ISOLATION AND CHARACTERIZATION OF SALT-ASSOCIATED PROTEIN IN CITRUS

A significant increase in the amount of a protein, whose migration in two-dimensional gel electrophoresis corresponds to an apparent molecular weight of 23-25 kDa and pI = 6.1, was observed in adapted salt-tolerant cultured cells derived from Shamuti orange (Citrus sinensis L. Osbeck) ovular callus cells. An increase was also determined when these cells were grown in the presence of abscisic acid (ABA) or polyethylene glycol (PEG) instead of NaCl, similarly to what was previously observed for osmotin, the tobacco salt-associated protein. However, no similarity has been so far observed between these proteins in their biochemical and immunochemical properties. Pulse labeling of the salt-tolerant cells suggests a low turnover of the accumulated protein. Its salt-induced accumulation in the cells was also relatively slow and gradually increased during a period of several days following exposure of the cells to NaCl. Cell fractionation experiments suggest that the protein is allocated in the soluble fraction and is not an integral membrane protein. Examination of the presence of the protein in citrus plants, such as Etrog citron (C. medica L.) and Cleopatra mandarin (C. reticulata) cultivars, revealed an increase in its concentration in all organs examined, following irrigation of the plants with saline water. Thus, this protein is associated with stress conditions employed on growing plants as well and is not a unique phenomenon restricted to cultured cells only.