Sugar transport in leaf discs of Phaseolus coccinius

Kinetic profiles for sucrose, glucose and 3-OMG glucose were determined in leaf discs of Phaseolus coccinius L. (cv. Scarlet). All three sugars exhibited identical uptake kinetics. At sugar concentrations below 25 m M , transport was due to an active, carrier-mediated transport system. A linear component was the dominant mode of uptake at sugar concentrations above 25 m M . Sucrose and glucose carriers were specific for these sugars, since no uptake inhibition was observed from competing sugars. Sucrose was not hydrolyzed by leaf tissue because the label in asymetrically labeled sucrose was not randomized. Furthermore, no label was present in hexose fractions when tissue was incubated with [⁸⁴C]-sucrose. Therefore, [¹⁴C]-sucrose uptake did not reflect hexose uptake.
Both saturable and linear components of uptake contribute significantly to total uptake rates. The former, however, is more important when apoplastic sugar concentrations are low. The molecular nature of the linear component is not well understood but accounts for most of the uptake at high sugar concentrations.     

Development of pH sensitivity of sucrose uptake during ageing of Vicia faba leaf discs

Uptake of [U-¹⁴C]-sucrose (40 m M ) by fresh and aged peeled leaf discs of broad bean ( Vicia faba L. cv. Aguadulce) has been studied. In fresh discs, uptake was nearly insensitive to external pH, whereas the pH response of absorption in discs aged for 12 h was bell-shaped, with an optimum between pH 5 and 6. At this pH, uptake was nearly twice that in fresh tissue. The passive (insensitive to carbonyl cyanide m -chlorophenylhydrazone and to cold treatment) uptake was the same in fresh or aged discs. The development of pH sensitivity of absorption did not appear when ageing was performed in the presence of 10^−H M cycloheximide or 5.7 × 10⁻⁵ M actinomycin D. Similarly, when the tissues were treated with 10⁻³ M spermidine for 2 h after excision and then aged for 10 h, the development of the pH-sensitive uptake system was inhibited. Ca²⁺ (10⁻² M ) supplied together with spermidine prevented the inhibiting effect of spermidine. The appearance of the pH-sensitive system was also markedly reduced if ageing took place in the presence of 10⁻³ M aminoethoxyvinylglycine. Autoradiographs from fresh discs and from discs aged with or without the inhibitors suggest that pH sensitivity developed more intensively in the parenchyma than in the veins.
The results suggest some caution when using excised leaf discs for studies on sucrose uptake and phloem loading. Development of pH sensitivity of uptake may require the synthesis of both DNA-dependent RNA and protein and could be related to ethylene metabolism.     

Soluble acid invertase activity in leaves is independent of species differences in leaf carbohydrates, diurnal sugar profiles and paths of phloem loading

Leaf sucrose, starch, hexose and maximum extractable soluble acid invertase activity were compared throughout the day in source leaves of 13 plant species chosen for their putative phloem-loading type (apoplastic or symplastic). Four species which represent the different phloem-loading types (tomato, barley, maize and Fuchsia ) were studied in detail. Using this information we wished to determine whether a positive correlation between foliar carbohydrates and acid invertase activity exists in leaves from different species and, furthermore, whether this relationship is determined by phloem-loading type. Acid invertase activity was relatively constant throughout the day in all species. The extent of sucrose, hexose and starch accumulation and the sucrose: starch ratio measured at a given time were species-dependent. No correlations were found between foliar soluble acid invertase activity and the hexose, sucrose or starch content of the leaves in any of the species, regardless of phloem-loading type. The species examined could be divided into three distinct groups: (1) high sucrose, low invertase; (2) low sucrose, low invertase; and (3) low sucrose, high invertase. The absence of an inverse relationship between leaf sucrose, hexose or starch contents and endogenous soluble acid invertase suggests that this enzyme is not directly involved in carbon partitioning in leaves but serves an auxiliary function.     

Interaction of cell turgor and hormones on sucrose uptake in isolated Phloem of celery

Phloem tissue isolated from celery (Apium graveolens L.) was used to investigate the regulation of sucrose uptake by turgor (manipulated by 50-400 milliosomolal solutions of polyethylene glycol) and hormones indoleacetic acid (IAA) and gibberillic acid (GA₃). Sucrose uptake was enhanced under low cellular turgor (increase in the V_max). Furthermore, enhancement of sucrose uptake was due to a net increase in influx rates since sucrose efflux was not affected by cell turgor. Manipulations of cell turgor had no effect on 3-O-methyl glucose uptake. When 20 millimolar buffer was present in uptake solutions, low turgor-induced effects were observed only at low pH range (4.5-5.5). However, the effect was extended to higher external pH (up to 7.5) when buffer was omitted from uptake solutions. A novel interaction between cellular turgor and hormone treatments was observed, in that GA₃ (10 micromolar) and IAA (0.1-100 micromolar) enhanced sucrose uptake only at moderate turgor levels. The hormones elicited little or no response on sucrose uptake under conditions of low or high cell turgor. Low cell turgor, IAA, GA₃, and fusicoccin caused acidification by isolated phloem segments in a buffer-free solution. It is suggested that enhanced sucrose uptake in response to low turgor and/or hormones was mediated through the plasmalemma H⁺-ATPase and most likely occurred at the site of loading.     

Influence of cell turgor on sucrose partitioning in potato tuber storage tissues

Sucrose uptake and partitioning in potato (Solanum tuberosum L.) tuber discs were examined under a range of mannitol and ethylene-glycol concentrations. Mannitol caused the same changes in turgor over a wide range of incubation periods (90 min-6 h), indicating that it did not penetrate the tissue. In comparison, ethylene glycol reduced turgor losses but did not eliminate them, even after 6 h. Between 100 mM and 300 mM mannitol, turgor fell by 350 kPa, compared with 35 kPa in ethylene glycol. Uptake experiments in mannitol alone showed that total sucrose uptake was strongly correlated with both osmotic potential and with turgor potential. In subsequent experiments sucrose uptake and partitioning were examined after 3 h equilibration in 100 mM and 300 mM concentrations of mannitol and ethylene glycol. Total sucrose uptake and the conversion of sucrose to starch were enhanced greatly only at 300 mM mannitol, indicating an effect of turgor, rather than osmotic potential on sucrose partitioning. The inhibitors p-chloromercuribenzenesulfonic acid and carbonylcyanide m-chlorophenylhydrazone (CCCP) both reduced sucrose uptake, but in quite different ways. p-Chloromercuribenzenesulfonic acid reduced total sucrose uptake but did not affect the partitioning of sucrose to starch. By contrast, CCCP inhibited total uptake and virtually eliminated the conversion of sucrose to starch. Despite this, sucrose uptake in the presence of CCCP continued to increase as the mannitol concentration increased, indicating an increase in passive transport at higher mannitol concentrations. Increased sucrose uptake above 400 mM mannitol was shown to be the result of uptake into the free space. The data show that starch synthesis is optimised at low but positive turgors and the relation between sucrose partitioning and the changing diurnal water relations of the tuber are discussed.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - PCMBS p-chloromercuribenzenesulfonic acid     

Recognition of phlorizin by the carriers of sucrose and hexose in broad bean leaves

Phlorizin (1-[2-(β- d -glucopyranosyloxy)-4, 6-dihydroxyphenyl]-3-(4-hydroxyphenyl)-1-propanone) is a well-known non-transported inhibitor of glucose uptake in animal cells. The effects of this compound were studied on the transmembrane potential difference (PD) of broad bean ( Vicia faba L. cv. Aguadulce) mesophyll cells, and on the uptake of amino acids and sugars by the leaf tissues. Phlorizin (5 m M ) induced a marginal depolarization (7 to 10 mV) of the normal PD (-140 mV), and a slight decrease in the uptake of glycine and serine. By contrast, phlorizin induced a stronger inhibition of the uptake of glucose and 3–O-methylglucose, and more particularly of sucrose uptake and phloem loading. In tissues aged for 12 h, 5 m M phlorizin inhibited the absorption of sucrose from a 1 m M solution by 70%. Kinetic experiments demonstrated that phlorizin acted as a competitive inhibitor for the sucrose carrier and for the hexose carrier. Efflux experiments showed that the counter-exchange of sucrose and of 3–O-methylglucose was also phlorizin-sensitive. Overall, the data show that phlorizin is recognized by the hexose carrier and, more efficiently, by the sucrose carrier of the material investigated, but that it is not transported across the membrane.     

Phloem turgor and the regulation of sucrose loading in Ricinus communis L.

The influence of plant water relations on phloem loading was studied in Ricinus communis L. Phloem transport was maintained in response to bark incisions even at severe water deficits. Water stress was associated with a net increase in the solute content of the sieve tubes, which resulted in maintenance of a positive phloem turgor pressure _p. There was a significant increase in solute flux through the phloem with decreasing xylem water potential (). In addition, sugar uptake by leaf discs was examined in media adjusted to different water potentials with either sorbitol (a relatively impermeant solute) or ethylene glycol (a relatively permeant solute). The limitations in this experimental system are discussed. The results nevertheless indicated that sucrose uptake can be stimulated by a reduction in cell _p, but that it is little affected by cell or solute potential _s. On the basis of these data we suggest that sucrose loading is turgor-pressure dependent. This may provide the mechanism by which transport responds to changes in sink demand in the whole plant.Abbreviations water potential - _s solute potential - _p pressure potential     

Companion cell-specific inhibition of the potato sucrose transporter SUT1

In many plants, translocation of sucrose from mesnsophyll to phloem for long-distance transport is carrier-mediated. The sucrose H⁺-symporter gene SUT1 from potato is expressed at high levels in the phloem of mature, exporting leaves and at lower levels in other organs. Inhibition of SUT1 by expression of an antisense gene in companion cells under control of the rolC promoter leads to accumulation of high amounts of soluble and insoluble carbohydrates in leaves and inhibition of photosynthesis. The distribution of in situ localized starch does not correspond with areas of reduced photosynthesis as shown by fluorescence imaging. Dissection of antisense effects on sink and source organs by reciprocal grafts shows that inhibition of transporter gene expression in leaves is sufficient to produce chlorosis in leaves and reduced tuber yield. In contrast to the arrest of plasmodesmal development found in plants that express yeast invertase in the apoplast, in mature leaves of sucrose transporter antisense plants plasmodesmata are branched and have median cavities. These data strongly support an apoplastic mode of phloem loading in potato, in which the sucrose transporter located at the plasma membrane of the sieve element/companion cell complex represents the primary route for sugar uptake into the long-distance translocation pathway.     

Psychrotolerant bacteria isolated from the leaf apoplast of cold-adapted wild plants improve the cold resistance of bean (Phaseolus vulgaris L.) under low temperature

We have isolated psychrotolerant bacteria from the leaf apoplast of cold-adapted wild plants and aimed to investigate their effect on the cold resistance of bean (Phaseolus vulgaris L.). Based on the findings of 16S rRNA gene sequence analysis, 20 isolates belonging to 5 bacteria species (Pseudomonas fragi, P. chloropaphis, P. fluorescens, P. proteolytica and Brevibacterium frigoritolerans) were identified in the leaf apoplastic fluid of Draba nemorosa, Galanthus gracilis, Colchicum speciousum, Scilla siberica, Erodium cicutarium, respectively. We have determined that 6 of the 20 isolates have exhibited ACC (1-aminocyclopropane-1-carboxylate) deaminase activity and secreted different extracellular proteins under cold condition (+4 °C) compared to normal growth condition (28 °C). The six isolates were then inoculated independently of each other to the leaves of 10-day-old bean seedlings growing under normal conditions (25/22 °C, 16/8 h photoperiod), and the inoculated and uninoculated (control) seedlings were transferred to cold (9/5 °C, 16/8 h photoperiod) for 3 days. The bacterial inoculations have decreased freezing injury, ice nucleating activity and lipid peroxidation content in parallel with the decrease of reactive oxygen species level such as O₂^.- and H₂O₂ in the inoculated seedlings compared to the control. In addition, the inoculations of the isolates have stimulated the activity of apoplastic antioxidant enzymes including superoxide dismutase, catalase, peroxidase, and glutathione reductase. The results show that the inoculations improve the cold resistance of bean seedlings and the psychrotolerant bacterial isolates can be evaluated within the group of plant growth promoting bacteria (PGPB) which can increase tolerance of cold-sensitive crops.     

Diurnal feeding as a potential mechanism of osmoregulation in aphids

Diurnal variation in phloem sap composition has a strong infuence on aphid performance.The sugar-rich phloem sap serves as the sole diet for aphids and a suite of physiological mechanisms and behaviors allowv them to tolerate the high osmotic stress.Here,we tested the hypothesis that night-time feeding by aphids is a behavior that takes advantage of the low sugar diet in the night to compensate for osmotic stress incurred while feeding on high sugar diet during the day.Using the electrical penetration graph(EPG)technique.we examined the eiects of diurmal rhythm on feeding behaviors of bird cherry-oat aphid(Rhopalosiphurm padi L.)on wheat.A strong diurmal rhythm in aphids as indicated by the presence of a cyclical pattern of expression in a core clock gene did not impact aphid feeding and similar feeding behaviors were observed during day and night.The major difference observed between day and night feeding was that aphids spent significantly longer time in phloem salivation during the night compared to the day.In contrast,aphid hydration was reduced at the end of the day-time feeding compared to end of the night-time fepding.Gene expression analysis of R.padi osmoregulatory genes indicated that sugar break down and water transport into the aphid gut was reduced at night.These data suggest that while diumal variation occurs in phloem sap composition,aphids use night time feeding to overcome the high osmotic stress incurred while feeding on sugar-rich phloem sap during the day.     

渗透胁迫对绿豆叶圆片叶绿素荧光的影响

绿豆叶圆片用0、-0．3、-0．6、-1．2、-1．8、-2．4MPa等6个渗透梯度处理24h后,其叶绿素荧光的反应表明：光系统Ⅱ的潜在量子产量(FD／Fm)和电子传递受体(QA)的库容(pool size)均随胁迫强度的增大而减小,但QA库容的下降明显比Fg／Fm的下降缓和。结合原初荧光Fo和最大荧光Fm的变化,分析认为本实验中渗透胁迫并未导致明显的光系统Ⅱ反应中心的失活或破坏。造成光系统Ⅱ潜在量子效率降低的原因主要是通过光系统Ⅱ反应中心到Ⅱ的电子传递受到阻抑。     

Functional characterisation and cell specificity of BvSUT1, the transporter that loads sucrose into the phloem of sugar beet (Beta vulgaris L.) source leaves

• Sugar beet (Beta vulgaris L.) is one of the most important sugar‐producing plants worldwide and provides about one third of the sugar consumed by humans. Here we report on molecular characterisation of the BvSUT1 gene and on the functional characterisation of the encoded transporter.
• In contrast to the recently identified tonoplast‐localised sucrose transporter BvTST2.1 from sugar beet taproots, which evolved within the monosaccharide transporter (MST) superfamily, BvSUT1 represents a classical sucrose transporter and is a typical member of the disaccharide transporter (DST) superfamily.
• Transgenic Arabidopsis plants expressing the β‐GLUCURONIDASE (GUS) reporter gene under control of the BvSUT1‐promoter showed GUS histochemical staining of their phloem; an anti‐BvSUT1‐antiserum identified the BvSUT1 transporter specifically in phloem companion cells. After expression of BvSUT1 cDNA in bakers’ yeasts (Saccharomyces cerevisiae) uptake characteristics of the BvSUT1 protein were studied. Moreover, the sugar beet transporter was characterised as a proton‐coupled sucrose symporter in Xenopus laevis oocytes.
• Our findings indicate that BvSUT1 is the sucrose transporter that is responsible for loading of sucrose into the phloem of sugar beet source leaves delivering sucrose to the storage tissue in sugar beet taproot sinks.

     

Effects of temperature on the conformation of the endoplasmic reticulum and on starch accumulation in leaves with the symplasmic minor-vein configuration

The phloem-loading-related effects of temperature on leaf ultrastructure were studied in seven species having numerous plasmodesmatal connections between the mesophyll and phloem (symplasmic minor-vein configuration). The response to temperature (between 5 and 30 °C) was characterized by drastic changes in the endoplasmic-reticulum labyrinth (ER labyrinth) of intermediary cells, in the position of the vacuole in bundle-sheath cells, and in the starch content in the chloroplasts of bundle-sheath cells and mesophyll cells. At temperatures above 20 °C, the ER system in the intermediary cells reached its maximal volume, while the vacuole in bundlesheath cells was positioned centripetally (proximal to the intermediary cell). With decreasing temperature, the ER labyrinth in intermediary cells gradually contracted till the ER was fully collapsed at 10 °C and the vacuole in bundle-sheath cells moved to a more centrifugal position. The apparent elimination of photosynthate transport via the ER and plasmodesmata at temperatures lower than 10 °C led to starch accumulation in the chloroplasts of bundle-sheath cells and mesophyll cells. All of these changes were fully temperature-reversible and probably reflect changes in the balance between photosynthate transport and storage. The ultrastructural shifts appear to be correlated with the passage of photosynthate through the intermediary cells and, as a consequence, with the rate of phloem loading at various temperatures. A contraction of the ER/plasmodesmata system imposed by cytoskeletal reorganisation is discussed as the reason for the blockage of phloem loading at low temperatures in association with the general chilling sensitivity of these species.Abbreviations BSC bundle-sheath cell - IC intermediary cell - MC mesophyll cell - PD plasmodesmata - PFD photon flux density - SE/CC-complex sieve element/companion cell complex The authors gratefully acknowledge the financial support by NWO (Dutch Organization for Scientific Research).     

The effect of cell turgor on sugar uptake in strawberry fruit cortex tissue

A reduction in cell turgor has been shown to stimulate sugar uptake in several plant sink tissues and it may regulate the import of assimilate into the sink apoplast, as well as maintain cell turgor. To determine whether cell turgor influences sugar uptake by strawberry (Fragaria x ananassa Duch. cv. Brighton) fruit cortex tissue, disks were cut from greenhouse-grown primary fruit at the green-white stage of development and placed in buffered incubation solutions containing either mannitol or ethylene glycol as an osmoticum. Cell turgor of fruit disks was calculated from the difference between the water potential of bathing solution and tissue solute potential after incubation at various osmolarities. Cell turgor increased when tissue disks were placed into mannitol incubation solutions more dilute than the water potential of fresh tissue (about 415 mOsmol kg^?1). The rate of uptake of [¹⁴C]-sucrose or [¹⁴C]-glucose decreased as osmolarity of the incubation solution increased, i.e. as cell turgor declined. Cell turgor and the rate of [¹⁴C]-sucrose uptake were unaffected when rapidly permeating ethylene glycol was used as an osmoticum. A decrease in cell turgor reduced both the V_max of the saturable (carrier mediated) kinetic component of sucrose uptake, and the slope of the linear (diffusional) component. The sulfhydryl binding reagent p-chloromercuibenzenesulfonic acid, an inhibitor of the plasma membrane sucrose carrier, strongly inhibited only the saturable component of sucrose uptake. Increased uptake of the nonmetabolizable sugar, O-methyl-glucose, at high turgor was similar to that of glucose, indicating that carrier activity was influenced by cell turgor, not cell metabolism. Turgor did not influence efflux of [¹⁴C]-sucrose from disks and had no effect on cell viability. Strawberry fruit cells do not possess a sugar uptake system that is stimulated by a reduction in turgor.     

Ultrastructural effects in potato leaves due to antisense-inhibition of the sucrose transporter indicate an apoplasmic mode of phloem loading

To study the export of sugars from leaves and their long-distance transport, sucrose-proton/co-transporter activity of potato was inhibited by antisense repression of StSUT1 under control of either a ubiquitously active (CaMV 35S ) or a companion-cell-specific (rolC) promotor in transgenic plants. Transformants exhibiting reduced levels of the sucrose-transporter mRNA and showing a dramatic reduction in root and tuber growth, were chosen to investigate the ultrastructure of their source leaves. The transformants had a regular leaf anatomy with a single-layered palisade parenchyma, and bicollateral minor veins within the spongy parenchyma. Regardless of the promoter used, source leaves from transformants showed an altered leaf phenotype and a permanent accumulation of assimilates as indicated by the number and size of starch grains, and by the occurrence of lipid-storing oleosomes. Starch accumulated throughout the leaf: in epidermis, mesophyll and, to a smaller degree, in phloem parenchyma cells of minor veins. Oleosomes were observed equally in mesophyll and phloem parenchyma cells. Companion cells were not involved in lipid accmulation and their chloroplasts developed only small starch grains. The similarity of ultrastructural symptoms under both promotors corresponds to, rather than contradicts, the hypothesis that assimilates can move symplasmically from mesophyll, via the bundle sheath, up to the phloem. The microscopical symptoms of a constitutively high sugar level in the transformant leaves were compared with those in wild-type plants after cold-girdling of the petiole. Inhibition of sugar export, both by a reduction of sucrose carriers in the sieve element/companion cell complex (se/cc complex), or further downstream by cold-girdling, equally evokes the accumulation of assimilates in all leaf tissues up to the se/cc complex border. However, microscopy revealed that antisense inhibition of loading produces a persistently high sugar level throughout the leaf, while cold-girdling leads only to local patches containing high levels of sugar. Received: 4 March 1998 / Accepted: 7 April 1998     

The mechanism of phloem loading in rice (Oryza sativa)

Carbohydrates, mainly sucrose, that are synthesized in source organs are transported to sink organs to support growth and development. Phloem loading of sucrose is a crucial step that drives long-distance transport by elevating hydrostatic pressure in the phloem. Three phloem loading strategies have been identified, two active mechanisms, apoplastic loading via sucrose transporters and symplastic polymer trapping, and one passive mechanism. The first two active loading mechanisms require metabolic energy, carbohydrate is loaded into the phloem against a concentration gradient. The passive process, diffusion, involves equilibration of sucrose and other metabolites between cells through plasmodesmata. Many higher plant species including Arabidopsis utilize the active loading mechanisms to increase carbohydrate in the phloem to higher concentrations than that in mesophyll cells. In contrast, recent data revealed that a large number of plants, especially woody species, load sucrose passively by maintaining a high concentration in mesophyll cells. However, it still remains to be determined how the worldwide important cereal crop, rice, loads sucrose into the phloem in source organs. Based on the literature and our results, we propose a potential strategy of phloem loading in rice. Elucidation of the phloem loading mechanism should improve our understanding of rice development and facilitate its manipulation towards the increase of crop productivity.     

Antisense inhibition of the sucrose transporter in potato: effects on amount and activity

The sucrose proton-cotransporter gene from potato (StSUT1) is mainly expressed in the phloem of mature, exporting leaves. To study the in vivo role of the protein, potato plants were transformed with antisense constructs of the sucrose transporter cDNA under control of the CaMV35S and the rolC promoters, respectively. Both types of transgenic plant develop symptoms characteristic of an inhibition of phloem loading. To determine the level of inhibition, immunological and transport studies were performed. Purified antibodies directed against a peptide from the central loop of SUT1 recognized a transporter with an apparent molecular mass of 47 kDa in leaf plasma membrane vesicles. Antisense repression under control of the non-specific CaMV35S promoter led to a strong reduction in SUT1 protein, whereas no such reduction could be detected when the companion cell-specific rolC promoter was used. Similarily. sucrose uptake in plasma membrane vesicles was reduced by 50–75% in CaMV35S but not in rolC plants. These data suggest that, unlike the rolC promoter, the sucrose transporter is expressed not only in the companion cells but also in other leaf cells. However, inhibition of the transporter by rolC-controlled antisense repression is sufficient to impair phloem loading.     

Closure of plasmodesmata in maize (Zea mays) at low temperature: a new mechanism for inhibition of photosynthesis

Background and Aims

Photosynthesis is one of the processes most susceptible to low-temperature inhibition in maize, a tropical C4 crop not yet fully adapted to a temperate climate. C4 photosynthesis relies on symplasmic exchange of large amounts of photosynthetic intermediates between Kranz mesophyll (KMS) and bundle sheath (BS) cells. The aim of this study was to test the hypothesis that the slowing of maize photosynthesis at low temperature is related to ultrastructural changes in the plasmodesmata between KM and BS as well as BS and vascular parenchyma (VP) cells.

Methods

Chilling-tolerant (CT) KW 1074 and chilling-sensitive (CS) CM 109 maize (Zea mays) lines were studied. The effect of moderate chilling (14 °C) on the rate of photosynthesis, photosynthate transport kinetics, and the ultrastructure of plasmodesmata linking the KMS, BS and VP cells were analysed. Additionally, the accumulation of callose and calreticulin was studied by the immunogold method.

Key Results

Chilling inhibited photosynthesis, photosynthate transfer to the phloem and photosynthate export from leaves in both lines. This inhibition was reversible upon cessation of chilling in the CT line but irreversible in the CS line. Simultaneously to physiological changes, chilling induced swelling of the sphincters of plasmodesmata linking KMS and BS cells and a decreased lumen of the cytoplasmic sleeve of plasmodesmata at the BS/VP interface in the CS line but not in the CT line. Accumulation of calreticulin, which occurred near the neck region of the closed plasmodesmata was observed after just 4 h of chilling and over-accumulation of callose at the KMS/BS and BS/VP interfaces occurred after 28 h of chilling.

Conclusions

Stronger chilling sensitivity of the CM 109 maize line compared with the KW 1074 line, shown by decreased photosynthesis and assimilate export from a leaf, is related to changes in the ultrastructure of leaf plasmodesmata at low temperature. The chain of reactions to chilling is likely to include calreticulin action resulting in rapid and efficient closure of the plasmodesmata at both KMS/BS and BS/VP interfaces. Callose deposition in a leaf was a secondary effect of chilling.     

The role of the leaves in the regulation of internode elongation in Phaseolus vulgaris

Time-course patterns of leaf and internode elongation were studied in bean plants. Each leaf started its main elongation period when the leaf below reached half of its final length. The onset of leaf unfolding was nearly synchronous with the initiation of the elongation of the subjacent internode. Excision of young leaves increased the rate of stem elongation as a result of an earlier unfolding of the next upper leaves and the concomitant advancement in the elongation of their subjacent internodes. IAA or NAA (1% in lanolin) suppressed the enhancement effects of leaf excision on leaf and internode elongation. The excision of a young leaf increased the final length of internodes located below it, and at the same time decreased the final length of the internodes located above the excised leaf. The reduction was greater the younger the internode. Differences in internode elongation after leaf excision were related to changes during internode ontogenesis in their relative response to the availability of assimilates on the one hand, and on the other hand to hormonal factors transported acropetally from the young leaves to the growing internodes.