Abstract
Calcium (Ca2+) is a universal second messenger involved in various cellular processes, leading to plant development and to biotic and abiotic stress responses. Intracellular variation in free Ca2+ concentration is among the earliest events following the plant perception of environmental change. These Ca2+ variations differ in their spatio-temporal properties according to the nature, strength and duration of the stimulus. However, their conversion into biological responses requires Ca2+ sensors for decoding and relaying. The occurrence in plants of calmodulin (CaM) but also of other sets of plant-specific Ca2+ sensors such as calmodulin-like proteins (CMLs), Ca2+-dependent protein kinases (CDPKs) and calcineurin B-like proteins (CBLs) indicate that plants possess specific tools and machineries to convert Ca2+ signals into appropriate responses. Here, we focus on recent progress made in monitoring the generation of Ca2+ signals at the whole plant or cell level and their long distance propagation during biotic interactions. The contribution of CaM/CMLs and CDPKs in plant immune responses mounted against bacteria, fungi, viruses and insects are also presented.
Highlights
Like all living organisms, plants face environmental challenges that can be either of a biotic nature such as interactions with pathogens or of an abiotic nature such as drought, soil salinity, air pollution, extreme temperatures and mechanical injury [1]
The importance of Ca2+ signalling in symbiosis has been recently reviewed, as well as the function of calcineurin B-like proteins (CBLs), which are mainly related to the regulation of membrane proteins involved in plant development, nutrition and abiotic stress [65]
CaM/calmodulin-like proteins (CMLs) have been reported to interact with different nuclear proteins [68] but do these interactions help to recruit other actors into transcriptional complexes and regulate their activity? What is the contribution of Ca2+ in these interactions? Efforts are made to answer these questions and to better understand the contribution of these sensors in Ca2+ signalling with a particular interest on plant specific CMLs
Summary
Plants face environmental challenges that can be either of a biotic nature such as interactions with pathogens (e.g., bacteria, fungi, oomycetes, viruses, insects) or of an abiotic nature such as drought, soil salinity, air pollution, extreme temperatures and mechanical injury [1]. Using R-GECO as an intensity-based Ca2+ sensor, Keinath et al studied Ca2+ responses induced by Pathogen-Associated Molecular Patterns (PAMPS) such as flagellin and chitin [60] They showed that flagellin, known to induce monophasic Ca2+ variations in cultured cells when Ca2+ changes are monitored with aequorin, was able to induce Ca2+ variations in roots. They localized these variations to the elongation zone where callose deposition takes place and immune-responsive genes are upregulated supporting a clear correlation between Ca2+ changes and defence responses [60] Overall, these few examples highlight the complexity of Ca2+ signalling and illustrate how monitoring Ca2+ signals at the cell tissue or whole plant level and the use of alternative Ca2+ probes from the available panel can be very helpful to decipher both immune and symbiotic responses. The importance of Ca2+ signalling in symbiosis has been recently reviewed, as well as the function of CBLs, which are mainly related to the regulation of membrane proteins involved in plant development, nutrition and abiotic stress [65]
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