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5.4.6 - Follow the flow: unloading of water and its destination

Phloem unloading of nutrients follows water release from vascular system.

As discussed before, because of their low transpiration rates, developing sinks typically import water through phloem, not xylem.  Water unloaded from phloem is used for cell growth or recycled back to the parental bodies.  Water is unloaded from se-cc complexes symplasmically in majority of sinks by bulk flow. For growth sinks such as shoot or root apices, continued symplasmic flow of phloem-imported water can drive cells expansion. In post-phloem unloading pathways interrupted with an apoplasmic step, water must exit cells of the unloading path across cell membranes, facilitated by aquaporins (AQPs). AQPs responsible for water flow across cell membranes are plasma membrane intrinsic proteins (PIPs) and tonoplast intrinsic proteins (TIPs). Strong PIP expression in expanding post-veraison grape berries has been shown to correlate with water flows into, and from, the berry apoplasm. For sinks that stop expansion but continuously accumulate biomass, water transported to storage sink apoplasms is recycled back to the parent plant body through a xylem route. Important roles played by AQPs in water recycling are indicated by their high expression at this stage in developing seed, particularly in the vascular parenchyma cells.

In conclusion, this chapter has shown how growth and development of meristems and other sinks is determined by phloem unloading events, and metabolism of the assimilates within the recipient sink cells. These transport and transfer processes vary between specific sinks, and can alter during development. Molecular techniques that alter expression of membrane transporters  can be used to study the pathways and limitations of photoassimilate transport into sinks such as seeds that have an apoplasmic step in the phloem unloading pathway, with the possibility of enhancing the rate of grain growth and crop yield in the future.