https://www.cell.com/trends/plant-science/pdf/S1360-1385(08)00297-5.pdf

Shades of gray: the world of quantitative disease resistance.

Components of Resistance that Reduce the Rate of Epidemic Development

New diseases arise when agriculture moves toward higher productivity. With a change of rice ecology, some formerly less important pests emerge, others vanish, and new ones appear. Bacterial blight is not a new disease of rice. Its importance to rice production in tropical Asia, however, was recognized only after the introduction of modern cultivars, which are highly responsive to nitrogen fertilizer. It is now a major rice disease throughout Asia. Bacterial blight is especially prevalent in irrigated and rainfed lowland rice areas in Asia. Severly infected leaves tend to dry quickly, and many farmers attribute the effects of the disease to drought, thus letting it go unnoticed. In the last twenty years, epidemics have been reported prior to the introduction of modern rices. Since the introduction and widespread cultivation of high­ yielding but susceptible rice cultivars, bacterial blight has become one of the most serious diseases of rice in Asia. In Punjab, Haryana, and western Uttar Pradesh States of India, major epidemics occurred in 1979 and 1980 ( 14); severe kresek was observed; and total crop failure was reported. Estimates of yield losses caused by bacterial blight are not available. The extent of yield loss depends on locality, season, weather, and cultivar. An increase in the amount of nitrogen fertilizer application favors disease development and thus causes greater yield loss (60) . The amount of nitrogen applied did not increase the incidence and severity of the disease in a resistant cultivar. In recent years the disease has been observed not only in all major rice­ producing countries of Asia, but also in the Sahelian countries of Africa (1)

Current status and future prospects of research on bacterial blight of rice

We have identified genes for pathogenicity toward rice (Oryza sativa) and genes for virulence toward specific rice cultivars in the plant pathogenic fungus Magnaporthe grisea. A genetic cross was conducted between the weeping lovegrass (Eragrostis curvula) pathogen 4091-5-8, a highly fertile, hermaphroditic laboratory strain, and the rice pathogen O-135, a poorly fertile, female-sterile field isolate that infects weeping lovegrass as well as rice. A six-generation backcrossing scheme was then undertaken with the rice pathogen as the recurrent parent. One goal of these crosses was to generate rice pathogenic progeny with the high fertility characteristic of strain 4091-5-8, which would permit rigorous genetic analysis of rice pathogens. Therefore, progeny strains to be used as parents for backcross generations were chosen only on the basis of fertility. The ratios of pathogenic to nonpathogenic (and virulent to avirulent) progeny through the backcross generations suggested that the starting parent strains differ in two types of genes that control the ability to infect rice. First, they differ by polygenic factors that determine the extent of lesion development achieved by those progeny that infect rice. These genes do not appear to play a role in infection of weeping lovegrass because both parents and all progeny infect weeping lovegrass. Second, the parents differ by simple Mendelian determinants, "avirulence genes," that govern virulence toward specific rice cultivars in all-or-none fashion. Several crosses confirm the segregation of three unlinked avirulence genes, Avr 1-CO39, Avr 1-M201 and Avr1-YAMO, alleles of which determine avirulence on rice cultivars CO39, M201, and Yashiro-mochi, respectively. Interestingly, avirulence alleles of Avr1-CO39, Avr1-M201 and Avr1-YAMO were inherited from the parent strain 4091-5-8, which is a nonpathogen of rice. Middle repetitive DNA sequences ("MGR sequences"), present in approximately 40-50 copies in the genome of the rice pathogen parent, and in very low copy number in the genome of the nonpathogen of rice, were used as physical markers to monitor restoration of the rice pathogen genetic background during introgression of fertility. The introgression of highest levels of fertility into the most successful rice pathogen progeny was incomplete by the sixth generation, perhaps a consequence of genetic linkage between genes for fertility and genes for rice pathogenicity. One chromosomal DNA segment with MGR sequence homology appeared to be linked to the gene Avr1-CO39. Finally, many of the crosses described in this paper exhibited a characteristic common to many crosses involving M. grisea rice pathogen field isolates.(ABSTRACT TRUNCATED AT 400 WORDS)

https://www.genetics.org/content/genetics/127/1/87.full.pdf

Magnaporthe grisea genes for pathogenicity and virulence identified through a series of backcrosses.

This review distills recent information on drought resistance characteristics of grain legumes with a view toward developing appropriate genetic enhancement strategies for water-limited environments. First, the possible adaptations that allow grain legumes to better cope with drought stress are summarized. It is suggested that there are considerable gains to be made in increasing yield and yield stability in environments characterized by terminal drought stress by further exploiting drought escape, by shortening crop duration. Many traits conferring dehydration avoidance and dehydration tolerance are available, but integrated traits, expressing at a higher level of organization, are suggested to be more useful in crop improvement programs. Possible genetic improvement strategies are outlined, ranging from empirical selection for yield in droughted environments to a physiological genetic approach. It is suggested that in view of recent advances in understanding drought resistance mechanisms, the latter strategy is becoming more feasible. It is concluded that use of this recently derived knowledge in a systematic manner can lead to significant gains in yield and yield stability of the world's major grain legumes, as they are mainly grown (and will continue to be grown) under rain-fed conditions.

/pdf/strategies-for-improving-drought-resistance-in-grain-legumes-55rcuu7t96.pdf

Strategies for Improving Drought Resistance in Grain Legumes

Horizontal, uniform, race-non-specific or stable resistance can be discerned according to Van der Plank, from vertical, differential, race-specific or unstable resistance by a test in which a number of host genotypes (cultivars or clones) are tested against a number of pathogen genetypes traces of isolatest If the total non-environmental variance in levels of resistance is due to main effects only differences between cultivars and differences between isolates) the resistance and the pathogen many (in the broad sense) are horizontal in nature Vertical resistance and pathogenicity are characterized by the interaction between host and pathogen showing up as a variance compenent in this test due to interaction between cultivars and isolates A host and pathogen model was made in which resistance and pathogenicity are governed by live polygenic loci Within the host the resistance genes show additivity Two models were investigated in model I resistance and pathogenicity genes operate in an additive way as envisaged by Van der Plank in his horizontal resistance Model II is characterized by a gene-for-gene action between the polygenes of the host and those of the pathogen The cultivar isolate test in model I showed only main effect variance Surprisingly, the variance in model II was also largely due to main effects The contribution of the interaction to the variance uppeared so small, that it would be difficult to discern it from a normal error variance So-called horizontal resistance can therefore be explained by a polygenic resistance, where the individual genes are vertical and operating on a gene-for-gene basis with virulence genes in the pathogen The data reported so far support the idea that model II rather than model I is the realistic one The two models also revealed that populations with a polygenic resistance based on the gene-for-gene action have an increased level of resistance compared with the addition model, while its stability as far as mutability of the pathogen is concerned, is higher compared to those with an additive gene action Mathematical studies of Mode too support the gene-for-gene concept The operation of all resistance and virulence genes in a natural population is therefore seen as one integrated system All genes for true resistance in the host population, whether they are major or minor genes are considered to interact in a gene-for-gene way with virulence genes either major or minor, in the pathogen population The models revealed other important aspects Populations with a polygenic resistance based on a gene-for-gene action have an increased level of resistance compared to populations following the addition model The stability, as far as mutability of the pathogen is concerned, is higher in the interaction model than in the addition model The effect of a resistance gene on the level of resistance of the population consists of its effect on a single plant times its gene frequency in the population Due to the adaptive forces in both the host and the pathogen population and the gene-for-gene nature of the gene action an equilibrium develops that allows all resistance genes to remain effective although their corresponding virulence genes are present The frequencies of the resistance and virulence genes are such that the effective frequencies of resistance genes tend to be negatively related to the magnitude of the gene effect This explains why major genes often occur at low frequencies, while minor genes appear to be frequent It is in this way that the host and the pathogen, both as extremely variable and vigorous populations, can co-exist Horizontal and vertical resistance as meant by Van der Plank therefore do not represent different kinds of resistances, they represent merely polygenic and oligogenic resistances resp In both situations the individual host genes interact specifically with virulence genes in the pathogen Van der Plank's test for horizontal resistance appears to be a simple and sound way to test for polygenic inheritance of resistance The practical considerations have been discussed The agro-ecosystems should be made as diverse as possible Multilines, polygenic resistance, tolerance, gene deployment and other measures should be employed, if possible in combination

J. C. Zadoks

Papers

The integrated concept of disease resistance: A new view including horizontal and vertical resistance in plants