Arabidopsis Sucrose Transporter SUT4 Interacts with Cytochrome b5-2 to Regulate Seed Germination in Response to Sucrose and Glucose

It remains unknown whether a sucrose transporter mediates sugar signaling. Here, we report that the Arabidopsis （Arabidopsis thaliana） sucrose transporter SUT4 interacts with five members of the Arabidopsis cytochrome b5 （Cyb5） family, and sucrose represses the interaction between SUT4 and a Cyb5 member Cyb5-2/A. We observed that down- regulation of SUT4 and three cytochrome b5 members （Cyb5-2, Cyb5-4, and Cyb5-6） confers the sucrose- and glucose- insensitive phenotypes in the sucrose/glucose-induced inhibition of seed germination. The sut4 cybS-2 double mutant displays slightly stronger sucrose/glucose-insensitive phenotypes than either the sut4 or cyb5-2 single mutant. We showed that the SUT4/Cyb5-2-mediated signaling in the sucrose/glucose-induced inhibition of seed germination does not require ABA or the currently known ABI2/ABI4/ABI5-mediated signaling pathway（s）. These data provide evidence that the sucrose transporter SUT4 interacts with Cyb5 to positively mediate sucrose and glucose signaling in the sucrose/ glucose-induced inhibition of seed germination.