Biology Reference
In-Depth Information
soybean seed, which is recognized for its edible
oil and protein, is the reason it has become a
major oilseed crop during World War II and con-
tinually increasing into the present. This crop
has been bred for adaptation to various lati-
tudes and is now grown from temperate to tropi-
cal regions. North and South America presently
account for 86% of worldwide soybean produc-
tion. Among those, the United States, Brazil, and
Argentina represent 81% of the total world sup-
ply (
http://www.soystats.com). So
ybean vegeta-
tive growth, reproduction, and maturity are day-
length dependent (Carlson and Lersten 2004);
therefore, soybean cultivars are assigned to 12
maturity groups (MG) varying from MGs 00
through X. Soybean MGs 00, 0, and I are adapted
to the longer days of southern Canada and the
northern United States. Succeeding MGs are
adapted to lower latitudes as summer days pro-
gressively get shorter. Groups IX and X are
adapted and grown under the shorter days of
the subtropics and tropics (Carlson and Lersten
2004).
There are many diseases and pests that can
infect and damage soybean plants, resulting in
economically significant yield losses for soy-
bean producers. One class of these pests is plant-
parasitic nematodes, which are unsegmented
roundworms that often cannot be seen by the
naked eye. These nematode species are found
in all soybean-growing areas (Niblack et al.
2004). The importance of each nematode species
depends on the soybean-growing areas to which
each species is most adapted. Among the nema-
tode species, soybean cyst nematode (SCN,
Het-
erodera glycines
Ichinohe) and root-knot nema-
tode (RKN,
Meloidogyne
spp.) cause the greatest
soybean yield losses and are the most impor-
tant worldwide. Because of their agricultural
importance, these two nematode species and
their soybean host have been intensively inves-
tigated, leading to the identification and molec-
ular characterization of new resistance sources,
which then led to the development of new soy-
bean varieties resistant to nematodes (Anand
1992; Boerma and Hussey 1992; Diers and Arelli
1999; Hartwig and Epps 1973a, 1973b). Another
plant-parasitic nematode species, called reni-
form nematode (RN,
Rotylenchulus reniformis
Linford and Oliveira), also causes yield losses
in soybean (Robbins et al. 1999). Efforts have
been made to evaluate soybean germplasm for
new sources of resistance to RN. However, com-
pared to SCN and RKN species, information
on RN species is limited and warrants further
investigation of genetics and genomics of plant
resistance.
These three parasitic nematode species differ
in terms of their modes of plant parasitism. For
example, unique structures called syncytia are
formed following an initial penetration of SCN
and RN through root tissue; in contrast, RKN
penetration induces a giant cell near the root
tips (Mitchum et al. 2012). Advances in molecu-
lar genetics and genomic methodologies of soy-
bean and nematodes have provided efficient tools
to identify and characterize transcription factors
and genes at work during plant parasitism. These
advances have also engendered a better under-
standing of the complex soybean-nematode phy-
tosystem. Efforts have been made to identify
the genes coding for nematodes' stylet secre-
tions and to elucidate the functionality of the
secreted proteins (Davis et al. 2004; Huang et al.
2003, 2004). Microarray gene expression profil-
ing using Affymetrix GeneChip and laser cap-
ture microdissection (LCM) have been widely
adapted to detect transcriptional changes that
occur during plant parasitism in thousands of
soybean and nematode genes (Ibranhim et al.
2011a; Ithal et al. 2007b; Klink et al. 2007b;
Puthoff et al. 2007). These findings provided
new insights into host plant responses to nema-
tode infection, allowing soybean scientists to
develop novel strategies to bioengineer crops
with robust resistance to phytopathogenic nema-
todes (Mitchum et al. 2012).
For decades, breeding for soybean varieties
resistant to parasitic nematodes has been shown
to be an effective and practical method to man-
age these pests. However, developing new resis-
tant cultivars has been challenging because of
Search WWH ::
Custom Search