Nematode Resistance in Soybean - Translational Genomics for Crop Breeding: Biotic Stress - page 100

formation of a unique structure called a feeding
cell or syncytium. A syncytium consists of hun-
dreds of fused and highly metabolically active
root cells on which the nematode feeds as it
develops into a third- (J3) and fourth-stage (J4)
juvenile. The life cycle of SCN depends on the
successful formation of the feeding cells. In the
roots, the juvenile continues the developmental
process to the adult male or female life stages,
which are vastly different in size and shape. The
reproduction of SCN is completed by obligate
amphimixis that requires an adult male to regain
its vermiform shape and motility, migrating out
of the plant's roots to fertilize an adult female
protruding as a pearly white spheroid from the
root's surface. The fertilized female can pro-
duce up to several hundred eggs. When the adult
female dies, her large dead body serves as a cyst
to protect the eggs from adverse environmental
conditions in the absence of a host until condi-
tions are favorable. More useful information of
this subject was also presented in comprehensive
reviews by Niblack et al. (2006) and Mitchum
and Baum (2008).
For its parasitic development, the nematode
is essentially dependent on the functions of par-
asitism proteins and plant host-SCN interac-
tion. The nematode uses a stylet to secret pro-
teins originating in its esophageal gland cells
into the host's roots, where a syncytium is ini-
tially formed and serves as a major nutrient
sink for the feeding nematode. Several studies
have been made using genomic and proteomic
approaches to identify SCN parasitism genes and
host-SCN interactions (Davis et al. 2004; Gao
et al. 2003; Ithal et al. 2007a, 2007b; Wang et al.
2001; Wang et al. 2005). It was reported that
stylet-secreted proteins are encoded by nema-
tode parasitism genes expressed in the nema-
tode's three esophageal gland cells (Davis et al.
2004). Sequencing and bioinformatic analysis of
the gland-enriched cDNA libraries led to the con-
firmation of proteins with predicted secretion
signal peptides and the identification of more
than 60 SCN parasitism gene candidates (Gao
et al. 2001, 2003; Wang et al. 2001). Among

these, a small subset of the parasitism proteins
was shown to be involved in cell wall modifi-
cation during infection, metabolic, and develop-
mental reprogramming of host cells and manip-
ulation of host defense mechanisms (Baum et al.
2007; Davis and Mitchum 2005). The roles and
functions of several other parasitism proteins
involved in metabolic reprogramming, molecu-
lar mimicry of host pathway components, aviru-
lence and virulence proteins, and so forth were
also reviewed in detail by others (Baum et al.
2007; Davis and Mitchum 2005; Mitchum and
Baum 2008).

GeneticVariationforVirulence

Virulence of parasitic nematodes is the ability to
evolve to either escape or overcome host resis-
tance. Many studies showed that there is exten-
sive genetic diversity among and within SCN
populations in the field (Niblack et al. 2002;
Niblack et al. 2006). Overuse of the same sources
of SCN resistance in soybean has consequently
resulted in genetic shifts of SCN populations
and the loss of resistance among soybean cul-
tivars. Young (1998) reported a synthetic nema-
tode population called LY1, originating from a
mass mating of races 2 and 3, which can repro-
duce on the broad-based resistant soybean cul-
tivar Hartwig, as well as its primary resistance
source, PI 437654. Results of a recent survey
showed that most SCN populations collected
from soybean fields in Missouri were virulent
and could reproduce on indicator lines, such as
PI 88788, PI 209332, PI 548316, and PI 548402,
which are typically used in SCN bioassays and
as resistance sources for soybean cultivar devel-
opment (Mitchum et al. 2007).
Following the gene-for-gene hypothesis for
plant-pathogen interaction, Golden et al. (1970)
initially proposed a “races” classification scheme
to describe SCN genetic variability and virulence
using four soybean differentials: Pickett, Peking,
PI 88788, and PI 90762, along with Lee, a
susceptible check. The number of females pro-
duced by H. glycines population on each soybean