Basic HTML Version
Molecular Pathogens
MP2010, Vol.1, No.1
http://mp.sophiapublisher.com
Page 4 of 7
1.3 Phylogenetic relationships of the banana RGCs
The phylogenetic analysis for RGA nucleotide
sequences and the deduced amino acids revealed that
the 20 sequences could be divided into 5 distinct types
(Figure 2). All of The amino acids deduced from the
RGAs shared homology (28%~54%) with those
deduced from the known wilt resistance genes such as
Fom-2, I2C-1, I2C-2, and I2 which indicate the
conservation of disease resistance gene evolution.
Technically, these RGAs isolated in the present study
would lay a base for the further cloning of wilt
resistance genes in banana. And they could also be
used as molecular marker for screening of candidate
wilt resistance genes in banana.
2 Discussion
Resistance (R) genes in plants play a crucial role in
disease prevention. Many R genes are dominant, or
incompletely dominant, and require specific dominant
avirulence (Avr) genes in the pathogen for their
function (Flor, 1946). This genetic interaction between
plant and pathogen led to the current view that such R
genes encode receptors for Avr gene-dependent
pathogen mole-cules (Staskawicz et al., 1995). Upon
recognition of these molecules, R gene products
activate plant defense mechanisms. These defenses
include rapid production of an oxidative burst resulting
in cell wall cross-linking, localized cell death (the
hypersensitive response), salicylic acid biosynthesis,
and induction of genes characteristic of systemic
acquired resistance (Levine et al., 1994; Ward et al.,
1991). R genes have been shown to encode two broad
categories of leucine-rich-repeat (LRR) proteins that
can be distinguished by protein domain structure and
site of pathogen perception (Jones and Takemoto, 2004).
The NBS-LRR-containing R proteins mediate the
recognition of an intracellular pathogen-derived signal.
Thus far, NBS-LRR proteins have been shown to
function in resistance signaling only in response to
pathogen. The second category of R proteins is inserted
in the plasma membrane and minimally consists of an
extracellular LRR domain and a transmembrane (TM)
domain (Jones and Takemoto, 2004).
Many studies suggested that there were three main
relationships between RGA and resistance genes.
Figure 2 Phylogenetic tree of Musa RGAs and know plant R
genes based on NBS domain
First, RGA is part of R gene or pseudo gene. In
Arobidopsis,a sort of RGA was tightly linked with
Rpp5 and was proved to be part of Rpp5 (Kanazin et
al, 1996). Second, RGA was tightly linked with R
gene. For instance, RGA were found to be tightly
linked with different located R genes in rice (Nori et
al., 2001). Third, RGA is unrelated to target gene. For
example, some RGA did not link with located R-genes