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Abstract [Objectives] The identification of salt tolerant genetic loci in rice can provide a research basis for the molecular mechanism of salt tolerance and gene resources for improving salt tolerant cultivars.
[Methods] Recombinant inbred lines （RILs） derived from Zhaxima， an indica landrace variety from Yunnan Province and Nanjing 46， an elite japonica variety with superior grain quality from Jiangsu Province were used. The salt tolerance at seeding stage in the RIL population was investigated as the phenotypic value.
[Results] Combined with the linkage map， a total of 4 QTLs were detected： qSST1， qSST3， qSST5 and qSST11， located in rice chromosomes 1， 3， 5 and 11， respectively. All positive alleles were from the parent Nanjing 46. Three QTLs among them were not included in chromosome intervals the same as cloned rice salt tolerance genes， and thus were described as new candidate gene loci associated with seedingstage salt tolerance.
[Conclusions] This study provides important information for further exploration and utilization of new salt tolerant QTLs in rice. It is of great significance for improving the utilization of saline land in China and ensuring the stable rice production.
Key words Oryza sativa L.; Seeding stage; Salt tolerance; QTL
As one of the origin centers of cultivated rice in Asia， China has abundant types of rice seed resources[1]. The main part of Chinas rich rice seed resources is landrace rice. Landrace rice not only contain a large number of excellent genes such as high yield， high quality， disease resistance， and stress tolerance genes， but also have rich genetic diversity[2-4]， which are valuable resources in China. With the longterm utilization of core germplasms in rice breeding， the problems of single genetic basis of the bred varieties and poor diversity are becoming increasingly prominent[5-7]. In the future， the key to whether rice breeding can achieve a substantial breakthrough lies in the exploration and utilization of the genetic diversity of landrace rice. With the rapid growth of the worlds population and the continuous decline of arable land， stable yield has always been one of the traits most concerned by breeders. However， unreasonable irrigation and excessive fertilization have caused the global environment to continue to deteriorate and the area of salinealkali land to increase year by year， involving more than 30 countries around the world. Rice is a crop with moderate salt susceptibility， and soil salt concentration has a great impact on its yield and quality. Most of the rice grown in salinealkali land will suffer from a crop failure， and even total loss. Therefore， the exploration of new salttolerant genes from landrace rice varieties by methods such as genetic improvement and breeding of new salttolerant rice varieties is of important significance for improving the utilization of saline land and ensuring stable rice. As early as the 1960s， research has reported that the salt tolerance of rice is a quantitative trait which is controlled by polygenes， which is also affected by the external environment and development period[8-11]. The twoleaf and oneheart stage is the most sensitive period for salt tolerance in rice[12]. Many researchers have conducted QTL mapping using quantitative trait mapping populations and molecular marker linkage maps. The results showed that most salt tolerance QTLs were distributed on chromosomes 1， 2， 6， and 7. A small amount of salt tolerance QTLs were also detected on chromosomes 10 and 11. So far， only a few salt tolerance genes have been cloned. The salt tolerance gene SKC1 on chromosome 1 was first cloned. When rice is under high salt stress and the stalk accumulates a large amount of Na+， the cell membranelocalized Na+ transporter encoded by SKC1 can transport excess Na+ from the stalk back to the root system， thereby reducing the toxicity of Na+ to rice plants and increasing the tolerance of rice plants to salt stress[13]. DST is a new zinc finger transcription factor obtained through mapbased cloning by Huang et al. It is derived from salttolerant mutants and can enhance salt or drought tolerance in rice through negative regulation. The mutated DST reduces the expression of genes for hydrogen peroxide metabolism， which causes hydrogen peroxide to accumulate in guard cells， resulting in stomata closing， thereby reducing water evaporation， and plants exhibited salt or drought tolerance[14]. HST1 is a newly reported salttolerant gene that encodes a type B response regulator protein OsRR22， which is related to the expression of genes related to osmotic adjustment and ion transport. Compared with SKC1 gene， HST1 gene has stronger salt tolerance[15].
Although some progress has been made in the cloning of salt tolerance genes in rice， there are still few gene resources that have been truly utilized in breeding salttolerant varieties. Exploring and locating new QTLs for salt tolerance in rice and improving the utilizability of landrace rice have positive significance for improving the genetic diversity of existing varieties. In this study， recombinant inbred lines constructed from Yunnan indica rice variety Zaxima and Jiangsu highquality rice variety Nanjing 46 were used to analyze QTLs for salt tolerance at the seedling stage， so as to obtain new salt tolerance QTLs and provide a theoretical basis for salttolerant rice breeding. Materials and Methods
Experiment materials
The Zaxima/Nanjing 46 recombinant inbred lines （a total of 143 lines） were constructed using Yunnan indica rice variety Zahima and Jiangsu highquality rice variety Nanjing 46 as parents by single seed descend method （SSD）.
Salt stress treatment at seedling stage of rice
The salt tolerance in the seedlings of the 143 families and their parents Zahima and Nanjing 46 was identified using 0.5% NaCl. Petri dishes 9 cm in diameter were used for accelerating germination. First， 12 ml of dH2O was added into each petri dish， followed by incubation in the dark at 28 ℃ for 3 d. Seeds with substantially uniform bud lengths were placed on 96well plates with 8 seeds in each row and 5 rows for each material. The 96well plates were placed in a watercontaining turnover box which was placed in an illumination incubator. The incubator was set with a photoperiod of 14 L/8 D， at 28 ℃ during the day and 24 ℃ at night. When the seedlings reached the twoleaf and oneheart stage， 0.5% NaCl was added to the nutrient solution for salt stress treatment， and the nutrient solution was changed every 3 d. The duration of the salt stress treatment was 21 d， and after the salt stress treatment， the seedlings were cultured in normal medium condition for 10 d. The experiment was set up with three repetitions， each with 3 replicates， and the average seedling survival rate of the 3 repetitions was used as the statistical data.
Investigation of salt tolerance at seedling stage
The seedling survival rates of each line and the parents were investigated at 21 d of the treatment and 10 d after reculturing in normal medium condition， and the average values were used as the salt tolerance evaluation data.
Survival rate=Number of survived plants/Number of total plants×100%
QTL analysis
The QTL analysis used the QTL detection software IciMapping v3.2 developed by Wang et al.[16]， which is based on inclusive composite interval mapping with the LOD threshold set to be 2.0. The analysis for QTLs of salt tolerance at seedling stage was performed in each recombinant inbred line within the whole genome range， combining the molecular marker detection results and the survival rates of seedlings[17].
Results and Analysis
Analysis of salt tolerance in seedlings of the parents and the 143 families
The seedling survival rate of the parent Zaxima was relatively low at 4.93%， and the survival rate of Nanjing 46 was higher at 57%. The difference was extremely significant. The survival rates of the recombinant inbred line population at the seedling stage were in the range of 0-90%， showing a continuous partial normal distribution （Fig. 1）， with a skewness of 0.456 and a kurtosis of -1.041.The presence of a certain number of superparental types indicates that this trait is a quantitative trait controlled by polygenes. Analysis of QTLs for salt tolerance at seedling stage
In the early stage of this experiment， 202 pairs of SSR polymorphic molecular markers evenly distributed on 12 chromosomes of rice were used to construct a molecular linkage map of this recombinant inbred line population. The total length and average map distance of the map were 1 437.3 and 8.1 cM， respectively， which meet the QTL mapping requirements. The location and genetic effects of the QTLs for salt tolerance at seedling stage were analyzed using QTL IciMapping v3.2 software， combined with phenotypic data， and a total of 4 QTLsrelated to salt tolerance at seedling stage were detected： qSST1， qSST3， qSST5 and QSST11， located on chromosomes 1， 3， 5， and 11 （Fig. 2，Table 1）， respectively， explaining a total of 34.03%of the phenotypic variation. qSST3 exhibited highest contribution rate to salt tolerance at seedling stage， reaching 11%. The QTL additive effects detected in this study were all negative， indicating that the synergistic loci of salt tolerance at seedling stage were all derived from the parent Nanjing 46.
Agricultural Biotechnology 2020
Conclusions and Discussion
A large number of excellent genetic resources are contained in landrace rice， while the genetic basis of the bred varieties that have been widely promoted in production has become increasingly single. If the excellent genetic resources of landrace rice can be fully explored， it will be of positive significance to improve the genetic basis diversity of modern bred varieties. Rice varieties have many important agronomic traits such as yield， quality， resistance to diseases and pests， and stress tolerance. Most of them are quantitative traits controlled by polygenes， and their genetic basis is complex. In order to clone quantitative trait genes， the construction of suitable mapping populations， accurate identification of quantitative trait QTLs and analysis of QTL locations and effects are the first considerations for science and technology workers. In the second step， each QTL needs to be segregated by continuous backcrossing to obtain nearisogenic lines of corresponding gene， thereby decomposing the complex quantitative traits into single Mendelian factors. By this method， many important QTLs for rice were cloned[18-21].
In this study， we used a set of rice recombinant inbred lines constructed with Zaxima and Nanjing 46 as parents to study the salt tolerance at seedling stage， and analyzed QTLs for salt tolerance at seedling stage using IciMapping V 3.2 software， combining with a genetic map. One QTL was detected on each of chromosomes 1， 3， 5， and 11， explaining 34.03% of the phenotypic variation in total. All four QTLs showed an additive effect， indicating that their synergistic loci were from the highvalue parent Nanjing 46. Amongthem， qSST3 on chromosome 3 had the largest effect value， reaching 11%. By comparison with previous reports， qSST1 coincides with the QTL chromosome interval mapped by Wang et al.[22]， which is located in the RM11438RM5497 interval of the first chromosome. Other three salttolerant QTLs have not been reported in any interval. They are new QTLs identified in this study. To date， a large number of QTLs for salt tolerance in rice have been reported， but the results are mostly different[23]. We deem that salt stress treatment concentration and time will affect the salt tolerance phenotype identification results of rice at the bud or seedling stage， and genetic material， population type and genetic background will affect the QTL analysis results. All these will lead to inconsistent identification results of QTLs related to salt tolerance in rice. Different scholars have different views on the understanding of salt tolerance at different developmental stages of rice. Some studies have suggested that salt tolerance at rice bud stage， seedling stage and mature stage has a certain correlation[24-25]， while other scholars believe that the salt tolerance performance of the same variety is not consistent at the bud and seedling stages， and the correlation between the two is very low[26-27]. This study only provides preliminary data for genetic identification of salt tolerance at the seedling stage of recombinant inbred lines. Among them， qSST1 was detected by different researchers in different environments using different research materials， indicating that the RM11438RM5497 interval of the first chromosome may be an important region that affects salt tolerance at rice seedling stage， which has certain reference significance for molecular markerassistedbreeding of new salttolerant rice lines at seedling stage.
References
[1] HAN LZ， HUANG QG， SHENG JS， et al. Overview of agronomic characters， cataloging and breeding of rice seed resources in China[J]. Journal of Plant Genetic Resources， 2002， 3（2）： 40-45. （in Chinese）
[2] ZHANG H， SUN J， WANG M， et al. Genetic structure and phylogergraphy of rice landraces in Yunnan， China， revealed by SSR[J]. Genome， 2007， 50（1）： 72-83.
[3] YANG ZQ， YANG CG， TANG CF， et al. Evaluation and cluster analysis of cold tolerance at the booting stage of national japonica rice in China[J]. Journal of Plant Genetic Resources， 2008， 9（4）： 485-491. （in Chinese）
[4] SUN JC， CAO GL， LI YF， et al. Analysis of genetic diversity within populations of rice （Oryza sativa L.） landraces[J]， Journal of Northwest A & F University： Natural Science Edition， 2011， 39（12）： 145-152. （in Chinese）
[5] WEI XH， TANG SX， WANG YZ， et al. Genetic diversity of allozyme associated with morphological traits in Chinese improved rice varieties[J]. Chinese Journal of Rice Science， 2003， 17（2）： 123-128. （in Chinese）
[6] QI YW， ZHANG DL， ZHANG HL， et al. Genetic diversity of bred rice varieties in China and their trends in the past 50 years[J]. Chinese Science Bulletin， 2006， 51（6）： 693-699. （in Chinese）
[7] DENG HZ， WANG CH， XU Q， et al. Comparative analysis of genetic diversity in landrace and improved rice varieties in China[J]. Journal of Plant Genetic Resources， 2015， 16（3）： 433-442. （in Chinese）
[8] PEARSON GA， AYERS AD， EBERHARD DL. Relative salt tolerance of rice during germination and early seeding development[J]. Soil Sci， 1966， 102： 151-156. [9] GUO Y， CHEN SL， ZHANG GY， et al. Application of cell engineering to obtain rice salttolerant mutant lines controlled by major genes[J]. Journal of Genetics and Genomics， 1997， 24（2）： 122-126. （in Chinese）
[10] GONG JM， HE P， QIAN Q， et al. Mapping QTLs for salt tolerance in rice[J]. Chinese Science Bulletin， 1998， 43（17）： 1847-1850. （in Chinese）
[11] LIN HX ， ZHU MZ， YANO M， et al. QTLs for Na and K uptake of the shoots and roots controlling rice salt tolerance[J]. Theoretical and Applied Genetics， 2004， 108（2）： 253-260.
[12] FANG XW， TANG LH， WANG YP. Selection on rice germplasm tolerant to salt stress[J]. Journal of Plant Genetic Resources， 2004， 5（3）： 295-298. （in Chinese）
[13] REN ZH， GAO JP， LI LG， et al. A rice quantitative trait locus for salt tolerance encodes a sodium transporter[J]. Nature Genetics， 2005， 37（10）： 1141-1146.
[14] HUANG XY， CHAO DY， GAO JP， et al. A previously unknow zinc finger protein， DST， regulates drought and salt tolerance in rice via stomatal aperture control[J]. Genes & Development， 2009， 23（15）： 1805-1817.
[15] TAKAGI H， TAMIRU M， ABE A， et al. MutMap accelerates breeding of a salttolerant rice cultivar[J]. Nature Biotechnology， 2015， 33（5）： 445-449.
[16] WANG JK， WAN XY， LI HH， et al. Application of identified QTLmarker associations in rice quality improvement through a designbreeding approach[J]. Theor Appl Genet，2007， 115： 87-100.
[17] MCCOUCH SR， CHO YG， PANL E， et al. Report on QTL nomenclature[J]. Rice Genet Newsl， 1997， 14： 11-13.
[18] WENG JF， GU SH， WAN XY， et al. Isolation and initial characterization of GW5， a major QTL associated with rice grain width and weight[J]， Cell Research， 2008， 18（12）： 1199-1209.
[19] ZHANG XJ， WANG JF， HUANG J， et al. Rare allele of OsPPKL1 associated with grain length causes extralarge grain and a significant yield increase in rice[J]. Proceedings of the National Academy of Sciences， 2012， 109（52）： 21534-21539.
[20] ISHIMARU K， HIROTSU N， MADOKA Y， et al. Loss of function of the IAAglucose hydrolase gene TGW6 enhances rice grain weight and increases yield[J]. Nature Genetics， 2013， 45（6）： 707-711.
[21] LI YB， FAN CC， XING YZ， et al.， Chalk5 encodes a vacuolar H+translocating pyrophosphatase influencing grain chalkiness in rice[J]. Nature Genetics， 2014， 46（4）： 398-404.
[22] WANG B， LAN T， WU WR. Mapping of QTLs for Na+ content in rice seedlings under salt stress[J]. Chinese Journal of Rice Science， 2007， 21（6）： 585-590. （in Chinese） [23] XING J， CHANG HL， WANG JG， et al. QTL analysis of Na+ and K+ concentrations in japonica rice under salt and alkaline stress[J]. Scientia Agricultura Sinica， 2015， 48（3）： 604-612. （in Chinese）
[24] ZANG J， SUN Y， WANG Y， et al. Dissection of genetic overlap of salt tolerance QTLs at the seedling and tillering stages using backcross introgression lines in rice. Science in China Series Clife[J]. Sciences， 2008， 51（7）： 583-591.
[25] CHEN YP. Evaluation of salt tolerance of japonica rice germplasms at different growth stages[D]. Yinchuan： Ningxia University， 2016. （in Chinese）
[26] XIE LJ， DUAN M， PAN XB， et al. identification and analysis of salt tolerance in different type rice cultivars during seedling and whole plant growth stage[J]. Acta Agriculturae Universitis Jiangxiensis， 2015， 37（3）： 404-410. （in Chinese）
[27] GUO WM， FU YP， SUN ZX， et al. Correlation analysis of different rice germplasm morphological indexes and salt tolerance under salt stress[J]. Journal of Plant Genetic Resources， 2003， 4（3）： 245-251. （in Chinese）
[Methods] Recombinant inbred lines （RILs） derived from Zhaxima， an indica landrace variety from Yunnan Province and Nanjing 46， an elite japonica variety with superior grain quality from Jiangsu Province were used. The salt tolerance at seeding stage in the RIL population was investigated as the phenotypic value.
[Results] Combined with the linkage map， a total of 4 QTLs were detected： qSST1， qSST3， qSST5 and qSST11， located in rice chromosomes 1， 3， 5 and 11， respectively. All positive alleles were from the parent Nanjing 46. Three QTLs among them were not included in chromosome intervals the same as cloned rice salt tolerance genes， and thus were described as new candidate gene loci associated with seedingstage salt tolerance.
[Conclusions] This study provides important information for further exploration and utilization of new salt tolerant QTLs in rice. It is of great significance for improving the utilization of saline land in China and ensuring the stable rice production.
Key words Oryza sativa L.; Seeding stage; Salt tolerance; QTL
As one of the origin centers of cultivated rice in Asia， China has abundant types of rice seed resources[1]. The main part of Chinas rich rice seed resources is landrace rice. Landrace rice not only contain a large number of excellent genes such as high yield， high quality， disease resistance， and stress tolerance genes， but also have rich genetic diversity[2-4]， which are valuable resources in China. With the longterm utilization of core germplasms in rice breeding， the problems of single genetic basis of the bred varieties and poor diversity are becoming increasingly prominent[5-7]. In the future， the key to whether rice breeding can achieve a substantial breakthrough lies in the exploration and utilization of the genetic diversity of landrace rice. With the rapid growth of the worlds population and the continuous decline of arable land， stable yield has always been one of the traits most concerned by breeders. However， unreasonable irrigation and excessive fertilization have caused the global environment to continue to deteriorate and the area of salinealkali land to increase year by year， involving more than 30 countries around the world. Rice is a crop with moderate salt susceptibility， and soil salt concentration has a great impact on its yield and quality. Most of the rice grown in salinealkali land will suffer from a crop failure， and even total loss. Therefore， the exploration of new salttolerant genes from landrace rice varieties by methods such as genetic improvement and breeding of new salttolerant rice varieties is of important significance for improving the utilization of saline land and ensuring stable rice. As early as the 1960s， research has reported that the salt tolerance of rice is a quantitative trait which is controlled by polygenes， which is also affected by the external environment and development period[8-11]. The twoleaf and oneheart stage is the most sensitive period for salt tolerance in rice[12]. Many researchers have conducted QTL mapping using quantitative trait mapping populations and molecular marker linkage maps. The results showed that most salt tolerance QTLs were distributed on chromosomes 1， 2， 6， and 7. A small amount of salt tolerance QTLs were also detected on chromosomes 10 and 11. So far， only a few salt tolerance genes have been cloned. The salt tolerance gene SKC1 on chromosome 1 was first cloned. When rice is under high salt stress and the stalk accumulates a large amount of Na+， the cell membranelocalized Na+ transporter encoded by SKC1 can transport excess Na+ from the stalk back to the root system， thereby reducing the toxicity of Na+ to rice plants and increasing the tolerance of rice plants to salt stress[13]. DST is a new zinc finger transcription factor obtained through mapbased cloning by Huang et al. It is derived from salttolerant mutants and can enhance salt or drought tolerance in rice through negative regulation. The mutated DST reduces the expression of genes for hydrogen peroxide metabolism， which causes hydrogen peroxide to accumulate in guard cells， resulting in stomata closing， thereby reducing water evaporation， and plants exhibited salt or drought tolerance[14]. HST1 is a newly reported salttolerant gene that encodes a type B response regulator protein OsRR22， which is related to the expression of genes related to osmotic adjustment and ion transport. Compared with SKC1 gene， HST1 gene has stronger salt tolerance[15].
Although some progress has been made in the cloning of salt tolerance genes in rice， there are still few gene resources that have been truly utilized in breeding salttolerant varieties. Exploring and locating new QTLs for salt tolerance in rice and improving the utilizability of landrace rice have positive significance for improving the genetic diversity of existing varieties. In this study， recombinant inbred lines constructed from Yunnan indica rice variety Zaxima and Jiangsu highquality rice variety Nanjing 46 were used to analyze QTLs for salt tolerance at the seedling stage， so as to obtain new salt tolerance QTLs and provide a theoretical basis for salttolerant rice breeding. Materials and Methods
Experiment materials
The Zaxima/Nanjing 46 recombinant inbred lines （a total of 143 lines） were constructed using Yunnan indica rice variety Zahima and Jiangsu highquality rice variety Nanjing 46 as parents by single seed descend method （SSD）.
Salt stress treatment at seedling stage of rice
The salt tolerance in the seedlings of the 143 families and their parents Zahima and Nanjing 46 was identified using 0.5% NaCl. Petri dishes 9 cm in diameter were used for accelerating germination. First， 12 ml of dH2O was added into each petri dish， followed by incubation in the dark at 28 ℃ for 3 d. Seeds with substantially uniform bud lengths were placed on 96well plates with 8 seeds in each row and 5 rows for each material. The 96well plates were placed in a watercontaining turnover box which was placed in an illumination incubator. The incubator was set with a photoperiod of 14 L/8 D， at 28 ℃ during the day and 24 ℃ at night. When the seedlings reached the twoleaf and oneheart stage， 0.5% NaCl was added to the nutrient solution for salt stress treatment， and the nutrient solution was changed every 3 d. The duration of the salt stress treatment was 21 d， and after the salt stress treatment， the seedlings were cultured in normal medium condition for 10 d. The experiment was set up with three repetitions， each with 3 replicates， and the average seedling survival rate of the 3 repetitions was used as the statistical data.
Investigation of salt tolerance at seedling stage
The seedling survival rates of each line and the parents were investigated at 21 d of the treatment and 10 d after reculturing in normal medium condition， and the average values were used as the salt tolerance evaluation data.
Survival rate=Number of survived plants/Number of total plants×100%
QTL analysis
The QTL analysis used the QTL detection software IciMapping v3.2 developed by Wang et al.[16]， which is based on inclusive composite interval mapping with the LOD threshold set to be 2.0. The analysis for QTLs of salt tolerance at seedling stage was performed in each recombinant inbred line within the whole genome range， combining the molecular marker detection results and the survival rates of seedlings[17].
Results and Analysis
Analysis of salt tolerance in seedlings of the parents and the 143 families
The seedling survival rate of the parent Zaxima was relatively low at 4.93%， and the survival rate of Nanjing 46 was higher at 57%. The difference was extremely significant. The survival rates of the recombinant inbred line population at the seedling stage were in the range of 0-90%， showing a continuous partial normal distribution （Fig. 1）， with a skewness of 0.456 and a kurtosis of -1.041.The presence of a certain number of superparental types indicates that this trait is a quantitative trait controlled by polygenes. Analysis of QTLs for salt tolerance at seedling stage
In the early stage of this experiment， 202 pairs of SSR polymorphic molecular markers evenly distributed on 12 chromosomes of rice were used to construct a molecular linkage map of this recombinant inbred line population. The total length and average map distance of the map were 1 437.3 and 8.1 cM， respectively， which meet the QTL mapping requirements. The location and genetic effects of the QTLs for salt tolerance at seedling stage were analyzed using QTL IciMapping v3.2 software， combined with phenotypic data， and a total of 4 QTLsrelated to salt tolerance at seedling stage were detected： qSST1， qSST3， qSST5 and QSST11， located on chromosomes 1， 3， 5， and 11 （Fig. 2，Table 1）， respectively， explaining a total of 34.03%of the phenotypic variation. qSST3 exhibited highest contribution rate to salt tolerance at seedling stage， reaching 11%. The QTL additive effects detected in this study were all negative， indicating that the synergistic loci of salt tolerance at seedling stage were all derived from the parent Nanjing 46.
Agricultural Biotechnology 2020
Conclusions and Discussion
A large number of excellent genetic resources are contained in landrace rice， while the genetic basis of the bred varieties that have been widely promoted in production has become increasingly single. If the excellent genetic resources of landrace rice can be fully explored， it will be of positive significance to improve the genetic basis diversity of modern bred varieties. Rice varieties have many important agronomic traits such as yield， quality， resistance to diseases and pests， and stress tolerance. Most of them are quantitative traits controlled by polygenes， and their genetic basis is complex. In order to clone quantitative trait genes， the construction of suitable mapping populations， accurate identification of quantitative trait QTLs and analysis of QTL locations and effects are the first considerations for science and technology workers. In the second step， each QTL needs to be segregated by continuous backcrossing to obtain nearisogenic lines of corresponding gene， thereby decomposing the complex quantitative traits into single Mendelian factors. By this method， many important QTLs for rice were cloned[18-21].
In this study， we used a set of rice recombinant inbred lines constructed with Zaxima and Nanjing 46 as parents to study the salt tolerance at seedling stage， and analyzed QTLs for salt tolerance at seedling stage using IciMapping V 3.2 software， combining with a genetic map. One QTL was detected on each of chromosomes 1， 3， 5， and 11， explaining 34.03% of the phenotypic variation in total. All four QTLs showed an additive effect， indicating that their synergistic loci were from the highvalue parent Nanjing 46. Amongthem， qSST3 on chromosome 3 had the largest effect value， reaching 11%. By comparison with previous reports， qSST1 coincides with the QTL chromosome interval mapped by Wang et al.[22]， which is located in the RM11438RM5497 interval of the first chromosome. Other three salttolerant QTLs have not been reported in any interval. They are new QTLs identified in this study. To date， a large number of QTLs for salt tolerance in rice have been reported， but the results are mostly different[23]. We deem that salt stress treatment concentration and time will affect the salt tolerance phenotype identification results of rice at the bud or seedling stage， and genetic material， population type and genetic background will affect the QTL analysis results. All these will lead to inconsistent identification results of QTLs related to salt tolerance in rice. Different scholars have different views on the understanding of salt tolerance at different developmental stages of rice. Some studies have suggested that salt tolerance at rice bud stage， seedling stage and mature stage has a certain correlation[24-25]， while other scholars believe that the salt tolerance performance of the same variety is not consistent at the bud and seedling stages， and the correlation between the two is very low[26-27]. This study only provides preliminary data for genetic identification of salt tolerance at the seedling stage of recombinant inbred lines. Among them， qSST1 was detected by different researchers in different environments using different research materials， indicating that the RM11438RM5497 interval of the first chromosome may be an important region that affects salt tolerance at rice seedling stage， which has certain reference significance for molecular markerassistedbreeding of new salttolerant rice lines at seedling stage.
References
[1] HAN LZ， HUANG QG， SHENG JS， et al. Overview of agronomic characters， cataloging and breeding of rice seed resources in China[J]. Journal of Plant Genetic Resources， 2002， 3（2）： 40-45. （in Chinese）
[2] ZHANG H， SUN J， WANG M， et al. Genetic structure and phylogergraphy of rice landraces in Yunnan， China， revealed by SSR[J]. Genome， 2007， 50（1）： 72-83.
[3] YANG ZQ， YANG CG， TANG CF， et al. Evaluation and cluster analysis of cold tolerance at the booting stage of national japonica rice in China[J]. Journal of Plant Genetic Resources， 2008， 9（4）： 485-491. （in Chinese）
[4] SUN JC， CAO GL， LI YF， et al. Analysis of genetic diversity within populations of rice （Oryza sativa L.） landraces[J]， Journal of Northwest A & F University： Natural Science Edition， 2011， 39（12）： 145-152. （in Chinese）
[5] WEI XH， TANG SX， WANG YZ， et al. Genetic diversity of allozyme associated with morphological traits in Chinese improved rice varieties[J]. Chinese Journal of Rice Science， 2003， 17（2）： 123-128. （in Chinese）
[6] QI YW， ZHANG DL， ZHANG HL， et al. Genetic diversity of bred rice varieties in China and their trends in the past 50 years[J]. Chinese Science Bulletin， 2006， 51（6）： 693-699. （in Chinese）
[7] DENG HZ， WANG CH， XU Q， et al. Comparative analysis of genetic diversity in landrace and improved rice varieties in China[J]. Journal of Plant Genetic Resources， 2015， 16（3）： 433-442. （in Chinese）
[8] PEARSON GA， AYERS AD， EBERHARD DL. Relative salt tolerance of rice during germination and early seeding development[J]. Soil Sci， 1966， 102： 151-156. [9] GUO Y， CHEN SL， ZHANG GY， et al. Application of cell engineering to obtain rice salttolerant mutant lines controlled by major genes[J]. Journal of Genetics and Genomics， 1997， 24（2）： 122-126. （in Chinese）
[10] GONG JM， HE P， QIAN Q， et al. Mapping QTLs for salt tolerance in rice[J]. Chinese Science Bulletin， 1998， 43（17）： 1847-1850. （in Chinese）
[11] LIN HX ， ZHU MZ， YANO M， et al. QTLs for Na and K uptake of the shoots and roots controlling rice salt tolerance[J]. Theoretical and Applied Genetics， 2004， 108（2）： 253-260.
[12] FANG XW， TANG LH， WANG YP. Selection on rice germplasm tolerant to salt stress[J]. Journal of Plant Genetic Resources， 2004， 5（3）： 295-298. （in Chinese）
[13] REN ZH， GAO JP， LI LG， et al. A rice quantitative trait locus for salt tolerance encodes a sodium transporter[J]. Nature Genetics， 2005， 37（10）： 1141-1146.
[14] HUANG XY， CHAO DY， GAO JP， et al. A previously unknow zinc finger protein， DST， regulates drought and salt tolerance in rice via stomatal aperture control[J]. Genes & Development， 2009， 23（15）： 1805-1817.
[15] TAKAGI H， TAMIRU M， ABE A， et al. MutMap accelerates breeding of a salttolerant rice cultivar[J]. Nature Biotechnology， 2015， 33（5）： 445-449.
[16] WANG JK， WAN XY， LI HH， et al. Application of identified QTLmarker associations in rice quality improvement through a designbreeding approach[J]. Theor Appl Genet，2007， 115： 87-100.
[17] MCCOUCH SR， CHO YG， PANL E， et al. Report on QTL nomenclature[J]. Rice Genet Newsl， 1997， 14： 11-13.
[18] WENG JF， GU SH， WAN XY， et al. Isolation and initial characterization of GW5， a major QTL associated with rice grain width and weight[J]， Cell Research， 2008， 18（12）： 1199-1209.
[19] ZHANG XJ， WANG JF， HUANG J， et al. Rare allele of OsPPKL1 associated with grain length causes extralarge grain and a significant yield increase in rice[J]. Proceedings of the National Academy of Sciences， 2012， 109（52）： 21534-21539.
[20] ISHIMARU K， HIROTSU N， MADOKA Y， et al. Loss of function of the IAAglucose hydrolase gene TGW6 enhances rice grain weight and increases yield[J]. Nature Genetics， 2013， 45（6）： 707-711.
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