High expression level of a gene coding for a chloroplastic amino acid selective channel protein is correlated to cold acclimation in cereals

A cold-regulated gene (cor tmc-ap3) coding for a putative chloroplastic amino acid selective channel protein was isolated from cold-treated barley leaves combining the differential display and the 5-RACE techniques. Cor tmc-ap3 is expressed at low level under normal growing temperature, and its expression is strongly enhanced after cold treatment. A positive correlation between the expression of cor tmc-ap3 and frost tolerance was found both among barley cultivars and among cereal species. The COR TMC-AP3 protein was expressed in vitro, purified and used to raise a polyclonal antibody. Western analysis showed that the cor tmc-ap3 gene product is localized to the chloroplastic outer envelope fraction, supporting its putative function. The frost-resistant winter cultivar Onice accumulated COR TMC-AP3 more rapidly and at a higher level than the frost-susceptible spring cultivar Gitane. After 28 days of cold acclimation the winter cultivar had about 2-fold more protein than the spring genotype. All these results suggest that an increased amount of a chloroplastic amino acid selective channel protein could be required for cold acclimation in cereals. Hypotheses about the role of COR TMC-AP3 during the hardening process are discussed.