Source-sink partitioning. Do we need Münch?

The simulation of phloem translocation by the Münch theory commonly uses resistances from sources to sinks: the resistances are therefore regarded as important in partitioning. Although resistance is generally a set constant, it is in fact strongly affected by viscosity, and thus the concentration of the transported solute. In this paper, the model of partitioning proposed by Minchin et al. was first corrected for variations in viscosity. The model was further modified, with the source considered as an activity of solute production rather than as a compartment concentration. When so defined, the source cannot differ from the sum of sink activities, largely outdating the source- or sink-limitation concepts. The corrected model confined the effect of resistances on the partitioning to low source activities. In the example of wheat grain filling analysed, such activities would be so low that they would correspond only to pathological conditions. In that case, the use of resistances in modelling is therefore just a mathematical burden, not even easily quantifiable since they are related to anatomical traits that are difficult to access. Leaving out resistances, it becomes easy to calculate the sink activities directly from the source activities, using an intuitive, accessible parametrization. The conditions for such a simplification are discussed.     

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.     

Improvement of pea biomass and seed productivity by simultaneous increase of phloem and embryo loading with amino acids

The development of sink organs such as fruits and seeds strongly depends on the amount of nitrogen that is moved within the phloem from photosynthetic‐active source leaves to the reproductive sinks. In many plant species nitrogen is transported as amino acids. In pea (Pisum sativum L.), source to sink partitioning of amino acids requires at least two active transport events mediated by plasma membrane‐localized proteins, and these are: (i) amino acid phloem loading; and (ii) import of amino acids into the seed cotyledons via epidermal transfer cells. As each of these transport steps might potentially be limiting to efficient nitrogen delivery to the pea embryo, we manipulated both simultaneously. Additional copies of the pea amino acid permease PsAAP1 were introduced into the pea genome and expression of the transporter was targeted to the sieve element‐companion cell complexes of the leaf phloem and to the epidermis of the seed cotyledons. The transgenic pea plants showed increased phloem loading and embryo loading of amino acids resulting in improved long distance transport of nitrogen, sink development and seed protein accumulation. Analyses of root and leaf tissues further revealed that genetic manipulation positively affected root nitrogen uptake, as well as primary source and sink metabolism. Overall, the results suggest that amino acid phloem loading exerts regulatory control over pea biomass production and seed yield, and that import of amino acids into the cotyledons limits seed protein levels.     

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.     

Partitioning Control by Water Potential Gradient: Evidence for Compartmentation Breakdown in Grape Berries

Xylem exudate was obtained from berries of Riesling grapes atdifferent stages of development after the onset of ripeningusing a pressure bomb technique. The osmotic potential of theexudate bore a 1:1 relationship to that of juice from the sameberries which were afterwards crushed and centrifuged. Thisresult provides the first direct evidence of compartmentationbreakdown in grape berries after the onset of ripening. Changesin berry deformability which occur at the same time and measurementsof the dynamics of exudation flow lead to the same conclusionregarding compartmentation breakdown. The breakdown in compartmentation occurs at the same time asthe rate of phloem translocation to the fruit suddenly increases.A mechanism was recently proposed to account for this increase.It required the existence of a water potential difference betweensource and sink such as would result from compartmentation breakdownin the sink tissues. The results, therefore, may be taken toindicate that this mechanism is indeed involved in the controlof assimilate partitioning in Vitis. Evidence in other publicationssuggests that the mechanism may be reasonably widespread inplants. Key words: Assimilate partitioning, phloem translocation mechanism, Vitis vinifera L., water potential gradients     

Changes in apoplastic and intracellular leaf sugars induced by the blocking of export in Vicia faba

Sugar export by broadbean ( Vicia faba L. cv. Aguadulce) was blocked by a cold jacket (1 cm-width, 1°C) applied on the petiole of a mature leaf or by heat-girdling the petiole. A time course study was made on the effects of these treatments on apoplastic and intracellular soluble sugars of the leaf in relation to phloem loading and photosynthesis. Blocking of export by heat-girdling induced an inhibition of phloem loading within 10 min, an accumulation of starch within 30 min and a rise in apoplastic sucrose within 60 min. By contrast, apoplastic hexoses and photosynthesis were not affected by this treatment within 8 h and intracellular sugars were not affected within 2 h. The cold jacket also increased the sucrose content of the apoplast. The increase in apoplastic sucrose induced by the cold barrier is reversed upon rewarming and less marked when the sink/source ratio is increased by defoliating all but the leaves studied. The results are discussed in terms of sink/source relationships. They show that the increase in apoplastic sucrose resulting from inhibition of loading is not part of the events leading from blocking of transport to change in carbon partitioning.     

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.     

Carbon allocation in fruit trees: from theory to modelling

Carbon allocation within a plant depends on complex rules linking source organs (mainly shoots) and sink organs (mainly roots and fruits). The complexity of these rules comes from both regulations and interactions between various plant processes involving carbon. This paper presents these regulations and interactions, and analyses how agricultural management can influence them. Ecophysiological models of carbon production and allocation are good tools for such analyses. The fundamental bases of these models are first presented, focusing on their underlying processes and concepts. Different approaches are used for modelling carbon economy. They are classified as empirical, teleonomic, driven by source–sink relationships, or based on transport and chemical/biochemical conversion concepts. These four approaches are presented with a particular emphasis on the regulations and interactions between organs and between processes. The role of plant architecture in carbon partitioning is also discussed and the interest of coupling plant architecture models with carbon allocation models is highlighted. As an illustration of carbon allocation models, a model developed for peach trees, describing carbon transfer within the plant, and based on source–sink and Münch transport theory is presented and used for analyzing the link between roots, shoots and reproductive compartments. On this basis, the consequences of fruit load or plant pruning on fruit and vegetative growth can be evaluated.     

土壤有机碳汇内涵与核算方法辨析

在"碳达峰、碳中和"战略需求下,土壤有机碳汇作为生态系统碳汇的重要组成部分,土壤碳库容量以及如何开展土壤有机碳汇核算日益成为生态碳汇的研究热点。梳理了国内外土壤有机碳汇及核算相关研究成果,解析了土壤有机碳汇的概念内涵,提出了以稳定性有机碳作为土壤有机碳汇的表征指标及获取方法。从土壤发生学角度提出了土壤碳汇阈值的概念,土壤中有机碳的含量随着分解转化最终会达到动态平衡,此时稳定有机碳含量值约是常数,这个常数就是稳定碳库的库容,在特定的成土因素下,碳库的核算值不会超过平衡时的常态值。在客观上,体现在非人类干扰状态下不同土壤类型自然状态下的稳定性有机碳含量。参照土壤有机质平衡理论,提出了土壤碳汇核算的定量化方法,为土壤碳汇的度量和核算提供了一套技术思路。下一步土壤有机碳汇的核算应在科学研究基础上多角度凝聚共识,制定碳汇核算标准,确定不同尺度下可操作、可重复以及可复制的土壤有机碳汇核算技术与方法。     

Generalized Münch coupling between sugar and water fluxes for modelling carbon allocation as affected by water status

A model of within-plant carbon allocation is proposed which makes a generalized use of the Münch mechanism to integrate carbon and water functions and their involvement in growth limitations. The plant is envisioned as a branched network of resistive pathways (phloem and xylem) with nodal organs acting as sources and sinks for sucrose. Four elementary organs (leaf, stem, fruit, root) are described with their particular sink functions and hydraulic attributes. Given the rates of photosynthesis and transpiration and the hydraulic properties of the network as inputs, the model calculates the internal fluxes of water and sucrose. Xylem water potential (Psi), phloem sucrose concentration (C) and turgor pressure (P) are calculated everywhere in the network accounting for osmotic equilibrium between apoplasm and symplasm and coupled functioning of xylem and phloem. The fluxes of phloem and xylem saps are driven by the gradients of P and Psi, respectively. The fruit growth rate is assumed as turgor pressure dependent. To demonstrate its ability to address within-plant competition, the model is run with a simple-branched structure gathering three leaves, eight stem segments, three competing growing fruits and one root. The model was programmed with P-Spice, a software specifically designed for simulating electrical circuits but easily adaptable to physiology. Simulations of internal water fluxes, sucrose concentrations and fruit growth rates are given for different conditions of soil water availability and hydraulic resistances (sensitivity analysis). The discussion focuses on the potential interest of this approach in functional--structural plant models to address water stress-induced effects.     

Phloem unloading and ‘sink strength’: The parallel between the site of attachment of Cuscuta and developing legume seeds

Sink regions play a central role in determining assimilate distribution patterns. Two factors are discussed which have a strong effect on the sink strength of a sink, viz. phloem unloading and turgor-sensitive transport. Sink strength may be defined as the capacity of phloem in the sink region to import assimilates from other parts of the plants and to release the imported substances into the sink apoplast.A stem parasitized by Cuscuta represents a very strong sink. A review is presented of data on enhanced phloem unloading, at the site of attachment of Custuta. Recent data on metabolically controlled sucrose and amino acid unloading into the seed coat apoplast of developing legume seeds show a remarkable parallel with phloem unloading in a parasitized Vicia faba stem. Data on turgor-sensitive sucrose and amino acid transport into developing seeds are presented, which throw new light on the pressure flow theory of phloem transport.     

Functional characterization of the Arabidopsis AtSUC2 Sucrose/H+ symporter by tissue-specific complementation reveals an essential role in phloem loading but not in long-distance transport

AtSUC2 (At1g22710) encodes a phloem-localized sucrose (Suc)/H(+) symporter necessary for efficient Suc transport from source tissues to sink tissues in Arabidopsis (Arabidopsis thaliana). AtSUC2 is highly expressed in the collection phloem of mature leaves, and its function in phloem loading is well established. AtSUC2, however, is also expressed strongly in the transport phloem, where its role is more ambiguous, and it has been implicated in mediating both efflux and retrieval to and from flanking tissues via the apoplast. To characterize the role of AtSUC2 in controlling carbon partitioning along the phloem path, AtSUC2 cDNA was expressed from tissue-specific promoters in an Atsuc2 mutant background. Suc transport in this mutant is highly compromised, as indicated by stunted growth and the accumulation of large quantities of sugar and starch in vegetative tissues. Expression of AtSUC2 cDNA from the 2-kb AtSUC2 promoter was sufficient to restore growth and carbon partitioning to nearly wild-type levels. The GALACTINOL SYNTHASE promoter of Cucumis melo (CmGAS1p) confers expression only in the minor veins of mature leaves, not in the transport phloem of larger leaf veins and stems. Mutant plants expressing AtSUC2 cDNA from CmGAS1p had intermediate growth and accumulated sugar and starch, but otherwise they had normal morphology. These characteristics support a role for AtSUC2 in retrieval but not efflux along the transport phloem and show that the only vital function of AtSUC2 in photoassimilate distribution is phloem loading. In addition, Atsuc2 mutant plants, although debilitated, do grow, and AtSUC2-independent modes of phloem transport are discussed, including an entirely symplastic pathway from mesophyll cells to sink tissues.     

Sink feedback regulation of photosynthesis in vines: measurements and a model.

An experimental and modelling study of source-sink interactions in Vitis vinifera L., cv. Cabernet Sauvignon, rooted cuttings under non-limiting environmental conditions with a 12 h photoperiod is presented here. After 4 h, measured photosynthesis, stomatal conductance and leaf carbohydrate content reached maximum values. Over the remainder of the photoperiod, photosynthesis and stomatal conductance decreased continuously, whereas leaf carbohydrate content remained relatively constant. Because the experiment took place in a non-limiting environment, the results suggest that stomatal regulation of photosynthesis was mediated by an internal factor, possibly related to sink activity. A simple 1-source, 2-sink model was developed to examine the extent to which the data could be explained by a hypothetical sink-to-source feedback mechanism mediated by carbohydrate levels in either the mesophyll, the source phloem or the phloem of one of the two sinks. Model simulations reproduced the data well under the hypothesis of a phloem-based feedback signal, although the data were insufficient to elucidate the detailed nature of such a signal. In a sensitivity analysis, the steady-state response of photosynthesis to sink activity was explored and predictions made for the partitioning of photosynthate between the two sinks. The analysis highlights the effectiveness of a phloem-based feedback signal in regulating the balance between source and sink activities. However, other mechanisms for the observed decline in photosynthesis, such as photoinhibition, endogenous circadian rhythms or hydraulic signals in the leaf cannot be excluded. Nevertheless, it is concluded that the phloem-based feedback model developed here may provide a useful working hypothesis for incorporation into plant growth models and for further development and testing.     

MassFlowDyn I: A Carbon Transport and Partitioning Model for Root System Architecture

Carbon partitioning is important for understanding root developmentbut little is known about its regulation. Existing models suggestthat partitioning is controlled by the potential sink strength.They cannot, however, simulate hierarchical uptake other thanby using absolute priorities. Moreover, they cannot explainthat the changes in photoassimilate partitioning result fromchanges in photosynthesis. In this paper we present a modelof phloem sieve circulation, based on the model of Minchin etal. (Journal of Experimental Botany44: 947–955, 1993).The root system was represented by a network of segments towhich meristems were connected. The properties of the segmentswere determined by the differentiation stage. Photoassimilateimport from each organ was assumed to be limited by a metabolicprocess and driven by Michaelis–Menten kinetics. The axialgrowth was proportional to meristem respiration, which drivesthe flux of new cells required for root elongation. We usedthe model to look at trophic apical dominance, determinate andindeterminate root growth, the effect of the activity of a rooton competition with its neighbours, and the effect of photoassimilateavailability on changes in partitioning. The simulated phloemmass flow yielded results of the same order of magnitude asthose generally reported in the literature. For the main wellvascularized axis, the model predicted that one single apicalmeristem larger than its neighbouring laterals, was enough togenerate a taprooted system. Conversely, when the meristem oflaterals close to the collar had a volume similar to that ofthe taproot, the predicted network became fibrous. The modelpredicted a hierarchical priority for organ photoassimilateuptake, similar to that described in the literature, duringthe decline in photosynthetic activity. Our model suggests thatdeterminate growth of the first laterals resulted from a localshortage of photoassimilate at their meristem, as a result ofthe limited transport properties of the developed roots. Copyright2000 Annals of Botany Company Münch theory, phloem transport model, photoassimilate-partitioning, root growth, root system architecture, translocation     

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.     

Phloem transport of amino acids is differentially altered by phosphorus deficiency according to the nitrogen availability in young barley plants

The phloem transport of amino acids is a key step in the efficient use of nitrogen (N). Despite the importance of this issue, little information is known about the regulation of phloem transport of amino acids in plants with low phosphorus (P) supply and even less in relation to N availability. To this end, we studied not only the assimilate partitioning in young barley plants grown with low N or/and P supply, but also we analyzed the implications of the different isoforms of glutamine synthetase, cytokinin oxidase/dehydrogenase 2 and several senescence-related proteases. Our results demonstrated that low P supply causes an accumulation of different nitrogenous compounds in expanded leaves depending on N availability and an inhibition of the phloem exudation rate of amino acids only in high-N plants, indicating an interaction between N and P in the establishment of N-partitioning. The accumulation of nitrogenous compounds in leaves of low-P plants without the accompaniment in amino acid export to the phloem was not related to an increase in nitrate assimilation pathway neither with the modulation of glutamine synthetase 1_1 expression, as it had been observed for N availability. But, these results could be explained as a consequence of a delay in the transition from sink to source of leaves, thus keeping the older leaves as sink organs, as indicated by the increase in cytokinin oxidase/dehydrogenase 2 expression and the repression of several senescence-related proteases in low-P plants with good availability of N.     

Overexpression of GmAAP6a enhances tolerance to low nitrogen and improves seed nitrogen status by optimizing amino acid partitioning in soybean

Amino acid transport via phloem is one of the major source‐to‐sink nitrogen translocation pathways in most plant species. Amino acid permeases (AAPs) play essential roles in amino acid transport between plant cells and subsequent phloem or seed loading. In this study, a soybean AAP gene, annotated as GmAAP6a, was cloned and demonstrated to be significantly induced by nitrogen starvation. Histochemical staining of GmAAP6a:GmAAP6a‐GUS transgenic soybean revealed that GmAAP6a is predominantly expressed in phloem and xylem parenchyma cells. Growth and transport studies using toxic amino acid analogs or single amino acids as a sole nitrogen source suggest that GmAAP6a can selectively absorb and transport neutral and acidic amino acids. Overexpression of GmAAP6a in Arabidopsis and soybean resulted in elevated tolerance to nitrogen limitation. Furthermore, the source‐to‐sink transfer of amino acids in the transgenic soybean was markedly improved under low nitrogen conditions. At the vegetative stage, GmAAP6a‐overexpressing soybean showed significantly increased nitrogen export from source cotyledons and simultaneously enhanced nitrogen import into sink primary leaves. At the reproductive stage, nitrogen import into seeds was greatly enhanced under both sufficient and limited nitrogen conditions. Collectively, our results imply that overexpression of GmAAP6a enhances nitrogen stress tolerance and source‐to‐sink transport and improves seed quality in soybean. Co‐expression of GmAAP6a with genes specialized in source nitrogen recycling and seed loading may represent an interesting application potential in breeding.     

基于植株拓扑结构的生物量分配的玉米虚拟模型

依据植物结构—功能相互作用机理,建立了能模拟玉米生长发育与形态结构建成的虚拟模型。该模型的重要部分为基于植株拓扑结构的生物量分配模块。叙述了该模块的构建原理,以2000年田间试验数据提取了玉米的发育、生物量生产和生物量分配参数。模型模拟了2001年的玉米生长发育与生物量分配过程,模拟结果与田间试验结果比较吻合。应用该模型模拟了2001年玉米不同生育阶段植株的生物量分配和各器官生物量积累动态。