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The transient elevation of cytosolic free calcium concentration ([Ca2+]cyt) induced by cold stress is a well‐established phenomenon; however, the underlying mechanism remains elusive. Here, we report that the Ca2+‐permeable transporter ANNEXIN1 (AtANN1) mediates cold‐triggered Ca2+ influx and freezing tolerance in Arabidopsis thaliana. The loss of function of AtANN1 substantially impaired freezing tolerance, reducing the cold‐induced [Ca2+]cyt increase and upregulation of the cold‐responsive CBF and COR genes. Further analysis showed that the OST1/SnRK2.6 kinase interacted with and phosphorylated AtANN1, which consequently enhanced its Ca2+ transport activity, thereby potentiating Ca2+ signaling. Consistent with these results and freezing sensitivity of ost1 mutants, the cold‐induced [Ca2+]cyt elevation in the ost1‐3 mutant was reduced. Genetic analysis indicated that AtANN1 acts downstream of OST1 in responses to cold stress. Our data thus uncover a cascade linking OST1‐AtANN1 to cold‐induced Ca2+ signal generation, which activates the cold response and consequently enhances freezing tolerance in Arabidopsis. Synopsis: How cold stress generates and controls transient elevation of calcium in the cytosol of plant cells is ill‐defined. Cold‐activated OST1 kinase promotes freezing tolerance in Arabidopsis by potentiating ANNEXIN1‐dependent calcium signaling. ANNEXIN1 (AtANN1) mediates the generation of cold‐triggered Ca2+ signals.Cold‐activated OST1 kinase phosphorylates AtANN1 to enhance its Ca2+ transport activity, thereby amplifying Ca2+ signaling.OST1‐AtANN1 module regulates the expression of cold responsive CBF and COR genes and plant freezing tolerance. [ABSTRACT FROM AUTHOR]

Abstract: The Arabidopsis thaliana RNA binding protein UBA2a is the closest homologue of the Vicia faba AKIP1 (56% identity). Like AKIP1, UBA2a is a constitutively-expressed nuclear protein and in response to ABA it is also reorganized within the nucleus in “speckles” suggesting a possible role of this protein in the regulation of mRNA metabolism during ABA signaling. AKIP1 interacts with, and is phosphorylated by, the upstream ABA-activated protein kinase AAPK. We have investigated if a pathway similar to that described in Vicia faba also exists in Arabidopsis. Our results showed that despite the resemblance between the corresponding Vicia and Arabidopsis proteins, it appears that the function of UBA2a is independent of OST1 phosphorylation. [Copyright &y& Elsevier]

OST1 (open stomata 1) protein kinase plays a central role in regulating freezing tolerance in Arabidopsis; however, the mechanism underlying cold activation of OST1 remains unknown. Here, we report that a plasma membrane‐localized clade‐E growth‐regulating 2 (EGR2) phosphatase interacts with OST1 and inhibits OST1 activity under normal conditions. EGR2 is N‐myristoylated by N‐myristoyltransferase NMT1 at 22°C, which is important for its interaction with OST1. Moreover, myristoylation of EGR2 is required for its function in plant freezing tolerance. Under cold stress, the interaction of EGR2 and NMT1 is attenuated, leading to the suppression of EGR2 myristoylation in plants. Plant newly synthesized unmyristoylated EGR2 has decreased binding ability to OST1 and also interferes with the EGR2‐OST1 interaction under cold stress. Consequently, the EGR2‐mediated inhibition of OST1 activity is released. Consistently, mutations of EGRs cause plant tolerance to freezing, whereas overexpression of EGR2 exhibits decreased freezing tolerance. This study thus unravels a molecular mechanism underlying cold activation of OST1 by membrane‐localized EGR2 and suggests that a myristoyl switch on EGR2 helps plants to adapt to cold stress. Synopsis: OST1 kinase, the master freezing tolerance regulator in Arabidopsis, is inhibited at warm temperatures by interaction with the membrane‐localized phosphatase EGR2. This interaction depends on EGR2 N‐myristoylation, attenuation of which upon cold stress provides a mean of activating OST1 at low temperatures. Plant type 2C protein phosphatase EGR2 is myristoylated by N‐myristoyltransferase NMT1.EGR2 interacts with and inhibits OST1 kinase activity.NMT1‐mediated myristoylation of EGR2 is important for its interaction with OST1.Cold stress attenuates EGR2 myristoylation and decreases its interaction with OST1, thereby activating OST1. Attenuated myristoylation of PP2C family member EGR2 upon cold stress releases its inhibitory interaction with the master plant cold response factor OST1. [ABSTRACT FROM AUTHOR]