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Lecture10 - Phloem 1.Conduits are living cells Sieve cells...

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Unformatted text preview: Phloem 1.Conduits are living cells Sieve cells in gymnosperms ` Sieve tube members in angiosperms Xylem Conduits are dead cells Tracheids Vessel elements 2. transport of organic compounds transport of water and minerals 3.Bidirectional movement Unidirectional movement Transport of photosynthate occurs mainly in the phloem Two demonstrations 1. girdling a tree has no immediate effect on water transport, but sugar accumulates above the girdle and tissue swells, tissue below girdle dies 2. application of 14CO2 or 14C-sucrose, then visualization of path of radioactive tracer indicates that photosynthate moves through phloem sieve elements Sieve elements are specialized cells. 1. These cells are without a nucleus, tonoplast, Golgi bodies and ribosomes 2. but retain plasma membrane and modified mitochondria, plastids and endoplasmic reticulum 3. Unlike tracheary elements (xylem), no secondary wall thickening Companion cells these are specialized sieve elements, division of a mother cell produces a sieve element and a companion cell. 1. Transports photosynthates (solutes) into sieve elements at the source and from sieve elements at the sink 2. Companion cells may support sieve elements by carrying out protein synthesis and other metabolic functions. ordinary companion cell companion wall ingrowth Intermediate cell transfer cell sieve element Materials Translocated in the Phloem Materials Source and Sink Translocation (solution/sap) moves from source (supply) to sink (metabolism or storage) and can be upwards or downwards Source: an exporting organ because carbon availability is greater than need Sink: nonphotosynthetic organ or an organ that does not produce enough photosynthates for its metabolic demand, Source-to-sink pathways specific pathways are complex but the following factors seem to determine translocation in herbaceous plants Proximity: upper leaves provide photosynthates to the shoot apex and lower leaves provide for the root Development: fruits become a dominant sink during reproduction, storage roots export as vegetative tissue starts to develop after over-wintering Vascular connections: source leaves preferentially supply sinks with which they have direct vascular connection, often directly above or below. Plasticity of the translocation pathway The Mechanism of Translocation in the Phloem The The Pressure-Flow Model Solute loading leads to an increased leads turgor pressure turgor Solute unloading leads to an decreased leads turgor pressure turgor A demonstration of open sieve plate pores in intact tissues demonstration The contents of the sieve tubes must be under pressure. This is difficult to measure because when a sieve tube is punctured with a measuring probe, the holes in its end walls quickly plug up. However, aphids can insert their mouth parts without triggering this response. Left: when it punctures a sieve tube, sap enters the insect's mouth parts under pressure and some soon emerges at the other end. Right: honeydew will continue to exude from the mouth parts after the aphid has been cut away from them. Phloem Loading: chloroplasts to sieve elements Triose phosphates formed by photosynthesis (in mesophyll cells) are transported from the chloroplasts to cytosol where these are converted to sucrose Sucrose moves from the mesophyll cell to the vicinity of the sieve elements Sieve element loading ミ solutes concentrate at the site of loading Photosynthate moves from mesophyll cells to sieve element-companion cell complex via apoplastic and/or symplastic pathways Sucrose is transported primarily through an apoplastic pathway sucrose concentrates in the sieve elements (relative to mesophyll cells) and is transported from the apoplast by a secondary active sucrose-H+ co-transporter (companion cell plasma membrane) The symporter and H+-ATPase are localized to side of the companion cell that faces the bundle sheath and parenchyma cells. Transport into sieve elements (from the companion cell) is through plasmodesmata, i.e. is passive How to experimentally prove the operation of the apoplast pathway in phloem loading? Phloem loading appears to be symplastic in plants with intermediary cells intermediary The Polymer-Trapping Model explains symplastic loading 1. Sucrose, synthesized in the mesophyll, diffuse from the bundle sheath cells into the intermediary cells through the abundant plasmodesmata the 2. In the IC, raffinose are synthesized, thus maintaining the diffusion gradients for both sucrose and raffinose. Because of their larger sizes, raffinose cannot diffuse both back to the mesophylls. back Branched plasmodesmata Branched How can you exprimentally prove the polymer-trap model? Phloem Unloading Phloem 1. Sieve element unloading 2. Short-distance transport 3. Storage and metabolism Transport into sink tissues Requires metabolic energy 1. Sucrose metabolism in sink cell maintain a sugar concentration gradient 2. Apoplastic loading means that sugars must cross at least two membrane, which requires energy-dependent transporters Developing seeds since there are no symplastic connections Developing between maternal tissues and the embryo between starting exporting sugar at the leaf tip Gradual transition of a leaf from Gradual sink to source 1. The symplastic unloading pathway is closed 2. The leaf is synthesizing sufficient amount of photosynthate photosynthate 3. The sucrose-synthesizing genes are expressed 4. The H+-sucrose symporter is in place in the plasmalemma of the SE/CC complex 5. The cessation of import and initiation of export are 5. independent processes. independent 6. Sugars are unloaded and loaded almost entirely 6. via different veins. via sink source Photosynthate Allocation and Partitioning Determined by the key enzymes involved in different metabolic pathways 1. Leaf metabolism . 1. and biomass --> other biomolecules 2. Synthesis of export compounds and export from leaves and 3. Synthesis of storage 3. compound (starch) compound Sinks are competitive depending on 3 factors 1. Nature of connection 2. Proximity 3. Sink strength = 3. [sink size x sink activity] [sink ...
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