Disease resistance in transgenic plants has been improved, for the first time, by the insertion of a gene from a biocontrol fungus. The gene encoding a strongly antifungal endochitinase from the mycoparasitic fungus Trichoderma harzianum was transferred to tobacco and potato. High expression levels of the fungal gene were obtained in different plant tissues, which had no visible effect on plant growth and development. Substantial differences in endochitinase activity were detected among transformants. Selected transgenic lines were highly tolerant or completely resistant to the foliar pathogens Alternaria alternata, A. solani, Botrytis cinerea, and the soilborne pathogen Rhizoctonia solani. The high level and the broad spectrum of resistance obtained with a single chitinase gene from Trichoderma overcome the limited efficacy of transgenic expression in plants of chitinase genes from plants and bacteria. These results demonstrate a rich source of genes from biocontrol fungi that can be used to control diseases in plants.

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Genes from mycoparasitic fungi as a source for improving plant resistance to fungal pathogens.

Alternaria solani causes diseases on foliage (early blight), basal stems of seedlings (collar rot), stems of adult plants (stem lesions), and fruits (fruit rot) of tomato. Early blight is the most destructive of these diseases and hence receives considerable attention in breeding. For over 60 years, breeding for early blight resistance has been practiced, but the development of cultivars with high levels of resistance has been hampered by the lack of sources of strong resistance in the cultivated tomato and by the quantitative expression and polygenic inheritance of the resistance. In some accessions of wild species, high levels of early blight resistance have been found, but breeding lines still have unfavorable horticultural traits from the donor parent. Recently, the first linkage maps with loci controlling early blight resistance have been developed based on interspecific crosses. These maps may facilitate marker-assisted selection. This overview presents the current knowledge about the A. solani–tomato complex with respect to its biology, genetics, and breeding.

Tomato early blight (Alternaria solani): the pathogen, genetics, and breeding for resistance

Roots of most terrestrial plants form symbiotic associations (mycorrhiza) with soil- borne arbuscular mycorrhizal fungi (AMF) Many studies show that mycorrhizal colonization enhances plant resistance against pathogenic fungi However, the mechanism of mycorrhiza-induced disease resistance remains equivocal In this study, we found that mycorrhizal inoculation with AMF Funneliformis mosseae significantly alleviated tomato (Solanum lycopersicum Mill) early blight disease caused by Alternaria solani Sorauer AMF pre-inoculation led to significant increases in activities of β-1,3-glucanase, chitinase, phenylalanine ammonia-lyase (PAL) and lipoxygenase (LOX) in tomato leaves upon pathogen inoculation Mycorrhizal inoculation alone did not influence the transcripts of most genes tested However, pathogen attack on AMF-inoculated plants provoked strong defense responses of three genes encoding pathogenesis-related (PR) proteins, PR1, PR2 and PR3, as well as defense-related genes LOX, AOC and PAL, in tomato leaves The induction of defense responses in AMF pre-inoculated plants was much higher and more rapid than that in un-inoculated plants in present of pathogen infection Three tomato genotypes: a Castlemart wild-type (WT) plant, a jasmonate (JA) biosynthesis mutant (spr2), and a prosystemin-overexpressing 35S::PS plant were used to examine the role of the JA signaling pathway in AMF-primed disease defense Pathogen infection on mycorrhizal 35S::PS plants led to higher induction of defense-related genes and enzymes relative to WT plants However, pathogen infection did not induce these genes and enzymes in mycorrhizal spr2 mutant plants Bioassays showed that 35S::PS plants were more resistant and spr2 plants were more susceptible to early blight compared with WT plants Our finding indicates that mycorrhizal colonization enhances tomato resistance to early blight by priming systemic defense response, and the JA signaling pathway is essential for mycorrhiza-primed disease resistance

/pdf/enhanced-tomato-disease-resistance-primed-by-arbuscular-34l1c45blq.pdf

Enhanced tomato disease resistance primed by arbuscular mycorrhizal fungus.

Late blight (LB), caused by the oomycete Phytophthora infestans, and early blight (EB), caused by the fungi Alternaria solani and A. tomatophila, are two common and destructive foliar diseases of the cultivated tomato (Solanum lycopersicum) and potato (Solanum tuberosum) in the United States and elsewhere in the world. While LB can infect and devastate tomato plants at any developmental stages, EB infection is usually associated with plant physiological maturity and fruit load where older senescing plants exhibit greater susceptibility and a heavy fruit set enhances the disease. At present, cultural practices and heavy use of fungicides are the most common measures for controlling LB and EB. Genetic resources for resistance have been identified for both diseases, largely within the tomato wild species, in particular the red-fruited species S. pimpinellifolium and the green-fruited species S. habrochaites. A few race-specific major resistance genes (e.g., Ph-1, Ph-2 and Ph-3) and several race-nonspecific r...

Genetics, Genomics and Breeding of Late Blight and Early Blight Resistance in Tomato

β-1,3-Glucanase (EC 3.2.1.39) and chitinase (EC 3.2.1.14) mRNAs, proteins, and enzyme activities were expressed specifically in the micropylar tissues of imbibed tomato (Lycopersicon esculentum Mill.) seeds prior to radicle emergence. RNA hybridization and immunoblotting demonstrated that both enzymes were class I basic isoforms. β-1,3-Glucanase was expressed exclusively in the endosperm cap tissue, whereas chitinase localized to both endosperm cap and radicle tip tissues. β-1,3-Glucanase and chitinase appeared in the micropylar tissues of gibberellin-deficient gib-1 tomato seeds only when supplied with gibberellin. Accumulation of β-1,3-glucanase mRNA, protein and enzyme activity was reduced by 100 μM abscisic acid, which delayed or prevented radicle emergence but not endosperm cap weakening. In contrast, expression of chitinase mRNA, protein, and enzyme activity was not affected by abscisic acid. Neither of these enzymes significantly hydrolyzed isolated tomato endosperm cap cell walls. Although both β-1,3-glucanase and chitinase were expressed in tomato endosperm cap tissue prior to radicle emergence, we found no evidence that they were directly involved in cell wall modification or tissue weakening. Possible functions of these hydrolases during tomato seed germination are discussed.

http://www.plantphysiol.org/content/plantphysiol/126/3/1299.full.pdf

Class I β-1,3-Glucanase and Chitinase Are Expressed in the Micropylar Endosperm of Tomato Seeds Prior to Radicle Emergence

Differential induction of pathogenesis-related proteins in tomato by Alternaria solani and the association of a basic chitinase isozyme with resistance.

M.H.A.J. Joosten

Papers

Differential induction of pathogenesis-related proteins in tomato by Alternaria solani and the association of a basic chitinase isozyme with resistance.