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Polygenic resistance is usually considered to be non-race-specific and durable. However, in several host-pathogen systems with demonstrated durability, polygenic resistance race-specific effects were demonstrated Parlevliet, Parlevliet and Zadoks realized that in gene-for-gene relationships the race-specific effects are of the same size as the gene effects.

Major resistance genes therefore are associated with clear, identifiable races, while polygenes result in only small race-specific effects, insufficient for unambiguously identifying races. In this latter case, one gets a general impression of non-race-specificity. Parlevliet and Zadoks also compared two models of polygenic pathosystems with one another. In system I the host polygenes operated in a gene-for-gene interaction with the pathogen polygenes, causing small race-specific effects.

In system II the host polygenes did not operate in a gene-for-gene way with the pathogen polygenes, representing true horizontal resistance. The two models showed that the polygene-for-polygene system was more durable less liable to adaptation in the pathogen population than the system where the polygenes were not operating in a gene-for-gene system, assuming all other variables, such as resistance mechanisms, are the same.

The durability of resistance also may depend on the circumstances.

Background

The major gene resistance of flax to flax rust was not durable in the USA, but it has been durable in The Netherlands Parlevliet, In the latter situation, flax has been a small crop and all flax grown is completely resistant. The pathogen population was, therefore, reduced so strongly that new races have little chance to emerge.


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Conscious breeding for disease resistance started early in this century and concentrated on major resistance genes. Soon after resistant cultivars were released farmers were confronted with the versatility of the pathogen; new forms of the pathogen neutralized the introduced resistance the resistance was said to be "broken".

This did not stop breeders from using such non-durable resistance. On the contrary, the rapid increase of man's effort to produce resistant crops based on major resistance genes was accompanied by an equally rapid increase in "broken" resistance. Even today the situation has not changed much. Screening and selection methods favouring major gene resistance are still widely used by breeders. As a result of the widespread use of such screening and selection methods QR has been used considerably less than major gene resistance, although abundantly present to most pathogens Parlevliet, Some susceptible plants become systemically resistant in response to localized infections, a phenomenon known as acquired resistance.

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This is best known in cucurbits and tobacco. When a lower leaf is infected, the whole plant becomes resistant to the same and to other pathogens and remains so for weeks. Plants with acquired resistance have high levels of pathogenesis - related proteins, salicylic acid, peroxidase, and other factors Scheffer, Obviously, there is a signalling mechanism that carries information to distant parts of the plant, but the nature of the signals is unknown Hammerschmidt and Kuc, Sequeira pointed out that the terminology used in this field is confusing.

The terms acquired resistance, activated resistance, acquired immunity, pre-munity, immunization, sensitisation as well as cross protection have been used as synonym for induced resistance to describe a range of related phenomena.

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Induced resistance means the enhancement of resistance in plants that are otherwise susceptible, in response to an extrinsic stimulus without alterations of the genome. Induced resistance exists in two different forms. It may be localized at the site of the inducing treatment or it may be systemic. The early pioneer research with phytoalexins established many basic principles for induced resistance. Plants respond to pathogens and non-pathogens by synthesizing low molecular weight, generally lipophilic compounds which inhibit the growth of fungi and bacteria in vitro and accumulate at the sites of infection to levels that inhibit the development of some pathogens Kuc, It is clear that phytoalexin structures and activity do not explain the often high specificity in plant-pathogen interaction.

Biotic and abiotic agents cause phytoalexin synthesis and accumulation.

Breeding for disease resistance in wheat - R.P. Singh, S. Rajaram

Certain fungal pathogens, especially in the genera Alternaria and Cochliobolus , are known to produce host-specific toxins or host-selective toxins HSTs as agents of virulence or pathogenicity. Subsequently with the advance of molecular biology, one of the main interests in HST research is the molecular analysis of the genes involved in HST biosynthesis, because these genes correspond to genes for pathogenicity or virulence.

However, investigations into the genetic control of HST biosynthesis and pathogenicity in Alternaria pathogens have been limited due to their lack of a sexual cycle Kodama et al. Gene tagging utilizing the heterologous integration of plasmids in now widely used to clone genes where little biochemical is available. The development of the so-called REMI restriction enzyme mediated integration method has substantially improved this technique and some genes important for fungal pathogenicity have already been tagged and cloned Kodama, Advances have been made in the discovery of host-specific toxins, their chemistry, their sites of action and physiological effects on host plants, their roles in fungal pathogenesis, and the genetics of toxin production by fungi Yoder, Small molecules have been implicated in systemic induced resistance, although their role appears to be more important for signal transduction than for directly inhibiting pathogens.

The best studied is salicylic acid SA , a compound derived from cinnamic acid Stermer, In the past 80 years resistance breeding has increased greatly to become a highly important, if not indispensable part of crop improvement.

Sustainable resistance management

Screening and selection methods have been were refined and have became more efficient. Our scientific knowledge has increased even more. Recently the molecular techniques have revolutionized our technical possibilities. With these new transformation techniques the questions arise what new perspectives are there for disease resistance breeding and how promising are they? For discussion purposes, they have been classified into three categories: i Transfer between plant species. Bread wheat Triticum aestivum L.

Several Triticum species and species of at least six other genera from the tribe Triticeae have been used successfully as donors of resistance genes Jones et al. Most of these transferred resistance genes were of the hypersensitive, non-durable type. However, the transfer of alien genes is often far from easy, always time consuming, some other alien DNA is always transferred in the process as well. When fully developed, transformation will not only be less time consuming, it will also be much more efficient than the conventional procedure, as in principal, no additional donor material will be transferred with the transformed gene.

The molecular transformation techniques not only allow the transfer of genes from related species, they can transfer genes from any organism to the crop species to be improved. Transformation of resistance genes of the non-durable type from one species to another does not seem very useful, as the non-durable character will not change.

The transformation of potentially durable resistance genes, however, is highly interesting. For example, the transfer of genes controlling the production of a phytoalexin in a given species to an unrelated crop species could make this latter species resistant to several of its pathogens, which are not adapted to this foreign phytoalexin. The genes for resveratrol, a phytoalexin of grapevine Vitis vinifera L. Hain et al , This pathogen-derived resistance PDR varies greatly from protecting against very high levels to very low levels of inoculum. The specificity also varies from highly specific, resistant only to the strain from which the gene was derived, to moderately specific, providing resistance to the virus from which the gene originated and to related viruses.

Initially, all attempts to use PDR were directed at transferring the viral genes coding for the coat nucleocapsid protein to the host plants. Such coat protein mediated resistance has been demonstrated against many viruses of widely different groups Singh et al. The resistance level obtained varies greatly from only slight to fairly strong.

Later other viral genes or parts of viral genes were transformed to host plants. From the diversity of results published so far, the conclusion seems justified that, through transformation to the host, any part of the viral genome can potentially give rise to PDR Lomonossoff, All resulting resistance had to be classified as quantitative. It is likely that PDR can be obtained from other pathogens. However, pathogens such as fungi have a much larger genome than viruses, where many genes are not directly involved in the pathogenicity process. It is quite possible that only the transformation of pathogenicity genes may result in PDR, which means that as a first step pathogenicity genes have to be identified, localized, marked and cloned, a considerably more complex affair than with the simple genomes of viruses.

The possibilities are almost infinite and each construct has to be tested. One could bring together two or three non-durable resistance genes, which are individually still effective, together in a construct to be introduced into a cultivar.

Plant Breeding for Disease Resistance

Constructs can also change the resistance expression from being induced upon attack to that of a constitutive expression. The invading pathogen is thus exposed to the resistance at an earlier stage. The first success was reported in An endo-chitinase gene of bean, its production normally induced after pathogen attack, was transformed with a constitutive promoter to tobacco. This increased the resistance to Rhizoctonia solani.

It was ineffective to Pythium aphanidermatum , a fungus lacking a chitin-containing cell wall Broglie et al. In summary one can conclude that molecular techniques of transferring genetic material with the aim of obtaining resistance are enormously diverse and very promising. The resistance obtained so far is largely of a quantitative nature.

In order to reduce costs and to increase the efficiency of identifying resistant plants or lines in segregating populations, breeders developed screening methods in which plants as young as possible were exposed to high concentrations of, preferably, a specified inoculum. This efficiently identifies complete resistance based on major genes but is inadequate for recognizing small differences in resistance.


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These screening approaches, together with the belief that polygenic resistance is difficult to select for and might not give a good level of resistance, led to the present situation where major gene resistance has been exploited very well, while QR has been used only sparingly. This is unfortunate as there is so much QR available.

Quantitative Resistance occurs to most of our important pathogens at various levels in nearly all our crops as discussed in the chapter "quantitative resistance". Since this QR does occur in the cultivars grown, it is genetic material that is related to what the breeders' desire. To select for QR means accumulating QR in much the same way as selecting for higher yields. The breeder selects the plants or lines with the lower levels of disease severity and by doing that continuously over the seasons, the level of QR will increase fairly rapidly as Parlevliet and Van Ommeren showed.

There is, however, one complication. If there is also non-durable major gene resistance around, it has to be taken into account. The QR is not visible when such an effective major gene is present. By using, preferably, local material, the frequency of such non-durable still effective major genes is often low, as the local pathogen population has adapted to these genes. Introducing plant material from elsewhere, especially from the centres of diversity, increases the frequency of such non-durable effective major genes considerably, as the local pathogen population has not yet adapted to the newly introduced resistance.

Therefore, to select QR stick as much as possible to local material as they will almost certainly carry QR. One can also avoid ending up with non-durable major resistance in the selected material by selecting against susceptibility, i. Plants or lines with complete resistance should also be removed in case of resistance breeding against specialized fungal pathogens, as such resistances can be assumed to be non-durable.


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