Breeding new cultivars for sustainable wheat production

Common wheat(Triticum aestivum L.)is a major staple food crop that feeds about 40% of the world's population. Wheat production and utilization accounts for ～28% of the global cereal crops(http://www.foodsecurityportal.org/)food-outlookbiannual-report-global-food-markets). Consequently, wheat supplies approximately one-fifth of human calories in a variety of forms,including leavened,flat and steamed breads,biscuits,cakes,different forms of noodles,pasta,couscous,and pies,as well as secondary products such as starch, gluten and nonflour milling fractions. Wheat consumption has been steadily increasing due to population expansion and urbanization. For example, wheat consumed in China increased more than sixfold (from 19 million tons to 123 million tons) in the 50 years from 1962 to 2012[1].

The annual global increase in wheat yield was estimated to be 1.0% [2].This does not meet the increasing demand for wheat. Breeding new cultivars is one of the most important drivers for increasing wheat production； however, breeding objectives vary based on local conditions and must be in accordance with the needs of consumers, processors and farmers. Cultivar development is highly competitive.Advances in high-throughput phenotyping and genotyping,genomic selection,molecular marker-assisted selection,and speed breeding improve the efficiency of wheat breeding.The deciphering and release of genome sequences for common wheat and its wild relatives have revolutionized wheat genetics and breeding research and opened the door for the exploitation of genomics in wheat breeding. Most of the world's wheat is produced in the Northern Hemisphere.This special issue, Breeding Wheat for the Global North,includes 13 review and research articles contributed by scientists from China, the USA, Canada, Mexico, and Australia.

Wheat breeding is a continuous effort of delivering new cultivars to farmers. A review by Li et al. [3]summarizes wheat production, progress of wheat breeding targeted to the northern part of the country, and technical improvements including dwarf-male-sterile wheat, speed breeding, and breeding chips. Increased grain yield is an ongoing objective of wheat breeding. Two contributions report the effects of genes TaGW8, Rht8 and Ppd-D1a on yield-associated traits [4,5].

Climate change will alter the relative importance of diseases. Fusarium head blight (FHB) is becoming a major threat to wheat production in many cereal production regions. It not only limits grain yield but also results in toxin-contaminated grain that is harmful to both humans and livestock. In recent years, epidemics of FHB and more northerly spread of occurrence has attracted attention by agricultural administrators, breeders, and growers in China and elsewhere. Breeding efforts are underway to improve FHB resistance in different wheat production regions. One review article and two research papers relate to improvements in FHB resistance. The contribution by Zhu et al. [6]reviewing breeding practices for improving resistance to FHB in China, the USA, and Canada describes the sources of FHB resistance currently used in these countries. A group of scientists from China and the USA,headed by Dr.G.H.Bai,report on the characterization of FHB resistance QTL in Chinese wheat landraces Haiyanzhong,Wangshuibai, Baisanyuehuang, Huangfangzhu, and Huangcandou [7]. In addition to the previously identified Fhb1 gene on chromosome 3BS several new QTL were detected on chromosomes 3A,2D,3D,and 4D.KASP markers have been developed for marker-assisted selection of these FHB resistance QTL. Fhb1 is currently the most effective gene conferring resistance to FHB.Three groups of scientists reported cloning of this important resistance gene [8-10]. A breeder-friendly marker for detection and selection of Fhb1 was based on a putative histidine-rich calcium-binding protein (TaHRC) [11]. Although Fhb1 has been successfully exploited by breeders in North America its use in the more northerly winter wheat regions in China has been problematic. As a consequence scientists at the Chinese Academy of Agricultural Sciences in Beijing have adopted a program of molecular marker-assisted backcrossing to leading cultivars and advanced breeding materials in the region. Many advanced lines with promising disease resistance are now in yield trials [12].

The discovery of new disease resistance genes contributes to sustainability of disease resistance. A major QTL for resistance to tan spot and Septoria nodorum blotch was detected on chromosome 5AL using two recombinant inbred line populations[13].Wheat relatives are important sources of genes for disease resistance. Pm64, a new gene conferring resistance to powdery mildew in a wheat derivative of wild emmer (Triticum dicoccoides) was identified on chromosome 2BL [14]. Its location was very close to that of stripe rust resistance gene Yr5. Several stripe rust resistant wheat-Thinopyrum intermedium introgression lines were developed by a group of scientists at Sichuan Agricultural University[15].The release of genome sequences of common wheat and its diploid (Aegilops tauschii) and tetraploid (T. dicoccoides)ancestral species as well as durum wheat expedites mapbased cloning of disease resistance genes.Pm2 that encodes a CC-NBS-LRR protein conferring resistance to powdery mildew was isolated through a positional cloning method. On the basis of the cloned sequence several previously named alleles of Pm2 were shown to be identical[16].Resistance genes used monogenically in agriculture are usually not durable.Responsible gene stewardship demands that resistance genes be used in effective combinations rather than being deployed singly. Laroche et al. [17]describe the practice pyramiding genes for resistance to the rusts in western Canada.

Abiotic stresses also pose major challenges to production.They are emerging in current production areas as climate change takes hold and also limit expansion of wheat production to new areas. Examples of these stresses include cold/heat, drought/waterlogging, salinity/alkalinity, and mineral deficiencies/toxicities. Low temperatures in certain wheat production areas during early spring can cause significant yield reductions in northern China. Zhao et al.[18]report on differentially expressed genes and metabolite profiles detected under freezing and cold acclimation treatments. The abscisic acid/jasmonic acid signaling pathway and proline metabolism were induced by cold treatment. Reduced production costs increase net returns to growers and can contribute to lower consumer prices.Herbicides are routinely used in weed control both in wheat and preceding crops and, when used responsibly, contribute to production and profitability. However, improper use of herbicides may cause damage to wheat. Resistance to herbicides in wheat, a major emerging problem worldwide,is covered in a review by Nakka et al. [19]. Mechanisms of herbicide resistance in wheat and weed populations in the wheat cropping systems and applications in weed management are also discussed.

Finally, the editors for this special issue thank all authors for their contributions, and reviewers for comments and suggestions for improving manuscripts. We thank the Editorial Office of The Crop Journal for their efficient handling and processing of the contributions selected for this special issue. We also acknowledge the support to this special issue by the National Key Research and Development Program of China(2017YFD0101000).