The role of tripartite interaction of calcium sensors and transporters in the accumulation of calcium in finger millet grain

Finger millet ( Eleusine coracana ) is one of important crops, and its grains contain an exceptionally high content of calcium. In order to investigate the molecular mechanism by which it orchestrate the accumulation of Ca 2+ during grain filling, some candidate genes encoding calcium transporters [calcium exchangers ( CAX1 , CAX3 )] and sensors [calcineurin-B like ( CBL4 and 10 )], a CBL-interacting protein kinase ( CIPK24 ), and calmodulin ( CaM ) were identified using transcriptomics and differential expression analysis in two genotypes of finger millet differing in grain calcium content. These transporters and sensors are highly expressed in leaves and developing spikes of the genotype with a high grain Ca 2+ indicating their potential role in Ca 2+ accumulation. Calcium transporters, mainly CAXs, pump Ca 2+ inside the cell through plasmalemma and tonoplast, and their activities are regulated by CaM dependent and independent Ca 2+ sensor proteins of CaM and CBL-CIPK networks. Abundance of CaM in a high grain Ca 2+ genotype is suggestive that CaM might also contribute for grain calcium accumulation by interaction with Ca 2+ ATPase. The upregulation of CAX1 in vegetative tissues and developing spikes and CAX3 only in developing spikes provides the most plausible clue for calcium transport and accumulation regulated by tripartite interaction in finger millet.