Maize Carbohydrate Partitioning Defective33 Encodes an MCTP Protein and Functions in Sucrose Export from Leaves

To sustain plant growth, development, and crop yield, sucrose must be transported from leaves to distant parts of the plant, such as seeds and roots. To identify genes that regulate sucrose accumulation and transport in maize (Zea mays), we isolated carbohydrate partitioning defective33 (cpd33), a recessive mutant that accumulated excess starch and soluble sugars in mature leaves. The cpd33 mutants also exhibited chlorosis in the leaf blades, greatly diminished plant growth, and reduced fertility. Cpd33 encodes a protein containing multiple C2 domains and transmembrane regions. Subcellular localization experiments showed the CPD33 protein localized to plasmodesmata (PD), the plasma membrane, and the endoplasmic reticulum. We also found that a loss-of-function mutant of the CPD33 homolog in Arabidopsis, QUIRKY, had a similar carbohydrate hyperaccumulation phenotype. Radioactively labeled sucrose transport assays showed that sucrose export was significantly lower in cpd33 mutant leaves relative to wild-type leaves. However, PD transport in the adaxial-abaxial direction was unaffected in cpd33 mutant leaves. Intriguingly, transmission electron microscopy revealed fewer PD at the companion cell-sieve element interface in mutant phloem tissue, providing a possible explanation for the reduced sucrose export in mutant leaves. Collectively, our results suggest that CPD33 functions to promote symplastic transport into sieve elements.