Source and Sink Leaf Metabolism in Relation to Phloem Translocation

Giaquinta, Robert T.

doi:10.1104/pp.61.3.380

Cited by 125 publications

(77 citation statements)

References 20 publications

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“…The results seem to contradict the general assumption that sucrose metabolizing enzymes such as sucrose synthase (14) and acid invertase (8,23,24,27) are lost as leaves develop. Consequently, we measured acid invertase activity during leaf development of two contrasting species, spinach and soybean.…”

Section: Changes In Acid Invertase During Leaf Developmentmentioning

confidence: 79%

Biochemical Mechanism for Regulation of Sucrose Accumulation in Leaves during Photosynthesis

Huber

1989

Plant Physiol.

174

136

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It is not known why some species accumulate high concentrations of sucrose in leaves during photosynthesis while others do not. To determine the possible basis, we have studied 10 species, known to differ in the accumulation of sucrose, in terms of activities of sucrose hydrolyzing enzymes. In general, acid invertase activity decreased as leaves expanded; however, activities remaining in mature, fully expanded leaves ranged from low (<10 micromoles per gram fresh weight per hour) to very high (>100 micromoles per gram fresh weight per hour). In contrast, sucrose synthase activities were low and relatively similar among the species (4-10 micromoles per gram fresh weight per hour). Importantly, leaf sucrose concentration, measured at midaftemoon, was negatively correlated with acid invertase activity. We propose that sucrose accumulation in vacuoles of species such as soybean and tobacco is prevented by acid invertase-mediated hydrolysis. Initial attempts were made to characterize the relatively high activity of acid invertase from mature soybean leaves. Two apparent forms of the enzyme were resolved by Mono Q chromatography. The two forms had similar affinity for substrate (apparent Km [sucrose] = 3 millimolar) and did not interconvert upon rechromatography. It appeared that the loss of whole leaf invertase activity during expansion was largely the result of changes in one of the enzyme forms. Overall, the results provide a mechanism to explain why some species do not accumulate sucrose in their leaves. Some futile cycling between sucrose and hexose sugars is postulated to occur in these species, and thus, the energy cost of sucrose production may be higher than is generally thought.In many common higher plants, the principal end products of leaf photosynthesis are the carbohydrates starch and sucrose. However, species differ in the partitioning of photosynthetically fixed carbon between starch and sucrose and the extent to which sucrose normally accumulates in leaves during the photoperiod (9, 13). Accumulation of carbohydrates in leaves is critical, as they are mobilized in darkness to support continued translocation (albeit at a reduced rate) and thereby maintain heterotrophic plant parts.Plant species such as soybean that accumulate starch typically do not accumulate soluble sugars; conversely, species ' Cooperative investigations ofthe U.S. Department ofAgriculture,

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Section: Changes In Acid Invertase During Leaf Developmentmentioning

confidence: 79%

Biochemical Mechanism for Regulation of Sucrose Accumulation in Leaves during Photosynthesis

Huber

1989

Plant Physiol.

174

136

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“…In sugar beet, accompanying the import-export transition is an increase in photosynthesis, an increase in the relative proportion of 14CO2-derived soluble compounds, and a change in sugar partitioning favoring sucrose synthesis. This change is paralleled by the development of sucrose phosphate synthase activity rather than by a decrease of the sucrose-hydrolyzing enzymes (12). In the same material, phloem loading begins 35 to 45 h prior to onset of export, and it has been suggested that this triggers the initiation of export by increasing sufficiently the solute concentration within the conducting cells (7).…”

mentioning

confidence: 98%

Plasma Membrane Vesicles from Source and Sink Leaves

Lemoine¹,

Gallet²,

Gaillard³

et al. 1992

Plant Physiol.

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Plasma membrane vesicles (PMVs) were prepared by phase partitioning from microsomal fractions of either sink or source leaves of sugar beet (Beta vulgaris L.). The purity, the internal volume, the sidedness, and the sealingness of PMVs prepared from sink leaves did not differ from those measured with PMVs from source leaves. Yet, in response to an imposed proton motive force, PMVs from source leaves accumulated about 4-fold more sucrose than PMVs from sink leaves. The developmental stage did not affect the uptake of glucose and valine in PMVs prepared from leaf tissues. It was concluded that the sink/source transition is accompanied either by the incorporation into the plasma membrane of leaf cells of proteins mediating proton-sucrose cotransport, or by their activation. N-ethylmaleimide and a polyclonal ascitic fluid directed against the 42-kD region of the plasma membrane containing a putative sucrose carrier inhibited the uptake of sucrose in PMVs from source leaves, but not in PMVs from sink leaves. Sodium dodecyl sulfate gel electrophoresis and western blot suggested that the 42 polypeptide was more abundant in the PMVs from source leaves than in the PMVs from sink leaves.

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“…Translocation in K-deficient leaves may be restricted by reduction in the rate of synthetic metabolism in general (5) or sucrose formation in particular (27); entry ofsucrose into the transport pool (1); or inhibition ofsome step involved in phloem loading (16,30). In unstressed soybean leaves, several studies have suggested that the rate of assimilate export may be related to the activity of SPS (7,10,11 25,26). However, studies have not been conducted with soybean plants to determine the relative impact of K-nutrition on export rate with respect to CER, or SPS activity.…”

mentioning

confidence: 99%