PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 63

et al. 2003 ). The leucine-rich repeat and the kinase domains of ERECTA were
specifically required for resistance to P . cucumerina , as er mutant alleles
impaired in any of these domains showed enhanced susceptibility to this fun-
gus (Llorente et al. 2005 ).

2.12.9 AtPHOS32, AtPHOS34, and AtPHOS43 Proteins

AtPHOS32, AtPHOS34, and AtPHOS43 are the other signaling components in
PAMP-triggered immunity. These proteins were shown to be rapidly phosphory-
lated upon fl g22 or chitin treatment (Peck et al. 2001 ; Merkouropoulos et al. 2008 ).
AtPHOS43 and related proteins in tomato and rice are phosphorylated within min-
utes after treatment with fl agellin or chitin fragments. Phosphorylation of
AtPHOS43 after fl agellin treatment was dependent on FLS2 (Peck et al. 2001 ).
AtPHOS32 and AtPHOS34 show similarity to bacterial universal stress protein
A.AtPHOS32 has been shown to be a substrate of the mitogen-activated kinases
MPK3 and MPK6 (Merkouropoulos et al. 2008 ). The target phosphorylation site in
AtPHOS32 is conserved in AtPHOS34 and among orthologues from many plant
species (Merkouropoulos et al. 2008 ).

2.12.10 BIR1

Upon recognition of the PAMP fl g22, the PRR FLS2 heterodimerizes with
BAK1 and activates the plant immune responses. Because constitutive activa-
tion of defense responses is detrimental, plant defense signaling pathways must
be negatively controlled (Gao et al. 2009a ). BAK1 forms a complex with BIR1
(for B RANCHING I NHIBITING R ECEPTOR 1 ) to negatively regulate defense
responses (Tang et al. 2008 ; Gao et al. 2009a ).
BIR1 is a BAK-1 interacting receptor-like kinase. Knocking out BIR1 leads to
activation of constitutive defense responses. A mutant, which suppresses the
activity of BIR1 has been obtained. The gene SOBIR1 (suppressor of bir1 )
encodes another receptor-like kinase whose over expression activates defense
responses (Gao et al. 2009a ). SOBIR1 functions as a specifi c regulator of resis-
tance activated by bir1 mutation. SOBIR1 is not required for fl g22-mediated
defense responses. SOBIR1 and BIR1 did not interact with each other. SOBIR1
has been shown to be a positive regulator of innate immunity and it activates
plant immune responses (Gao et al. 2009a ). Transgenic plants over expressing
SOBIR1 showed enhanced expression of both PR-1 and PR-2 genes. BIR1
appears to negatively regulate SOBIR1 signaling pathways in Arabidopsis (Gao
et al. 2009a ).