Phloem Loading in Two Scrophulariaceae Species. What Can Drive Symplastic Flow via Plasmodesmata?

Phloem loading is the initial step in photoassimilate export and the one that creates the driving force for mass flow. It has been proposed that loading occurs symplastically in species that translocate carbohydrate primarily as raffinose family oligosaccharides (RFOs). In these plants, dense fields of plasmodesmata connect bundle sheath cells to specialized companion cells (intermediary cells) in the minor veins. According to the polymer trap model, advanced as a mechanism of symplastic loading, sucrose from the mesophyll diffuses into intermediary cells and is converted there to RFOs. This process keeps the sucrose concentration low and, because of the larger size of the RFOs, prevents back diffusion. To test this model, the RFO pathway was down-regulated in Verbascum phoeniceum L. by suppressing the synthesis of galactinol synthase (GAS), which catalyzes the first committed step in RFO production. Two GAS genes (VpGAS1 and VpGAS2) were cloned and shown to be expressed in intermediary cells. Simultaneous RNAi suppression of both genes resulted in pronounced inhibition of RFO synthesis. Phloem transport was negatively affected, as evidenced by the accumulation of carbohydrate in the lamina and the reduced capacity of leaves to export sugars during a prolonged dark period. In plants with severe down-regulation, additional symptoms of reduced export were obvious, including impaired growth, leaf chlorosis, and necrosis and curling of leaf margins.plasmodesmata ͉ polymer trap ͉ stachyose ͉ galactinol ͉ RNAi U p to 80% of the carbon fixed in mature leaves by photosynthesis is exported to heterotrophic sinks to enable their growth and development. The first step in the transport pathway, and one that is highly regulated (1, 2), is the transfer of photoassimilate from mesophyll cells to the sieve elements (SEs) and companion cells (CCs) of minor veins (3-6). This process, known as phloem loading, creates the positive hydrostatic pressure difference between source and sink phloem that drives the mass flow of solution.Two loading mechanisms have been proposed. In one, photoassimilate enters the apoplast and is subsequently loaded into the phloem by specific transport proteins (3, 7). The second mechanism appears to be entirely symplastic (by plasmodesmata) (4-6). The first hint that loading might take place symplastically came from the discovery, in certain species, of specialized CCs (intermediary cells) in the minor veins, which are linked to bundle sheath cells by extremely high numbers of asymmetrically branched plasmodesmata (5).In all plants with intermediary cells, the phloem sap contains a substantial amount of raffinose family oligosaccharide (RFO), especially the tri-and tetrasaccharides, raffinose, and stachyose. The consistent association of RFOs with intermediary cells suggests that the synthesis of these sugars is an integral part of the phloem-loading mechanism.This reasoning is the basis of the polymer trap model of symplastic loading (8,9). According to the model, sucrose diffuses from bundle sheath cel...

show abstract

“…Other models of symplastic loading do not incorporate RFO synthesis into the proposed mechanisms (27,28).…”

Section: Discussionmentioning

confidence: 99%

Phloem loading in Verbascum phoeniceum L. depends on the synthesis of raffinose-family oligosaccharides

McCaskill

Turgeon

2007

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

show abstract

“…4). The reason for this correlation could be that the concentration of Suc is higher in the cytosol than in the vacuoles of leaf cells (Winter et al, 1993(Winter et al, , 1994Heineke et al, 1994;Voitsekhovskaja et al, 2006), which requires that other solutes accumulate in the vacuoles to balance the water potential in the two compartments. To explore these relationships, we plotted concentrations of other polar metabolites against transport sugars (Fig.…”

Section: Leaf Polar Metabolites and Inorganic Ionsmentioning

confidence: 99%

Phloem Loading Strategies and Water Relations in Trees and Herbaceous Plants

Cheng

Guo

et al. 2011

Plant Physiology

View full text Add to dashboard Cite

Most herbaceous plants employ thermodynamically active mechanisms of phloem loading, whereas in many trees, the mechanism is passive, by diffusion. Considering the different water transport characteristics of herbs and trees, we hypothesized that water relations play a role in the adoption of phloem loading strategies. We measured whole-plant hydraulic conductance (K p ), osmolality, concentrations of polar metabolites, and key inorganic ions in recently mature leaves of 45 dicotyledonous species at midafternoon. Trees, and the few herbs that load passively, have low K p , high osmolality, and high concentrations of transport sugars and total polar metabolites. In contrast, herbs that actively load sucrose alone have high K p , low osmolality, and low concentrations of sugars and total polar metabolites. Solute levels are higher in sugar alcohol-transporting species, both herbs and trees, allowing them to operate at lower leaf water potentials. Polar metabolites are largely responsible for leaf osmolality above a baseline level (approximately 300 mM) contributed by ions. The results suggest that trees must offset low K p with high concentrations of foliar transport sugars, providing the motivating force for sugar diffusion and rendering active phloem loading unnecessary. In contrast, the high K p of most herbaceous plants allows them to lower sugar concentrations in leaves. This reduces inventory costs and significantly increases growth potential but necessitates active phloem loading. Viewed from this perspective, the elevation of hydraulic conductance marks a major milestone in the evolution of the herbaceous habit, not only by facilitating water transport but also by maximizing carbon use efficiency and growth.

show abstract

“…leaves of species with uniform distribution of radiolabel were exposed to 14 CO 2 for 1 h and autoradiographed. Again, no vein pattern was seen in these plants.…”

Section: Autoradiographymentioning

confidence: 99%

“…These raffinose family oligosaccharides (RFOs) are larger than sucrose and are apparently unable to diffuse back to the mesophyll through the intermediary cell plasmodesmata. The RFOs accumulate in the phloem, a process known as polymer trapping (12), to a combined concentration that is similar to that of sucrose in apoplastic loaders (13,14). Thus, there are 3 recognized strategies of sucrose loading: Apoplastic and symplastic with or without polymer trapping.…”

mentioning

confidence: 99%

A comprehensive picture of phloem loading strategies

Rennie

Turgeon²

2009

Proc. Natl. Acad. Sci. U.S.A.

341

328

View full text Add to dashboard Cite

Mechanisms of phloem loading in the minor veins of leaves are known for only a few species. We propose that there are a limited number of loading strategies for the primary photoassimilates, sucrose and sugar alcohols. These strategies can be predicted based on thermodynamic and anatomical considerations and identified by autoradiography of veins following uptake of 14 C-labeled compounds, analysis of leaf solute composition and concentrations, and plasmodesmatal counting. Experiments on 45 dicotyledonous species identified the predicted loading patterns. Over 50-fold differences in concentrations of sucrose and sugar alcohols in leaves were measured. The cumulative concentrations of transport compounds in leaves correlated with loading mechanisms, a previously unrecognized association. Comparisons of solute concentrations and osmotic potentials of whole leaves suggest that sucrose and sugar alcohols are more concentrated in the cytosol than in the vacuoles of mesophyll cells, thus increasing the driving force for passive loading in species that employ this strategy. Passive loading is more widespread than previously thought, especially in trees. The results indicate that plants have exploited all thermodynamically feasible and structurally compatible loading strategies and that these strategies can be identified with straightforward protocols.plasmodesmata ͉ polymer trap ͉ raffinose ͉ stachyose ͉ sucrose P hloem loading is the starting point for export of carbohydrates and other nutrients from leaves (1-4). To date, mechanisms of sucrose loading have been established for a relatively small number of species. Less is known about sugar alcohols, which in some plants are present in phloem sap at higher concentrations than sucrose (5-7).In many species, loading involves an apoplastic step, driven by plasma-membrane transporters and energized by the proton motive force (8). In other plants, loading is symplastic, driven by a downhill concentration gradient from the mesophyll to the phloem and requiring high plasmodesmatal densities (2, 3). In some plants that load symplastically, the process is driven by diffusion alone and is passive (9, 10). In other species that load via the symplast, sucrose from the mesophyll diffuses into specialized companion cells (CCs) in the minor veins (11), known as intermediary cells, and is converted to raffinose and stachyose. These raffinose family oligosaccharides (RFOs) are larger than sucrose and are apparently unable to diffuse back to the mesophyll through the intermediary cell plasmodesmata. The RFOs accumulate in the phloem, a process known as polymer trapping (12), to a combined concentration that is similar to that of sucrose in apoplastic loaders (13,14). Thus, there are 3 recognized strategies of sucrose loading: Apoplastic and symplastic with or without polymer trapping.It is reasonable to assume that the anatomical features associated with sucrose loading in a given sieve element-companion cell (SE-CC) complex constrain available strategies of sugar alcohol loading. If s...

show abstract

Phloem Loading in Two Scrophulariaceae Species. What Can Drive Symplastic Flow via Plasmodesmata?

Cited by 80 publications

References 43 publications

Phloem loading in Verbascum phoeniceum L. depends on the synthesis of raffinose-family oligosaccharides

Phloem loading in Verbascum phoeniceum L. depends on the synthesis of raffinose-family oligosaccharides

Phloem Loading Strategies and Water Relations in Trees and Herbaceous Plants

A comprehensive picture of phloem loading strategies

Contact Info

Product

Resources

About