What do sieve tubes transport

This hypothesis accounts for several observations:. This video provides a concise overview of sugar sources, sinks, and the pressure flow hypothesis:. Symporters move two molecules in the same direction; Antiporters move two molecules in opposite directions. Photosynthates, such as sucrose, are produced in the mesophyll cells a type of parenchyma cell of photosynthesizing leaves. Sugars are actively transported from source cells into the sieve-tube companion cells, which are associated with the sieve-tube elements in the vascular bundles.

This active transport of sugar into the companion cells occurs via a proton-sucrose symporter ; the companion cells use an ATP-powered proton pump to create an electrochemical gradient outside of the cell.

The cotransport of a proton with sucrose allows movement of sucrose against its concentration gradient into the companion cells. From the companion cells, the sugar diffuses into the phloem sieve-tube elements through the plasmodesmata that link the companion cell to the sieve tube elements. Phloem sieve-tube elements have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap.

Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells. Image credit: OpenStax Biology. This increase in water potential drives the bulk flow of phloem from source to sink.

Unloading at the sink end of the phloem tube can occur either by diffusion , if the concentration of sucrose is lower at the sink than in the phloem, or by active transport , if the concentration of sucrose is higher at the sink than in the phloem.

If the sink is an area of active growth, such as a new leaf or a reproductive structure, then the sucrose concentration in the sink cells is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly metabolized for growth. If the sink is an area of storage where sugar is converted to starch, such as a root or bulb, then the sugar concentration in the sink is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly converted to starch for storage.

But if the sink is an area of storage where the sugar is stored as sucrose, such as a sugar beet or sugar cane, then the sink may have a higher concentration of sugar than the phloem sieve-tube cells.

In this situation, active transport by a proton-sucrose antiporter is used to transport sugar from the companion cells into storage vacuoles in the storage cells. Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements.

Phloem transports sugars and amino acids dissolved in water. The xylem transports water and minerals from the roots up the plant stem and into the leaves. In a mature flowering plant or tree, most of the cells that make up the xylem are specialised cells called vessels.

Transport in the xylem is a physical process. It does not require energy. Phloem moves sugar that the plant has produced by photosynthesis to where it is needed for processes such as:. Transport in the phloem is therefore both up and down the stem. Transport of substances in the phloem is called translocation.

Phloem consists of living cells. The cells that make up the phloem are adapted to their function:. The xylem and phloem are distributed differently in roots and stems.

A pressure probe was used to determine the turgor pressure in the sieve elements SE located in the main vein of source leaves located along the plant axis. In parallel, anatomical studies performed on SE at various sites along the stem yielded parameters used to compute sieve tube radius, sieve plate pore numbers and radii, SE lengths and local phloem conductivity. These data were then used to calculate the theoretical pressure gradient that would be required to achieve the measured flow velocities.

Based on data presented, the authors conclude that their study on morning glory provides experimental support for the phloem pressure flow model.

In addition, it is disappointing that, having gone to such pains to collect the anatomical data, the authors used assumptions from plants like Arabidopsis to design aspects of their experiments.

For example, they assumed that, as in Arabidopsis , symplasmic phloem unloading occurs in morning glory roots. Added to this, even if symplasmic unloading were to take place, making turgor pressure measurements on cortical cells, rather than on SEs located in the sink region of the root, confounds data interpretation.

Given that numerous cells are positioned between the SE and the cortical cell layer, the measured p value of 0. Putting this aside, one has to puzzle as to how the authors arrived at the value of 0. This gives a delta p of 0. So, the data are in the ballpark, but the authors have not yet knocked the ball out of the park!

The same can be said of the p-protein aspect of the manuscript. However, the data in Figure 4 suggests that for plants like trees, the manifold model might need to be revised. The authors aim to address two fundamental questions regarding pressure flow hypothesis that have been long debated and that are key to understand phloem physiology: the continuity of the flow are the pores blocked by p-proteins?

They argue that part of the controversy is caused by lack of reliable data due to technical constrains and develop new methods to avoid these problems.

Here they measured in situ sap viscosity and phloem pressure in a morning glory plant partly defoliated and also the red oak tree to show its phloem characteristics, by great increase of sieve tubes conductivity, is still in accordance with the Munch model, a matter of debate for long distance sap flow movement in trees. They also provide evidence in Arabidopsis that p-proteins, long thought to block phloem sap flow at sieve plates, are in fact able to diffuse through them and thus still allow pressure driven sap flow.

The authors, using a set of complementary approaches, are able here to apprehend the different parameters driving the sap flow in the sieve tube. The techniques developed by this team are a great technical advance for phloem studies. The data is clear, almost everywhere well explained and in accordance with the authors' assumptions.

Nevertheless, several points should be addressed:. This point need further clarification and may require a supplemental figure. It would have been better to confirm this result directly in the morning glory vine model to strengthen this result.

If FLIM experiment are not feasible in trees, why not try here to obtain an estimation of sap phloem viscosity through concentrations determination of extracted phloem sap contents previously used technique to estimate sap viscosity mentioned by the authors in the text?

It would have more strength than just an assumption. But we still don't know if the situation of P-proteins is same or similar in other plant species. May I ask the authors to discuss? We have followed all suggestions with a few exceptions that we have explained in the text.

The revised manuscript includes only the morning glory data and the text has been revised to emphasize the significance of these data to the long-standing question of phloem transport in large and long plants.

We believe that the removal of the P-protein data and oak data has made the story much less complex and easier to access. All sample sizes n were provided in the original manuscript in the figure legends as requested in the eLife author guide. However, we agree that it would be beneficial to provide an overview and to accomplish this we have added a sentence to the Introduction.

Because the oak data have been removed from the manuscript, the central point of the concern is obsolete. However, we would like to explain why we did not measure root pressure in partly defoliated long plants.

As described in the manuscript, measurement of root turgor pressure required the removal of the root system from the pot, putting the root system in a plastic bag, cutting a hole in the bag and pulling a root out, keeping the root moist and mounting it on the microscope stage. Doing this with a large plant appeared not feasible. A single minor crack in the highly delicate stems of morning glory would have jeopardized the project and likely have resulted in errors in the measured turgor pressures.

We had provided the source data, but we agree that it would be beneficial to have an easy accessible direct comparison of the data between the plants. We therefore have generated Figure 3—figure supplement 3 showing a graphical comparison as well as tables for the individual parameters. We have also included the error bars in this figure supplement as they appeared distracting in Figure 3. A statement in Figure 3 legend refers readers to Figure 3—figure supplement 3 for standard deviations.

For example, they assumed that, as in Arabidopsis, symplasmic phloem unloading occurs in morning glory roots. Not only in Arabidopsis , but in all plants studied so far including monocots, symplastic unloading has been shown in the root unloading zone. We have added Figure 2—figure supplement 3 showing proof of symplastic unloading in root tips in morning glory. We would certainly have preferred to take direct sink sieve tube measurements in addition to cortical measurements, but as noted in the original manuscript, this is impossible.

In roots the phloem is located in the central cylinder which would require splitting the root in half to access the sieve tubes for measurements. How this could be done without injury and major impacts on transport, unloading, and turgor is not clear to the authors.

This supports our statement in the original manuscript. Our calculations show how much pressure is needed to overcome frictions within the tube system. The results show that most of the pressure differential will be consumed and that there is not a large margin for a high-pressure manifold system.

Therefore, one has to assume that symplastic unloading does not require large pressure differentials as outlined in the Discussion and Figure 7. But the results do not contradict a pressure flow model. Figure 2 shows 5 individual measurements, which average 1. Subtracting 0. We do not see any problem with our calculations and the text clearly states what we have done. The only discrepancy is that we provided data with three digits 0.

We have changed this in the revised manuscript. According to our measurements, 0. As concluded in the original manuscript, the measured pressure is high enough to drive the flow to any sink in the plant as the maximum source to sink distance does not exceed 2 m.

It is our opinion, however, that we are permitted to claim that we have provided strong support for pressure driven mass flow. Certainly our data put to rest the idea that pressure driven flow is not capable of transporting photoassimilates over long distances.

We do not see why Figure 2 provides support for the high-pressure manifold model. The model requires significant pressures in the unloading zone as outlined in Patrick , and it appears that reviewer 1 agrees with us that the conclusions from our data does not support this model. We would prefer to keep the conclusions as presented.

The calibration curve shown in Figure 2—figure supplement 1C is generated by measuring 2-NDBG lifetime versus known viscosities of aqueous sucrose solutions. We have added better explanation in the figure legend to clarify this. All phloem sap viscosity values based on stylectomy or exudates are currently based on estimations and are not measured in situ. The small volume in stylectomy leads to rapid concentration and viscosity changes because of evaporation which can be limited, but not entirely prevented.

Exudates often contain contaminations from neighboring cells and the apoplast and oxidization may lead to gelling of the sap e. In addition, sieve tube viscosity is dependent on all solutes in the sap, not only on sucrose. Since the primary aim was to measure viscosity and not sucrose concentrations, we decided to develop a method for in situ measurements by FLIM, calibrated against known viscosities which we believe provides better values than invasive methods.

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication. This article is distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use and redistribution provided that the original author and source are credited. Article citation count generated by polling the highest count across the following sources: Crossref , Scopus , PubMed Central.

The movement of water by osmosis causes pressure differences that drive the transport of sugars over long distances in plants.

Plants develop new organs to adjust their bodies to dynamic changes in the environment. How independent organs achieve anisotropic shapes and polarities is poorly understood. To address this question, we constructed a mechano-biochemical model for Arabidopsis root meristem growth that integrates biologically plausible principles.

Computer model simulations demonstrate how differential growth of neighboring tissues results in the initial symmetry-breaking leading to anisotropic root growth. Furthermore, the root growth feeds back on a polar transport network of the growth regulator auxin. Model, predictions are in close agreement with in vivo patterns of anisotropic growth, auxin distribution, and cell polarity, as well as several root phenotypes caused by chemical, mechanical, or genetic perturbations.

Our study demonstrates that the combination of tissue mechanics and polar auxin transport organizes anisotropic root growth and cell polarities during organ outgrowth. Therefore, a mobile auxin signal transported through immobile cells drives polarity and growth mechanics to coordinate complex organ development. Temporal dynamics of gene expression underpin responses to internal and environmental stimuli. In eukaryotes, regulation of gene induction includes changing chromatin states at target genes and recruiting the transcriptional machinery that includes transcription factors.

As one of the most potent defense compounds in Arabidopsis thaliana , camalexin can be rapidly induced by bacterial and fungal infections. Though several transcription factors controlling camalexin biosynthesis genes have been characterized, how the rapid activation of genes in this pathway upon a pathogen signal is enabled remains unknown. By combining publicly available epigenomic data with in vivo chromatin modification mapping, we found that camalexin biosynthesis genes are marked with two epigenetic modifications with opposite effects on gene expression, trimethylation of lysine 27 of histone 3 H3K27me3 repression and acetylation of lysine 18 of histone 3 H3K18ac activation , to form a previously uncharacterized type of bivalent chromatin.

Mutants with reduced H3K27me3 or H3K18ac suggested that both modifications were required to determine the timing of gene expression and metabolite accumulation at an early stage of the stress response. Our study indicates that the H3K27me3-H3K18ac bivalent chromatin, which we name as kairostat, plays an important role in controlling the timely induction of gene expression upon stress stimuli in plants.

Cited 84 Views 7, Annotations Open annotations. The current annotation count on this page is being calculated. Cite this article as: eLife ;5:e doi: Figure 1 with 1 supplement see all. Download asset Open asset. Figure 1—source data 1 Source data of sieve tube geometrical parameters for Figure 1 and Figure 3—figure supplement 3. Figure 2 with 3 supplements see all.

Video 1. Download asset. Download as MPEG Download as WebM. Download as Ogg. Figure 3 with 3 supplements see all. Figure 3—source data 1 Source data of sieve tube geometrical parameters for Figure 3 and Figure 3—figure supplement 3. Figure 4 with 2 supplements see all. Figure 5. Figure 5—source data 1 Source data of sieve tube geometrical parameters for Figure 5. Figure 6. The following data sets were generated. Aikman DP Contractile proteins and hypotheses concerning their role in phloem transport Canadian Journal of Botany 58 — Patrick JW Does Don Fisher's high-pressure manifold model account for phloem transport and resource partitioning?

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