Calcium signalling components underlying NPK homeostasis: potential avenues for exploration.

Plants require the major macronutrients, nitrogen (N), phosphorus (P) and potassium (K) for normal growth and development. Their deficiency in soil directly affects vital cellular processes, particularly root growth and architecture. Their perception, uptake and assimilation are regulated by complex signalling pathways. To overcome nutrient deficiencies, plants have developed certain response mechanisms that determine developmental and physiological adaptations. The signal transduction pathways underlying these responses involve a complex interplay of components such as nutrient transporters, transcription factors and others. In addition to their involvement in cross-talk with intracellular calcium signalling pathways, these components are also engaged in NPK sensing and homeostasis. The NPK sensing and homeostatic mechanisms hold the key to identify and understand the crucial players in nutrient regulatory networks in plants under both abiotic and biotic stresses. In this review, we discuss calcium signalling components/pathways underlying plant responses to NPK sensing, with a focus on the sensors, transporters and transcription factors involved in their respective signalling and homeostasis.

Role of calcium sensor protein module CBL-CIPK in abiotic stress and light signaling responses in green algae.

Ca2+ signaling is an important biological process that enable to perceive and communicate information in the cell. Our current understanding of the signaling system suggests that plants and animals have certain differences in signal-sensing mechanisms. The Ca2+-mediated CBL-CIPK module has emerged as a major sensor responder network for Ca2+ signaling and has been speculated to be involved in plant terrestrial life adaptation. This module has previously been reported in Archaeplastids, Chromalveolates, and Excavates. In our experimental analysis of Chlamydomonas reinhardtii CBLs, we proved that the CrCBL1 protein interacts with Phototropin and Channelrhodopsin, and the expression of CrCBLs is modulated by light. Further analysis using chlorophyte and streptophyte algal sequences allowed us to identify the differences that have evolved in CBL and CIPK proteins since plants have progressed from aquatic to terrestrial habitats. Moreover, an investigation of Klebsormidium CBL and CIPK genes led us to know that they are abiotic stress stimuli-responsive, indicating that their role was defined very early during terrestrial adaptations. Structure-based prediction and Ca2+-binding assays indicated that the KnCBL1 protein in Klebsormidium showed a typical Ca2+-binding pocket. In summary, the results of this study suggest that these stress-responsive proteins enable crosstalk between Ca2+ and light signaling pathways very early during plant adaptation from aquatic to terrestrial habitats.