Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

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Role of protein kinase in gene-for-gene interaction
A remarkable difference between resistant and susceptible plants is the time
needed to recognize pathogen invasion and activate defence responses. The
ability to quickly induce defence mechanisms is a characteristic of
incompatible (resistant) plant-pathogen interactions. Resistant plants are
equipped with a molecular alert system, which allows them not only to
recognize pathogen intrusion, but also to amplify very efficiently the initial
alarm signal and to activate self-defence. In recent years it has become evident
that reversible protein phosphorylation plays a cardinal role in transducing
signals leading to disease resistance in plants. Protein kinase and phosphatase
in plants, as well as in animals, are implicated as key components in the
signalling mechanism critical for responses to environmental stresses and
attack by pathogens (Sessa and Martin, 2000).
Protein kinase and resistance to rusts
In plants, several types of receptor-like kinases have been isolated and
characterized based on the sequence of their extracellular domains, and some
have been demonstrated to be involved in plant development or in the reaction
to environmental signals. An RLK gene family in wheat (wlrk, wheat leaf rust
kinase) with a new type of extracellular domain has been described. A member
of this new gene family has previously been shown to co-segregate with the
leaf rust resistance gene Lr10. The diversity of the wlrk gene family was studied
by cloning the extracellular domain of different members of the family.
Sequence comparisons demonstrated that the extracellular domain consists of
three very conserved regions interrupted by three variable regions. Linkage
analysis indicated that the wlrk genes are specifically located on chromosome
group 1 in wheat and on the corresponding chromosomes of other members of
the Triticeae family. The wlrk genes are constitutively expressed in the aerial
parts of the plant, while no expression was detected in roots. Protein
immunoblots demonstrated that the WLRK protein coded by the Lrk10 gene is
an intrinsic plasma membrane protein. This is consistent with the hypothesis
that WLRK proteins are receptor protein kinases localized to the cell surface. In
addition, preliminary evidence is present that other disease resistance loci in
wheat contain genes that are related to WLRK (Feuillet et al., 1998). The
product of WLRK, which is a receptor-like protein kinase, recognizes the
invasion of specific leaf rust races, and starts its function as a protein kinase
and cause phosphorylation of downstream kinases such as MAPK (mitogenactivated
protein kinase) involved in signal transduction.
Signal transduction pathways link pathogen perception with the final
execution of defence action. Signalling mechanisms that mediate plant defence
response may be strongly conserved among different species of plants, and bypassing
the specific recognition events by permanently activating components
of key signalling pathways has the potential to enhance disease resistance in
general (Xing and Jordan, 2000). In other words, if we identify one of the key