Received Apr 13; Accepted Jul 2. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc. This article has been cited by other articles in PMC. Abstract Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose and some other sugars such as raffinose and polyols across plant organs through the phloem.
Published online Jul This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.
This article has been cited by other articles in PMC. Abstract Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose and some other sugars such as raffinose and polyols across plant organs through the phloem.
In this paper, we summarize current knowledge about the phloem transport mechanisms and review the effects of several abiotic water and salt stress, mineral deficiency, CO2, light, temperature, air, and soil pollutants and biotic mutualistic and pathogenic microbes, viruses, aphids, and parasitic plants factors.
Concerning abiotic constraints, alteration of the distribution of sugar among sinks is often reported, with some sinks as roots favored in case of mineral deficiency.
Many of these constraints impair the transport function of the phloem but the exact mechanisms are far from being completely known. Phloem integrity can be disrupted e.
Photosynthesis inhibition could result from the increase in sugar concentration due to phloem transport decrease.
Biotic interactions aphids, fungi, viruses… also affect crop plant productivity. Recent breakthroughs have identified some of the sugar transporters involved in these interactions on the host and pathogen sides.
The different data are discussed in relation to the phloem transport pathways. When possible, the link with current knowledge on the pathways at the molecular level will be highlighted. In all cases, sucrose is the main form of carbon found in the phloem.
In addition to sucrose, polyols mainly sorbitol and mannitol and oligosaccharides of the raffinose family can also be found.
In some species, both polyols and raffinose are found in the phloem Rennie and Turgeon, Hexose transport in the phloem has also been reported for a limited number of species van Bel and Hess, but these results were recently challenged Liu et al.
Raffinose and other members of the raffinose family oligosaccharides are indirectly involved in the building up of sugar concentrations in the phloem by polymer trapping Rennie and Turgeon, Conversely, polyols tend to behave exactly like sucrose as far as transport is concerned and thus, in apoplastic loaders, there are specific polyol transporters Noiraud et al.
Unless stated otherwise, sucrose is the main sugar we deal with in the following sections. The amount of sucrose available for export from source leaves depends on several parameters: The mechanism of active phloem loading from the apoplastic space involves sucrose and polyol transporters that have been identified in numerous species Noiraud et al.
The second mechanism for active phloem loading is polymer trapping, whereby sucrose is converted to raffinose or larger molecules through addition of galactose to sucrose in intermediary cells Rennie and Turgeon, Active phloem loading may not be universal as there are many indications of passive loading at least in tree species Rennie and Turgeon, ; Turgeon, b.
This is achieved by maintaining high solute concentrations in the mesophyll cells of such species.Translocation Assimilates (sucrose and amino acids) move between sources (leaves and storage organs) and sinks (buds, flowers, fruits, roots and storage organs) in phloem sieve tubes in a process called translocation.
The products from the source are usually translocated to the nearest sink through the phloem. The movement of the sap in the phloem occurs through mass flow,, the driving force for this movement being the entry of sucrose and subsequently water in the sieve tubes in the source organ while, at the other end of the conduit in the sink organs, the continuous unloading of solutes and water would maintain the flow.
Explain how translocation of sucrose occurs in the phloem as proposed by the mass flow hypothesis. Essay by transunited, April download word file, 1 pages download word file, 1 pages 3 votes3/5(3). Learn phloem transport with free interactive flashcards.
Choose from different sets of phloem transport flashcards on Quizlet. Unloading at the sink end of the phloem tube can occur either by diffusion, if the concentration of sucrose is lower at the sink than in the phloem, or by active transport, if the concentration of sucrose is higher at the sink than in the phloem.
If the sink is an area of active growth, such as a new leaf or a reproductive structure, then the. From s to the mids, the mechanism of phloem translocation was a subject of research.
Now, one theory is generally accepted as the correct explanation for translocation. This theory called the pressure flow hypothesis is favoured by most plant physiologists and was proposed by E. Munch in Germany in