A Previously Unknown Maltose Transporter Essential for Starch Degradation in Leaves

Abstract

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 MEX1is a maltose transporter that is unrelated to other sugar transporters. The severe mex1 phenotype demonstrates that MEX1is 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.

EST-analysis of the thermo-acidophilic red microalga Galdieriasulphuraria reveals potential for lipid A biosynthesis and unveils the pathway of carbon export from rhodoplasts

“…In support of this hypothesis, we did not find any ESTs with similarity to the plastidic maltose transporter MEX1 (Niittyla et al, 2004), and none of the 13 ESTs encoding putative monosaccharide transporters was closely related to the plastidic glucose translocator pGlcT (Weber et al, 2000). Furthermore, the export of glucose from plastids is likely to require the activity of hexokinase (Wiese et al, 1999;Weber et al, 2000), and antisense repression of hexokinase in potato plants caused a starch excess phenotype (Veramendi et al, 1999).…”

“…Up to 50% of the carbon dioxide assimilated during the day is stored as transitory starch within chloroplasts and exported to the cytosol during the night in the form of glucose and maltose (Niittyla et al, 2004;Weise et al, 2004). However, as outlined above, red algae store starch in the cytosol and produce floridosides as soluble sugars.…”

“…The G. sulphuraria genome encodes at least nine distinct putative monosaccharide transporters Seed plants possess three distinct families of sugar transporters: (i) disaccharide transporters that primarily catalyze sucrose transport; (ii) monosaccharide transporters mediate the transport of a variable range of monosaccharides (Bu¨ttner and Sauer, 2000;Williams et al, 2000); and (iii) a maltose transporter that represents a novel type of sugar transporter (Niittyla et al, 2004). In seed plants, transporters play crucial roles in the intra-and intercellular and longdistance transport of sugars throughout the plant.…”

The Import and Export Business in Plastids: Transport Processes across the Inner Envelope Membrane

“…One mutant showed, in addition, elevated levels of maltose and was shown to be impaired in the export of maltose generated in the plastid during starch degradation at night. The mutated locus that was named mex-1 (maltose excess) was shown to encode the plastid maltose transporter Mex-1 (Niittylä et al, 2004). A loss of Mex-1 leads to an accumulation of maltose in amyloplasts that in turn severely inhibits starch breakdown.…”

“…During the day, carbon is exported in the form of TPs by the TPT while at night chloroplasts export mainly maltose and-to a lesser extent-Glc, both derived from starch breakdown (Weber et al, 2000;Niittylä et al, 2004). An extensive analysis of the phylogeny of the pPT family has clearly revealed that these transporters are related to a large superfamily of transporters, the nucleotide sugar transporters that are localized in the Golgi or ER membranes and transport nucleotide sugars like GDP-Man or UDP-Gal into the ER and Golgi lumen (Knappe et al, 2003;Weber et al, 2006).…”