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Differences in membrane selectivity drive phloem transport to the apoplast from which maize florets develop
1College of Marine Studies (now College of Earth, Ocean and Environment), University of Delaware, Lewes, DE 19958, USA.
The apoplast, not the symplast, serves as the pathway for phloem-delivered nutrients to maize florets. This apoplastic pathway facilitates nutrient transport and isolates the developing embryo sac.
Area of Science:
- Plant Biology
- Plant Physiology
- Reproductive Biology
Background:
- Floral development relies on photosynthates transported via the phloem.
- Previous research debated whether nutrients reached the flower through apoplastic or symplastic pathways.
Purpose of the Study:
- To elucidate the transport pathway and mechanism of phloem-derived nutrients to maize florets.
- To determine if the apoplast or symplast is the primary route for nutrient delivery.
Main Methods:
- Maize plants were grown to pollination, with florets harvested pre-fertilization.
- Tissues were analyzed for glucose using enzyme-based assays after phloem unloading.
- Carboxyfluorescein diacetate was used to trace pathway movement and analyze membrane-mediated release.
Main Results:
- Phloem contents were released into the pedicel apoplast below the floret's nucellus.
- Glucose and carboxyfluorescein were detected in the apoplast, with release controlled by phloem parenchyma plasma membranes.
- The nucellus selectively absorbed glucose while rejecting carboxyfluorescein.
Conclusions:
- The apoplast serves as a crucial depot for phloem contents, refuting a strictly symplastic pathway.
- Nutrient release into the apoplast is non-selective, creating a pressure gradient for bulk flow.
- Selective nucellar absorption isolates the embryo sac, ensuring its development is independent of maternal tissues.
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