Colocalization of sucrose synthase expression and sucrose storage in the sugarbeet taproot indicates a potential role for sucrose catabolism in sucrose accumulation

Authors: Fugate, Karen; Eide, John; MARTINS, DANIEL - Universidade Federal De Vicosa; Grusak, Michael; DECKARD, EDWARD - North Dakota State University; FINGER, FERNANDO - Universidade Federal De Vicosa

Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2019
Publication Date: 7/24/2019
Citation: Fugate, K.K., Eide, J.D., Martins, D.N., Grusak, M.A., Deckard, E.L., Finger, F.L. 2019. Colocalization of sucrose synthase expression and sucrose storage in the sugarbeet taproot indicates a potential role for sucrose catabolism in sucrose accumulation. Journal of Plant Physiology. 240:153016. https://doi.org/10.1016/j.jplph.2019.153016.
DOI: https://doi.org/10.1016/j.jplph.2019.153016

Interpretive Summary: Sugarbeet plants synthesize sucrose in their leaves and transport it into the root. Although much of the incoming sucrose is stored in the taproot, a portion of the incoming sucrose is degraded and used to support growth. It has been observed that the ability to degraded sucrose is closely related to the taproot’s ability to accumulate sucrose. How the ability to degrade sucrose could encourage its accumulation, however, is not obvious. To understand how degradation of sucrose in the sugarbeet taproot might promote sucrose accumulation, research was conducted to determine the sites of degradation and the sites of sucrose accumulation in the taproot. This research found that sucrose storage was not uniform throughout the root and not all cells in the root accumulate sucrose. Cells that store sucrose, however, occurred in the same tissue types and locations as those that degrade sucrose. This suggests that sucrose degradation is needed for sucrose storage. It is known that metabolic energy is needed to store sucrose. It is proposed that sucrose degradation provides this energy. It is also known that the transport of sugar from the leaves into the root is promoted by maintaining low sucrose concentrations in the regions of the root where sucrose enters. Sucrose degradation likely plays a key role in maintaining low sucrose concentrations in these regions, and therefore acts to drive sucrose transport from leaves into the root.

Technical Abstract: Sucrose metabolism is believed to have a central role in promoting sink strength and sucrose storage in the sugarbeet taproot. How sucrose accumulation is increased by sucrose-degrading enzymes, however, is a paradox. To elucidate roles for sucrose-degrading activities in sucrose accumulation, relationships between the intercellular location of sucrose-catabolizing enzymes and sites of sucrose accumulation were determined in the sugarbeet taproot. Sucrose storage was evident in parenchyma cells of the outer cortex, rays, and rings of parenchyma tissue, but was absent in phloem, the vascular cambium, cells surrounding these tissues, or cells surrounding xylem. Sucrose synthase, which was primarily responsible for sucrose catabolism throughout the taproot, was expressed in similar cell and tissue types to those accumulating sucrose. Co-localization of sucrose synthase with sucrose accumulation, as well as sucrose synthase localization near the tonoplast, suggests a role for the enzyme in generating metabolic energy to fuel sucrose sequestration in the vacuole. Localization near the plasma membrane also suggests a role for sucrose synthase in supplying substrates for cell wall biosynthesis. By utilizing sucrose for ATP or cell wall biosynthesis, sucrose synthase likely maintains the source-to-sink sucrose gradient that drives sucrose transport into the root, thereby promoting sugarbeet root sink strength.