Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 337

7.3.2 Arabidopsis MKK2 Differentially Induces Defense

Responses Against Different Pathogens

AtMKK2 overexpressing transgenic Arabidopsis thaliana plants showed enhanced
susceptibility to the fungal pathogen Alternaria brassicicola , whereas these plants
were more resistant to the bacterial pathogens Pseudomonas syringae pv. tomato
Dc3000 and Erwinia carotovora subsp. carotovora (Brader et al. 2007 ). To assess
the downstream events in the MAPK cascade, transgenic Arabidopsis plants over-
expressing the MAPKK MKK2 were developed (Teige et al. 2004 ). The MKK2
overexpressing plants showed altered expression of 152 genes involved in
transcriptional regulation, signal transduction, and defense responses (Teige et al.
2004 ). The activated genes included transcription factors such as WRKY tran-
scription factors, Myb DNA binding protein, and ethylene-responsive element
binding factor. The activated genes in the MKK2 overexpressing plants also
included several genes involved in signal transduction pathways. Several calmod-
ulin and calcium binding proteins were strongly upregulated in the MKK2 overex-
pressor lines. Genes involved in phosphorylation/dephosphorylation systems
were also activated in the MKK2 overexpressing plants. These included MAPKK5
gene and protein phosphatase 2C genes. The MKK2 overexpressor lines also
showed upregulation of several enzymes and targets of the ethylene and jasmo-
nate pathways. The increased expression included JA-mediated defense gene
PDF1.2a encoding defensin, ethylene biosynthesis gene AtACS-6 encoding ACC
synthase, and lipoxygenase gene involved in JA biosynthesis (Teige et al. 2004 ).
Downstream events include activation of calcium/calmodulin signaling system
(Teige et al. 2004 ), and JA-, SA-, ET-, and ABS- mediated signaling systems
(Takahashi et al. 2007a ). In Arabidopsis , the constitutively active AtMKK2
increases the expression levels of genes that encode enzymes in ET/JA signaling
system (Brader et al. 2007 ).

7.3.3 Arabidopsis MKK3 Positively Regulates Immune

Responses

MKK3 is an upstream activator of MPK1, MPK2, MPK7, and MPK14 (Dóczi
et al. 2007 ). Arabidopsis plants overexpressing AtMKK3 show increased expres-
sion of PR genes (Dóczi et al. 2007 ). These plants showed enhanced disease
resistance against Pseudomonas syringae pv. tomato (Dóczi et al. 2007 ). MKK3
has been shown to play important role in plant innate immunity (Dóczi et al.
2007 ). The growth of virulent bacterial pathogen P . syringae pv. tomato DC3000
was increased in mkk3 knockout plants and decreased in MKK3 -overexpressing
plants. MKK3 overexpression lines showed increased expression of several PR
genes (Dóczi et al. 2007 ). These results suggest that MKK3 positively regulate
plant immune responses.