The sucrose transporter SlSUT2 from tomato interacts with brassinosteroid functioning and affects arbuscular mycorrhiza formation

Mycorrhizal plants benefit from the fungal partners by getting better access to soil nutrients. In exchange, the plant supplies carbohydrates to the fungus. The additional carbohydrate demand in mycorrhizal plants was shown to be balanced partially by higher CO2 assimilation and increased C metabolism in shoots and roots. In order to test the role of sucrose transport for fungal development in arbuscular mycorrhizal (AM) tomato, transgenic plants with down-regulated expression of three sucrose transporter genes were analysed. Plants that carried an antisense construct of SlSUT2 (SlSUT2as) repeatedly exhibited increased mycorrhizal colonization and the positive effect of plants to mycorrhiza was abolished. Grafting experiments between transgenic and wild-type rootstocks and scions indicated that mainly the root-specific function of SlSUT2 has an impact on colonization of tomato roots with the AM fungus. Localization of SISUT2 to the periarbuscular membrane indicates a role in back transport of sucrose from the periarbuscular matrix into the plant cell thereby affecting hyphal development. Screening of an expression library for SlSUT2-interacting proteins revealed interactions with candidates involved in brassinosteroid (BR) signaling or biosynthesis. Interaction of these candidates with SlSUT2 was confirmed by bimolecular fluorescence complementation. Tomato mutants defective in BR biosynthesis were analysed with respect to mycorrhizal symbiosis and showed indeed decreased mycorrhization. This finding suggests that BRs affect mycorrhizal infection and colonization. If the inhibitory effect of SlSUT2 on mycorrhizal growth involves components of BR synthesis and of the BR signaling pathway is discussed.