References

7 Signal Perception and Transduction in Plant Innate Immunity 103
(Eulgem et al. 1999; Kroj et al. 2003). Moreover, Asai et al. (2002) identified
a flagellin-induced MAPK cascade and WRKY transcription factors that act
downstream of FLS2, and described a role of MAPK in activating early defense
gene transcription likely through WRKY transcription factor activity.
Although transcription factors are likely to be phosphorylation substrates
of plant MAPKs, the only proteins known to be directly phosphorylated
by AtMPK6 are two isoforms of 1-aminocyclopropane-1-carboxylic acid
synthase (ACS), the rate-limiting enzyme of ethylene biosynthesis (Liu and
Zhang 2004). MPK6-catalyzed phosphorylation of ACS2 and ACS6 results
in accumulation of ACS protein, elevated levels of cellular ACS activity,
ethylene production and ethylene-induced plant defense phenotypes. Further
functional links between MAPK activation and the plant immune
response, that is PR gene expression and the initiation of programmed cell
death, were established in Arabidopsis and tobacco, respectively (Jonak et
al. 2002; Menke et al. 2004; Nürnberger et al. 2004; Ren et al. 2002; Zhang
and Klessig 2001). For example, one study provided evidence that species
immunity in Nicotiana benthamiana was crucially dependent on NbSIPK
andNbWIPK(orthologoustoAtMPK6andAtMPK3).Virus-inducedgene
silencing of either of the two isoforms resulted in significant reduction of
transcript levels that allowed massive growth of P. cichorii,whichwouldnot
multiply on wild-type N. benthamiana. Intriguingly, silencing of the heatshock
proteins HSP70 and HSP90 in the same system also compromised
nonhost resistance to P. cichorii, suggesting a crucial role of chaperone
activity in this particular type of plant resistance (Kanzaki et al. 2003). As
NbHSP90 was shown to interact with NbSIPK in a yeast two-hybrid system,
it is conceivable that chaperones may constitute scaffolds that stabilize
plant MAPK cascades. A requirement for chaperone activity (a particular
HSP90 isoform of Arabidopsis) has recently been demonstrated also for
R-gene-dependent resistance (Takahashi et al. 2003).
Forward and reverse genetics approaches have contributed substantially
to our current understanding of the molecular requirements of plant immunity.
The analysis of mutants impaired in hormone homeostasis revealed
that jasmonic acid, ethylene and salicylic acid are not only crucial
to cultivar-specific host plant resistance, but may also be indispensable for
the maintenance of nonhost resistance in specific plant–microbe combinations
(Mysore and Ryu 2004). NPR1, a protein that modulates expression
of PR genes in a salicylic acid dependent manner, was recently shown to
be regulated by its redox status which facilitated complex formation with
transcription factors in the nucleus and subsequent binding to PR gene
promoters and PR gene expression (Mou et al. 2003). EDS1 and PAD4
are lipase-like proteins that were previously shown to be important for
cultivar-specific host resistance governed by TIR-NBS-LRR genes, while
NDR1, a protein of unknown function is essential for cultivar-specific host