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.

为了维持植物的生长，发育和农作物产量，必须将蔗糖从叶子运输到植物的远处，例如种子和根。为了鉴定调节玉米蔗糖积累和运输（基因玉蜀黍），我们分离碳水化合物分区defective33（cpd33），隐性突变体累积过量的淀粉和可溶性糖在成熟的叶子。所述cpd33突变体也显示出在叶片萎黄，大大减少的植物生长，和生育力降低。第33章编码包含多个C2域和跨膜区域的蛋白质。亚细胞定位实验显示CPD33蛋白定位于浆膜（PD），质膜和内质网。我们还发现，拟南芥（QUIRKY）中CPD33同源物的功能丧失突变体具有相似的碳水化合物超积累表型。放射性标记的蔗糖转运分析表明，相对于野生型叶片，cpd33突变体叶片中的蔗糖出口量显着降低。但是，cpd33中沿前后轴方向的PD传输不受影响突变叶。有趣的是，透射电子显微镜显示突变韧皮部组织中伴随细胞-筛子界面处的PD较少，这为突变叶片中蔗糖输出减少提供了可能的解释。总的来说，我们的结果表明CPD33的作用是促进共生运输到筛子中。