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          Institute: MPI für molekulare Pflanzenphysiologie     Collection: Publikationen Pflanzenphysiologie     Display Documents

ID: 127520.0, MPI für molekulare Pflanzenphysiologie / Publikationen Pflanzenphysiologie
Phloem metabolism and function have to cope with low internal oxygen
Authors:van Dongen, J. T.; Schurr, U.; Pfister, M.; Geigenberger, P.
Date of Publication (YYYY-MM-DD):2003-04
Title of Journal:Plant Physiology
Journal Abbrev.:Plant Physiol
Issue / Number:4
Start Page:1529
End Page:1543
Review Status:not specified
Audience:Not Specified
Abstract / Description:We have investigated the consequences of endogenous limitations in oxygen delivery for phloem transport in Ricinus communis. In situ oxygen profiles were measured directly across stems of plants growing in air (21% [v/v] oxygen), using a microsensor with a tip diameter of approximately 30 gm. Oxygen levels decreased from 21% (v/v) at the surface to 7% (v/v) in the vascular region and increased again to 15% (v/v) toward the hollow center of the stem. Phloem sap exuding from small incisions in the bark of the stem was hypoxic, and the ATP to ADP ratio (4.1) and energy charge (0.78) were also low. When 5-cm stem segments of intact plants were exposed to zero external oxygen for 90 min, oxygen levels within the phloem decreased to approximately 2% (v/v), and ATP to ADP ratio and adenylate energy charge dropped further to 1.92 and 0.68, respectively. This was accompanied by a marked decrease in the phloem sucrose (Suc) concentration and Suc transport rate, which is likely to be explained by the inhibition of retrieval processes in the phloem. Germinating seedlings were used to analyze the effect of a stepwise decrease in oxygen tension on phloem transport and energy metabolism in more detail. Within the endosperm embedding the cotyledons-next to the phloem loading sites-oxygen decreased from approximately 14% (v/v) in 6-d-old seedlings down to approximately 6% (v/v) in 10-d-old seedlings. This was paralleled by a similar decrease of oxygen inside the hypocotyl. When the endosperm was removed and cotyledons incubated in a 100 mm Suc solution with 21%, 6%, 3%, or 0.5% (v/v) oxygen for 3 h before phloem sap was analyzed, decreasing oxygen tensions led to a progressive decrease in phloem energy state, indicating a partial inhibition of respiration. The estimated ratio of NADH to NAD(+) in the phloem exudate remained low (approximately 0.0014) when oxygen was decreased to 6% and 3% (v/v) but increased markedly (to approximately 0.008) at 0.5% (v/v) oxygen, paralleled by an increase in lactate and ethanol. Suc concentration and translocation decreased when oxygen was decreased to 3% and 0.5% (v/v). Falling oxygen led to a progressive increase in amino acids, especially of alanine, gamma-aminobutyrat, methionine, and isoleucine, a progressive decrease in the C to N ratio, and an increase in the succinate to malate ratio in the phloem. These results show that oxygen concentration is low inside the transport phloem in planta and that this results in adaptive changes in phloem metabolism and function.
Free Keywords:ricinus-communis l
; growing potato-tubers
; wheat leaf protoplasts
; sucrose synthase
; starch synthesis
; companion-cell
; amino-acids
; l seedlings
; nucleotide levels
; solute transport
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für molekulare Pflanzenphysiologie/Metabolische Netzwerke/AG Geigenberger
External Affiliations:Max Planck Inst Mol Plant Physiol, Muhlenberg 1, D-14476 Golm, Germany
Max Planck Inst Mol Plant Physiol, D-14476 Golm, Germany
Forschungszentrum Julich, Inst Phytosphere Res, D-52425 Julich, Germany
Univ Heidelberg, Inst Bot, D-69120 Heidelberg, Germany
Identifiers:ISI:000185076400004 [ID No:1]
ISI:000185076400004 [ID No:2]
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