Detection of calmodulin and calmodulin-binding proteins in pure phloem sap of rice plants

Calcium-ligand blots using Ca-45(2+) revealed that there were at least four calcium-binding proteins (17, 18.5, 48 and 52 kDa) in the phloem sap of rice plants. Furthermore, an anti-spinach calmodulin antibody cross-reacted strongly with an 18.5 kDa protein from the phloem sap. When electrophoresed on native gels, the anti-calmodulin cross-reacting protein migrated more slowly in the presence of calcium than in its absence. Based on these results, we concluded that the cross-reacting protein was a calmodulin. An I-125-CaM overlay assay revealed that two calmodulin-binding proteins (20 kDa and 40 kDa) were present in the rice phloem sap. Based on these data, it is possible that a calmodulin signal cascade is present in the sieve tubes of rice plants.