15
Genes Involved in Pathogenesis and
Defense Responses
Tomomi Nakagawa, Shin Okazaki, and Naoto Shibuya
Abstract
The legume family includes important grain, forage, and agroforestry
species. One of the major constraints for cultivation of these legumes is
obviously production loss by disease (Graham and Vance in Plant Physiol
131(3):872–877, 2003 ). Thus,Lotus japonicusis utilized not only as a
symbiotic model but also as a research platform for studying serious
diseases in legumes. However, most of our knowledge about the defense
mechanism inL. japonicuscomes from the study of legume–rhizobia
symbiosis. In this section, we mainly focus on the regulation of defense
responses in host symbiotic process, the pathogenic aspect of symbiotic
microbial partners, and then illustrate theLotuspathogens.15.1 General Introduction of Plant
Defense Mechanism
In the natural environment, land plants are con-
stantly exposed to the threats of pathogenic
microbes. To defend themselves from the infec-
tion of these hostile microbes, plants have evolved
two layers of inducible defense mechanisms
(Jones and Dangl 2006 ). Thefirst layer of defense
mechanisms relies on the detection of microbe-
associated molecular patterns (MAMPs), which
are common components of various microbes but
not present in higher plants/animals, and activates
basal defense responses such as generation of
reactive oxygen species (ROS), induction of
defense genes, and production of antimicrobial
compounds such as phytoalexins. Rapid activation
of these basal defense responses is generally suf-
ficient to block microbial invasions. The common
and invariant nature of the MAMP molecules such
as fungal cell wall chitin and bacterialflagellin for
various microbes also makes MAMPs-triggered
immunity (MTI) effective for the prevention of
most pathogenic infections (Monaghan and Zipfel
2012 ; Segonzac and Zipfel 2011 ). Recent studies
revealed that microbial strategies for successful
infection often intend to avoid or perturb the plant
recognition for MAMPs (Dodds and Rathjen
2010 ). For example, many Gram-negative bacteria
inject a repertoire of“virulence effector”proteins
around or into the host cells through type III or IV
secretion system (T3SS or T4SS) and manipulate
host immunity to enhance their own pathogenicity
(Hueck 1998 ; Christie and Vogel 2000 ). To copeT. Nakagawa (&)S. OkazakiN. Shibuya
Department of Life Sciences, School of Agriculture,
Meiji University, 1-1-1 Higashi-Mita, Tama-Ku,
Kawasaki, Kanagawa 214-8571, Japan
e-mail: [email protected]
S. Tabata and J. Stougaard (eds.),The Lotus japonicus Genome, Compendium of Plant Genomes,
DOI 10.1007/978-3-662-44270-8_15,©Springer-Verlag Berlin Heidelberg 2014
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