Salt Tolerance Gene Mapping Of Wild Soybean(Glycine Soja. Sieb. And Zucc) And Effect Of GmSALT3 On Soybean Yield

Soil salinization is one of major abiotic stress that threatening agricultural production in the world. Studies on salt tolerance gene（s） from germplasm resources will be useful on both theory and practice for developing soybean cultivars with high salt tolerance. In this study, we used two F2:3 populations separately derived from the cross of a salt-tolerant wild soybean NY36-87 and two salt-sensitive cultivated soybean Zhonghuang 39 and Zhonghuang 35 to map the salt tolerance gene of wild soybean. In our previous study, a dominant gene, Gm SALT3, responsible for salt tolerance was cloned from a salt-tolerant variety Tiefeng 8. To clarify the effect of Gm SALT3 on soybean growth, and to determine whether it had equivalent salinity tolerance in different genetic backgrounds, we developed three sets of near isogenic lines（NIL） derived from progenies of 85-140×Tiefeng 8 using marker-assisted selection for the target allele Gm SALT3 or Gmsalt3. These three sets of NIL were used to evaluate the accumulation of Na+, K+ and Cl-, the salt-tolerancec at germination stage and the yield. The results are as follows: 1. Mapping of salt tolerance gene in wild soybeanTwo F2:3 segregated populations were developed from the cross of a salt-tolerant wild soybean NY36-87 and 2 salt-sensitive cultivated soybean Zhonghuang 39 and Zhonghuang 35. An Zhonghuang 39×NY36-87 F2:3 family with 182 lines was screened for salt tolerance identification, the salt tolerance phenotype segregated with a ratio of 1 : 2 : 1（tolerant : heterozygous : sensitive）, which meaned the salt tolerance was controlled by a single dominat gene. Markers linked with the salt-tolerant gene were found on chromosome 3 by using bulked segregant analysis（BSA）. Recombinants were screened from other 467 individuals of the same F2 population. According to the phenotype and genotype of these recombinants, the salt tolerance gene was further mapped in a 98-kb region between ssr₃₁307 and ssr₃₁313, which including the salt tolerance locus of Gm SALT3. We map the salt tolerant gene in the same region using F2:3 population derived from Zhonghuang 35×NY36-87, which indicated that the salt tolerance gene in NY36-87 might be Gm SALT3. 2. Gm SALT3 modulates shoot Na+ and Cl- in soybean under salt stressA SSR marker ssr₃₁310, closely related to the salt tolerance gene was used to screen heterozygous individuals from the F4 to F6 generation of a cross 85-140×Tiefeng 8. To compare the function of Gm SALT3/Gmsalt3 alleles, three sets of NIL（782-T/782-S, 820-T/820-S, 860-T/860-S） carried salt-tolerant（Gm SALT3） or salt-sensitive（Gmsalt3） allelle were selected at F7 generation. The similarity of genetic background between NIL-T and NIL-S of each pair of NIL was about 95.6%-99.3% according to the result of 147 polymorphic markers between Tiefeng 8 and 85-140. Under Na Cl stress, three NIL-T lines showed significantly stronger salt tolerance than three NIL-S lines. Leaf and stem Na+ content in NIL-T lines were significantly lower than their corresponding NIL-S lines. No significant differences in leaf K+ content were observed in each pair of NIL. Shoot Na+/K+ ratio in NIL-T lines was significantly lower than their corresponding NIL-S lines. This indicated that shoot Na+/K+ balance was mostly dependent on shoot Na+ accumulation. Shoot Na+ content in 782-T was higher than that in 820-T and 860-T, indicated that the regulation of shoot Na+ accumulation by Gm SALT3 was affected by the genetic background. In addition, a detailed analysis of NIL-820 and NIL-860 was made to study how Gm SALT3 affected ion accumulation. Results showed that Na+ accumulation occurred later than Cl– accumulation in the aerial tissues of soybean. Accumulation of Cl– was consistently significantly greater in the leaves and stems of both NIL-S compared to their respective NIL-T after three days of salt treatment, whereas this was only the case for Na+ after five days. These results indicate that Gm SALT3 modulates shoot Na+ and Cl- accumulation in different ways. 3. Gm SALT3 does not increase salt tolerance at germination stageWe compared the relative germination rate, early vigour and fresh weight of three sets of NIL at 100 mmol L-1 and 200 mmol L-1 Na Cl stress, and no significant difference was observed in each pair of NIL-T and NIL-S. No difference was observed in each pair of NIL-T and NIL-S on salt tolerance at germination stage. Under 200 mmol L-1 Na Cl stress, NIL-820 had significantly higher relative germination rate than NIL-782 and NIL-860, indicated NIL-820 may have salt-tolerance gene（s） at germination stage. 4. Gm SALT3 has no penalty to the yield under normal condition and confers a yield advantage to soybean plants on saline fieldsIn pot assays, there was no significant difference of pod number and seed number between NIL-T and NIL-S under control condition. While significantly lower seed number and pod number was observed for NIL-S plants when compared with their corresponding NIL-T after plants were treated with 100 mmol L-1 Na Cl solution. In standard field condition, NIL-T lines had similar plant height, pod number, seed number, seed weight of per plant and 100-seed weight with NIL-S lines; when plants were grown on saline field, seed weight of per plant and 100-seed weight of NIL-T lines were higher than their corresponding NIL-S lines. Thus, the Gm SALT3 has no penalty to the yield under normal soil condition and contributes to developing the yield of NIL-T lines under saline stress.