Principles of Plant Genetics and Breeding

Host plant resistance to disease and insect pests
BREEDING WHEAT 479
Genetic resistance is the primary method of choice for controlling diseases in wheat, and has been proven repeatedly as an effective
and environmentally sound method to control serious yield-reducing pathogens. The use of disease-resistant cultivars
reduces the use of pesticides and thus contributes to a reduction in environmental contamination (Anderson 2000). Losses to all
pests and diseases currently average 10–20% annually, and therefore potential savings to growers, not counting the elimination
of costs of applying pesticides, are in the hundreds of millions of dollars (10% of the US wheat production has been worth about
US$500 million annually since 2000).
Genetic resistance has frequently resulted in selection pressure on the pathogen population, which then mutates to overcome
the resistance. Two strategies, either a pyramid or a multiline approach, have been used to increase the durability of resistance
genes. Gene pyramiding is the combining of two or more resistance genes. Resistance is more durable because additional mutations
in the pathogen population are needed to overcome the resistance in the host plant. Multiline cultivars are made up of a
series of closely related genotypes, each carrying different sources of resistance to a pest. The genetic diversity of the multiline
results in balancing selection against the pest that enhances the durability of the resistance genes deployed in the multiline cultivar.
Both strategies have been difficult to accomplish without MAS because once an effective resistance gene is present in a
breeding line, it is difficult to screen for the incorporation of additional resistance genes using the plant phenotype. Few multilines
have been released, in practice, because of the difficulty in introgressing so many different sources of resistance into visually and
agronomically similar genotypes. MAS can be used effectively to combine different resistance genes into elite lines while maintaining
pre-existing, effective resistance genes and to introgress several resistance sources into a recurrent parent.
On a global basis, the three rusts – leaf rust (Puccinia recondita), stem rust (P. graminis), and yellow or stripe rust (P. striiformis)
– are among the most damaging diseases of wheat and other small grain crops. Besides reducing yield, the rust diseases also seriously
affect the milling and baking qualities of wheat flour. Multimillion dollar yield losses have been attributed to leaf and stripe
rust every year since 2000. Stripe rust alone caused a US$360 million loss to the wheat crop in 2004 (updates are available online
(Long 2005)). New races of leaf and stripe rust have been identified in the USA since 2000 that are virulent in many of the previously
resistant cultivars. Fortunately, new resistance genes have been identified in wheat cultivars, and wild relatives of wheat
have been introgressed into hexaploid wheat. Using MAS, six new leaf rust resistance genes and five stripe rust resistance genes
have been introgressed into 58 lines and cultivars from eight different market classes. Leaf rust resistance genes include Lr21,
Lr39, and Lr40 from Triticum tauschii, Lr37 from T. ventricosum, Lr47 from T. speltoides, and LrArm from T. timopheevi subsp.
armeniacum. Four major stripe rust resistance genes Yr5, Yr8, Yr15, and Yr17 have been combined with the high temperature
adult plant resistance gene identified in “Stephens”. This resistance has been durable for over 25 years in the Pacific northwest. A
quantitative trait locus (QTL) for this resistance has recently been identified, probably located on chromosome 6BS (K Campbell,
unpublished data). A new stripe rust resistance gene Yr36 has also been identified on chromosome 6BS, closely linked to the
HPGC gene. Both genes can be simultaneously selected using PCR (polymerase chain reaction) markers Xuhw89 and Xgwm193
(J. Dubcovsky, personal communication). PCR-specific markers are available for Lr2, Lr47, and the linked group of resistance
genes Lr37-Yr17-Sr38 (Helguera et al. 2003). Microsatellite marker Xgwm210 is linked to Lr39 and Xgwm382 to LrA m .
Microsatellite markers Xgwm18 and Xgwm264 flank stripe rust resistance gene Yr15 (Chen et al. 2003).
Fusarium head blight (FHB) is an important disease in common and durum wheat producing areas of the United States and
Canada. An epidemic of FHB from 1993 to 1997 resulted in devastating economic losses to the wheat industry of the region, with
1993 estimates alone surpassing $1 billion. FHB causes both severe yield reduction and decreased grain quality. In addition,
infected grain may contain harmful levels of mycotoxins that prevent its use for human consumption or feed. Control of FHB has
been difficult due to the ubiquitous nature and wide host range of the pathogen, and dependence of the disease upon unpredictable
climatic conditions. In some parts of the USA, fungicides have been used to reduce losses, but this practice adds to
grower costs, poses significant environmental risks, and is not always effective. Available resistance to FHB in wheat is quantitatively
expressed, with a continuous distribution among progeny. Two major QTLs have been identified on chromosome 3BS from
“Sumai 3” and on chromosome 3A from T. dicoccoides. Microsatellite markers, Xgwm533 and Xgwm493, bracketing both QTL
regions have been used to introgress these genes into 28 durum and common wheat cultivars. The selected QTL region from the
“Sumai 3” chromosome arm 3BS explains up to 40% of the phenotypic variation in FHB resistance in one cross. Selection for the
QTL region from chromosome arm 3AS in durum wheat has been done using SSR markers Xgwm2 and Xgwm674. The QTL
region on 3AL explains more than 37% of the phenotypic variation in a durum cross. Both FHB QTL regions are robust and are
expressed in well-adapted genetic backgrounds (Liu & Anderson 2003).
Practical use of MAS in forward breeding programs
MAS selection programs must be integrated at all times into existing breeding programs. Recurrent parents are selected from
high-yielding, elite germplasm. Intermediate products of MAS are returned to the breeding programs for evaluation and crossing
purposes. After each generation of backcrossing, selected heterozygous plants are self-pollinated and the BC 1–3 F 2 seeds planted
as additional segregating populations. A strict backcrossing strategy is not expected to increase yield, except for the reduction of
yield losses due to pathogens. Therefore, this backcrossing strategy should be used only as a complement of active “forward
breeding” programs.