Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 356

In contrast to many members of the MAPK family, plant MKPs form only a
small gene family. Only fi ve MKPs have been reported in Arabidopsis (Kerk et al.
2002 ). This disproportionate ratio of MAPK to MKP suggests that one MKP regu-
lates multiple MAPKs in plants. The Arabidopsis MAPK phosphatase AtMKP1
specifi cally interacts with AtMPK3, AtMPK4, and AtMPK6 (Ulm et al. 2002 ). The
tobacco MKP NtMKP1 inactivates salicylic acid-induced protein kinase (SIPK)
through dephosphorylation of the TEY motif of SIPK (Katou et al. 2005 ). The phos-
phatase activity of NtMKP1 was increased strongly by the binding of SIPK and
only weakly by another MAPK, WIPK, revealing the specifi city of NtMKP1 (Katou
et al. 2005 ).
MKPs have been shown to negatively regulate JA and ET signaling systems in
Arabidopsis thaliana (Schweighofer et al. 2007 ). An A. thaliana Ser/Thr phospha-
tase of type 2C, AP2C1, inactivates the MAPKs MPK4 and MPK6. Mutant ap2c1
plants produce signifi cantly higher amounts of JA in response to external stimulus.
Plants with increased AP2C1 levels display lower activation of MAPKs, reduced
ethylene production, and compromised innate immunity against Botrytis cinerea .
These results suggest that the phosphatase negatively regulates the MAPK pathway
and ET and JA signaling system (Schweighofer et al. 2007 ).

7.19 MAP Kinase Cascades Modulate Phosphorylation

of Transcription Factors to Trigger Transcription
of Defense Genes

MAP kinase cascades have been shown to be involved in phosphorylation of tran-
scription factors, which are involved in transcription of defense genes activated
through SA- or JA-, or ET- dependent signaling systems. BWMK1 (Blast- and
wounding- activated MAPK 1), a rice mitogen-activated protein kinase is targeted
to the nucleus. This protein phosphorylates the rice transcription factor OsEREBP1
( O ryza s ativa e thylene- r esponsive e lement- b inding p rotein 1 ). EREBPs are known
to bind to the GCC box DNA motif (AGCCGCC) that is located in the promoter of
several PR genes. In vitro phosphorylation of OsEREBP1 by BWMK1 enhanced its
ability to bind to the GCC box. Ectopic expression of the BWMK1 in tobacco plant
induced the expression of a broad spectrum of PR genes (Cheong et al. 2003 ).
Ethylene response factor6 (ERF6) is another substrate of MPK3/MPK6 and it
regulates Arabidopsis defense gene expression and resistance against Botrytis
cinerea. Phosphorylation of ERF6 by MPK3/MPK6 in either the gain-of-function
transgenic plants or in response to B. cinerea infection increases ERF6 protein sta-
bility in vivo (Meng et al. 2013 ). Chitin elicitors induced the kinase activity of two
MAPK genes, AtMPK6 and AtMPK3 in Arabidopsis (Wan et al. 2004 ). In addition,
WRKY22 , WRKY29 , WRKY33 , and WRKY53 , which encode four WRKY transcrip-
tion factors that recognize TTGAC(C/T) W-box elements in promoters of several
defense-related genes, were up-regulated by the elicitor treatment (Wan et al. 2004 ).
WRKY33 is a pathogen-inducible transcription factor, whose expression is