Unit 7 Wheat - Triticum aestivum and Related Species

Origin, Taxonomy, Genetic and Reproductive Systems

Wheat is grown on more hectares than any other food crop, and is one of the most important sources of nutrients for humans in many areas of the world. The appearance of the plant is typical of the grass family, but the head (a spike) is densely packed with grains. Most cultivars have awns, which are the appendages attached to each spikelet (unit of the inflorescence), giving the heads a bearded look.

Important wheat species

Wheat was domesticated in the Near East at least 9,000 years ago. It originated through hybridization and polyploidy of several species of the genus Triticum, all with a basic chromosome number of 7 (n=7). Some species of Triticum are cultivated and some are noncultivated, or rarely so. The wild species of wheat can still be found in northern Iraq, Iran, and Turkey. The most important contemporary Triticum species and subspecies are described in this section.

Common wheat - Triticum aestivum L.

Common bread wheat is an allohexaploid, with 2n=42 chromosomes that are thought to derive from three wild ancestors, each with a unique genome of n=7 chromosomes. The genome of wheat is designated 'ABD' to signify the ancestral sources of its 21 pairs of chromosomes.

Durum - Triticum turgidum subsp. durum Durum wheat is a tetraploid species with 2n=28 chromosomes derived from two genomes (AB). It is widely grown in the northern US, Canada, southern Europe, and parts of India. Due to its high protein content, it is the preferred type of wheat for making spaghetti, macaroni, and noodles.

Club - Triticum aestivum subsp. compactum Club wheat is a hexaploid (ABD) with 2n=42 chromosomes, belonging to the same species as common bread wheat. The heads of this subspecies are more compact, but the difference can be attributed to changes in just two genes controlling spikelet density. Most of the commercial production of club wheat occurs in the Pacific Northwest of the US, with limited production in Australia.

Related species and wheat progenitors

Einkorn - Triticum monococcum Einkorn was one of the first species of wheat that was grown for food, and wild forms still exist. There is evidence that it was cultivated in the Stone Age (10,000 BC) in the Near East and southwestern Europe. Einkorn is diploid and its 2n=14 chromosomes are derived from a single genome (designated AA). Unlike modern wheat, the seeds of the wild forms of Einkorn do not thresh free of the chaff, and the rachis of the spike is brittle and breaks apart at maturity.

http://www.hort.purdue.edu/newcrop/proceedings1996/V3-156.html

Emmer / Polonicum - Triticum turgidum subsp. dicoccum

Emmer was the dominant type of wheat produced in the Near East and Europe between 10,000 and 4,000 BC. Like einkorn, it also has a brittle rachis, with the hull remaining attached to the grain during threshing. It is a tetraploid species (AABB) with 2n=14 chromosomes derived from hybridization of einkorn wheat and a type of goat grass. Formerly the various species of goat grasses were considered to be in the genus Aegilops, but now they are included in the genus Triticum. Limited amounts of emmer are grown today, primarily as feed for animals. Durum wheat is closely related to emmer, but the grain of Durum can be readily separated from the chaff.

Spelt - Triticum aestivum subsp. spelta

http://www.hort.purdue.edu/newcrop/afcm/spelt.html

Spelt is thought to have originated from hybridization between emmer wheat and another species of goat grass (Triticum tauschii), followed by doubling of the chromosomes to produce a hexaploid (2n=42 with three genomes: ABD). Spelt was the dominant wheat grown throughout the Near East, Balkans and Europe during the Bronze age (4,000 to 1,000 BC). Some of the earliest references to spelt wheat are from the Bible.

Spelt is an alternative feed grain to oat and barley, with nutritional value similar to oats. Unlike modern wheat, the grain remains attached to the hull during threshing. The spelt hull has nearly as much feed value as the grain. Spelt can be used as food grain after removal of the hull and can be obtained through organic or health food outlets. There are a few hundred thousand acres of production in the US.

Spelt is thought to be easy to digest and to impart a unique taste in bread products. Spelt is sometimes suggested as an alternative for individuals with allergies to wheat or gluten proteins. However, for the majority of sufferers of Celiac disease, spelt is not an alternative to wheat, because it has gluten proteins similar to those of common wheat.

Polyploid origin and domestication of wheat

The diagram below provides a summary of important aspects of wheat domestication, highlighting the importance of polyploidy as a mechanism for crop evolution.

Cytogenetic and molecular studies have shown that there is a high level of homoeology among the three genomes. Corresponding chromosomes from the A,B, and D genomes have many of the same genes, and the genes tend to occur in a similar order along the chromosomes. This suggests that the three ancestral species also evolved from a common ancestor.

Why is polyploidy important?

Genetic redundancy (duplication of genes) permits wheat to tolerate major structural changes in chromosomes.

Cytogenetic relationships among the chromosomes were determined by Dr. Ernie Sears (University of Missouri) through his classic work on the development and analyses of aneuploid genetic stocks (aneuploids have one more or one less than the normal, euploid number of chromosomes.)

Genetic stocks for gene mapping

• Aneuploid series
• Chromosome substitution series
• Chromosome translocations from related species
  • Example - 1B/1R and 1A/1R from rye
• Gene introgression from related species
• Major genes for disease and insect resistance
  • Example: T. tauschii was donor for resistance to leaf rust; Septoria; Hessian fly
  • Example - Aegilops ventricosa was donor for footrot resistance
• Wide array of related species
• Continued importance of progenitor species as genetic resource
• Similar ‘location’ of genes among homoeologues
  • Advantage for genetic mapping
  • Suggests targets for genetic investigations and gene introgressions

Difficulties in molecular genetic manipulations of wheat

• Three genomes to map
• Three copies of many genes
• High level of redundancy
• Most traits are under polygenic control and quantitative inheritance
  • Example: Three genes for seed coat color
  • In Red x White cross, few as 1/64 white seeds occur in an F2 population - must have very large populations to determine locations of genes on the chromosome map
• Difficult to identify and manipulate ‘minor genes’
• Gene regulation, gene interactions are more complicated in polyploids
• Direct impact on transformation and genetic engineering efficiency - more complex
• Expression of inserted genes may be masked or silenced, even if native genes are reinserted
  • example: glutenin protein alleles in ARS research

Growth Requirements, Physiological and Adaptive Traits

Wheat is a cool season crop that grows best at temperatures around 60 °F. The minimum length of the growing season is about 100 days, depending on the cultivar and environmental conditions. From 15 to 20 or more inches of precipitation are necessary for annual cropping. In some dryer areas wheat is grown once every 2 years, with the land fallowed (kept free of vegetation) in alternate years to accumulate moisture in the soil.

Physiological traits of wheat

• Broadly adapted
• C3 photosynthesis
• Tolerant of low temperatures
• Winter dormancy and cold tolerant
• Less tolerant of hot, humid environments
• Sensitive to acid soils (Aluminum toxicity)

Growth stages of wheat

Review at least one of the sites below to learn about wheat morphology and growth stages:

The Extension Service at Kansas State University has developed a website called "Adopt a Wheat Field" that takes a step-by-step look at the growth and development of a wheat crop throughout the growing season in Kansas. Although intended for kids, the site provides a close-up view of the crop from a grower's perspective that can also be appreciated by adults: http://www.oznet.ksu.edu/pr_aawf/welcome.htm

For more technical information, see "The Growth and Development Guide for Spring Wheat" from the University of Minnesota Extension Service: http://www.extension.umn.edu/distribution/cropsystems/DC2547.html

Additional information on growth stages is available from the North Dakota State University Extension Service:
http://www.ext.nodak.edu/extpubs/plantsci/weeds/w564w.htm

Compensatory growth pattern - ‘plasticity’

Components of Yield:

• number of tillers
• number of spikes
• number of florets/spike
• number of seeds/floret
• number of seeds/spike
• seed weight

The plant may compensate for a decrease in one component by increasing another yield component. For example, a plant that is stressed early in the season and produces few tillers may have larger kernels (higher seed weight). This plasticity contributes to yield stability over environments and facilitates response to changing environmental conditions during the growing season.

Major traits impacting adaptation

Vernalization response

• Winter vs spring wheats - winter wheats require a period of cold for flowering to occur
• Facultative - development ‘hastened’ by vernalization

Photoperiod response - daylength

• Wheat is a long-day plant requiring long days (short nights) to flower. Some cultivars are photoperiod insensitive.

Abiotic stress tolerances

• Limited genetic variation for these traits
• Drought, heat, frost
• Soil conditions: acid soils, mineral stress, salt tolerance
• Winterhardiness, cold tolerance

C3 Photosynthesis

• Less efficient in hot climates or under tropical conditions

Earliness per se

• Grain filling duration and rate
• Important for varietal adaptation within a growing region

Grain Composition, Processing and End-use Quality

Grain composition

http://www.smallgrains.org/WHFACTS/kernel.htm

The endosperm makes up about 83 percent of the kernel weight. It consists of starch granules embedded in a protein matrix and is the source of white flour. The greatest share of total grain protein and carbohydrates occur in the endosperm. The endosperm is also a good source of iron, and many B-complex vitamins, such as riboflavin, niacin, and thiamine.

The bran constitutes about 14 1/2 percent of the kernel weight. Bran is included in whole-wheat flour and is also available separately. Of the nutrients in whole wheat, the bran contains a small amount of protein, larger quantities of the B-complex vitamins listed above, trace minerals, and indigestible cellulose material also called dietary flour.

The germ contributes about 2 1/2 percent of the kernel weight. The germ is the embryo or sprouting section of the seed. Of the nutrients in whole wheat, the germ contains minimal quantities of protein, but a greater share of B-complex vitamins and trace minerals. Wheat germ can be purchased separately and is included in whole-wheat flour. However, the germ is usually separated because it is high in fat, which limits the keeping quality of flour.

Nutrient Comparison of Selected Wheat Foods

Gluten proteins

Wheat is the only plant species to possess gluten proteins and the only species to have proteins with these unique physical properties.

The definitive trait of wheat is its ability to form a visco-elastic gluten network when hydrated in doughs.

Cystine disulfide bonds in endosperm storage proteins have the ability to form protein networks (polymers) in doughs. Proteins are linked via disulfide bonds to form a macro-polymer network which holds gas bubbles during fermentation and baking.

Gluten protein quality is defined by genes for:

High- and low- molecular weight glutenins

• Linear molecules with extensive branching
• Contributes to dough ‘strength’
• LMW (low molecular weight) glutenins have less branching - Contributes ‘viscous’ property
• Highly repetitive structure with disulfide S-S covalent bonds on ends
• Non-covalent hydrogen bonds between strands
• Contributes elastic properties
• Macro-polymers
• Among largest naturally occurring molecules: >100,000 mw

Gliadins (omega and gamma)

• Circular protein bodies
• 25,000-100,000 mw
• Contribute extensible properties to gluten and dough

Non-gluten proteins

• <25,000 mw
• Albumins, globulins; flour enzymes

Ratios of HMW and LMW glutenins, gliadin, and non-gluten proteins are important to meet functionality and product quality requirements.

Milling

Milling is the separation of the bran and germ from the endosperm and the reduction of the endosperm to uniform particle size (flour). This is done by a sequence of breaking, grinding and separating operations.

Breaking involves passing the wheat through a series of grinding rolls which break the wheat up into a bran fraction (which is removed), large mainly bran-free endosperm chunks, and a small amount of flour. The endosperm chunks are then passed through a smooth set of rolls which reduce the endosperm to finer and finer particles. After each break and reduction roll, the ground material is sieved and free flour is removed, leaving only large particles to go forward into the next set of rollers, where they are further reduced to produce more flour. A typical flour mill will have up to four break rolls and twelve reduction rolls, which lead to the production of some 16 flour streams, a nearly pure bran stream, a germ stream and a bran/flour/germ wheat feed stream.

The milling process is common to the production of all flours. The quality of the wheat going into the mill, e.g. protein content, will determine the types of flour being produced. By blending together the many different flour streams produced by the mill, a miller can create further variations in features such as flour color. Very white flours would come from the early streams only, while most streams produce brown products. Whole meal flour is produced when all the streams, bran, germ and flours are blended back together with nothing removed.

U.S. Wheat Market classes

http://www.smallgrains.org/whfacts/6CLASSWH.HTM

Hard red winter - HRW
High protein, strong gluten
Pan breads, yeast breads, buns, rolls

Hard red spring - HRS
Highest in protein, med-strong gluten, blending wheat
Pan breads, yeast breads, buns, hard rolls

Soft red winter - SRW
Med-low protein, weak gluten
Flat breads, cakes, pastries, crackers

Soft white - SW
Spring and winter types are not differentiated
Low protein, very weak gluten
Flat breads, cakes, pastries, crackers, noodles, batters and thickener

Hard white - HW
Medium to high protein, med-strong gluten
Yeast breads, Asian noodles, steam bread

Durum
High protein, medium to strong gluten
Pasta, macaroni, spaghetti

‘Industry acceptability’ is required.

‘Value-added’ quality traits desired for IP marketing (sale as patented varieties).

The production of new wheat varieties requires intensive testing of baking quality:

Hard wheats - breads, noodles

	Soft wheat – cookies, cakes

Wheat Production and Marketing Statistics

Minnesota Association of Wheat Growers. Wheat Facts. http://www.smallgrains.org/Toolshed/Wheat_Facts/wheat_facts.htm

United States Department of Agriculture
National Agricultural Statistics Service
http://www.usda.gov/nass/aggraphs/cropmap.htm#wheat

Winter wheat production is concentrated in the central Great Plains states and in the Pacific Northwest. These states have sufficient cold in the winter to satisfy vernalization requirements. Spring wheat production is concentrated in the northern Great Plains where temperatures are too low to sustain a crop through the winter. Soft white winter and soft red winter varieties are grown in areas with relatively high rainfall.

Major Wheat Producing Countries, 2002

Major Diseases and Insect Pests

Wheat Diseases and Pests: a guide for field identification

http://wheat.pw.usda.gov/ggpages/wheatpests.html

Wheat diseases

The rusts are obligate parasites, which means that they must have the appropriate host in order to survive. The sexual stage for Stem rust is known to occur on Barberry. The alternate hosts required for the sexual stage of Leaf rust do not occur in North America. There is no known alternative host or sexual stage for Stripe rust.

Host plant resistance is the most effective mode of control for the rusts. Major race changes occur frequently for leaf and stripe rust in response to resistance genes deployed in production areas.

Leaf spotting diseases
Leaf blotch - Septoria tritici
Tan Spot - Helminthosporium tritici

Bunts or smuts Common bunt Loose smut Karnal Bunt Dwarf smut The bunts and smuts have a major impact on international trade, grain and seed shipments.       Smut

Head Scab (Fusarium) Fungus produces a toxin (DON)

Root diseases Cephalosporium stripe (Cephalosporium gramineum) Take-all (Ophiobolus graminis) Eyespot or Strawbreaker footrot (Psuedocercosporella herpotrichoides) Dryland crown rot (Fusarium graminearum and F. culmorum) Sharp eyespot (Rhizoctonia) Psuedocercosporella strawbreaker footrot

Viruses Wheat streak mosaic (transmitted by curl mite) Barley Yellow Dwarf (transmitted by aphid)   Barley Yellow Dwarf Virus

Insect pests Hessian Fly Russian Wheat Aphid Bird-Cherry Oat Aphid and Greenbug Transmit Barley Yellow Dwarf Virus

Genetic Resources and Breeding

Wheat reproduction and breeding

• Predominantly self-pollinated
• Most varieties are inbred, purelines, with limited internal variation
• Seed can be saved and maintained by farmer - no loss in productivity
• Limited heterosis or hybrid vigor
• Hybrid seed market unsuccessful to-date
• Very few multilines, composites, or landraces are grown
• Genetic technology and breeding mostly in public sector

Extensive world collections Land races, modern cultivars, related species National Plant Germplasm System - Aberdeen, ID working collection; 60,000 accessions National Center for Genetic Resources Preservation (NCGRP) - Ft. Collins, CO CIMMYT - Mexico Vavilov Institute - Russia Australian collection National and regional breeding programs

Wheat Breeding Contributions and Impact Life-span of a wheat variety: 4 - 6 years Average cost of variety development: $1+ million per variety Average yield increase per year from breeding : 0.5 to 1% Total yield increase from breeding since 1950 : 2X to 3X, depending on growing region

New directions in variety development:
public-private partnerships to deliver biotechnology products and market novel traits

Assignments

Review some of the web sites listed in the References section below. You will find useful information to reinforce material presented in this lecture.

Quiz

Take the quiz on this Unit on the Blackboard.

References

Center for New Crops & Plant Products, Purdue University. 2003. Triticum species.
http://www.hort.purdue.edu/newcrop/nexus/Triticum_spp_nex.html

Curtis, B.C., S. Rajaram, and H. Gómez Macpherson. 2002. Bread wheat: improvement and Production. FAO Plant Production and Protection Series No. 30.
http://www.fao.org/DOCREP/006/Y4011E/y4011e00.htm

Fowler, D.B. 2002. Winter Cereal Production. University of Saskatchewan. http://www.usask.ca/agriculture/plantsci/winter_cereals/index.php

Kansas State University Research and Extension. Adopt a Wheat Field. http://www.oznet.ksu.edu/pr_aawf/welcome.htm

Kansas State University Research and Extension. 2005. Wheat Page.
http://www.oznet.ksu.edu/pr%5Fwheatpage/

Levetin, E. and K. McMahon. 2005. The grasses. Chapter 12 in Plants and Society, 4th edition. McGraw-Hill, New York, NY. Additional on-line notes and references:
http://highered.mcgraw-hill.com/sites/0072528427/student_view0/chapter12/chapter_outline.html

Minnesota Association of Wheat Growers. 2004. Small Grains. The internet source for small grain growers. http://www.smallgrains.org/

Minnesota Association of Wheat Growers. 2004. Wheat Facts.
http://www.smallgrains.org/Toolshed/Wheat_Facts/wheat_facts.htm

NABIM. Flour milling.
http://www.nabim.org.uk/flourmilling.asp

Oplinger, E.S., E.A. Oelke, A.R. Kaminski, K.A. Kelling, J.D. Doll, B.R. Durgan, and R.T. Schuler. 2000. Spelt. In Alternative Field Crops Manual
http://www.hort.purdue.edu/newcrop/afcm/spelt.html

Simmons,S.R., E.A. Oelke, and P.M. Anderson. 1995. Growth and development guide for spring wheat. University of Minnesota Extension Service.
http://www.extension.umn.edu/distribution/cropsystems/DC2547.html

Stallknecht, G.F., K.M. Gilbertson, and J.E. Ranney. 1996. Alternative wheat cereals as food grains: Einkorn, emmer, spelt, kamut, and triticale. p. 156-170. In: J. Janick (ed.), Progress in new crops. ASHS Press, Alexandria, VA.
http://www.hort.purdue.edu/newcrop/proceedings1996/V3-156.html

USDA. Wheat Diseases and Pests: a guide for field identification
http://wheat.pw.usda.gov/ggpages/wheatpests.html