Abstract
In Arabidopsis, the salt overly sensitive (SOS) pathway, consisting of calcineurin B-like protein 4 (CBL4/SOS3), CBL-interacting protein kinase 24 (CIPK24/SOS2) and SOS1, has been well defined as a crucial mechanism to control cellular ion homoeostasis by extruding Na+ to the extracellular space, thus conferring salt tolerance in plants. CBL10 also plays a critical role in salt tolerance possibly by the activation of Na+ compartmentation into the vacuole. However, the functional relationship of the SOS and CBL10-regulated processes remains unclear. Here, we analyzed the genetic interaction between CBL4 and CBL10 and found that the cbl4 cbl10 double mutant was dramatically more sensitive to salt as compared to the cbl4 and cbl10 single mutants, suggesting that CBL4 and CBL10 each directs a different salt-tolerance pathway. Furthermore, the cbl4 cbl10 and cipk24 cbl10 double mutants were more sensitive than the cipk24 single mutant, suggesting that CBL10 directs a process involving CIPK24 and other partners different from the SOS pathway. Although the cbl4 cbl10, cipk24 cbl10, and sos1 cbl10 double mutants showed comparable salt-sensitive phenotype to sos1 at the whole plant level, they all accumulated much lower Na+ as compared to sos1 under high salt conditions, suggesting that CBL10 regulates additional unknown transport processes that play distinct roles from the SOS1 in Na+ homeostasis.
Highlights
Soil salinity imposes ion toxicity, hyperosmotic stress, and secondary stresses such as oxidative damage and nutritional disorders on plants [1]
To investigate the functional interaction between CBL10 and three components of the salt overly sensitive (SOS) pathway (SOS1, CIPK24 and CBL4), a series of double mutants were created by genetic crossing and their genotypes were confirmed by reverse transcription-polymerase chain reaction (RT-PCR) (Figure S1)
To test the salt sensitivity of the cbl4 cbl10 mutant, five-day-old mutant and wild-type plants were transferred to the Murashige and Skoog (MS) medium with a series of NaCl concentrations
Summary
Soil salinity imposes ion toxicity, hyperosmotic stress, and secondary stresses such as oxidative damage and nutritional disorders on plants [1]. Plants have evolved several mechanisms to respond to the harsh environment and adjust their growth under high salt conditions [2]. One critical mechanism involves calcium elevation and calcium-dependent signaling pathways in plant cells [3]. The SOS (salt overly sensitive) pathway represents a calcium-dependent signaling pathway responsible for Na+ homeostasis and salt tolerance in Arabidopsis [4]. The pathway starts from CBL4 (calcineurin B-like 4), a calcium sensor protein that is supposed to respond to the specific Ca2+ signals triggered by excess Na+ [5]. The CBL4 protein interacts with a serine/threonine protein kinase (CIPK24) that activates the SOS1, a Na+/H+ antiporter, leading to the Na+ efflux from the cytosol [6,7,8]. CBL4 plays a critical role in the development of lateral roots through the modulation of auxin gradients and maxima in roots under mild salt conditions [14,15]
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