Calcium-dependent protein kinase genes in corn roots

Two cDNAs encoding Ca(2+)-dependent protein kinases (CDPKs), CRPK1 and CRPK2 (corn root protein kinase 1 and 2), were isolated from the root tip library of corn (Zea mays L., cv. Merit) and their nucleotide sequences were determined. Deduced amino acid sequences of both the clones have features characteristic of plant CDPKs, including all 11 conserved serine/threonine kinase subdomains, a junction domain and a calmodulin-like domain with four Ca(2+)-binding sites. Northern analysis revealed that CRPK1 mRNA is preferentially expressed in roots, especially in the root tip; whereas, the expression of CRPK2 mRNA was very low in all the tissues tested. In situ hybridization experiments revealed that CRPK1 mRNA is highly expressed in the root apex, as compared to other parts of the root. Partially purified CDPK from the root tip phosphorylates syntide-2, a common peptide substrate for plant CDPKs, and the phosphorylation was stimulated 7-fold by the addition of Ca2+. Our results show that two CDPK isoforms are expressed in corn roots and they may be involved in the Ca(2+)-dependent signal transduction process.