Wheat breeding in northern China: Achievements and technical advances

Hongji Li*,Yng Zhou Wnli Xin,Yiqin Wi, Junling Zhng, Lili Guo

aThe National Engineering Laboratory of Crop Molecular Breeding,Institute of Crop Sciences,Chinese Academy of Agricultural Sciences,Beijing 100081,China

bInstitute of Crop Breeding,Heilongjiang Academy of Agricultural Sciences,Harbin 150086,Heilongjiang,China

cInstitute of Crop Research,Ningxia Academy of Agricultural and Forestry Sciences,Yinchuan 750100,Ningxia,China

dInstitute of Foxtail Millet,Shanxi Academy of Agricultural Sciences,Changzhi 046000,Shanxi,China

eNational Agricultural Technology Extension and Service Center,Beijing 100125,China

Keywords:Common wheat Dwarf male sterile wheat Speed breeding SNP chips Triticum aestivum

ABSTRACT Common wheat is the major cereal crop that underpins the food safety of China. Both winter wheat and spring wheat are grown on ～24 million ha. This review aims to summarize the current status of wheat production and breeding progress in the northern wheat production areas of the country, and to review recently advanced technologies being applied in wheat breeding,including the use of dwarf-male-sterile (DMS) wheat,speed breeding and specialized wheat breeding SNP chips.Crossing is the initial step in most breeding programs.DMS wheat is a convenient tool for large scale production of hybrid seed. Speed breeding or accelerated generation turnover attempts to reduce the time taken in cultivar development.Several different SNP chips are high-throughput,genome-wide genotyping platforms for breeding and research.

Contents

1. Current status of wheat production in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719

2. Breeding wheat for northern China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719

2.1. Northern Winter Wheat Zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 720

2.2. Northeastern Spring Wheat Zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 721

2.3. Northern and Northwestern Spring Wheat Zones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722

2.4. Xinjiang Winter and Spring Wheat Zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722

3. Technical advances in wheat breeding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724

3.1. Dwarf-male-sterile wheat,an efficient method for breeding and commercialization of hybrid cultivars . . . . 724

3.2. Speed breeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724

3.3. Development and application of wheat breeding chips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 725

4. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726

1. Current status of wheat production in China

Common hexaploid wheat(Triticum aestivum)is grown in at least 43 countries[1,2].It provides the bulk of calories for about 40%of the global population.Wheat has been widely grown as a staple food source in China for about 4500 years,mostly in the northern part of the country [3,4]. Historical records and archaeological evidence demonstrate that wheat was grown from northwestern to eastern,southern and southwestern China[5,6].After analyzing 717 Chinese landraces with DArTseq and the Wheat660K single nucleotide polymorphism (SNP) markers, Zhou et al. [4]concluded that wheat had spread from the northwestern Caspian Sea region to South China. China is the world's leading wheat producer,with approximately 17%of global production[7].With urbanization,population growth,and rising income,the demand for wheat continues to increase[8-10].

The global yield increase per annum of wheat was estimated to be 1.0%and the figure for China was 1.7%[11].Norse et al.[12]predicted that China will be self-sufficient in wheat production by 2020. Currently China is about 95% self-sufficient in wheat[10]. During the last two decades wheat production increased from 99.6 million tons in 2000 to 130.2 million tons in 2018 (Fig.1).This increase was achieved during a period of decreasing area,especially a sharp decline that occurred from 2000 to 2004 caused by lower economic returns to farmers from wheat compared to other cereal and oil seed crops such as rice,maize,soybean,and rapeseed [13]. Grain production policies set by the Government played an important role in these changes.

Wheat is grown in 31 Chinese provinces but the areas in Liaoning, Jilin, Fujian, Jiangxi, Guangdong, and Guangxi provinces are quite small. Since 2000, the wheat areas in 23 provinces have declined but there were increases in 7 provinces, including Henan, Anhui, Jiangsu, Shandong,Hubei, Xinjiang, and Inner Mongolia. The largest areas and production of decline occurred in southwestern,northeastern and northwestern provinces (except for Xinjiang and Gansu)(Fig. 2-A, B). During the period production increased from 99.6 million tons in 2000 to 134.3 million tons in 2018. The yield of wheat increased from 3.7 to 5.4 t ha-1over the past 20 years, and yield was the major contributor to the continuous improvement in production. It was estimated that the yield increase achieved by cultivar improvement during the 1990s was about 24.7%,and during 2001-2012 it was 52%[14].

Wheat with winter growth habit(i.e.,winter wheat)requires vernalization and can be sown only in autumn. Wheat with spring growth habit(i.e.,spring wheat)is able to flower without a need of vernalization,so it can be sown in autumn or spring.The area and production of winter wheat far exceeds that of spring wheat.Winter wheat occupies about 95%of the total wheat area of China [15]. Spring-sown wheat is mainly grown in higher latitude or higher elevation areas of Inner Mongolia, Heilongjiang,Qinghai,Ningxia,parts of Xinjiang,Hebei,Tianjin,Shanxi,Gansu,and Tibet,and small areas in Liaoning and Jilin.The area and production of spring wheat in 2015 were significantly lower than in 2000. These reductions mainly occurred in Inner Mongolia, Gansu, Qinghai, Shanxi, Ningxia, Heilongjiang, Liaoning and Jilin (Fig. 3-A, B) to be replaced by increased areas of maize,soybean,and rice.However,spring wheat planting areas increased in Hebei,Tianjin,and Xinjiang.

2. Breeding wheat for northern China

The wheat growing regions in China are divided into ten agroecological zones [16,17], which reflect differences in climatic conditions, soil type, cultivar adaptation, and crop management (Fig. 4). In 2017, the China Ministry of Science and

Fig.1-Planting areas,total production,and yield of wheat in China from 2000 to 2018.Data are provided in the China Statistical Yearbook available at http://www.stats.gov.cn/.

Fig.2- Planting areas(A)and production(B) of wheat in different provinces in 2000 and 2017.Data are provided in the China Statistical Yearbook available at http://www.stats.gov.cn/.

Technology launched a wheat breeding manifesto. Based on the different wheat cultivation zones,there are five programs:(1) Northern China Wheat Breeding Program, (2) Northern Yellow and Huai River Valley Wheat Breeding Program, (3)Southern Yellow and Huai River Valley Wheat Breeding Program, (4) Middle and Low Yangtze River Valley Winter Wheat Breeding Program,and(5)Southwestern Winter Wheat Breeding Program. The targeted area of the Northern China Wheat Breeding Program included the Northern Winter Wheat Zone, Northeastern Spring Wheat Zone, Northern Spring Wheat Zone, Northwestern Spring Wheat Zone, and Xinjiang Winter-Spring Wheat Zone.In this article,we review progress in wheat breeding in the areas targeted by this program. The characteristics of the most widely grown cultivars in this region are listed in Table 1.

2.1. Northern Winter Wheat Zone

This wheat zone is located at the northern edge of the autumn-sown wheat area where winter-habit is obligatory. It includes Beijing, Tianjin, north central Hebei, southeastern and central Shanxi,northern Shaanxi,and eastern Gansu,and accounts for about 9%of the total wheat area in China.

The average temperature in January ranges from-3.2 °C to-10.9 °C and the minimum temperature ranges from-22 °C to-27 °C. Winter killing of seedlings is a major problem and strong cold tolerance is essential. The annual precipitation is 300-600 mm of which only 30% occurs during the wheat growing season. Low precipitation and mandatorily limited use of underground water are major constraints to production.Lodging resistance is important in the irrigated fields. High temperature and low moisture during grain filling often causes reductions in grain weight and forced maturity, and hence reduced grain yields. Resistance to stripe rust and powdery mildew are prioritized among disease problems.

Lunxuan 987, released in 2003 [18], was a breakthrough in wheat breeding after 2000. It was developed from a recurrent selection population that involved crossing among a dwarfmale-sterile (DMS) line and ＞20 wheat cultivars [19]. In the national registration trial,the yield of Lunxuan 987 exceeded the Jingdong 8 control,the most widely grown cultivar in the zone at that time, by 12.7%. The record yield of Lunxuan 987 was 10.1 t ha-1in a farm field. Compared to Jingdong 8 and other cultivars widely grown in this zone, the height of Lunxuan 987 was ～15 cm shorter and contributed to better lodging resistance.Late maturity and poor quality of Lunxuan 987 are traits that need to be improved. Lunxuan 987 was not only widely grown,but also became a parent of many derived cultivars such as Lunxuan 167 (released in Beijing, 2014), Lunxuan 169 (National level,2014),Zhongmai 996(Tianjin,2014),Zhongmai 998(Tianjin,2015), Zhongmai 1062 (National level, 2016), Nongda 5133(Beijing, 2017), Nongda 5181 (National level, 2017), Lunxuan 310(Hebei, 2018), Zhongmai 122 (Beijing, Tianjin, and Hebei, 2018),Zhongmai 123(Beijing,2018),and Zhongmai 1312(Beijing,2019).

Fig.3-Spring wheat planting areas(A)and production(B)in different provinces in 2000 and 2015.Data are provided inZhao et al.[4].

Zhongmai 175 (BPM27/Jing 411), released in 2007 [20], is 3-5 days earlier in maturity than Lunxuan 987. This whitegrained cultivar is competitive in the wheat market in northern China. It has good dough extensibility that makes it suitable for Chinese noodles.Zhongmai 175 is currently the most widely grown cultivar in this zone. The other attributes of Zhongmai 175 include its high water and nitrogen use efficiency and fast grain filling rate [21]. Zhongmai 175 has weak lodging resistance under irrigated,high yield conditions and is vulnerable to powdery mildew.

Lunxuan 266(Jimai 19/Jimai 22//Jimai 22),released in 2018,had a yield advantage of 6.0% over Zhongmai 175 in the joint provincial registration trial of Beijing,Tianjin,and Hebei from 2014 to 2017. Compared to Zhongmai 175, Lunxuan 266 has shorter stature and a stiffer culm,more kernels per spike,and higher thousand-kernel weight, making it attractive to farmers. It has better powdery mildew resistance than Zhongmai 175 and acceptable winter killing tolerance.

The earliest work on improvement of wheat grain quality in China occurred in this wheat zone in the early 1980s. The initial work was to characterize the quality characteristics of existing cultivars and breeding lines.A small number of lines with promising bread making quality were identified.Zhongzuo 8131-1 was released as a bread making cultivar in Beijing and Tianjin in 1987 and 1989, respectively. It was widely used as a parent in breeding programs not only in the Northern Winter Wheat Zone, but also in other zones.Zhengzhou 8603, Zhongyou 9507, and Liken 2 were selected directly from Zhongzuo 8131-1 and had promising bread making quality. Wanmai 33 (Zhongzuo 8131-1/Annong 8326)was also developed using Zhongzuo 8131-1 as a parent.

Steamed bread and Chinese noodle are the main products consumed in northern China； both products require moderate dough strength. Standardized laboratory protocols and evaluation methods for steamed bread and white dry noodle were established[22].As most wheat cultivars in the Northern Winter Wheat Zone(as well as other zones)have weak dough strength,a large proportion of the grain of wheat cultivars classified as having bread making quality is used in blending with grain of other varieties to increase dough strength. Increased dough strength is the most important target for breeding programs in northern China.

2.2. Northeastern Spring Wheat Zone

This spring wheat zone, the largest spring-sown wheat area,accounts for about 7% of the wheat area in China. It includes Heilongjiang, eastern Inner Mongolia, and small areas of Jilin and Liaoning. The annual temperature ranges from -3 °C to 7 °C. The wheat growing duration is 75-95 days, the shortest in China. Drought at sowing in March or April and waterlogging during the grain filling stage in July or August are the major abiotic constraints.Leaf rust,root rot caused by Bipolaris sorokiniana, Fusarium head blight and powdery mildew are the major diseases. The majority of wheat cultivars are photoperiod sensitive； they develop very slowly in spring but develop very rapidly after stem elongation.

Kehan 16 (Jiusan 79F5-5416/Ke 80 Yuan 229//Ke 76-750/Ke 76F4-779-5///Ke 76-413)is a spring wheat cultivar with high yield potential and wide adaptability [23]. Since its release in 2000 Kehan 16 remains a dominant cultivar in the Northeast Spring Wheat Zone. Its wide adaptability is attributed to good seedling drought resistance and waterlogging tolerance during the grain filling stage. Kehan 16 has resistance to lodging, stem rust and leaf rust, Fusarium head blight, root rot, and pre-harvest sprouting. The average yield of Kehan 16 in the provincial registration trial was 5.4 tons ha-1.

Kenjiu 10 (Jiusan 84-7251/Jiusan 87148//Ke 85-33) is another spring wheat cultivar with high yield potential and wide adaptability [24]. The yield and quality of Kenjiu 10 are better than Kehan 16. Kenjiu 10 currently serves as the control cultivar in national registration trials in the Northeast Spring Wheat Zone. Longmai 33 (Longmai 26/Jiusan 3u92) and Longmai 35 (Ke 90-513/Longmai 26), released in 2012 and 2013,respectively[25,26],are the most widely grown cultivars in this zone. Both cultivars have better bread making quality and higher yield than Kehan 16 and Kenjiu 10.

Improvement of wheat quality in the Northeast Spring Wheat Zone started in the late 1980s.Screening of the existing cultivars and advanced breeding lines identified several accessions with acceptable quality, including Longmai 11 and Liaochun 10 that have been used as parents for quality improvement. Much attention was given to relationship between high molecular weight glutenin subunits (HMW-Gs) and baking quality as originally described by Payne et al. [27,28]in the UK. HMW-Gs analysis and sodium dodecyl sulfate (SDS)-sedimentation tests were widely used in parental and early generation selection in breeding. Currently, most cultivars with good bread baking quality carry HMW-Gs 5 + 10. Wheat varieties from this zone do not carry the wheat-rye T1BL·1RS translocation chromosome associated with lower bread-making quality.

Fig.4- Ten agro-ecological zones of wheat in China.I, Northern Winter Wheat Zone；II,Yellow and Huai River Valleys Facultative Wheat Zone；III, Middle and Low Yangtze River Valley Autumn-Sown Spring Wheat Zone；IV, Southern Autumn-Sown Spring Wheat Zone；V,Southwestern Autumn-Sown Spring Wheat Zone；VI, Northeastern Spring Wheat Zone；VII,Northern Spring Wheat Zone；VIII,Northwestern Spring Wheat Zone；IX,Qinghai-Tibetan Plateau Spring-Winter Wheat Zone；X,Xinjiang Winter-Spring Wheat Zone.The map of China is available at http://bzdt.ch.mnr.gov.cn/index/html and the wheat zones are redrawn from He et al.[17].

2.3. Northern and Northwestern Spring Wheat Zones

The Northern Spring Wheat Zone accounts for about 4%of the total wheat area and includes northern suburban areas of Beijing and Tianjin,northern Hebei,Shanxi,and Shaanxi,and the central western Inner Mongolia. In Tianjin, progress has been made in developing spring wheat cultivars with good bread making quality. Cultivars Jinqiang 11 (S06K-16/S08 Yuanjian 109) and Jinqiang 12 (S06K-16/S08 Yuanjian 51) are mainly produced for local millers； they were derived from Liaochun 10 (released in the 1980s) and Canadian cultivar Wildcat. Wheat cultivars grown in the central west part of Inner Mongolia overlap the Northwestern Spring Wheat Zone.

The Northwestern Spring Wheat Zone consists of eastern Qinghai, and most of Gansu and Ningxia. Spring wheat cultivar Ningchun 4 was developed from the cross Sonora 64 × Hongtu in 1976 [29]. Sonora 64 is a Mexican spring wheat cultivar, and Hongtu is derived from an Abbondanza × Quality cross.Abbondanza was widely grown following its introduction from Italy in 1956. Quality, an early introduction from Australia, was not only widely grown in Qinghai,Gansu,and Ningxia,but was also used as a parent in many breeding programs.Ningchun 4 is still widely grown and continues to serve as the control cultivar in registration trials in this spring wheat zone.The average yield of Ningchun 4 in farmer's fields was about 5.0 t ha-1and over 7.5 t ha-1under intensive managements. This early maturity,short stature, stiff culmed cultivar is well-known for its outstanding noodle-making quality.

Ningchun 55 (Ningchun 32/TK106), released in 2017 [30],has potential to replace Ningchun 4. The grain yield of Ningchun 55 exceeded Ningchun 4 by 8.1%, 8.2%, and 6.7% in provincial registration trials in 2014,2015,and 2016.Its record yield was 9.8 t ha-1in 2016. It has similar adaptive traits and better quality than Ninchan 4.

Southern and eastern Gansu and southeastern Ningxia in the Northwestern Spring Wheat Zone are important regions for over-summer survival of the stripe rust and powdery mildew pathogens and serve as sources of primary inocula that infect seedlings of autumn-sown materials [31,32]. New races of the pathogens are also frequently reported from the region. Historically, many cultivars carried the T1BL·1RS translocation inherited from Lovrin 13,Kavkaz and Predgornia 2[33-35].

2.4. Xinjiang Winter and Spring Wheat Zone

Xinjiang is the largest wheat producer in northwest China with an annual wheat growing area of about 1.13 million ha,or 4% of the total wheat area in China. It is classified as a separate wheat zone due to its unique geographic location.There are approximately equal proportions of winter and spring wheat.In northern Xinjiang snow-covering of autumnsown winter wheat is adequate to prevent winter killing in most years. Wheat cultivars with facultative growth habit predominate in southern Xinjiang. Xinjiang is characterized by cold winters, hot summers, and low precipitation. The temperatures in January range from -12 °C to -20 °C, and in July from 18 °C to 24 °C.The annual precipitation in the north ranges from 150 to 500 mm, and ＜100 mm in the south. The difference in temperature between day and night is 11 °C.The annual accumulated sunshine duration ranges from 2500 to 3600 h. All autumn-sown wheat and ＞90% of spring-sown wheat are irrigated.

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In 2017, the top three wheat cultivars were Xindong 20,Xinchun 6, and Xindong 22 [36-38]. The facultative cultivar Xindong 20, released in 1995 and grown mainly in southern Xinjiang,is characterized by early maturity,short stature and stiff culms. The average yield of Xindong 20 in Xinjiang provincial registration trials was 6.8 t ha-1and the record yield was 9.1 t ha-1. Winter wheat cultivar Xindong 22,released in 1999, remains a dominant cultivar in winter wheat areas in northern Xinjiang. It is characterized by early maturity,high thousand-kernel weight,stiff culms,resistance to stripe rust and powdery mildew,fast grain filling,and good quality.The average yield is about 7.5 t ha-1.

Xinchun 6 is a spring-sown wheat cultivar released in 1993.It has high yield potential, high thousand-kernel weight, early maturity, good drought tolerance, and stiff culms. The average yield of Xinchun 6 was 6.8 t ha-1in the national registration trials and its record yield was 10.5 t ha-1in a farm field.

3. Technical advances in wheat breeding

3.1. Dwarf-male-sterile wheat, an efficient method for breeding and commercialization of hybrid cultivars

Fig.5-An F2 population from a cross between DMS-Zhoumai 16 and Jimai 22.Dwarf plants are sterile；and the tall plants are fertile and can be advanced for cultivar development.

Increased yield is the ongoing most important objective in cultivar development. In modern hybridization breeding, crossing between desired parents is the initial step to obtain hybrid seeds. Manual emasculation is generally used to make crosses.However, for some breeding programs such as recurrent selection where large numbers of hybrid seeds are required this is very labor-intensive. If hybrid cultivars are to be utilized methods of mass hybrid seed production must be developed.In 1976,a male sterile plant with the single dominant gene(Ta1 =Ms2) on chromosome 4DS was discovered in Taigu County,Shanxi province [39,40]. However, the difficulty in identification of male sterile plants limited the use of Ms2 in recurrent selection. Moreover, the sterile plants were prone to being pollinated by plants with higher stature,which led to increased height of recurrently selected populations. A few years later, a dwarf wheat line Ai-bian 1(Rht10 = Rht-D1c on chromosome 4DS)was identified as a natural mutant plant in landrace Aiganzao[41,42].Genes Ms2 and Rht-D1c were recombined to produce the dwarf male sterile line[43].Since the two genes are closely linked there is always clear cut segregation (1:1) for dwarf male sterile versus tall male fertile plants in the F1generation when any heterozygous DMS line is crossed with any normal height male fertile parent (Fig. 5). The DMS plants can be used as maternal parents in crossing to produce hybrid seeds, and the tall male fertile plants can be advanced to pure lines for developing cultivars or can be differentially harvested as a hybrid cultivar.DMS wheat has been used in both conventional breeding programs to improve the efficiency of crossing by replacing manual emasculation and in recurrent selection programs[44,45]. Improvements in yield potential, certain quality parameters, and resistance to lodging and diseases were obtained in breeding programs using DMS wheat. Lunxuan 987 and a number of other wheat cultivars were developed using DMS wheat[45-47].

The efficiency of DMS wheat in cultivar development depends on the availability of the Ms2 Rht-D1c combination in specific genetic backgrounds.Because use of the original DMS line could involve multiple parents there could be excessive segregation in progenies of DMS hybrids. To circumvent this problem, the DMS trait was transferred into a number of common,currently used parents,including Jimai 22,Jimai 20,Zhoumai 16, Zhoumai 18, Zhengmai 9023, Lunxuan 061,Yanzhan 4110, Lunxuan 987, Shiluan 02-1, Shi 4185, Shijiazhuang 8, Gaocheng 8901, Shixin 733, and Aikang 58 by backcrossing (Y. Zhou, unpublished data). Those selected DMS lines are obviously preferred by breeders.

3.2. Speed breeding

The wide variation of ecotypes and climatic conditions allows growing of both winter wheat and spring wheat in China.Because wheat cultivars with winter growth habit require an obligatory period of low temperature for vernalization, they are usually sown from late September to October, and harvested in June the following year, with a growth duration of about 240 days.Wheat cultivars with spring growth habit do not need obligatory vernalization,and can be sown either in autumn with a growth duration of about 200 days in southern provinces or in spring with about 100 days growth duration in certain northern provinces, respectively. Most traditional breeding programs operate with a single generation each year.

Hybridization breeding of self-pollinated crops requires several generations of selfing to establish sufficient genetic and phenotypic uniformity prior to agronomic evaluation and varietal release. Several strategies have been used to accelerate the production of uniform lines.These include(1)doubled haploid populations developed from the original hybrid plants[48]； (2) growing wheat plants locally in the off-season either in controlled environments or in the fields； (3) moving segregating populations to suitable alternative field locations,a system well known as‘shuttle breeding'as used by CIMMYT and several North American breeding programs.Where more than two generations are required each year,the process can be assisted by tissue culture, seed treatment to ensure absence of dormancy [49], growth under continuous light,and forced seed ripening. Vernalization poses a problem but there are suggestions that this too can be overcome by specific environmental regimes[50].

Several groups in China have well established shuttle breeding systems that address some of the above constraints.Yuanmou County in Yunnan province(101°35′-102°05′E,25°25′-26°07′N)has served as the winter breeding base for spring wheat since 1973 [51,52]. It is located in the Jinsha River basin in the northern part of the Yunnan-Guizhou Plateau,where the annual mean temperature is 22 °C and the lowest temperature in December is 13.8 °C； annual precipitation is 648 mm (approximately 75% in summer) and the average daily sunshine hours are 7.3 h. More recently winter wheat has been cycled through Yuanmou County. It is planted following artificial vernalization in September to October and harvested in February. It is sown again in a high-altitude location at Guyuan in Hebei province(115°07′E, 41°67′N, North Winter Wheat Zone), where temperatures are low enough for vernalization in the field,and harvested in late August. Alternatively, potted seedlings vernalized under natural conditions can be transplanted at Yuanmou in December and harvested in March. Sanya in Hainan province is another location that allows wheat growth during the winter [53]. Other areas with cool summers allowing generation increase include Kunming in Yunnan province [54], Xining in Qinghai province[55], Dali in Shaanxi province [56], and Zhangbei in Hebei province.

Immature embryos following fertilization can be cultured in vitro to form plantlets on selective media in order to circumvent the grain filling stage. This method was used to enable five generations per year in pyramiding high molecular weight glutenin sub-units and powdery mildew resistance gene Pm21[57,58].

Methods for creating haploid plants include anther culture and the wheat × maize system[59].Chromosomes of haploid plants are doubled by colchicine treatment. The doubled haploids derived from anther culture had higher recombination frequency than those derived from wheat by maize hybridization [60]. Doubled haploid wheat technology has been used in wheat breeding and germplasm enhancement in Canada, USA, Europe, Brazil, China, and India [61-63]. In China, anther culture has been incorporated in wheat breeding programs since the late 1970s [48]. More than 40 cultivars derived from anther culture have been commercialized [64]. This method is more suitable for shortening the breeding cycle rather than developing cultivars because of strong genotype-specific selection in response to anther culture.Albinism is a major constraint in developing haploids through anther culture [62]. Wheat-maize system is extensively used in cultivar development in Canada, because it is genotype independent, and a number of doubled haploidderived cultivars have been widely grown [61-63]. Doubled haploid wheat breeding at CIMMYT has not been preferred to the long-standing shuttle breeding selection system, but is used in developing populations for genetic studies[65].

More recently, scientists from the University of Queensland,Australia and John Innes Centre,UK proposed a‘speed breeding'method that permits up to 6 generations per year of spring bread wheat [66]. This was realized by fine-tuning the functional expression of circadian clock genes through continuous photoperiod treatment. Speed breeding reduced the time for several selfing generations in developing recombinant inbred line populations within one to two years. In shortening breeding time this technology not only increases genetic gain over time,but also has potential for cultivar development in combination with recently emerged genomic technologies, such as highthroughput genotyping, genome editing, and genomic selection[67]. Several wheat research programs in China have developed laboratories based on the Australia-UK report.

3.3. Development and application of wheat breeding chips

Molecular marker-assisted selection (MAS) provides the possibility of selection at the genotype level. It is powerful and efficient because it is independent of environment and makes trait selection more accurate provided the information on which it is based is accurate and independent of genetic background. Single nucleotide polymorphism (SNP) markers are far more abundant than earlier classes of molecular markers based on DNA hybridization and PCR [68]. Microarray-based genotyping arrays of large numbers of SNP probes now available as Illumina Infinium BeadChip or Affymetrix Axiom arrays allow powerful and cost-effective highthroughput genotyping of breeding materials[69].

The first SNP chip for wheat was the Wheat 9K iSelect SNP array based on an Illumina Infinium platform[70].A total of 9000 SNPs was assembled in the Illumina iSelect SNP array and 7594 polymorphic loci were used to construct a consensus map with an average SNP density of 1.9 ± 1.0 SNP/cM. However, the SNP loci in this array were not evenly distributed across the subgenomes or chromosomes.The numbers of loci for the A and B genomes far exceeded those for the D genome.The Wheat 90 K iSelect SNP array,also based on the Illumina Infinium platform,comprised many more SNP loci (81,587) [71]. As the complete wheat genome sequence was not available at the time of development only 46,977 SNPs (57.6%) were mapped to specific chromosomes based on eight mapping populations. About half(49.5%)of the mapped SNPs were for the B genome,35.5%for the A genome and only 14.7%for the D genome.

The Affymetrix Axiom platform was used in the assembly of two further SNP chips, the Wheat 660 K Axiom [72]and the Wheat HD Genotyping array (820K) [73], developed independently in China and the UK.These SNP chips contain more SNP loci than the previous versions.However,the cost of using these SNP chips is high, thus limiting their application in breeding.Recently,Rimbert et al.[74]in France developed the TaBW280K,a 280K SNP array,using the Affymetrix Axiom platform.

Ongoing efforts were made to develop cost-effective SNP chips for wheat breeding. A subset of SNP markers based on the 90K Infinium array was used to develop the KWS 15K Wheat Infinium array that was used in several studies on agronomic traits, such as male floral traits and the reduced height gene Rht24 [75,76]. In Europe, the 819,571 SNP markers in the Wheat HD Genotyping array were also used to develop the 35K SNP chip, Wheat Breeder's Array specifically for breeding[77].These SNP platforms have been used in Europe.In China, an Axiom Wheat55K SNP Array containing 53,063 SNP markers was developed by the Institute of Crop Sciences,Chinese Academy of Agricultural Sciences, based on a subset of the Wheat660K SNP Array. The SNP markers in this array were selected to ensure a uniform distribution on all wheat chromosomes,each of which contain ～2600 SNP markers at a genetic distance of ～0.1 cM, an average physical distance of 300 kb. Another 55K wheat chip consisting of SNP and functional markers was developed by China Golden Marker(Beijing)Biotechnology Co.,Ltd.,and more recently,a 15K SNP chip containing many functional wheat markers was developed.A breeder SNP chip,the KPS Wheat 90K Chip,composed of 84,611 SNP markers, was jointly developed by the Beijing Hybrid Wheat Engineering Technology Research Center,Beijing Academy of Agriculture and Forestry Sciences and Beijing Compass Biotechnology Co., Ltd. All of these works were designed to provide affordable specialized chips with informative SNP markers for wheat breeding.They have been used in construction of genetic linkage maps, genome-wide association studies, genetic mapping, parental selection, and progeny genotyping in wheat breeding[4,78-81].

4. Concluding remarks

Increased wheat production on a falling planting area is attributed to genetic improvement in yield of new cultivars and agronomic measurement. Increased yield is the main objective of wheat breeding, but quality is also becoming increasingly important to meet the demands of more discriminatory consumers. High quality cultivars that are water efficient,widely adapted,and resistant to diseases and abiotic stresses are favored by seed companies and growers. Some old cultivars possess beneficial characteristics that prolong their production, for example, Ningchun 4 in the Northwest Spring Wheat Zone for its wide adaptability and promising noodle-making quality,and Xindong 20 in the Xinjiang Winter and Spring Wheat Zone for its early maturity that makes it suitable for the double cropping system in that region.Breeders must meet the demands of growers and markets in order for a new cultivar to be successful.

The standards for cultivar registration in China have been amended to meet market demands. The major changes in standards are increased weightings for quality and resistance to diseases and abiotic stress. For example, a cultivar can be certified on the basis of improved quality and/or disease resistance even though the yield has not increased relative to the control. These amendments will encourage breeders to shift their breeding objectives to produce cultivars with greater market value and requiring less protection by chemicals while maintaining yield levels.

Production conditions have also changed in many wheat production regions in northern China. For example, ＞80% of winter wheat fields in southern Xinjiang must include fruit trees；winter wheat cultivars must have better tolerance to shading when intercropped with fruit trees [82]. Higher levels of stripe rust and powdery mildew resistances will be required in oversummering areas for the pathogens in the high attitude regions of northwestern China.The objective of this strategy is to reduce the inoculum spread to other areas. Speed breeding strategies are receiving increased attention due to the high competitiveness of wheat breeding.This is being assisted by well-equipped greenhouse facilities now available to many breeding programs,as well as routine use of off-season nurseries in southern China and high-altitude areas in northern China. High-throughput genotyping through use of affordable wheat-specific SNP breeding chips will replace at least some field testing but breeders must continue to be cognisant of the effects of environment and background genotypes.

Bulked segregant analysis, in combination with highthroughput genotyping techniques, such as RNA-seq,genome-resequencing, and SNP arrays, provides a powerful and rapid method to map genes or QTL for traits of agronomic importance [83]. This technique is commonly used in large scale development of molecular markers for localizing wheat genes [81,84-90]. The availability of genome sequences for common wheat[91]and closely related diploid and tetraploid species [92-96]offers a resource to increase the efficiency of wheat breeding through genotype-based selection.It will also facilitate the molecular mapping and map-based cloning of wheat genes,and finally in breeding of new cultivars.

Acknowledgments

The authors are grateful to Dr. Xuexu Hu, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Beijing,China, for providing information on wheat quality. Financial support provided by the National Key Research and Development Program of China (2017YFD0101000) and the Agricultural Science and Technology Innovation Program is acknowledged.