Faculty
Isgouhi Kaloshian
Associate Professor of Nematology
Office: 2251 Speith Hall
Phone: (951)827-3913
Fax: (951)827-3719
E-mail: isgouhi.kaloshian@ucr.edu
Root-knot nematodes (RKNs) are obligate plant parasites causing major economic damage to many crops including tomato. These nematodes have evolved a complex and intimate relationship with their hosts. This communication is highly specific, suggesting that the plant nematode interactions involve a continual exchange of signals between the two organisms. Initiation of pathogenic interaction depends on the nematode ability to recognize, penetrate, and initiate an elaborate feeding site. Plants have evolved effective mechanisms to recognize pathogens and to stop their growth. A common mechanism of race-specific resistance to a large number of pathogens, including nematodes, is the induction of hypersensitive response (HR) or cell death.
The tomato gene, Mi-1, is the first cloned plant resistance gene with dual specificity to two unrelated organisms, a nematode and an aphid. Despite this novel function, Mi-1 encodes a protein sharing molecular features, nucleotide binding site and leucine rich repeats, with described single-specificity resistance genes. Resistance to nematodes is characterized by HR in roots, however no HR is detected in leaves in the Mi-1-mediated resistance to the potato aphid. We are employing both genetic and molecular approaches to dissect the Mi-1-mediated resistance pathway to both organisms. We are using mutational analysis to identify and characterize plant genes that are required for nematode and aphid recognition and the subsequent expression of resistance. We have identified a gene Rme1 (for resistance to Meloidogyne), that is required for both nematode and aphid resistance. Recent experiments indicated that Rme1 acts early in the Mi-1 resistance signal transduction pathway, either at the Mi-1 level or before Mi-1. We are also using microarray analysis to study aphid and nematode feeding responses in resistant and susceptible tomato.
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Fig. 1. Potato aphid, Macrosiphum euphorbiae, feeding on tomato leaflet. |
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Fig. 2. Resistant tomato root tip infected with J2 of root-knot nematode, stained pink with acid fuchsin. Dark stained areas indicate dead cells (HR). The second project in the lab is to isolate tomato resistance genes with novel specificities. Currently, only one RKN resistance gene, Mi-1, is present in cultivated tomato. Mi-1 is not effective at soil temperatures above 28 C. A heat-stable resistance to RKNs have been identified in the wild tomato species, Lycopersicon peruvianum accession LA2157. The resistance in LA2157 is mediated by Mi-9 which is localized to chromosome 6. We have fine mapped Mi-9 to a similar genetic interval as Mi-1 (Mi-1 is also located on the short arm of chromosome 6). |
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Fig. 3. Tomato (right), Lycopersicon esculentum, and its relative, L. peruvianum (left), the source of the heat-stable RKN resistance. |
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Fig. 4. Our glasshouse where the mutant screen is carried out. In a typical experiment 120 M2 families are planted (25 seeds/ family). Experiments overlap to allow maximum use of space and resources. One round of mutant screens with RKNs takes about 4 months. |