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ACA2 through phosphorylation at position Ser 45 in the N-terminal regulatory
domain. Thus there may be a crosstalk between two different Ca 2+ signaling path-
ways (calmodulin and CDPKs), providing a mechanism to control Ca 2+ effl ux
through opposing inhibitory and stimulatory activities. Factors that shift this balance
may alter the rate of Ca 2+ effl ux, and thereby alter the magnitude or duration of a
Ca 2+ signal (Hwang et al. 2000b ).
The Arabidopsis Ca 2+ -ATPases ACS8 and ACS10 have been shown to form a
complex with the PRR FLS2 in planta. The mutant aca8 and aca10 plants showed
decreases in the PAMP fl g22-induced Ca 2+ transient increase in cytosol (Frey et al.
2012 ), suggesting that Ca 2+ -ATPases modulate the Ca 2+ signaling.
Ca 2+ -ATPases have been shown to be involved in plant innate immune responses.
Programmed cell death (PCD) initiated at the infection sites is a defense response
against pathogens (Zhu et al. 2010 ). The ER-localized type IIb Ca 2+ -ATPase of
Nicotiana benthamiana ( NbCA1 ) has been shown to function as a regulator of PCD.
Silencing of NbCA1 accelerated the PAMP cryptogein-induced cell death.
Downregulation of NbCA1 resulted in the modulation of intracellular calcium sig-
naling in response to the PAMP cryptogein treatment (Zhu et al. 2010 ). The results
indicate that ER-Ca 2+ -ATPase is a component of the calcium effl ux pathway that
controls PCD in an innate immune response. Disruption of the vacuolar calcium-
ATPases in Arabidopsis results in the activation of salicylic acid signaling pathway
involved in innate immune responses, suggesting the role of Ca 2+ -ATPases in innate
immunity (Boursiac et al. 2010 ).
4.6
Plasma Membrane H + -ATPases in Ca 2+ Signaling
H + -ATPases belong to the large P-type ATPases superfamily. The plasma membrane
H + -ATPases generate an H + -gradient across the plant plasma membrane. The proton
gradient energizes many important transport systems in plants. The proton gradient
also creates an electrical potential, which is used to drive cation uptake through ion
channels. The plasma membrane H + -ATPase is an H + pump and is electrogenic, i.e.
it can establish a membrane potential: positive on the outside, negative on the inside.
It establishes the pH gradient (acidic on the outside) and creates the plasma mem-
brane potential (typically −100 to −200 mV; negative on the inside) (Palmgren
1998 ; Palmgren and Harper 1999 ; Elmore and Coaker 2011 ). H + -ATPases use
energy from ATP hydrolysis to pump protons from the cytosol to the extracellular
space (Sondergaard et al. 2004 ). Activation or inhibition of the H + -ATPase modu-
lates membrane potential (Ward et al. 2009 ). Changes in membrane potential alter
the activities of voltage-gated channels and control ion fl ux at the plasma membrane
(Haruta et al. 2010 ; Elmore and Coaker 2011 ).
Plant H + -ATPases become activated as the C-terminal domain is degraded by
controlled proteolysis. C-terminus of the ATPase of these pumps may be a regula-
tory domain inhibiting pump activity (Palmgren et al. 1991 ). It has been suggested
that the C-terminus of the ATPase might block the active site, and that following
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