How are leaves plumbed inside a branch? Differences in leaf-to-leaf hydraulic sectoriality among six temperate tree species

The transport of water, sugar, and nutrients in trees is restricted to specific vascular pathways, and thus organs may be relatively isolated from one another (i.e. sectored). Strongly sectored leaf-to-leaf pathways have been shown for the transport of sugar and signal molecules within a shoot, but not previously for water transport. The hydraulic sectoriality of leaf-to-leaf pathways was determined for current year shoots of six temperate deciduous tree species (three ring-porous: Castanea dentata, Fraxinus americana, and Quercus rubra, and three diffuse-porous: Acer saccharum, Betula papyrifera, and Liriodendron tulipifera). Hydraulic sectoriality was determined using dye staining and a hydraulic method. In the dye method, leaf blades were removed and dye was forced into the most proximal petiole. For each petiole the vascular traces that were shared with the proximal petiole were counted. For other shoots, measurements were made of the leaf-area-specific hydraulic conductivity for the leaf-to-leaf pathways (k(LL)). In five out of the six species, patterns of sectoriality reflected phyllotaxy; both the sharing of vascular bundles between leaves and k(LL) were higher for orthostichous than non-orthostichous leaf pairs. For each species, leaf-to-leaf sectoriality was determined as the proportional differences between non-orthostichous versus orthostichous leaf pairs in their staining of shared vascular bundles and in their k(LL); for the six species these two indices of sectoriality were strongly correlated (R2=0.94; P <0.002). Species varied 8-fold in their k(LL)-based sectoriality, and ring-porous species were more sectored than diffuse-porous species. Differential leaf-to-leaf sectoriality has implications for species-specific co-ordination of leaf gas exchange and water relations within a branch, especially during fluctuations in irradiance and water and nutrient availability.     

Implications of nitrogen phloem loading for carbon metabolism and transport during Arabidopsis development

Metabolite transport processes and primary metabolism are highly interconnected. This study examined the importance of source-to-sink nitrogen partitioning, and associated nitrogen metabolism for carbon capture, transport and usage. Specifically, Arabidopsis aap8(AMINO ACID PERMEASE 8) mutant lines were analyzed to resolve the consequences of reduced amino acid phloem loading for source leaf carbon metabolism,sucrose phloem transport and sink development during vegetative and reproductive growth phase. Results showed that decreased amino acid transport had a negative effect on sink development of aap8 lines throughout the life cycle, leading to an overall decrease in plant biomass. During vegetative stage, photosynthesis and carbohydrate levels were decreased in aap8 leaves, while expression of carbon metabolism and transport genes, as well as sucrose phloem transport were not affected despite reduced sink strength.However, when aap8 plants transitioned to reproductive phase, carbon fixation and assimilation as well as sucrose partitioning to siliques were strongly decreased. Overall,this work demonstrates that phloem loading of nitrogen has varying implications for carbon fixation, assimilation and source-to-sink allocation depending on plant growth stage. It further suggests alterations in source-sink relationships, and regulation of carbon metabolism and transport by sink strength in a development-dependent manner.     

Induced sink strength as a prerequisite for induced tannin biosynthesis in developing leaves of Populus

Induced defenses occur predominately in young, developing plant tissues that rely upon carbohydrate import to support their growth and development. To test the hypothesis that the induced production of carbon-based defenses is dependent upon photoassimilate import, we examined the response of developing leaves of hybrid poplar (Populus deltoides 2 P. nigra) saplings to wounding by gypsy moth caterpillars (Lymantria dispar L.) and exogenous jasmonic acid (JA). Growth rates, condensed tannin contents and acid invertase activities were measured for individual leaves and the translocation of ¹³C-labeled resources between orthostichous source-sink pairs was quantified. Results showed a substantial increase in the activity of cell wall invertase in sink leaves wounded by gypsy moth caterpillars and treated with JA. JA-induced sink leaves also imported 3-4 times as much ¹³C-labeled carbon from orthostichous source leaves relative to controls and allocated a significant portion of this imported ¹³C to condensed tannin biosynthesis. Reduced carbohydrate flow to these leaves, caused by source leaf removal, resulted in reduced condensed tannin levels and the emergence of a growth-defense tradeoff. These results indicate that (1) induced sink strength is elicited by insect wounding and JA application in hybrid poplar foliage, (2) imported resources are allocated to the production of carbon-based defenses, and (3) the level of induced defense in leaves can be constrained by the ability of leaves to import carbohydrates from source tissues. Together, these results suggest that within-canopy variations in induced resistance may arise in part because of uneven distribution of resources to induced foliage.     

The effects of developmental stage and source leaf position on integration and sectorial patterns of carbohydrate movement in an annual plant, Perilla frutescens (Lamiaceae)

A well-integrated plant shows extensive carbohydrate translocation through the plant body. Even in highly integrated plants, however, translocation patterns will be sectorial if vascular tissue restricts carbon movement to sectors along stems. Both integration and sectorial translocation patterns are sensitive to plant architecture and thus may change as a plant develops. These patterns should vary also with the position of the source leaf because leaves at each node are unique in age and vascular relationship to the rest of the plant. I measured the effects of developmental stage and location of the source leaf on integration and sectoriality in an annual plant, Perilla frutescens, by labeling plants with C at one of three leaves and four developmental stages. Stage and source leaf affected both integration and sectoriality. Most notably, integration declined and sectoriality increased during seed fill, when resource demand at each node was high. Furthermore, translocation was least extensive from the leaf supporting the largest number of seeds on its axillary branch. These results suggest that plants are not homogeneous collections of subunits; rather, the role of each leaf in a plant's carbon budget is a function of its age and location on the plant.     

Modeling the influence of differential sectoriality on the photosynthetic responses of understory saplings to patchy light and water availability

Exploitation of patchy light is a key determinant of plant performance in the forest understory. While many adaptive traits are known, the role of stem vasculature in understory photosynthesis is not established. Sectoriality—the degree of vascular constraint to long distance transport—has been hypothesized to limit growth in heterogeneous light. We simulated the photosynthetic potential of sectored and integrated plants in patchy light, as a function of soil water potential (patchy or uniform). We used hydraulic parameters typical of temperate woody species in an Ohm’s law model including a tangential resistance parameter, and simulated cavitation by varying axial resistance of leaves, leaves and roots, or the whole plant. Our results suggest that differential sectoriality will not affect photosynthesis when water is plentiful, but can constrain stomatal conductance at more negative soil water potentials, especially when only a small portion of the crown receives light. This effect is strongest just below the turgor loss point, and depends on axial resistance and soil water heterogeneity. Increased resistance in high light leaves decreases photosynthesis regardless of sectoriality. However, when resistance is increased for leaves and roots or the whole plant, photosynthesis decreases more for sectored than for integrated plants. Moreover, the simulations suggest that sectoriality can further depress photosynthesis when water availability is asymmetrical. These results might explain why integrated species, such as Betula lenta, B. alleghaniensis, and Acer saccharum thrive in the forest understory and grow rapidly into canopy gaps, while sectored species, such as Quercus rubra, do not.     

Jasmonic acid treatment to part of the root system is consistent with simulated leaf herbivory, diverting recently assimilated carbon towards untreated roots within an hour

It is known that shoot application of jasmonic acid (JA) leads to an increased carbon export from leaves to stem and roots, and that root treatment with JA inhibits root growth. Using the radioisotope ¹¹C, we measured JA effects on carbon partitioning in sterile, split-root, barley plants. JA applied to one root half reduced carbon partitioning to the JA-treated tissue within minutes, whereas the untreated side showed a corresponding – but slower – increase. This response was not observed when instead of applying JA, the sink strength of one root half was reduced by cooling it: there was no enhanced partitioning to the untreated roots. The slower response in the JA-untreated roots, and the difference between the effect of JA and temperature, suggest that root JA treatment caused transduction of a signal from the treated roots to the shoot, leading to an increase in carbon allocation from the leaves to the untreated root tissue, as was indeed observed 10 min after the shoot application of JA. This supports the hypothesis that the response of some plant species to both leaf and root herbivores may be the diversion of resources to safer locations.     

Variation in the export of 13C and 15N from soybean leaf: the effects of nitrogen application and sink removal

Translocation of carbon and nitrogen within a single source-sink unit, comprising a trifoliated leaf, the axillary pod and the subtending internode, and from this unit to the rest of the plant was examined in soybean (Glycine max L. cv. Akishirome) plant by feeding ¹³CO₂ and ¹⁵NO₃. The plants were grown at two levels of nitrogen in the basal medium, i.e. low-N (2 g N m^–2) and high-N (35 g N m^–2) and a treatment of depodding was imposed by removing all the pods from the plant, except the pod of the source sink unit, 13 days after flowering. The plants at high-N accumulated more biomass in its organs compared to low-N and pod removal increased the weight of the vegetative organs. When the terminal leaflet of the source-sink unit was fed with ¹³CO₂, almost all of the radioactive materials were retained inside the source-sink unit and translocation to rest of the plants was insignificant under any of the treatments imposed. Out of the¹³C exported by the terminal leaflet, less than half went into the axillary pod, as the lateral leaflets claimed equal share and very little material was deposited in the petiole. Pod removal decreased ¹³C export at high-N , but not at low-N. Similar to ¹³C, the source-sink unit retained all the ¹⁵N fed to the terminal leaflet at high-N. At low-N, the major part of ¹⁵N partitioning occurred in favour of the rest of the plant outside the source-sink unit, but removal of the competitve sinks from the rest of the plants nullified any partitioning outside the unit. Unlike the situation in ¹³C, no partitioning of ¹⁵N occurred in favour of the lateral leaflets from the terminal leaflet inside the unit. It is concluded that sink demand influences partitioning of both C and N and the translocation of carbon is different from that of nitrogen within a source-sink unit. The translocation of the N is more adjustive to a demand from other sink units compared to the C.     

Short-term control of root: shoot partitioning

We present data showing that the fraction of the available photosynthatepartitioned between the root and the shoot of a barley seedlingis affected by the supply of photosynthate from the source leaf:an increased fraction of the exported photosynthate goes tothe shoot when supply is reduced. Also, if the roots are cooleda short time before reducing the supply of photosynthate, thenthe effect of a reduced supply upon partitioning is reversedwith an increased fraction then going to the root. We concludethat the distribution of available photosynthate between competingsinks is influenced by source supply as well as sink function.The reported source-sink interactions are consistant with thepredictions of a recently pro posed model of source-sink interaction(Minchin et al., 1993). The concept of marginal partitioningis introduced to describe the distribution, between all of thesinks, of a small change in photosynthate supply. Key words: Carbohydrate partitioning, shoot : root ratio, source-sink interactions     

Carbon allocation to shoots and roots in relation to nitrogen supply is mediated by cytokinins and sucrose: Opinion

In this paper we firstly show some general responses of biomass partitioning upon nitrogen deprivation. Secondly, these responses are explained in terms of allocation of carbon and nitrogen, photosynthesis and respiration, using a simulation model. Thirdly, we present a hypothesis for the regulation of biomass partitioning to shoots and roots.Shortly after nitrogen deprivation, the relative growth rate (RGR) of the roots generally increases and thereafter decreases, whereas that of the shoot decreases immediately. The increased RGR of the root and decreased RGR of the shoot shortly after a reduction in the nitrogen supply, cause the root weight ratio (root weight per unit plant weight) to increase rapidly.We showed previously that allocation of carbon and nitrogen to shoots and roots can satisfactorily be described as a function of the internal organic plant nitrogen concentration. Using these functions in a simulation model, we analyzed why the relative growth rate of the roots increases shortly after a reduction in nitrogen supply. The model predicts that upon nitrogen deprivation, the plant nitrogen concentration and the rate of photosynthesis per unit plant weight rapidly decrease, and the allocation of recently assimilated carbon and nitrogen to roots rapidly increases. Simulations show that the increased relative growth rate of the root upon nitrogen deprivation is explained by decreased use of carbon for root respiration, due to decreased carbon costs for nitrogen uptake. The stimulation of the relative growth rate of the root is further amplified by the increased allocation of carbon and nitrogen to roots. Using the simple relation between the plant nitrogen concentration and allocation, the model describes plant responses quite realistically.Based on information in the literature and on our own experiments we hypothesize that allocation of carbon is mediated by sucrose and cytokinins. We propose that nitrogen deprivation leads to a reduced cytokinin production, a decreased rate of cytokinin export from the roots to the shoot, and decreased cytokinin concentrations. A reduced cytokinin concentration in the shoot represses cell division in leaves, whereas a low cytokinin concentration in roots neutralizes the inhibitory effect of cytokinins on cell division. A reduced rate of cell division in the leaves leads to a reduced unloading of sucrose from the phloem into the expanding cells. Consequently, the sucrose concentration in the phloem nearby the expanding cells increases, leading to an increase in turgor pressure in the phloem nearby the leaf's division zone. In the roots, cell division continues and no accumulation of sugars occurs in dividing cells, leading to only marginal changes in osmotic potential and turgor pressure in the phloem nearby the root's cell division zone. These changes in turgor pressure in the phloem of roots and sink leaves affect the turgor pressure gradients between source leaf-sink leaf and source leaf-root in such a way that relatively more carbohydrates are exported to the roots. As a consequence RWR increases after nitrogen deprivation. This hypothesis also explains the strong relationship between allocation and the plant nitrogen status.     

Altered xylem-phloem transfer of amino acids affects metabolism and leads to increased seed yield and oil content in Arabidopsis

Seed development and nitrogen (N) storage depend on delivery of amino acids to seed sinks. For efficient translocation to seeds, amino acids are loaded into the phloem in source leaves and along the long distance transport pathway through xylem-phloem transfer. We demonstrate that Arabidopsis thaliana AMINO ACID PERMEASE2 (AAP2) localizes to the phloem throughout the plant. AAP2 T-DNA insertion lines showed changes in source-sink translocation of amino acids and a decrease in the amount of seed total N and storage proteins, supporting AAP2 function in phloem loading and amino acid distribution to the embryo. Interestingly, in aap2 seeds, total carbon (C) levels were unchanged, while fatty acid levels were elevated. Moreover, branch and silique numbers per plant and seed yield were strongly increased. This suggests changes in N and C delivery to sinks and subsequent modulations of sink development and seed metabolism. This is supported by tracer experiments, expression studies of genes of N/C transport and metabolism in source and sink, and by phenotypic and metabolite analyses of aap2 plants. Thus, AAP2 is key for xylem to phloem transfer and sink N and C supply; moreover, modifications of N allocation can positively affect C assimilation and source-sink transport and benefit sink development and oil yield.     

Carbon allocation and partitioning in Vigna radiata (L.) Wilczek as affected by additional carbon gain

Carbon allocation to the source leaf, export and partitioning to the sink were studied in mungbean supplied by additional carbon from the source leaves subjected to high CO2 concentrations (600 and 900 cm3 m-3) in three metabolic and functional source-sink combinations. The plants were pruned to a source-path-sink system. With CO2 enrichment there was an appreciable increase in net photosynthetic CO2 uptake in earlier formed and physiologically younger leaves. Most of the carbon fixed as a result of enrichment was translocated out of the source leaf within one diurnal cycle. The carbon remaining in the source leaf was unchanged. Partitioning of extra carbon into starch or sugar depended upon the amount of extra carbon synthesized. The unloading of the extra carbon into sinks depended on whether it was used for growth or stored. Under increased carbon content, the leaf as a sink was able to reorganize its metabolic reactions more rapidly to maintain the required gradient for unloading than the pod acting as the sink.     

Carbon allocation and partitioning in Vigna radiata (L.) Wilczek as affected by additional carbon gain

Carbon allocation to the source leaf, export and partitioning to the sink were studied in mungbean supplied by additional carbon from the source leaves subjected to high CO2 concentrations (600 and 900 cm3 m-3) in three metabolic and functional source-sink combinations. The plants were pruned to a source-path-sink system. With CO2 enrichment there was an appreciable increase in net photosynthetic CO2 uptake in earlier formed and physiologically younger leaves. Most of the carbon fixed as a result of enrichment was translocated out of the source leaf within one diurnal cycle. The carbon remaining in the source leaf was unchanged. Partitioning of extra carbon into starch or sugar depended upon the amount of extra carbon synthesized. The unloading of the extra carbon into sinks depended on whether it was used for growth or stored. Under increased carbon content, the leaf as a sink was able to reorganize its metabolic reactions more rapidly to maintain the required gradient for unloading than the pod acting as the sink.     

Photoassimilate partitioning in nodulated soybean II. The effect of changes in photoassimilate availability shows that nodule permeability to gases is not linked to the supply of solutes or water

It is concluded that the permeability of the soybean nodule to gases is not linked to the supply of solutes or water via the phloem to the nodule. Nodule respiration and nitrogenase activity were less affected by diel variation and shading treatments than partitioning to the nodule, as assessed using a non-invasive ¹¹C-based technique. Thus C import to the nodule was not matched to C requirement by the nodule. Transit times of tracer to, and within, the nodulated root increased under conditions of reduced photosynthetic rate. The increase in transit time was interpreted as a reduction in the flux of phloem sap. Thus the fluxes of both water and C to the nodule decreased following a reduction in photosynthetic rate. The change in partitioning of recent photosynthate to soybean roots and nodules in response to changes in photoassimilate availability was also used to assess the 'priority' of these sinks. Partitioning from the leaf to the root system was greatly decreased when photoassimilate availability was limited, indicating that root system priority is lower than that of the shoot, as reported for other systems. However, partitioning of tracer arriving in the root system between the nodulated and non-nodulated zones of the root was not affected by changes in photoassimilate availability, as caused by diel change, shading, or steaming of branch roots. Thus although nodules are sinks of high sink 'activity', they have 'priority' equal to that of other root sinks. It is suggested that there are similar phloem unloading kinetics, despite the very different metabolic destiny of the carbohydrate within the two organs.     

Manipulation of sucrose phloem and embryo loading affects pea leaf metabolism,carbon and nitrogen partitioning to sinks as well as seed storage pools

Seed development largely depends on the long‐distance transport of sucrose from photosynthetically active source leaves to seed sinks. This source‐to‐sink carbon allocation occurs in the phloem and requires the loading of sucrose into the leaf phloem and, at the sink end, its import into the growing embryo. Both tasks are achieved through the function of SUT sucrose transporters. In this study, we used vegetable peas (Pisum sativum L.), harvested for human consumption as immature seeds, as our model crop and simultaneously overexpressed the endogenous SUT1 transporter in the leaf phloem and in cotyledon epidermal cells where import into the embryo occurs. Using this ‘Push‐and‐Pull’ approach, the transgenic SUT1 plants displayed increased sucrose phloem loading and carbon movement from source to sink causing higher sucrose levels in developing pea seeds. The enhanced sucrose partitioning further led to improved photosynthesis rates, increased leaf nitrogen assimilation, and enhanced source‐to‐sink transport of amino acids. Embryo loading with amino acids was also increased in SUT1‐overexpressors resulting in higher protein levels in immature seeds. Further, transgenic plants grown until desiccation produced more seed protein and starch, as well as higher seed yields than the wild‐type plants. Together, the results demonstrate that the SUT1‐overexpressing plants with enhanced sucrose allocation to sinks adjust leaf carbon and nitrogen metabolism, and amino acid partitioning in order to accommodate the increased assimilate demand of growing seeds. We further provide evidence that the combined Push‐and‐Pull approach for enhancing carbon transport is a successful strategy for improving seed yields and nutritional quality in legumes.     

Compound-specific differences in (13)C of soluble carbohydrates in leaves and phloem of 6-month-old Eucalyptus globulus (Labill)

Movement of photoassimilates from leaves to phloem is an important step for the flux of carbon through plants. Fractionation of carbon isotopes during this process may influence their abundance in heterotrophic tissues. We subjected Eucalyptus globulus to 20, 25 and 28 °C ambient growth temperatures and measured compound-specific δ(13)C of carbohydrates obtained from leaves and bled phloem sap. We compared δ(13)C of sucrose and raffinose obtained from leaf or phloem and of total leaf soluble carbon, with modelled values predicted by leaf gas exchange. Changes in δ(13)C of sucrose and raffinose obtained from either leaves or phloem sap were more tightly coupled to changes in c(i)/c(a) than was δ(13)C of leaf soluble carbon. At 25 and 28 °C, sucrose and raffinose were enriched in (13)C compared to leaf soluble carbon and predicted values - irrespective of tissue type. Phloem sucrose was depleted and raffinose enriched in (13)C compared to leaf extracts. Intermolecular and tissue-specific δ(13)C reveal that multiple systematic factors influence (13)C composition during export to phloem. Predicting sensitivity of these factors to changes in plant physiological status will improve our ability to infer plant function at a range of temporal and spatial scales.     

The Effect of Root Formation on Photosynthesis of Detached Leaves

The CO₂ exchange of fully expanded detached primary leaves ofdwarf bean (Phaseolus vulgaris) with roots on the petioles hasbeen measured. Rates of apparent photosynthesis and respirationincreased as roots grew, decreased when roots were removed,and increased again as root regenerated. Rates of photosynthesisof different leaves were highly correlated with the dry weightof root on their petioles. Photosynthesis and respiration weredecreased when root growth was restricted by kinetin, and wereincreased when root growth was stimulated by IAA. Photosynthesisof an attached leaf declined with time while that of a comparabledetached leaf increased. The results suggest that photosynthesisis correlated with the size of the roots that are a sink forphotosynthates.     

Experimental evidence for diel variations of the carbon isotope composition in leaf, stem and phloem sap organic matter in Ricinus communis

Carbon isotope fractionation in metabolic processes following carboxylation of ribulose-1,5-bisphosphate (RuBP) is not as well described as the discrimination during photosynthetic CO(2) fixation. However, post-carboxylation fractionation can influence the diel variation of delta(13)C of leaf-exported organic matter and can cause inter-organ differences in delta(13)C. To obtain a more mechanistic understanding of post-carboxylation modification of the isotopic signal as governed by physiological and environmental controls, we combined the modelling approach of Tcherkez et al., which describes the isotopic fractionation in primary metabolism with the experimental determination of delta(13)C in leaf and phloem sap and root carbon pools during a full diel course. There was a strong diel variation of leaf water-soluble organic matter and phloem sap sugars with relatively (13)C depleted carbon produced and exported during the day and enriched carbon during the night. The isotopic modelling approach reproduces the experimentally determined day-night differences in delta(13)C of leaf-exported carbon in Ricinus communis. These findings support the idea that patterns of transitory starch accumulation and remobilization govern the diel rhythm of delta(13)C in organic matter exported by leaves. Integrated over the whole 24 h day, leaf-exported carbon was enriched in (13)C as compared with the primary assimilates. This may contribute to the well-known--yet poorly explained--relative (13)C depletion of autotrophic organs compared with other plant parts. We thus emphasize the need to consider post-carboxylation fractionations for studies that use delta(13)C for assessing environmental effects like water availability on ratio of mole fractions of CO(2) inside and outside the leaf (e.g. tree ring studies), or for partitioning of CO(2) fluxes at the ecosystem level.     

Phloem Unloading in Sink Leaves of Nicotiana benthamiana: Comparison of a Fluorescent Solute with a Fluorescent Virus

Using noninvasive imaging techniques, we compared phloem unloading of the membrane-impermeant, fluorescent solute carboxyfluorescein (CF) with that of potato virus X expressing the gene for the green fluorescent protein. Although systemic virus transport took considerably longer to occur than did CF transport, unloading of both solute and virus occurred predominantly from the class III vein network, a highly branched veinal system found between class II veins. The minor veins (classes IV and V) played no role in solute or virus import but were shown to be functional in xylem transport at the time of import by labeling with Texas Red dextran. After virus exit from the class III phloem, the minor veins eventually became infected by cell-to-cell virus movement from the mesophyll. During the sink/source transition, phloem unloading of CF was inhibited from class III veins before the cessation of phloem import through them, suggesting a symplastic isolation of the phloem in class III veins before its involvement in export. The progression of the sink/source transition for carbon was unaffected by the presence of the virus in the sink leaf. However, the virus was unable to cross the sink/source boundary for carbon that was present at the time of viral entry, suggesting a limited capacity for cell-to-cell virus movement into the apical (source) region of the leaf. A functional model of the sink/source transition in Nicotiana benthamiana is presented. This model provides a framework for the analysis of solute and virus movement in leaves.     

Alteration in carbon partitioning induced by the movement protein of tobacco mosaic virus originates in the mesophyll and is independent of change in the plasmodesmal size exclusion limit

The influence of the 30 kDa movement protein of tobacco mosaic virus (TMV-MP) on carbon partitioning in trans-genie tobacco plants (Nicotiana tabacum cv. Xanthi) expressing the TMV-MP was investigated. Using reciprocal grafting of transgenic tobacco plants expressing this movement protein and vector control plants, as well as transgenic tobacco plants expressing the TMV-MP in phloem cells only, we showed that the interactive site involved in carbon allocation to roots is localized to the mesophyll tissue. Biomass partitioning experiments conducted on transgenic plants, in which various deletion mutant forms of the TMV-MP (two of which included deletions in the domain responsible for increasing the size exclusion limit) were expressed, revealed that the TMV-MP exerts its influence on carbon allocation via a mechanism that is completely independent of the TMV-MP-induced increase in the plasmodesmal size exclusion limit. Furthermore, small N- and C-terminal deletions in the MP revealed the complexity of the interactions likely to be involved between the MP and an endogenous regulatory mechanism. We propose that the TMV-MP interferes with an endogenous signal transduction pathway that involves macromolecular trafficking through plasmodesmata to regulate biomass partitioning between the source and various sink tissues.     

Tie-dyed1 encodes a novel, phloem-expressed transmembrane protein that functions in carbohydrate partitioning

Carbon is partitioned between export from the leaf and retention within the leaf, and this process is essential for all aspects of plant growth and development. In most plants, sucrose is loaded into the phloem of carbon-exporting leaves (sources), transported through the veins, and unloaded into carbon-importing tissues (sinks). We have taken a genetic approach to identify genes regulating carbon partitioning in maize (Zea mays). We identified a collection of mutants, called the tie-dyed (tdy) loci, that hyperaccumulate carbohydrates in regions of their leaves. To understand the molecular function of Tdy1, we cloned the gene. Tdy1 encodes a novel transmembrane protein present only in grasses, although two protein domains are conserved across angiosperms. We found that Tdy1 is expressed exclusively in phloem cells of both source and sink tissues, suggesting that Tdy1 may play a role in phloem loading and unloading processes. In addition, Tdy1 RNA accumulates in protophloem cells upon differentiation, suggesting that Tdy1 may function as soon as phloem cells become competent to transport assimilates. Monitoring the movement of a fluorescent, soluble dye showed that tdy1 leaves have retarded phloem loading. However, once the dye entered into the phloem, solute transport appeared equal in wild-type and tdy1 mutant plants, suggesting that tdy1 plants are not defective in phloem unloading. Therefore, even though Tdy1 RNA accumulates in source and sink tissues, we propose that TDY1 functions in carbon partitioning by promoting phloem loading. Possible roles for TDY1 are discussed.