Arabidopsis thaliana branching enzyme 1 is essential for amylopectin biosynthesis and cesium tolerance

Arabidopsis thaliana BRANCHING ENZYME 1 (AtBE1) is a chloroplast-localized embryo-lethal gene previously identified in knockout mutants. AtBE1 is thought to function in carbohydrate metabolism; however, this has not been experimentally demonstrated. Chlorosis is a typical symptom of cesium (Cs) toxicity in plants. The genetic target of Cs toxicity is largely unknown. Here, we isolated a Cs⁺-tolerant and chlorophyll-defective Arabidopsis ethyl methanesulfonate (EMS) mutant, atbe1-5. Mapping by sequencing and genetic complementation confirmed that a single amino acid change (P749S) in a random coil motif of AtBE1 confers the mutant's Cs⁺-tolerant and chlorophyll-defective phenotype. An isothermal titration calorimetry assay determined that the 749th residue is the Cs⁺-binding site and hence likely the target of Cs⁺ toxicity. We hypothesized that binding of Cs⁺ to the 749th residue of AtBE1 inhibits the enzyme's activity and confers Cs⁺ toxicity, which in turn reduces photosynthetic efficiency. In support with this hypothesis, atbe1-5 leaves have a reduced photosynthetic efficiency, and their amylose and amylopectin contents are ∼60 % and ∼1%, respectively, of those in Col-0 ecotype leaves. Leaves of the mutant have a lower sucrose, but higher maltose, concentration than those of Col-0. This study demonstrated that AtBE1 is an essential gene for amylopectin and amylose biosynthesis, as well as the target of Cs⁺ toxicity; therefore, it can serve as a genetic locus for engineering plants to extract Cs⁺ from contaminated soil while maintaining growth.