Martínez, F.1, 2 * and D. Rubiales1
1 Instituto de Agricultura Sostenible (CSIC), Departamento de Agronomía y Mejora Genética Vegetal. Apartado 4084. 14080 Córdoba, Spain.
2 Laboratory of Plant Breeding. Building Number 512 (Wageningen University). Binnenhaven 5, 6709 PD Wageninen, The Netherlands.
* Correspondence author: Email: firstname.lastname@example.org
Accepted for publication: 30 November 2002
Citation: Cereal Rusts and Powdery Mildews Bulletin [www.crpmb.org/] 2002/1130martinez
Puccinia triticina (wheat leaf rust) is an important disease of cultivated wheat worldwide. It can infect bread and durum wheat as well as triticale. The use of genetic resistance is the most economical and environmentally friendly way to control this disease. Breeders have relied mostly on resistance that is governed by major genes. This is usually associated with plant cell necrosis around the infection site (low infection type) and therefore called hypersensitivity. This kind of resistance may be overcome soon after cultivars carrying the resistance are grown on a large acreage. There is a great concern about the lack of durability of disease resistance (Johnson, 1992). Several strategies can be adopted to prolong the durability of the ephemeral Lr resistance genes, such as gene pyramiding, diversification and application of cultivar mixtures (McDonald and Linde, 2002). Another possibility would be to introduce resistance types that are intrinsically durable like partial resistance, which is characterised by a slow epidemic build-up despite a high infection type (IT) (non hypersensitive type of resistance) (Parlevliet and Van Ommeren, 1975).
Most of the studies on resistance to leaf rust on wheat have been carried out on bread wheat. The resistance to leaf rust in durum wheat has been little studied. There are indications that the resistance to leaf rust in durum wheat can be based on genes different to the Lr genes reported on bread wheat cultivars (Zhang and Knott, 1990; 1993). There are 46 durum wheat cultivars of differential resistance to leaf rust reported (Cereal Disease Laboratory of USDA 2001).
Leaf rust is gaining importance in durum
wheat (Martínez, 2002) which makes it necessary to search for novel
sources of resistance and their characterisation. The Italian durum wheat
cultivar Creso possesses a high level of resistance of P. triticina
that has turned out to be effective since 1975 and hence may be considered
durable (Pasquini and Casulli, 1993). For this reason this cultivar has
been selected to study the genetic of its resistance.
Materials and Methods
116 F2 plants from a cross between the resistant cultivar Creso and the susceptible cultivar Pedroso (durum wheat as well) were studied for resistance to P. triticina. These were sown in plastic boxes (35x20x8 cm) containing each box about forty F2 plants, four plants of Creso and Pedroso and one F1 plant.
Seedlings were inoculated in the third leaf stage, DC
13 (Zadoks et al., 1974), with the isolate Flamingo of P. triticina
(virulence/avirulence avr2b, 2c, 11, 12, 14a, 14b, 15, 18, 20, 21,
22, 23, 33, 34, 35, 44(I), B(I), B(II)/Avr 1, 2a, 3, 3bg, 3ka, 9, 10,
13, 16, 17, 24, 25, 26, 28, 29, 30, 32, 37, 38(I), 38(II), 44(II), W).
Five milligrams of freshly collected spores were used per box resulting
in a deposition of about 100 spores/cm2. The mixture of spores and talcum
powder (1:20 v/v) was dusted over the plants of which the third leaves
were placed in a horizontal position adaxial side upward. Afterwards plants
were incubated during 24 hours in humidity at saturation and darkness
at 20º C. Then plants were transferred to a growth chamber at 20ºC,
14 h of photoperiod and 112 µmol/m2.sec of light intensity.
Daily counting of the number of uredia in a marked area was made from
the beginning of sporulation until the number of uredia no longer increased.
These countings were used to calculate the latency period. The latency
period was estimated as the time period between the beginning of incubation
until the moment that 50 % of the uredia had appeared (Rubiales and Niks,
1995). Twelve days after inoculation the infection type (IT) of the infection
sites on each leaf was evaluated on a 0-9 scale (McNeal et al.,
1971). IT8 (large pustules not associated with necrosis or chlorotic plant
tissue) were considered the threshold value separating resistance and
Cultivar Creso displayed a very low IT (IT1-3) whereas Pedroso showed a very high IT (IT9) (Table 1). All F1 plants displayed an intermediate IT (IT5) (uredia surrounded by necrosis). F2 segregated discontinuously into plants showing IT from 1 to 7 (71 %), which is considered resistant, and plants showing IT9 (29 %), which is considered fully susceptible. These percentages fit to a proportion 3:1 according to a Chi-square analysis (Chi-square = 0.99, significant to 95 %). A high percentage of lines displayed an IT greater than or equal to 8 (Figure 1).
There was substantial variation for latency period of the F2 plants displaying IT ³ 8 (compatible or susceptible reaction). The latency period ranged between 91 and 122 % of that of cultivar Pedroso (Figure 2).
The resistance of Creso is due to the combination of a
hypersensitive reaction, visible macroscopically as a low IT, and to a
long latency period. The segregation in the F2 plants shows that this
hypersensitive resistance is based on a single gene. The dominance is
not complete since F1 plants displayed an intermediate IT, higher than
that of the resistant parent Creso. This indicates that the heterozygous
genotype, with only one allele expressing resistance, does not display
the resistance as well as the homozygous genotype. Partial dominance of
the resistance has been reported in wheat to leaf rust (Kolmer and Dyck,
The combination of different mechanisms of resistance that occurs in Creso is a valuable strategy to increase durability in the resistance (Rubiales and Niks, 2000). Similar combinations of mechanisms are reported in some resistant durum wheat (Singh et al., 1993). However the combinations must have a solid basis of partial resistance. In this way, Pretorius & Roelfs (1996) tried to reproduce the resistance of bread wheat "Era", with durable resistance to P. triticina combining its postulated genes (Lr10, Lr13 and Lr34) in one susceptible background. Some of these lines were more susceptible than "Era" leading to the conclusion that the additional resistance of this wheat was probably due to extra genes for partial resistance expressed in "Era".
Durable resistance in Creso appears to be due to a combination
of a single dominant gene conferring hypersensitive resistance and one
or more additional factors conferring partial resistance. This cultivar
might be used to increase durability to P. triticina in durum wheat.
We gratefully acknowledge CYCIT project AGF99-1036 for
financial support, Ana Moral for technical assistance and Dr Rients Niks
for the critically reading of this manuscript.
Cereal Disease Laboratory. 2001. www.cdl.umn.edu/res_gene/durlr.html, University of Minnesota St. Paul, MN 55108.
Jacobs T, Broers L, 1989. The inheritance of host plant effect on latency period of wheat leaf rust in spring wheat. I: Estimation of gene action and number of effective factors in F1, F2 and backcross generations. Euphytica 44, 197-206.
Johnson R, 1992. Reflections of a plant pathologist on breeding for disease resistance, with emphasis on yellow rust and eyespot of wheat. Plant Pathology 41, 239-54.
Kolmer JA, Dyck PL, 1994. Gene expression in the Triticum aestivum-Puccinia recondita f. sp. tritici gene-for-gene system. Phytopathology 84, 437-40.
Martínez F, 2002. Mecanismos de resistencia a roya en trigo y cebada. University of Córdoba, Spain. PhD thesis.
McNeal FH., Konzak CF, Smith EP, Tate WS, Russell TS, 1971. A uniform system for recording and processing cereal research data. USDA, Agricultural Research Service, Washington, D.C. ARS, 34-121.
McDonald BA, Linde C, 2002. The population genetics of plant pathogen and breeding strategies for durable resistance. Euphytica 124, 163-80.
Niks RE, 2002. Too long incubation may lead to poor expression of resistance to rust fungi in barley. Cereal Rust and Powdery Mildew Bulletin http://www.crpmb.org/2002/0128niks/.
Parlevliet JE, Van Ommeren A. (1975). Partial resistance of barley to leaf rust, Puccinia hordei. II. Relationship between field trials, micro plot tests and latent period. Euphytica 24, 293-303.
Pasquini M, Casulli F, 1993. Resistenza "durevole" a Puccinia recondita f. sp. tritici Ed Erysiphe graminis f. sp. tritici in frumenti duri italiani. Phytopathoogial Mediterranea 32, 135-42.
Pretorius ZA, Roelfs AP, 1996. The Role of Lr10, Lr13, and Lr34 in the expression of adult plant resistance to leaf rust in the wheat cultivar Era. Plant Disease 80, 199-202.
Rubiales D, Niks RE, 1995. Characterization of Lr34, a major gene conferring nonhypersensitive resistance to wheat leaf rust. Plant Disease 79, 1208-12.
Rubiales D, Niks RE, 2000. Combinations of mechanisms of resistance to rust fungi as a strategy to increase durability. Durum wheat improvement in the Mediterranean region: New challenges. Options Mediterranéennes. C. Royo, M. M. Nachit, N. Di Fonzo and J. L. Araus. Zaragoza (Spain). 40, 333-39.
Singh RP,Bechere E, Abdalla O, 1993. Genetic analysis of resistance to leaf rust in 9 Durum wheat. Plant Disease 77, 460-63.
Zadoks JC, Chang TT, Konzak CF, 1974. A decimal code for the growth stages of cereals. Weed Res. 14, 415-421.
Zhang H, Knott DR, 1990. Inheritance of leaf rust resistance in durum wheat. Crop Science 30, 1218-1222.
Zhang HT, Knott DR, 1993. Inheritance of adult plant resistance to leaf rust in 6 durum wheat cultivars. Crop Science 33, 694-697.