The Role of Phloem Loading Reconsidered

Turgeon, Robert

doi:10.1104/pp.110.153023

Cited by 143 publications

(148 citation statements)

References 48 publications

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“…One possibility is that active loading allows herbaceous plants to maintain low inventories of nonstructural carbohydrates in leaves to maximize the efficiency of carbon use, which has a strong positive effect on growth rate (Turgeon, 2010a). This is consistent with the fact that actively loading herbaceous plants are almost devoid of nonstructural carbohydrates in their leaves at dawn (Fondy and Geiger, 1982;Qiu and Israel, 1992;Camacho-Cristobal and González-Fontes, 1999;Niittylä et al, 2004).…”

supporting

confidence: 62%

“…This leads to lower total nonstructural carbohydrate inventories at dawn during active growth, largely because the starch accumulated during the day is almost all used up by the end of the night (Fondy and Geiger, 1982;Qiu and Israel, 1992;Camacho-Cristobal and González-Fontes, 1999;Niittylä et al, 2004). By reducing foliar carbohydrate inventories, they free up carbon to fuel rapid growth (Smith and Stitt, 2007;Turgeon, 2010a). High relative growth rate is a critically important, adaptive characteristic of herbaceous plants (Grime and Hunt, 1975;Hunt and Cornelissen, 1997).…”

Section: Discussionmentioning

confidence: 99%

“…Indeed, a recent analysis indicates that passive loading is more highly restricted to the woody habit than previously recognized; very few herbs use this strategy (Davidson et al, 2011). Both active loading mechanisms elevate the sugar concentration, and the hydrostatic pressure, of the phloem to higher levels than passive loading (Turgeon, 2010a(Turgeon, , 2010b). The striking difference in phloem pressure has fundamental implications with regard to the mechanism of longdistance transport, carbon partitioning, defense, and responses to elevated CO 2 (Kö rner et al, 1995;Long et al, 2006;Turgeon, 2010b).…”

mentioning

confidence: 99%

“…In the passive loading mechanism, which is not as well studied as the active strategies, diffusion alone is apparently sufficient. According to this model, the concentration of Suc in the cytosol of mesophyll cells is high enough that there is no need for any further elevation in concentration in the phloem to drive long-distance transport by Mü nch pressure flow (Turgeon and Medville, 1998;Reidel et al, 2009;Turgeon, 2010a).…”

mentioning

confidence: 99%

“…Three phloem loading strategies have been identified for Suc, two of which are thermodynamically active and the third passive (Reidel et al, 2009;Rennie and Turgeon, 2009;Turgeon, 2010a). In the more common active mechanism, apoplastic loading, Suc enters the apoplast and is pumped into sieve element-companion cell complexes by membrane-bound transporters, using energy derived from the proton motive force (Sauer, 2007).…”

mentioning

confidence: 99%

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Phloem Loading Strategies and Water Relations in Trees and Herbaceous Plants

Cheng

Guo

et al. 2011

Plant Physiology

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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.

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supporting

confidence: 62%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Phloem Loading Strategies and Water Relations in Trees and Herbaceous Plants

Cheng

Guo

et al. 2011

Plant Physiology

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Phloem Parenchyma

Kim,

Frommer

2024

Encyclopedia of Life Sciences

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Phloem parenchyma is one of the key cell types constituting the phloem of angiosperms. Phloem parenchyma cells play essential roles in the transfer of diverse compounds, such as sucrose and amino acids, into the sieve element‐companion cell complex (SECCC). A fraction of the phloem parenchyma cells trans ‐differentiate into phloem parenchyma transfer cells (PPTC) by developing plasma membrane amplification associated with cell wall ingrowth. The increased membrane surface area in PPTCs may create additional space for the transporters to localise thereby enhancing phloem loading efficiency. Recent advances in single‐cell sequencing and imaging techniques have provided new information on the potential roles of the phloem parenchyma, not only in the transport of nutrients but also in plant metabolism and defence mechanisms. Key Concepts The phloem consists of sieve elements, companion cells and phloem parenchyma cell types. The phloem parenchyma cells play pivotal roles during phloem loading. Certain phloem parenchyma cells undergo trans ‐differentiation and develop cell wall ingrowth. The entry of sucrose into the SECCC follows at least one of the two paths – the apoplasmic and/or the symplasmic pathway. In Arabidopsis , SWEET11, SWEET12 and SWEET13, sucrose transporters are localised to the phloem parenchyma and efflux sucrose to the apoplasm. Sucrose in the apoplasm is actively taken up into the SECCC complex by SUT1/SUC2 proton‐coupled sucrose symporters for long‐distance transport. Metabolic activities in the phloem parenchyma and the adjacent companion cells likely contribute to reshaping the metabolomes of the phloem and xylem. The ontogenetic development of phloem parenchyma remains elusive, requiring future research activities.

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Fundamentals of Phloem Transport Physiology

Patrick

2012

Phloem

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The Role of Phloem Loading Reconsidered

Cited by 143 publications

References 48 publications

Phloem Loading Strategies and Water Relations in Trees and Herbaceous Plants

Phloem Loading Strategies and Water Relations in Trees and Herbaceous Plants

Phloem Parenchyma

Fundamentals of Phloem Transport Physiology

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