Calcium-Independent Activation of Salicylic Acid-Induced Protein Kinase and a 40-Kilodalton Protein Kinase by Hyperosmotic Stress

Abstract

Reversible protein phosphorylation/dephosphorylation plays important roles in signaling the plant adaptive responses to salinity/ drought stresses. Two protein kinases with molecular masses of 48 and 40 kD are activated in tobacco cells exposed to NaCl. The 48-kD protein kinase was identified as SIPK (salicylic acid-induced protein kinase), a member of the tobacco MAPK (mitogen-activated protein kinase) family that is activated by various other stress stimuli. The activation of the 40-kD protein kinase is rapid and dose-dependent. Other osmolytes such as Pro and sorbitol activate these two kinases with similar kinetics. The activation of 40-kD protein kinase is specific for hyperosmotic stress, as hypotonic stress does not activate it. Therefore, this 40-kD kinase was named HOSAK (high osmotic stress-activated kinase). HOSAK is a Ca 2؉ -independent kinase and uses myelin basic protein (MBP) and histone equally well as substrates. The kinase inhibitor K252a rapidly activates HOSAK in tobacco cells, implicating a dephosphorylation mechanism for HOSAK activation. Activation of both SIPK and HOSAK by high osmotic stress is Ca 2؉ and abscisic acid (ABA) independent. Furthermore, mutation in SOS3 locus does not affect the activation of either kinase in Arabidopsis seedlings. These results suggest that SIPK and 40-kD HOSAK are two new components in a Ca 2؉ -and ABA-independent pathway that may lead to plant adaptation to hyperosmotic stress.

SALT AND DROUGHT STRESS SIGNAL TRANSDUCTION IN PLANTS

“…In addition to hyperosmotic stress, hypoosmotic stress, salicylic acid, or fungal elicitors also could activate the tobacco MAP kinase (13). An Arabidopsis protein crossreacting with antibodies against the tobacco MAP kinase was found to be activated by hyperosmotic stress (29). A MAP kinase kinase that interacts with and activates the alfalfa SIMK was reported (41).…”

Osmotic Signaling in Plants. Multiple Pathways Mediated by Emerging Kinase Families

“…In tobacco, a rapid and transient activation of the MAPK salicylic acidinduced protein kinase (SIPK) was demonstrated in response to hyperosmolarity induced by several osmolytes (Droillard et al, 2000;Hoyos and Zhang, 2000;Mikolajczyk et al, 2000). The upstream MAPKK could be NtMEK2, which was shown to phosphorylate and activate SIPK in vitro, as well as in vivo, in a tobacco leaf transient expression assay (Yang et al, 2001).…”

Nitrate Efflux Is an Essential Component of the Cryptogein Signaling Pathway Leading to Defense Responses and Hypersensitive Cell Death in Tobacco

“…Consistent with this hypothesis, kinases have been shown to be activated in response to osmotic shock. In particular, Hoyos and Zhang (2000) recently proposed that the cryptogein-induced 40-kD kinase is likely to be the same kinase as HOSAK (high osmotic stress-activated kinase), a tobacco kinase activated by hyperosmotic stress caused by the exposure of cells to high concentrations of NaCl, Pro, or sorbitol. A tobacco 40-to 42-kD kinase showing identical characteristics (substrate preference and hyperosmotic stress sensitivity) and identified as a Ser/Thr kinase also was described by Mikolajczyk et al (2000).…”