Passive symplastic phloem loading in the duckweed Spirodela polyrhiza
The lemnoidae, commonly called duckweeds, are a group of small, rapidly growing aquatic plants that play an important role in pond ecosystems and are used in biotechnological and remediation applications. While small, duckweeds feature phloem tissue in fronds and roots. To gain insight on duckweed phloem function, we investigated how sugar is loaded into the phloem sieve elements in the giant duckweed Spirodela polyrhiza. Genomes of S. polyrhiza and three other duckweeds do not feature genes for the sucrose transporters typically associated with active apoplastic phloem loading. Neither did a sucrose transporter inhibitor affect sucrose concentration in phloem exudate. Active symplastic phloem loading was excluded based on the conventional plasmodesmata configuration and absence of oligosaccharides in S. polyrhiza phloem. Instead, uniform plasmodesmata density along the phloem loading pathway indicated a passive symplastic phloem loading type. When plasmodesmata permeability was artificially reduced by hormone treatment, the sucrose concentration in the phloem exudate was reduced, highlighting the potential role of plasmodesmata regulation in setting carbon export rates in species with passive phloem loading. Our results identify S. polyrhiza as the first monocot species with passive phloem loading. Moreover, they indicate opportunities for optimization of duckweed growth.