Recently, Pan Zhiyong and his research team from Key Laboratory of Horticultural Plant Biology (Ministry of Education) published an online article entitled “A Medicago truncatula SWEET transporter implicated in Arbuscule maintenance during arbuscular mycorrhizal symbiosis” in New Phytologist, identifying the SWEET transporter that was involved in sugar export of host plants and affected arbuscular mycorrhizal(AM) symbiosis.
80% of terrestrial plants on the earth can form mutualistic symbiosis with AM fungi. The formation of AM depends on the equal exchange of nutrients between the fungi and plants. The roots obtain water and mineral nutrients provided by the AM fungi, while the AM fungi obtain carbohydrates, namely sugar and fatty acids, provided by the plants. However, it remains elusive that how sugar is transported from host plants to fungi.
In previous studies of AM symbiosis of citrus, researchers found that sugar transporter SWEET might be related to AM symbiosis. Based on this, the expression of SWEET gene in mycorrhiza was studied through the model system of Medicago truncatula. Research showed that the transcripts of SWEET 1b, a member of SWEET gene family, were elicited by AM fungi and enriched in arbuscular cells. Subcellular localization analysis revealed that SWEET 1b-GFP fusion protein was accurately located in the anterior arbuscular membrane, also known as the nutrient exchange membrane of the plants and fungi. The complementary experiment of heterologous expression in yeast hexose transport mutant demonstrated that SWEET 1B transported glucose; Overexpression of SWEET 1b promoted the growth of intraradical mycelium in AM fungi. Interestingly, two SWEET1b gene insertion mutants exhibited no significant defects in AM symbiosis, but overexpression of MtSWEET1bY57A/G58D, acting in a dominant‐negative manner, resulted in enhanced collapse of arbuscules, suggesting that another redundant genes were involved in the sugar export process of the AM host plants. This study has unveiled the role of SWEET transporters of the plants in exporting sugar to fungi from host plants, and served as important clues for the understanding of the nutrient exchange mechanism of AM symbiosis.
An Jianyong, Ph.D. Candidate of College of Horticulture & Forestry Sciences of HZAU, is the first author of this article, and Associate Professor Pan Zhiyong from HZAU and Erik Limpens from Wageningen University are the correspondent authors. The research has been funded by National Key R&D Program of China, Independent Innovation Foundation of HZAU and the High-end Expert Project of State Administration of Foreign Exports Affairs.
Translated by: Zhou Shuai
Supervised by: Zhang Juan