Seed-specific overexpression of a potato sucrose transporter increases sucrose uptake and growth rates of developing pea cotyledons

During the storage phase, cotyledons of developing pea seeds are nourished by nutrients released to the seed apoplasm by their maternal seed coats. Sucrose is transported into pea cotyledons by sucrose/H+ symport mediated by PsSUT1 and possibly other sucrose symporters. PsSUT1 is principally localised to plasma membranes of cotyledon epidermal and subepidermal transfer cells abutting the seed coat. We tested the hypothesis that endogenous sucrose/H+ symporter(s) regulate sucrose import into developing pea cotyledons. This was done by supplementing their transport activity with a potato sucrose symporter (StSUT1), selectively expressed in cotyledon storage parenchyma cells under control of a vicilin promoter. In segregating transgenic lines, enhanced [(14)C]sucrose influx into cotyledons above wild-type levels was found to be dependent on StSUT1 expression. The transgene significantly increased (approximately 2-fold) transport activity of cotyledon storage parenchyma tissues where it was selectively expressed. In contrast, sucrose influx into whole cotyledons through the endogenous epidermal transfer cell pathway was increased by only 23% in cotyledons expressing the transgene. A similar response was found for rates of biomass gain by intact cotyledons and by excised cotyledons cultured on a sucrose medium. These observations demonstrate that transport activities of sucrose symporters influence cotyledon growth rates. The attenuated effect of StSUT1 overexpression on sucrose and dry matter fluxes by whole cotyledons is consistent with a large proportion of sucrose being taken up at the cotyledonary surface. This indicates that the cellular location of sucrose transporter activity plays a key role in determining rates of sucrose import into cotyledons.     

Inorganic ions modulate the path of phloem loading of sucrose in Ricinus communis L. seedlings

The impact of inorganic ions on sucrose fluxes in the cotyledons and on the pathway of phloem loading was studied in Ricinus communis L. seedlings. The cotyledons were incubated in defined solutions which contained either potassium, sodium, magnesium or calcium as chloride salts, or the sodium salts of sulphate or phosphate. Sucrose uptake from the medium into the cotyledons was only slightly affected by the salts. Sucrose efflux to the medium was increased by phosphate and sulphate and to a lesser extent by sodium and potassium. Phloem loading from the apoplasm and the symplasm was analysed by addition of labelled sucrose to the medium, determination of the specific radioactivity of sucrose in sieve-tube exudate and quantification of export into the seedling axis. Potassium and sodium stimulated the apoplasmic route of phloem loading of sucrose, mostly at the expense of loading from cotyledon sucrose pools. In contrast, sulphate and phosphate strongly inhibited the apoplasmic route whereas the (small) symplasmic flux from the cotyledon sucrose pools was less affected. Magnesium ions inhibited phloem loading by both pathways. The potential of ions in modulating the pathways of sucrose export in day to day operation of plants is discussed.     

The Cellular Pathway of Short-distance Transfer of Photosynthates and Potassium in the Elongating Stem ofPhaseolus vulgarisL. A Physiological Assessment

Plasmolytic disruption of plasmodesmata interconnecting metaphloemsieve element-companion cell complexes with small and largephloem parenchyma cells in the elongating region of internode2 ofPhaseolus vulgarisL. seedlings did not affect accumulationof phloem-imported¹⁴C-photosynthates and⁸⁶rubidium. The membrane-impermeantdye, 5(6) carboxyfluorescein, loaded into leaf phloem as themembrane-permeant diacetate ester, was found not to move radiallyout of the importing sieve elements in the internode elongationregion. In contrast, the apoplasmic tracer, Calcuofluor White,rapidly moved laterally throughout all tissues of the elongationzone. Hexoses, sucrose and potassium were identified as themain osmotica in internode apoplasmic sap. Label asymmetry in[¹⁴C](fructosyl)sucrose was retained on accumulation by excisedstem segments. Uptake of [¹⁴C]sucrose and⁸⁶rubidium by stemsegments exhibited saturation kinetics. Sucrose uptake was inhibitedby the slowly penetrating sulphydryl reagent, para-chloromercuribenzenesulphonicacid.In vitrorates of sucrose uptake, at apoplasmic concentrations,corresponded to its predictedin vivorate of delivery to thestem ground tissues from mature sieve elements when respiratorylosses were assumed to be confined to the stem phloem. For potassium,the total delivery rate could be accounted for by itsin vitrorateof uptake. Overall, it was concluded that radial transport,in the elongation zone of internode 2 ofPhaseolus vulgarisL.seedlings, follows an apoplasmic route from mature sieve elementsto stem ground tissues.Copyright 1998 Annals of Botany Company PhaseoluLes vulgaris, apoplasm, elongating stem, French bean, photosynthates, potassium, radial transfer, symplasm.     

Functional Characterization of Two Ripening-related Sucrose Transporters from Grape Berries

The ripening of grape (Vitis vinifera L.) berries is associatedwith a large accumulation of glucose and fructose in the vacuolesof the fruit cells. These hexoses are derived from sucrose,which is released from the phloem and may be taken up by parenchymacells prior to hydrolysis. We have expressed two putative ripening-relatedsucrose transporters from grape berries, VvSUC11 (synonymouswith VvSUT1) and VvSUC12, in an invertase deficient yeast strainto characterize their transport activities. Sucrose was takenup by yeast transformed with either transporter at an optimumpH of <4.5 and with a Michaelis constant (K_m) of 0.9–1.4m M. The uptake of sucrose through VvSUC11 and VvSUC12 was inhibitedby protonophores and by vanadate. This is consistent with anactive uptake mechanism involving proton cotransport, typicalof sucrose/H⁺symporters. The transporters from grape berrieswere functionally similar to Scr1, a sucrose transporter fromRicinus cotyledons. It is likely that in grape berries VvSUC11and VvSUC12 facilitate the loading of sucrose from the apoplastinto the parenchyma cells. Copyright 2001 Annals of Botany Company Fruit, grape berries, plasma membrane, sugars, sucrose transporters, Vitis vinifera     

Phloem loading and unloading of sugars and amino acids

In terrestrial higher plants, phloem transport delivers most nutrients required for growth and storage processes. Some 90% of plant biomass, transported as sugars and amino nitrogen (N) compounds in a bulk flow of solution, is propelled though the phloem by osmotically generated hydrostatic pressure differences between source (net nutrient export) and sink (net nutrient import) ends of phloem paths. Source loading and sink unloading of sugars, amino N compounds and potassium largely account for phloem sap osmotic concentrations and hence pressure differences. A symplasmic component is characteristic of most loading and unloading pathways which, in some circumstances, may be interrupted by an apoplasmic step. Raffinose series sugars appear to be loaded symplasmically. However, sucrose, and probably certain amino acids, are loaded into minor veins from source leaf apoplasms by proton symporters localized to plasma membranes of their sieve element/companion cell (se/cc) complexes. Sucrose transporters, with complementary kinetic properties, are conceived to function as membrane transporter complexes that respond to alterations in source/sink balance. In contrast, symplasmic unloading is common for many sink types. Intervention of an apoplasmic step, distal from importing phloem, is reserved for special situations. Effluxers that release sucrose and amino acids to the surrounding apoplasm in phloem loading and unloading are yet to be cloned. The physiological behaviour of effluxers is consistent with facilitated membrane transport that can be energy coupled. Roles of sucrose and amino acid transporters in phloem unloading remain to be discovered along with mechanisms regulating symplasmic transport. The latter is hypothesized to exert significant control over phloem unloading and, in some circumstances, phloem loading.     

Sucrose carrier RcSCR1 is involved in sucrose retrieval, but not in sucrose unloading in growing hypocotyls of Ricinus communis L

The transport of assimilates from source to sink tissues is mediated by the phloem. Along the vascular system the phloem changes its physiological function from loading phloem to transport and unloading phloem. Sucrose carrier proteins have been identified in the transport phloem, but it is unclear whether the physiological role of these transporters is phloem unloading of sucrose or retrieval of apoplasmic sucrose back into the sieve element/companion cell complex. Here, we describe the dynamic expression of the Ricinus communis sucrose carrier RcSCR1 in the hypocotyl at different sink strengths. Our results indicate that phloem unloading in castor bean is not catalysed by the phloem loader RcSCR1. However, this sucrose carrier represents the molecular basis of the sucrose retrieval mechanism along the transport phloem, which is dynamically adjusted to the sink strength. As a consequence, we assume that other release carrier(s) exist in sink tissues, such as the hypocotyl, in R. communis.     

Pathways of Uptake and Accumulation of Sugars in Tomato Fruit

The route of sucrose unloading from the conducting tissue, theregulation of sucrose hydrolysis and the uptake and subsequentmetabolism of sugars were investigated in the rapidly growingtomato fruit. During the first two weeks of fruit enlargement, the vacuoleaccounted for more than 85% of the protoplast volume and theintercellular space accounted for 20% of the fruit placentaltissue. The plasmodesmatal frequency was highest between phloemparenchyma cells and lowest between phloem sieve cells and phloemparenchyma. The total invertase activity was about 8 µmolglucose g^–1 d. wt min^–1 during the rapid growingperiod and increased six-fold at ripening. The wall-bound invertaseaccounted for less than 11% of the total activity. Invertaseactivity increased with increasing sucrose concentrations (upto 50 mM) in the incubation medium, but decreased at higherconcentrations. Sucrose synthase activity could only be detectedwhen fruit was older than 19 d. The uptake and metabolism of sugars by fruit cells were investigatedby incubation of fruit slices with ¹⁴C-sugars for 3 h. The uptakeof sucrose increased with the sucrose concentration up to 200mM. The rate of glucose uptake and its conversion to the ethanol-insolublefraction were higher than those of sucrose. The uptake of sucrosedid not compete with that of glucose or vice versa, providedthe osmotic potential of the incubation solution was maintainedconstant. The uptake of sucrose was not inhibited by metabolicinhibitors such as PCMBS, CCCP, sodium azide or vanadate. TheATPase activity in the fruit tissue was low. These findings did not identify conclusively the mode of sucroseunloading. However, the uptake of sugars by fruit cells is non-specificand does not appear to require a membrane carrier or plasmalemmaATPase to provide energy for sucrose uptake. Fruit, invertase, Lycopersicon esculentum, phloem unloading, plasmodesmata, sucrose     

Hexose transporter CsSWEET7a in cucumber mediates phloem unloading in companion cells for fruit development

In the fleshy fruit of cucumbers (Cucumis sativus L.), the phloem flow is unloaded via an apoplasmic pathway, which requires protein carriers to export sugars derived from stachyose and raffinose into the apoplasm. However, transporter(s) involved in this process remain unidentified. Here, we report that a hexose transporter, CsSWEET7a (Sugar Will Eventually be Exported Transporter 7a), was highly expressed in cucumber sink tissues and localized to the plasma membrane in companion cells of the phloem. Its expression level increased gradually during fruit development. Down-regulation of CsSWEET7a by RNA interference (RNAi) resulted in smaller fruit size along with reduced soluble sugar levels and reduced allocation of ¹⁴C-labelled carbon to sink tissues. CsSWEET7a overexpression lines showed an opposite phenotype. Interestingly, genes encoding alkaline α-galactosidase (AGA) and sucrose synthase (SUS) were also differentially regulated in CsSWEET7a transgenic lines. Immunohistochemical analysis demonstrated that CsAGA2 co-localized with CsSWEET7a in companion cells, indicating cooperation between AGA and CsSWEET7a in fruit phloem unloading. Our findings indicated that CsSWEET7a is involved in sugar phloem unloading in cucumber fruit by removing hexoses from companion cells to the apoplasmic space to stimulate the raffinose family of oligosaccharides (RFOs) metabolism so that additional sugars can be unloaded to promote fruit growth. This study also provides a possible avenue towards improving fruit production in cucumber.

Transporter CsSWEET7a removes hexose from companion cells to the apoplasmic space to stimulate fruit phloem unloading so that additional sugars can be unloaded to promote fruit growth.     

The importance of root carbohydrate abundance in ammonium uptake

The influence of carbohydrates on ammonium uptake and ammonium transporter (AMT1) expression was investigated in roots of field pea (Pisum arvense) and rutabaga (Brassica napus var. rapifera). Ammonium transport into field pea seedlings diminished markedly following cotyledon removal, which indicated that uptake of ammonium was under control of reserves stored in the cotyledons. Excision of cotyledons decreased also the level of some amino acids, glucose and total reducing sugars in field pea roots. To investigate the importance of the sugar supply for the regulation of ammonium uptake at low external NH ₄ ⁺ level, 1 mM glucose or sucrose was supplied for several hours to the field pea seedlings deprived cotyledons or to intact rutabaga plants. Supply of both sugars resulted in a substantial increase in ammonium uptake by both plant species and enhanced markedly the expression of AMT1 in rutabaga roots. The results indicate that sugars may regulate ammonium transport at the genetic level.     

Import of Sucrose and its Partitioning in the Synthesis of Galactomannan and Raffinose-oligosaccharides in the Developing Guar (Cyamopsis tetragonolobus) Seed

Import of sucrose and its transformation to galactomannan andraffinose-oligosaccharides have been studied in the developingguar seed. The amount of galactomannan gradually increased withthe ageing of the seed. During the entire period of pod development,sucrose constituted the major portion of the free sugars inthe seed (both endosperm and cotyledons) as well as in the podwall. Besides myo-inositol, the free sugars detected in thedeveloping endosperm and cotyledons were glucose, fructose,raffinose and stachyose. Some compounds, possibly glycosides(RG values higher than that of fructose), were also detectedin the endosperm. In the later stages of seed development, therelative proportion of raffinose in the free sugars increased,reaching 50% of the total free sugars in 77-d-old cotyledons.With pod maturity, the activities of soluble acid and boundacid invertases in the pod wall increased manifold with a concomitantdecline in the non-reducing sugar content. These enzymes seemto be involved in the mobilization of sucrose from this fruitingstructure into the seed. An increased synthesis of raffinose-oligosaccharidesboth in the endosperm and cotyledons was associated with highactivities of soluble acid invertase (pH 4.8) and sucrose-UDPglucosyl transferase in these tissues. Feeding uniformly labelled¹⁴C-sugars to the detached intact pods as well as to the isolatedendosperm and cotyledons resulted in labelling of all endogenousfree sugars and galactomannan. The uptake and incorporationinto galactomannan of ¹⁴C was stimulated by Co²⁺, Mn²⁺ and Mg²⁺.Except for mannose, a major proportion of the ¹⁴C from glucose,fructose and sucrose appeared in sucrose in both endosperm andcotyledons indicating a fast reconstitution of sucrose in situ.Based on the present results, a possible mode of transformationof sucrose to galactomannan and raffinose-oligosaccharides hasbeen proposed. Key words: Sucrose, galactomannan, raffinose-oligosaccharides, invertase, sucrose-UDP glucosyl transferase, ¹⁴C-incorporation, guar seed     

Role of phloem in sucrose transport by Ricinus cotyledons

Sucrose is taken up and accumulated by cotyledons of Ricinus communis L. Autoradiographic studies reveal a predominant accumulation of sucrose in the phloem of the cotyledons. The export of sucrose from the cotyledons to hypocotyl and roots proceeds in the phloem by mass flow. These results, taken together with previous data, are experimental evidence for proton-sucrose symport as the mechanism of phloem loading.     

Sucrose transport by seedlings of Ricinus communis L: the export of sucrose from the cotyledons to the hypocotyl as a function of sucrose concentration in the cotyledons

Abstract Using seedlings of Ricinus communis L. sucrose export from the cotyledons to the hypocotyl and roots was measured at different levels of sucrose concentration in the cotyledons. Sucrose export followed Michaelis-Menten kinetics with a half-saturation of export at 35 mM sucrose in the cotyledons. A maximal export flux of 90 μmol h^?l g^?1 fresh weight of the cotyledons was obtained. Both these figures coincide with those obtained for sucrose uptake into the cotyledons. It is postulated that sucrose uptake and sucrose export occurs by the same mechanism and possibly by the same cells which then would have to be part of the phloem. Since sucrose uptake has been shown to proceed as proton-sucrose co-transport, phloem loading might also be energized by the protonmotive potential difference. The data, furthermore, are difficult to reconcile with the symplastic route of phloem loading.     

A suite of sucrose transporters expressed in coats of developing legume seeds includes novel pH-independent facilitators

A suite of newly discovered sucrose transporter genes, PsSUF1, PsSUF4, PvSUT1 and PvSUF1, were isolated from the coats of developing pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.) seeds. Sequence analysis indicated that deduced proteins encoded by PsSUF1, PvSUT1 and PvSUF1 clustered in a separate sub-group under sucrose transporter Clade I, whereas the deduced protein encoded by PsSUF4 clustered in Clade II. When expressed in yeast, these genes were shown to encode sucrose transporters with apparent Michaelis Menten constant (Km) values ranging from 8.9 to 99.8 mm. PvSUT1 exhibited functional characteristics of a sucrose/H+ symporter. In contrast, PsSUF1, PvSUF1 and PsSUF4 supported the pH- and energy independent transport of sucrose that was shown to be bi-directional. These transport properties, together with that of counter transport, indicated that PsSUF1, PvSUF1 and PsSUF4 function as carriers that support the facilitated diffusion of sucrose. Carrier function was unaffected by diethylpyrocarbonate and by maltose competition, suggesting that the sucrose binding sites of these transporters differed from those of known sucrose/H+ symporters. All sucrose transporters were expressed throughout the plant and, of greatest interest, were co-expressed in cells considered responsible for sucrose efflux from seed coats. The possible roles played by the novel facilitators in sucrose efflux from seed coats are discussed.     

Long-term Metabolism of [14C]Sugars in Stored Potatoes

[¹⁴C]Sucrose, [¹⁴C]glucose and [¹⁴C]fructose were introducedinto potato tubers held at 10 °C and the redistributionof label chased over a 65 d period in storage. Respiratory losseswere identical in all treatments, as was the partitioning of¹⁴C between soluble and insoluble forms. Sucrose was the predominantlabelled sugar in the tubers after 20 h, regardless of the original[¹⁴C]sugar introduced, and was loaded and distributed throughoutthe tubers by the internal phloem system. After 20 h the proportionsof labelled sugars bore no relationship to those of the unlabelledendogenous sugars. However, with time the percentage of ¹⁴Cin sucrose fell while that in glucose increased and by 65 dthe proportions of the labelled sugars more closely resembledthe endogenous pools. Fructose represented a consistently lowproportion of both the labelled and unlabelled sugars. By 21d a considerable proportion of the soluble ¹⁴C had been convertedto starch (approx. 25% of the total tuber 14C), this value remainingrelatively constant for the remainder of the storage period.Sprouts which formed on the tubers contained up to 6% of thetotal tuber ¹⁴C but less than 0.2% of the tuber dry matter.It is suggested that the bulk of the translocated [¹⁴C]sucroseentered the symplast and exchanged slowly with the bulk of thesugars in the storage cell vacuoles. [¹⁴C]sugars, phloem loading, starch, potato tuber, Solunum tuberosum, cold storage     

Changes in phloem export of sucrose in leaves in response to phosphorus, potassium and magnesium deficiency in bean plants

The effect of varied phosphorus (10 and 250 mmol P m^–3potassium (50 and 2010 mmol K m^–3) and magnesium (20 and1000 mmol Mg m^–3 supply on sucrose, reducing sugars, aminoacids, P, K, and Mg in phloem exudate was studied in bean (Phaseolusvulgaris L.) plants over a 12 d growth period in nutrient solution.Phloem exudates were collected from detached primary leavesusing the EDTA-promoted exudation technique. Compared with controlnutrient-sufficient plants, sucrose export in the phloem exudatewas drastically decreased by K deficiency and, particularly,by Mg deficiency, whereas P deficiency either had no effector stimulated sucrose export. In Mg-deficient plants the rateof sucrose export was decreased to 10–20% of the controlplants. There was a close Inverse relationship between phloemexport and leaf concentration of sucrose: higher leaf concentrationsof sucrose were accompanied by lower phloem export of sucrose.In contrast to sucrose, reducing sugars in the exudates werevery low and not affected by P, K and Mg deficiency. The phloemexport of amino acids was strongly depressed by Mg deficiency,but only slightly by P and K deficiency. Resupplying Mg to Mg-deficientplants for 12 h during the dark or light periods rapidly stimulatedsucrose export. After resup ply of Mg for 24 h and 48 h therate of sucrose export was comparable with the rate in the controlplants. The results demonstrate a key role for Mg in phloem loadingand export of photosynthates from source leaves, especiallysucrose. Inhibition of root growth and development of visualsymptoms of chlorosis in Mg-deficient plants are suggested asconsequences of Impaired phloem loading. In agreement with thisin P-deficient plants where phloem loading was not impaired,chlorosis was absent and root growth was maintained at a highlevel. Key words: Bean, carbon partitioning, magnesium nutrition, phloem transport, phosphorus nutrition, potassium nutrition     

Identification and characterization of a sucrose transporter isolated from the developing cotyledons of soybean

Reduced carbon produced in mature leaves is distributed throughout plants in the form of sucrose. Sucrose transporter proteins (SUT) play a crucial role in transporting sucrose. We isolated a cDNA encoding a sucrose transporter, GmSUT1, which is expressed in the developing cotyledons of soybean (Glycine max). [14C]sucrose uptake assays demonstrate that GmSUT1 has a K(m) of 5.6mM and a V(max) of 5.8 nmol sucrose min(-1)(mg cells)(-1), which are similar to those of the low-affinity-high-capacity sucrose transporter family. GmSUT1 protein accumulates gradually during cotyledon development, correlating with increasing sucrose levels in the maturing cotyledons. Collectively, these data suggest that GmSUT1 plays an active role in the movement of sucrose into the developing seeds.     

The Cellular Pathway of Radial Transfer of Photosynthates in Stems of Phaseolus vulgaris L.: Effects of Cellular Plasmolysis and p-Chloromercuribenzene Sulphonic Acid

Cellular plasmolysis with l M solutions of mannitol appearedto sever plasmodesmatal interconnections between all cells ofthe stems of Phaseolus vulgaris plants except the sieve element-companioncell (se—cc) complexes. Phloem loading and uptake of [¹⁴C]sucroseby the storage cells of the stems was unimpaired by cellularplasmolysis followed by rehydration of the stem tissues. Accumulationof phloem-transported ¹⁴C-photosynthates of the treated stemswas inhibited in summer-grown plants and unaffected in winter-grownplants indicating that phloem unloading follows a symplasticand a free-space route respectively depending on growth season.At a concentration that did not interfere with cellular metabolism,p-chloromercuribenzene sulphonic acid (PCMBS) applied to thestems blocked [¹⁴C]sucrose loading into the phloem and storagecells of the stem, but had no effect on the pool size of free-spacesugars. This latter response is consistent with a facilitatedmechanism of sugar unloading to the stem free-space. Accumulationof phloem-transported ¹⁴C-photosynthates was stimulated by PCMBSand this effect was most pronounced in winter-grown plants.Cellular plasmolysis followed by rehydration abolished the PCMBSaction on ¹⁴C-photosynthate accumulation. This effect is consistentwith a PCMBS induction of phloem unloading through the stemsymplast. It is proposed that phloem unloading in bean stemsmay follow either a free-space or symplastic route and thatthe latter route is entrained under sink-limited conditions. Phaseolus vulgaris, french bean, stem, phioem unloading, free-space, symplast     

Phloem unloading follows an extensive apoplasmic pathway in cucumber (Cucumis sativus L.) fruit from anthesis to marketable maturing stage

The phloem unloading pathway remains unclear in fruits of Cucurbitaceae, a classical stachyose-transporting species with bicollateral phloem. Using a combination of electron microscopy, transport of phloem-mobile symplasmic tracer carboxyfluorescein, assays of acid invertase and sucrose transporter, and [(14)C]sugar uptake, the phloem unloading pathway was studied in cucumber (Cucumis sativus) fruit from anthesis to the marketable maturing stage. Structural investigations showed that the sieve element-companion cell (SE-CC) complex of the vascular bundles feeding fruit flesh is apparently symplasmically restricted. Imaging of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands of the vascular bundles in the whole fruit throughout the stages examined. A 37 kDa acid invertase was located predominantly in the cell walls of SE-CC complexes and parenchyma cells. Studies of [(14)C]sugar uptake suggested that energy-driven transporters may be functional in sugar trans-membrane transport within symplasmically restricted SE-CC complex, which was further confirmed by the existence of a functional plasma membrane sucrose transporter (CsSUT4) in cucumber fruit. These data provide a clear evidence for an apoplasmic phloem unloading pathway in cucumber fruit. A presumption that putative raffinose or stachyose transporters may be involved in soluble sugars unloading was discussed.     

The Pathway of Radial Transfer of Photosynthate in Decapitated Stems of Phaseolus vulgaris L.

[¹⁴C]Sucrose was found to be the predominant component of the¹⁴C-photosynthates that accumulated in the free space of decapitatedstems of P. vulgaris plants. The ¹⁴C-photosynthates appearedto occupy the entire free-space volume of the stems at totalsugar concentrations in the range of 3–12 mM. The free-spacesugar levels were found to rapidly decline once photosynthatetransfer to the stems was halted. Moreover, it was found thatestimates of the rate of in vitro sucrose uptake by the stemscould account fully for the decline in free-space sugar levels.Overall, the evidence indicated that at least part of the radialpathway of photosynthate transfer in bean stems involved thestem apoplast. It is tentatively proposed that, based on celland tissue distribution of ¹⁴C-photosynthates, the apoplasticpathway extends from the membrane boundary of the sieve element/companion-cellcomplex to all other cells of the stem. Apoplast, Phaseolus vulgaris L., bean, phloem unloading, photosynthates, symplast