Cross-talk of calcium-dependent protein kinase and MAP kinase signaling

Plants use different signaling pathways to acclimate to changing environmental conditions. Fast changes in the concentration of free Ca2+ ions-so called Ca2+ signals-are among the first responses to many stress situations. These signals are decoded by different types of calcium-dependent protein kinases, which-together with mitogen-activated protein kinases (MAPK)-present two major pathways that are widely used to adapt the cellular metabolism to a changing environment. Ca2+-dependent protein kinase (CDPK) and MAPK pathways are known to be involved in signaling of abiotic and biotic stress in animal, yeast and plant cells. In many cases both pathways are activated in response to the same stimuli leading to the question of a potential cross-talk between those pathways. Cross-talk between Ca2+-dependent and MAPK signaling pathways has been elaborately studied in animal cells, but it has hardly been investigated in plants. Early studies of CDPKs involved in the biotic stress response in tobacco indicated a cross-talk of CDPK and MAPK activities, whereas a recent study in Arabidopsis revealed that CDPKs and MAPKs act differentially in innate immune signaling and showed no direct cross-talk between CDPK and MAPK activities. Similar results were also reported for CDPK and MAPK activities in the salt-stress response in Arabidopsis. Different modes of action are furthermore supported by the different subcellular localization of the involved kinases. In this review, we discuss recent findings on CDPK and MAPK signaling with respect to potential cross-talk and the subcellular localization of the involved components.