Abstract

Creso is a durum wheat cultivar considered to carry durable resistance to leaf rust. The inheritance of the macroscopic components of its resistance was studied in a F2 population of 116 plants coming from a cross with the susceptible durum wheat cultivar Pedroso. The proportion of plants showing low and high infection type (IT) fitted a segregation 3:1 indicating a single dominant gene controlling hypersensitive resistance. The latency period of plants showing high IT displayed a large variation indicating also the existence of a high level of partial resistance in Creso. Therefore the durable resistance of Creso may be attributable to a combination of hypersensitive and partial resistance.

Introduction

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 susceptibility.

Results

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).

Discussion

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, 1994).
On the other hand there are evidence that Creso has a high level of partial resistance since some F2 plants displaying a compatible reaction possess a high latency period. It is remarkable that this long latency period appears even after a long period of incubation in darkness (24 h) that can minimise the effect of partial resistance (Niks, 2002). A certain number of F2 plants have been found with a slightly shorter latency period than the susceptible check Pedroso. This short latency period is probably due to environmental variation and not to transgressive segregation. In fact the latency period of Pedroso in seedlings is similar to the well-known susceptible check to wheat leaf rust cv. Little Club (Martínez 2002). Partial resistance of Creso appears to be recessive since the number of lines with short latency period is higher than number of lines with a long latency period. Jacobs & Broers (1989) reported that partial resistance in bread wheat to wheat leaf rust is recessive.

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.

Acknowledgements

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.

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