Plants cannot move to escape environmental challenges. Biotic stresses result from a battery of potential pathogens: fungi, bacteria, nematodes and insects intercept the photosynthate produced by plants, and viruses use replication machinery at the host's expense. Plants, in turn, have evolved sophisticated mechanisms to perceive such attacks, and to translate that perception into an adaptive response. Here, we review the current knowledge of recognition-dependent disease resistance in plants. We include a few crucial concepts to compare and contrast plant innate immunity with that more commonly associated with animals. There are appreciable differences, but also surprising parallels.

Plant pathogens and integrated defence responses to infection.

Trichoderma spp. are free-living fungi that are common in soil and root ecosystems. Recent discoveries show that they are opportunistic, avirulent plant symbionts, as well as being parasites of other fungi. At least some strains establish robust and long-lasting colonizations of root surfaces and penetrate into the epidermis and a few cells below this level. They produce or release a variety of compounds that induce localized or systemic resistance responses, and this explains their lack of pathogenicity to plants. These root-microorganism associations cause substantial changes to the plant proteome and metabolism. Plants are protected from numerous classes of plant pathogen by responses that are similar to systemic acquired resistance and rhizobacteria-induced systemic resistance. Root colonization by Trichoderma spp. also frequently enhances root growth and development, crop productivity, resistance to abiotic stresses and the uptake and use of nutrients.

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Trichoderma species--opportunistic, avirulent plant symbionts.

Inducible defense-related proteins have been described in many plant species upon infection with oomycetes, fungi, bacteria, or viruses, or insect attack. Several types of proteins are common and have been classified into 17 families of pathogenesis-related proteins (PRs). Others have so far been found to occur more specifically in some plant species. Most PRs and related proteins are induced through the action of the signaling compounds salicylic acid, jasmonic acid, or ethylene, and possess antimicrobial activities in vitro through hydrolytic activities on cell walls, contact toxicity, and perhaps an involvement in defense signaling. However, when expressed in transgenic plants, they reduce only a limited number of diseases, depending on the nature of the protein, plant species, and pathogen involved. As exemplified by the PR-1 proteins in Arabidopsis and rice, many homologous proteins belonging to the same family are regulated developmentally and may serve different functions in specific organs or tissues. Several defense-related proteins are induced during senescence, wounding or cold stress, and some possess antifreeze activity. Many defense-related proteins are present constitutively in floral tissues and a substantial number of PR-like proteins in pollen, fruits, and vegetables can provoke allergy in humans. The evolutionary conservation of similar defense-related proteins in monocots and dicots, but also their divergent occurrence in other conditions, suggest that these proteins serve essential functions in plant life, whether in defense or not.

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Significance of Inducible Defense-related Proteins in Infected Plants

Host-microbe interactions: Shaping the evolution of the plant immune response

▪ Abstract We hypothesize that the evolutionary potential of a pathogen population is reflected in its population genetic structure. Pathogen populations with a high evolutionary potential are more likely to overcome genetic resistance than pathogen populations with a low evolutionary potential. We propose a flexible framework to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure. According to this framework, pathogens that pose the greatest risk of breaking down resistance genes have a mixed reproduction system, a high potential for genotype flow, large effective population sizes, and high mutation rates. The lowest risk pathogens are those with strict asexual reproduction, low potential for gene flow, small effective population sizes, and low mutation rates. We present examples of high-risk and low-risk pathogens. We propose general guidelines for a rational approach to breed durable resistance according to the evolutionary potential of the pathogen.

Pathogen population genetics, evolutionary potential, and durable resistance

HOST genotype specificity in interactions between biotrophic pathogens and plants in most cases complies with the gene-for-gene model1; success or failure of infection is determined by absence or presence of complementary genes, avirulence and resistance genes, in the pathogen and host plant, respectively. Resistance, expressed by the induction of a hypersensitive response in the host, is envisaged to be based on recognition of the pathogen, mediated through direct interaction between products of pathogen avirulence genes (the so-called race-specific elicitors) and receptors in the host plant, the putative products of resistance genes1. The interaction between the biotrophic fungus Cladosporium fulvum and its only host, tomato (Lycopersicon esculentum), is a well-established model system for studying fungus-plant gene-for-gene relationships1. Here we report the isolation of race-specific elicitor AVR4 of C. fulvum and the cloning of its encoding avirulence gene. We present evidence that, in nature, a single base-pair change in this avirulence gene leads to virulence of races previously avirulent on tomato genotypes carrying the complementary Cf4 resistance gene.

Host resistance to a fungal tomato pathogen lost by a single base-pair change in an avirulence gene

A race-specific peptide elicitor from Cladosporium fulvum induces a hypersensitive response on Cf9 tomato genotypes. We have hypothesized that the avirulence of fungal races on Cf9 genotypes is due to the production of this elicitor by an avirulence gene, avr9. To obtain cDNA clones of the avr9 gene, oligonucleotide probes were designed based on the amino acid sequence determined previously. In northern blot analysis, one oligonucleotide detected an mRNA of 600 nucleotides in tomato-C. fulvum interactions involving fungal races producing the elicitor. A primer extension experiment indicated that the probe hybridized to a region near position 270 of the mRNA. The probe was used to screen a cDNA library made from poly(A)+ RNA from an appropriate compatible tomato-C. fulvum interaction. One clone was obtained corresponding to the mRNA detected by the oligonucleotide probe. Sequence analysis revealed that this clone encoded the avr9 elicitor. By isolating longer clones and by RNA sequencing, the primary structure of the mRNA was determined. The mRNA contains an open reading frame of 63 amino acids, including the sequence of the elicitor at the carboxyterminus. A time course experiment showed that the avr9 mRNA accumulates in a compatible tomato-C. fulvum interaction in correlation with the increase of fungal biomass. The avr9 gene is a single-copy gene that is absent in fungal races which are virulent on tomato Cf9 genotypes. Possible functions of the avirulence gene are discussed.

Cloning and characterization of cDNA of avirulence gene avr9 of the fungal pathogen Cladosporium fulvum, causal agent of tomato leaf mold.

The avirulence genes Avr9 and Avr4 from the fungal tomato pathogen Cladosporium fulvum encode extracellular proteins that elicit a hypersensitive response when injected into leaves of tomato plants carrying the matching resistance genes, Cf-9 and Cf-4, respectively We successfully expressed both Avr9 and Avr4 genes in tobacco with the Agrobacterium tumefaciens transient transformation assay (agroinfiltration) In addition, we expressed the matching resistance genes, Cf-9 and Cf-4, through agroinfiltration By combining transient Cf gene expression with either transgenic plants expressing one of the gene partners, Potato virus X (PVX)-mediated Avr gene expression, or elicitor injections, we demonstrated that agroinfiltration is a reliable and versatile tool to study Avr/Cf-mediated recognition Significantly, agroinfiltration can be used to quantify and compare Avr/Cf-induced responses Comparison of different Avr/Cf-interactions within one tobacco leaf showed that Avr9/Cf-9-induced necrosis developed slower than necrosis induced by Avr4/Cf-4 Quantitative analysis demonstrated that this temporal difference was due to a difference in Avr gene activities Transient expression of matching Avr/Cf gene pairs in a number of plant families indicated that the signal transduction pathway required for Avr/Cf-induced responses is conserved within solanaceous species Most non-solanaceous species did not develop specific Avr/Cf-induced responses However, co-expression of the Avr4/Cf-4 gene pair in lettuce resulted in necrosis, providing the first proof that a resistance (R) gene can function in a different plant family

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Agroinfiltration is a versatile tool that facilitates comparative analyses of Avr9/Cf-9-induced and Avr4/Cf-4-induced necrosis

Although Mendel published his genetic studies on peas in 1866 it was not until the end of the last century that his work was understood and taken up by other scientists. In 1898, Farrer (48) discovered that resistance to rust in wheat is inherited. Biffen's discovery in 1905 (9) that resistance to yellow rust (Puccinia striiformis) in wheat obeys Mendel's laws caused a breakthrough in breeding for disease resistance and the understanding of the genetics of plant-fungus interactions. The optimism induced by Biffen's studies led him and other scientists to the conclusion that resistance to this fungus would be durable, as it was expected that the fungus would evolve too slowly to overcome the introduced resistance gene. Only a decade later, however, Stakman and coworkers (126, 127), studying resistance in wheat to wheat stem rust (Puccinia graminis f . sp. trifici), were able to show that resistance

Molecular characterization of gene-for-gene systems in plant-fungus interactions and the application of avirulence genes in control of plant pathogens.

Successful colonisation of plants by pathogens requires efficient utilisation of nutrient resources available in host tissues. Several bacterial and fungal genes are specifically induced during pathogenesis and under nitrogen-limiting conditions in vitro. This suggests that a nitrogen-limiting environment may be one of the cues for disease symptom development during growth of the pathogens in planta. Here we review recent literature on the effect of nitrogen and nitrogen-regulated genes on disease development, caused by phytopathogenic bacteria and fungi. Furthermore, the potential influence of nitrogen-limitation or general nutrient limitation on several in planta-induced bacterial and fungal pathogenicity, virulence and avirulence genes will be discussed.

P.J.G.M. de Wit

Papers

Host resistance to a fungal tomato pathogen lost by a single base-pair change in an avirulence gene

Cloning and characterization of cDNA of avirulence gene avr9 of the fungal pathogen Cladosporium fulvum, causal agent of tomato leaf mold.

Agroinfiltration is a versatile tool that facilitates comparative analyses of Avr9/Cf-9-induced and Avr4/Cf-4-induced necrosis

Molecular characterization of gene-for-gene systems in plant-fungus interactions and the application of avirulence genes in control of plant pathogens.

The effect of nitrogen on disease development and gene expression in bacterial and fungal plant pathogens