Maize Chlorotic Leaf Spot1 encodes a fumarylacetoacetate hydrolase essential for carbohydrate partitioning.

Carbohydrate partitioning from photosynthetic sources to non-photosynthetic sinks is essential for plant development and crop yield. Using a maize-teosinte BC₂S₃ population, we identify Chlorotic Leaf Spot1 (CLS1), a fumarylacetoacetate hydrolase (FAH) in the tyrosine degradation pathway that plays an essential role in carbohydrate partitioning in maize. CLS1 localizes to the plasma membrane, cytoplasm, and nucleus. Allelic tests and sequence analysis reveal that the teosinte parent CIMMYT8759 carries a weak allele of CLS1, likely due to rare amino acid substitutions at residues 175 and 355. Loss-of-function mutants of CLS1 develop chlorotic leaf spots accompanied by carbohydrate hyperaccumulation, reduced photosynthetic efficiency, chloroplast damage, and impaired transient starch conversion. Critically, cls1 mutants exhibit ectopic callose accumulation and aberrant plasmodesmata ultrastructure at the mesophyll-bundle sheath and bundle sheath-vascular parenchyma interfaces. This defect causes starch granule and soluble sugar accumulation in chlorotic leaf tissues, indicating a disruption of the symplastic transport pathway. Collectively, our results uncover an important role for FAH in plant development and identify CLS1 as a key regulator of symplastic carbohydrate partitioning.