Journal
JOURNAL OF EXPERIMENTAL BOTANY
Volume 72, Issue 10, Pages 3723-3738Publisher
OXFORD UNIV PRESS
DOI: 10.1093/jxb/erab085
Keywords
Oryza sativa (Rice); phloem loading; apoplastic; invertase; sucrose transporter; photosynthesis
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Funding
- Natural Science Foundation of Jiangsu Province for Excellent Young Schloars [BK20200079]
- National Natural Science Foundation of China [31901454]
- National Key Research and Development Program of China [2017YFD0301200]
- Jiangsu Collaborative Innovation Center for Modern Crop Production
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The research on apoplastic phloem loading in rice, conducted by overexpressing yeast invertase in the cell wall to block sugar transmembrane loading, revealed the potential phloem loading mechanism. The study showed that OsSUT1 and OsSUT5 are expressed in phloem companion cells, supporting the conclusion that apoplastic phloem loading plays a major role in sugar export from rice leaves.
Phloem loading is the first step in sucrose transport from source leaves to sink organs. The phloem loading strategy in rice remains unclear. To determine the potential phloem loading mechanism in rice, yeast invertase (INV) was overexpressed by a 35S promoter specifically in the cell wall to block sugar transmembrane loading in rice. The transgenic lines exhibited obvious phloem loading suppression characteristics accompanied by the accumulation of sucrose and starch, restricted vegetative growth and decreased grain yields. The decreased sucrose exudation rate with p-chloromercuribenzenesulfonic acid (PCMBS) treatment also indicated that rice actively transported sucrose into the phloem. OsSUT1 (SUCROSE TRANSPORTER 1) showed the highest mRNA levels of the plasma membrane-localized OsSUTs in source leaves. Cross sections of the OsSUT::GUS transgenic plants showed that the expression of OsSUT1 and OsSUT5 occurred in the phloem companion cells. Rice ossut1 mutants showed reduced growth and grain yield, supporting the hypothesis of OsSUT1 acting in phloem loading. Based on these results, we conclude that apoplastic phloem loading plays a major role in the export of sugar from rice leaves.
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