Page 7 - Legume Genomics and Genetics

Legume Genomics and Genetics (online), 2010, Vol. 1, No.8, 41-46
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44
RSC -14 and RSC-11 from Qihuang No.1 were also previ-
ously mapped on linkage group F between Satt334
and Sct_033 by Li et al. (2006) and Bai et al. (2009),
respectively. The resistance to SMV in Qihuang22 and
Qihuang No.1 may be controlled by a same gene due
to the latter as one of the parents of the former. And
there also was the report that the region densely covered
with R genes was closely linked to Satt334 and
Sct_033 (Liu et al., 2000, Progress in Natural Science,
10(11): 1012-1017). The truth that whether the resis-
tance to the 3 different SMV strains was controlled
only by the same gene or by 3 different linked genes
still need to be further explored.
The ideal markers used in MAS are based on PCR by
considering experimental cost and technical feasibility.
Zheng et al. (1997) considered that the available gen-
etic distance between markers used in MAS and the
target gene should be less than 5.0 cM. In the previous
reports, most of the markers used to locate the resis-
tance genes to SMV were RAPD and RFLP markers
(Yu et al., 1994; Jeong et al., 2003; Hayes et al., 2000;
Zhang et al., 1996; Zheng et al., 2001; Wang et al.,
2004), which were applied limited in breeding program
for their poor repeatability, complicated operation and
high labor and time consumed. The co-dominant SSR
markers Satt334 and Sct_033 identified in this research,
closely linked to the resistance gene to SMV, can be
used to screen the homozygous resistant plant in early
generations by convenient experimental operation.
Furthermore, the markers used in MAS showed high
efficiency, and the efficiency for the resistance plant
selection was more than 94% when one marker beside
RSC-12 was used and 100% when the two markers were
co-used. Therefore, it is feasible for the two SSR
markers to be used as a tool in SMV resistance breed-
ing program.
In the recent years, breeders try their best to integrate
several different resistance genes into a same elite
variety, so as to improve resistance, broaden resistance
spectrum, and prolong its service life in agricultural
production. There is no chance to inoculate the same
plant with different SMV strains simultaneously by
conventional pathogen inoculation method, which is
inconvenient to screen and identify the individual
plant possessing multi-resistance from hybrid proge-
nies. Fortunately, the molecular markers, especially
SSR markers, closely linked to the target gene can be
used in MAS to pyramid multi genes into a same
variety quickly and accurately. The markers closely
linked to the resistance gene to SMV identified in this
research provide the basic information in pyramiding
the resistance genes in soybean.
3 Materials and Methods
3.1 materials
The cultivar Qihuang22, obtained from multiple cross
of Yishuipingdinghuang, Qihuang No.1, Juxuan23 and
Nongza9- 3 (Xu et al., 2004), is resistant to SMV strain
SC-12, and is also multi resistant to some other
different SMV strains (Shang et al., 1999; Li et al.,
2006; Wang et al., 2003). F1, F 2, F2:3, F3, F4 were all
obtained from the cross of Qihuang22 (R)×Nanno-
ng1138-2 (S).
The SMV strain SC -12 is a prevalent low virulent
strain distributed in Northern China Spring Planting
Soybean Region, Middle and Lower Huang-Huai and
Changjiang Valleys (Guo et al., 2005; Wang et al.,
2005), and whose reaction on differential hosts was
similar with No.2 strain groups in northeast classified
by Lv et al.(1985).
3.2 Inoculation identification and resistance eva-
luation
The parents and their offspring were planted in plastic
pots in an aphid-free greenhouse. All the young plants
in pots were inoculated with the inoculum containing
SC-12 by gently rubbing the new leaves when the
primary leaves unfolded, and once more when the first
trifoliate leaves expanded. The observations were taken
1- week after the first inoculation, and then disease
reactions to SC- 12 were evaluated at 3-days interval
for 3 weeks. At the same time the plants with symp-
toms were marked to avoid disturbance from latency of
symptoms. The pesticide were sprayed on time to avoid
the cross infection by aphid.
Any plant with mosaic symptoms on leaves above
those inoculated was regarded as susceptible. Then,
the number of plants with different symptomatic
reaction was calculated, respectively. Chi-square tests
were performed to determine the goodness-of-fit of
observed segregation ratios in F2 and F2:3.
3.3 Preparation of the resistant bulk and susceptible
bulk
Soybean genomic DNA was extracted from sampled