Expression and localization of calcium-dependent protein kinase isoforms in chickpea

Calcium-dependent protein kinases (CPKs) play important roles in multiple signal transduction pathways but the precise role of individual CPK is largely unknown. We isolated two cDNAs encoding two CPK isoforms ( Cicer arietinum CPKs—CaCPK1 and CaCPK2) of chickpea. Their expression in various organs and in response to various phytohormones, and dehydration, high salt stress and fungal spore in excised leaves as well as localization in leaf and stem tissues were analyzed in this study. CaCPK1 protein and its activity were ubiquitous in all tissues examined. In contrast, CaCPK2 transcript, CaCPK2 protein and its activity were almost undetectable in flowers and fruits. Both CaCPK1 and CaCPK2 transcripts and proteins were abundant in roots but in minor quantities in leaves and stems. Of the three phytohormones tested, viz. indole-3-acetic acid (IAA), gibberellin (GA 3) and benzyladenine (BA), only BA increased both CaCPK1 and CaCPK2 transcripts, proteins and their activities. GA 3 induced accumulation of CaCPK2 transcript and protein but CaCPK1 remained unaffected. The expression of CaCPK1 and CaCPK2 in leaves was enhanced in response to high salt stress. Treatments with Aspergillus sp. spores increased expression of CaCPK1 in chickpea leaf tissue but had no effect on CaCPK2. Excised leaves subjected to dehydration showed increase in CaCPK2 expression but not in CaCPK1. Both isoforms were located in the plasma membrane (PM) and chloroplast membrane of leaf mesophyll cells as well as in the PM of stem xylem parenchyma cells. These results suggest specific roles for CaCPK isoforms in phytohormone/defense/stress signaling pathways.