Sucrose export defective1 encodes a novel protein implicated in chloroplast-to-nucleus signaling

The Sucrose export defective1 (Sxd1) gene of maize was cloned and shown to encode a novel protein conserved between plants and cyanobacteria. The stru cture of the Sxd1 locus was determined in wild-type plants and two independ ent sxd1 alleles. Expression analysis demonstrated that the gene was transc ribed in all green tissues, with highest levels in maturing leaf blades. In situ hybridization studies revealed high levels of Sxd1 mRNA in bundle she ath cells, with lower levels within the mesophyll, The SXD1 protein was loc alized to chloroplasts, in both bundle sheath and mesophyll cells, Levels o f sucrose, glucose, and fructose were compared between wild-type and sxd1 p lants. Mutant plants were fully capable of producing sucrose and accumulate d all three sugars at concentrations above those measured in wild-type plan ts. Despite these increased sugar concentrations, photosynthetic gene expre ssion was not significantly downregulated in affected areas of sxd1 leaf bl ades. These results are consistent with photosynthate being trapped within anthocyanin-accumulating regions of sxd1 leaves due to plasmodesmal occlusi on at the bundle sheath-vascular parenchyma boundary of the minor veins. A model for SXD1 function is proposed in which the protein is involved in a c hloroplast-to-nucleus signaling pathway necessary for proper late-stage dif ferentiation of maize bundle sheath cells, including the developmentally re gulated modification of plasmodesmata.