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Published on June 9, 2006; 10.1104/pp.106.082719

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Received April 27, 2006
Returned for revision May 25, 2006
Accepted May 25, 2006

The pathway of sugar transport in germinating wheat seeds

Naohiro Aoki , Graham N. Scofield , Xin-Ding Wang , Christina E. Offler , John W. Patrick , and Robert T. Furbank *

CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia
School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia

* Corresponding author; email: robert.furbank{at}csiro.au.

Three homeologous genes encoding a sucrose transporter in hexaploid wheat (Triticum aestivum), TaSUT1A, 1B and 1D, were expressed in germinating seeds, where their function is unknown. All three TaSUT1 proteins were confirmed to be capable of transporting both sucrose and maltose, by complementation test with the SUSY7/ura3 yeast mutant strain. The role of sucrose transporters in germinating grain was examined by combining in situ hybridisation, immuno-localisation, fluorescent dye tracer movement, and metabolite assays. TaSUT1 transcript and SUT protein were detected in cells of the aleurone layer, the scutellar epidermis, scutellar ground cells and sieve element-companion cell complexes located in the scutellum, shoot and root. Ester loading of the membrane-impermeable fluorescent dye carboxyfluorescein into the scutellum epidermal cells of germinating seeds showed that a symplasmic pathway connects the scutellum to the shoot and root via the phloem. However, the scutellar epidermis provides an apoplasmic barrier to solute movement from the endosperm tissue. Measurements of sugars in the root, shoot, endosperm and scutellum suggest that, following degradation of endosperm starch, the resulting hexoses are converted to sucrose in the scutellum. Sucrose was found to be the major sugar present in the endosperm early in germination whilst maltose and glucose predominate during the later stage. It is proposed that loading the scutellar phloem in germinating wheat seeds can proceed by symplasmic and apoplasmic pathways, the latter facilitated by SUT activity. In addition, SUTs may function to transport sucrose into the scutellum from the endosperm early in germination and later transport maltose.




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