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Spider Mite Resistance Mechanisms in Lycopersicon hirsutum Accession LA2329
Department of Horticulture
Insect damage is responsible for a significant loss of crop yield. Relatives of some crop plants possess genes that deter or resist insects. These genes can be used to improve crop productivity The purpose of this study is to provide a better understanding of how a wild relative of tomato resists a broad array of insects. The knowledge generated will lead to creation of crop plants that are insect resistant.
2010 Project Description
L. hirsutum (L.h.), LA2329 has been crossed with inbred lines of L. esculentum (L.e.) and F1, F2 and F3, BC1F1 and most recently BC2F1 generations have been obtained. For the parents, trichome secretions concentrations ranged from 31-55 for L.e. and from 26-102 mg/cm2 for L.h. In the F1, concentrations ranges were 16-34 mg/cm2, considerably lower than either parent. For the F2, concentrations ranged from 14-66 and in the F3, from 17-67 mg/cm2.
F1 hybrids were uniformly very resistant to spider mites (T. urticae). The F2 segregated 3:1 for resistance:susceptibility to spider mites and to leaf miners. One F3 family arising from a susceptible F2 was susceptible. Other F3 families arising from resistant F2 hybrids were uniformly resistant or segregated, with a majority of segregants resistant. Trichome secretions from the resistant parent were highly repellent to spider mites.
In the BC1F1, frequency of resistance to spider mites was high indicating a dominate mode of resistance inheritance. Trichome secretions of the BC1F1 contained sesquiterpenes similar to those present in L.h. However, concentrations were lower. And, toxicity and repellency to spider mites were less.
Evaluation of four distinct BC2F1 populations, using the BC1F1s selected for resistance as parents, were evaluated. However, only a small number of individuals from each population were sampled. L.e. recurrent parents included NC EBR-1 and Summit as well as Zaofen 2. Only one highly resistant BC1F1 individual was identified. Frequency of resistance in these populations was low.
Subsequently, a small BC1F2 population was evaluated for mite resistance. In a whole leaf bioassay average susceptibility rating was 3.5 (0 = complete resistance, 7 = complete susceptibility), and the range of ratings among individuals was 0.5 to 7. Repellency was present as a resistance mechanism, as determined by the thumbtack bioassay in 20% of the individuals. Resistant individuals were not self-fertile. The most resistant BC1F2 individual was backcrossed with L.e. to obtain a BC2F1. Characterization of this BC2F1 is incomplete at this time. However, in this generation we know that at least 20% of the individuals have high levels of resistance to spider mites, and of these at least one individual has large fruit (>125 g) and is self-fruitful.
Since the initiation of the project, collaborators in China have been active contributors to this project (Chai Min, tomato breeder, Beijing Vegetable Research Center; Gao Jianchang, tomato breeder, Chinese Academy of Agriculture) by performing similar crosses, and conducting evaluation of resistance to insects including leaf miners (Liriomyza sativae) and whiteflies (Bemisia tabaci). At the time this was written, the PI was a Research Professor at the Chinese Academy of Agriculture, actively engaged in characterization of whitefly resistance in BC1 and BC2 hybrid populations.
The reported change in knowledge allows agricultural scientists to reevaluate the genetic potential of insect resistance in Lycopersicon hirsutum. Most of the insect resistance reported in this species has been associated with leaf trichomes. Trichome-based resistance, while effective, has been difficult to utilize by plant breeders.
Results of this research when completed may lead to tomato crops that are better able to withstand insect attack. By providing a better understanding of the components of resistance, causes of resistance, and mechanisms of resistance, breeding efficiency may be improved. This would result in more tomato varieties that are resistance to mites or insects.
Recovery of BC2F1 individuals having high levels of mite resistance that are self-fruitful and have large fruit is particularly promising, further motivating attempts at genetic improvement of insect resistance in tomato. A better understanding of how wild relatives of tomato defend themselves against insects will ultimately lead to genetic improvement of the pest resistance of tomato. Such knowledge may lead to a more precise measure of resistance or lead to methods that increase selection efficiency. In the future, it may be possible to grow tomatoes without application of toxic pesticides. Identification of chemicals involved in resistance may lead to development of tomato as a source of botanical pesticides.
Snyder, J. and G. F. Antonious (2009) "Trichomes -- importance in plant defense and plant breeding." CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources 2009, No. 076 4, 1-16 DOI: 10.1079/PAVSNNR20094076.