tips, of bean, investigated by Thorne (1985). In ultrastructural data and various other this material, unloading from the con- approaches (use of impermeant inhibitors) ducting complex in the seed coat (i.e., indicate that unloading is symplastic (Fig. unloading sensu stricto) is symplastic and 4A). In this case, the rate of import is then the assimilates are also released into directly dependent upon the metabolic the apoplast at the interface between the 2 activity of the tissue, which will consume generations (Fig. 4D). Sucrose is not split the imported assimilates. before being absorbed by the cotyledons. In the stems of various herbaceous spe- In the fruit of maize, investigated by Shan- non et al. (1986), unloading from the sieve cies (sugar cane, broadbean, bean), un-
element-companion cell complex is also These data show that storage in receiving symplastic (Fig. 4E). Assimilates then cells may be regulated by hormones. As apparently enter the apoplast of the pla- regards unloading from the phloem, centa-chalaza. However, in contrast to the sensu stricto, Clifford, et aL (1986) have case described above, they are hydro- reported that import of [ in C]assimilates 14 lyzed in the apoplastic compartment. bean pods was promoted by benzylamino- Indeed, hexoses constitute over 80% of purine and abscisic acid. However, this the carbohydrate released into the apop- stimulation was rather weak and did not last interface between the 2 generations. last for a long time. As in the case of load- Assimilates are then taken up by the albu- ing, the hormonal effects on unloading are men, presumably as hexoses, and this is still poorly understood. facilitated by the conversion of the outer In summaiy, long distance transport layer of albumen into transfer cells, which and, therefore, the growth of the plant are are characterized by extensive wall in- dependent upon membrane activities at growths. It must be stressed that sucrose the source and the sink levels, but we still hydrolysis, even when it occurs, may not know little about the details of some of be a necessary prerequisite for sugar these activities, especially in trees. It is accumulation by the sink cells, as has clear that va.rious strategies have been been demonstrated for the taproot of developed in the plant kingdom (apoplas- sugar beet (Lemoine et al., 1988). In- tic or symplastic loading, apoplastic or version of sucrose by a cell wall invertase symplastic unloading, chemical continuity prevents its retrieval by the conducting or non-continuity of the transported sub- complex (Eschrich, 1980) and it increases strates) to ensure the transport and the the osmotic pressure in that cell wall. compartmentation of nutrients in the plant. Possible regulation of unloading may be osmotic or hormonal, as for loading. For example, Aloni et aL (1986) have shown References that unloading of assimilates from the stem of broadbean decreased when was the mannitol concentration of the medium Aloni B., Wyse R.E. & Griffith S. (1986) Sucrose was changed from 0 to 400 mM mannitol, transport and phloem unloading in stem of Vicia faba: possible involvement of a but opposite results have been reported sucrose carrier and osmotic regulation. Plant PhysioL with legume fruits (Wolswinkel, 1985). Stu- 81, 482-486 dies made with different sink organs agree Canny M.J. (1987) Locating active proton extru- that high solute concentration in the apo- sion pumps in leaves. Plant Cell Environ. 10, plast promotes assimilate uptake into the 271-274 receiving cells (Wolswinkel, 1985). Cliford P.E., Offler C. & Patrick J.W. (1986) Growth regulators have rapid effects on photo- hormonal control, Saftner Concerning synthate unloading from seed coats of Phaseo- and Wyse (1984) showed that treatment lus vulgaris L. Plant Physiol. 80, 635-637 by abscisic acid enhanced the active com- Daie J. llnteraction of cell turgor and hor- (1987) ponent of sucrose uptake in sugar beet uptake in isolated phloem of on sucrose mones root discs, while auxin decreased this celery. P/anrP/ys/oA 84, 1033-1037 uptake 2-fold. These effects, clearly visible Delrot S. (198i’) Phloem loading: apoplastic or within 30 min of treatment, were optimal at symplastic? Plant Physiol. Biochem. 25, 667- 1-10 pM for both hormones. K or auxin + 676 the response to abscisic acid prevented Delrot S. (1989) Loading of photoassimilates. but cytokinins and gibberellic acid did not. In: Transport of Assimilates. (Baker D.A. & Mil-
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