In this study,differential display technique was used to compare the gene expression difference between Arabidopsis wild type and the crk45 mutant with the T-DNA inserted in the CRK45 gene under ABA treatment.The results showed that:(1)One gene showed 4fold higher expression in the mutant than that in the wild type.(2)Searches in the NCBI database revealed that this gene encoding a Ca²⁺ binding protein composed by 130 amino acid and with the typical EF hand,so it was dubbed as CICBP (CRK45 inhibited Ca²⁺ binding protein).(3)Northern blotting analysis further confirmed that CICBP was indeed significantly induced by ABA treatment in the crk45 mutant,and its expression was inhibited by CRK45.(4)After treated by 75 mmol/L exogenerous Ca²⁺,the germination rate of the crk45 mutant is 30.8%,which is obviously higher than that of the wild type (17.16%),indicating that CRK45 plays a role in seed germination inhibition under Ca²⁺ treatment.(5) qRT-PCR analysis showed that the expression of CRK45 was induced highly by Ca²⁺ in wild type,but keeping very low level in the mutant,indicated that the gene was completely knocked out.In addition,the transcript of CICBP gene was also upregulated in the crk45 mutant after Ca²⁺ treatment compared with wild type.These results indicated that CRK45 inhibited germination of Arabidopsis seeds by downregulating CICBP after ABA and Ca²⁺ treatment.