PURIFICATION OF A ZN-BINDING PHLOEM PROTEIN WITH SEQUENCE IDENTITY TOCHITIN-BINDING PROTEINS

In citrus blight, a decline disorder of unknown etiology, the tree can opy exhibits symptoms of Zn deficiency while Zn accumulates in the tru nk phloem. We have purified a Zn-binding protein (ZBP) from phloem tis sue of healthy and blight-affected citrus (Citrus sinensis [L.] Osbeck on Citrus jambhiri [L.]). The molecular weight of the ZBP was estimat ed to be 5000 by size-exclusion chromatography and sodium dodecyl sulf ate-polyacrylamide gel electrophoresis, ion-exchange chromatography at pH 8.0 demonstrated the 5-kD ZBP to be anionic. A partial N-terminal amino acid sequence revealed a cysteine-, glycine-rich domain with 45 to 80% identity with the chitin-binding domain of hevein, wheat germ a gglutinin, and several class I chitinases. That the abundance of this protein increased 2.5-fold in association with Zn accumulation in the phloem is characteristic of citrus blight. Tissue mass changes of the phloem suggests that altered tissue structure accompanies blight. Phlo em accumulation of the 5-kD ZBP may be in response to wounding or othe r stress of blight-affected citrus.