A Previously Unknown Maltose Transporter Essential for Starch Degradation in Leaves

A previously unknown maltose transporter is essential for the conversion of starch to sucrose in Arabidopsis leaves at night. The transporter was identified by isolating two allelic mutants with high starch levels and very high maltose, an intermediate of starch breakdown. The mutations affect a gene of previously unknown function, MEX1. We show that MEX1 is a maltose transporter that is unrelated to other sugar transporters. The severe mex1 phenotype demonstrates that MEX1 is the predominant route of carbohydrate export from chloroplasts at night. Homologous genes in plants including rice and potato indicate that maltose export is of widespread significance. The conversion of starch to sucrose in leaves in the dark is one of the largest metabolic fluxes in living organisms, amounting to tens of millions of tons of carbon every night. Sucrose and starch ar