Research Accomplishment Reports 2011

Arthropod Resistance of Lycopersicon hirsutum LA2329, A Wild Relative of Tomato

J. Snyder
Department of Horticulture

 

Non-Technical Summary

Modern agriculture has relied on application of pesticides to reduce or prevent damage caused by arthropods and disease. However, rising concerns with the direct and indirect costs of pesticide use have spurred searches for alternative approaches to prevent damage caused by these stresses.

Use of cultivars having innate resistance to pests provides one path for reducing pesticide application and, perhaps a more sustainable agriculture. Deployment of resistant cultivars could reduce energy-intensive practices such as the use of petroleum-based pesticides or could allow wider adaptation of the crop.

Furthermore, use of genetic-based mechanisms of resistance (host resistance) may lead to safer and more ecologically sound agricultural systems. Genetic transfer of host resistance is conceptually straightforward and requires the ability to recognize the trait in populations of genetic resources or in subsequent generations in which the trait may segregate. This requirement applies whether classical plant breeding methods based on pollination and fertilization or methods of genetic engineering are used to transfer the trait.

The more that is known about causal mechanisms of resistance, then usually the more efficient the ability to transfer resistance from one plant to another. The proposed approach will provide specific information about causes of insect resistance in tomato, making the production of insect resistant varieties of tomato easier.

2011 Project Description

A new bioassay for assessing tomato resistance to Bemesia tabaci has been developed. The assay uses leaves removed from the plant placed in flasks containing water. Genotypes are mass infested with unsexed whiteflies maintained on cucumber at a rate of 100 flies per leaf. After 8 hours leaves are removed and eggs counted. Precision and sensitivity of the bioassay was demonstrated by its ability to detect differences in oviposition among wild resistant and cultivated susceptible lines. Importantly, compared to assay of intact plants, the ex planta assay had greater precision and successfully demonstrated differences of whitefly oviposition among leaf positions.

Approximately 80 BC2F1 interspecific hybrids were evaluated for whitefly resistance with the recently developed ex planta bioassay. Approximately 10 individuals were identified as having resistance to whitefly, compared to the non-resistant recurrent parent. Five QTL's have been detected associated with reduced fecundity, two located on each of chromosomes 2 and 6, with one QTL located on chromosome 5. Additionally 3 QTL's associated with non-preference were located, two on chromosome 6 and one on chromosome 2. One QTL was shared for reduced fecundity and non-preference.

Approximately 400 F2 individuals resulting from selfing a BC2F1 were grown in the field during summer of 2010. This particular BC2F1 was an atypical plant, in that it was self-fruitful, and had very red, large fruit, unlike its small, pink -fruited recurrent parent. Growth of most individuals was vigorous.

During August, the field was successfully infested with two-spotted spider mites. Most of the F2 individuals were highly susceptible, and were killed by the mite infestation. Approximately 30 individuals demonstrated tolerance to the mite infestation. However some of these resistant individuals had few or no fruit.

Of those individuals having high fruit set, the 10 most spider mite-tolerant lines and 10 highly susceptible lines were clonally propagated and subsequently self-pollinated. Additionally, seed was collected from these 20 (10 tolerant and 10 highly susceptible) plants grown in the field. This plant material will form the basis for future research on genetics of tolerance to mite feeding in tomato.

Investigation of mite resistance in a small BC2F1 population detected resistance in about 15% of the plants. The mechanism of resistance was investigated in 6 BC2F1 individuals, three susceptible and three resistant. On abaxial surfaces, the number of eggs per live mite at 72 hours was 2.1, 3.7 and 6.1 for the susceptible individuals, comparable to 4.2 eggs/live mite obtained on the susceptible recurrent parent. For the resistant BC2F1 individuals, eggs per mite ranged from 0.9 to 4.2, all values higher than those obtained for the resistant parent, 0.1 eggs/live mite. On the adaxial surface eggs/live mite ranged from 0.9 to 1.5, lower than that observed for the recurrent parent, 2.5 eggs/mite but higher than the resistant parent, 0.4 eggs/mite.

2011 Impact

Whiteflies (Bemesia spp.) are important pests of tomato. These whiteflies cause direct damage to tomato, leading to several problems including loss of yield and problems with tomato ripening. Also these whiteflies serve as virus vectors. Some of these viruses cause very serious production problems. For example, tomato yellow leaf curl virus can cause a 100% crop loss if the infection occurs early in the development of the crop.

Whiteflies are difficult pests to control using insecticides because it develops resistance very quickly. Development of a whitefly resistant tomato may reduce the risks and costs of tomato production in areas where whiteflies and the viruses that they transmit are present. Development of bioassays and identification of genes (QTLs - quantitative trait loci) are important steps in transferring resistance from wild to cultivated tomato, which may ultimately lead to breeding tomatoes resistant to whitefly.

Spider mites are also important pests of tomato. However, their control, and especially control of associated economic damage is somewhat easier that that for whitefly. Thus, production of a mite resistant tomato would also be welcome by tomato producers, but they would likely put this goal somewhat lower on their priority needs list, certainly lower than resistance to whitefly.

That said, previous work has indicated that causal mechanisms of resistance to arthropods in wild tomato often confer resistance to a range of small arthropods. Thus mechanisms of arthropod resistance can be identified with spider mites, with the reasonable expectation that associated causal factors will also confer resistance to a range of arthropods. Thus the tolerance that segregates in the interspecific BC2F2 may be a useful character conferring resistance to spider mites, and well as other arthropods.

The additional work conducted with spider mites revealed differential resistance between leaf surfaces, indicating the likely involvement of trichomes in the resistance mechanism. All these observations provide potential paths leading to the production of insect resistant tomato cultivars.

2011 Publications

Snyder, J., G. Antonious, and R. Thacker. (2011). A sensitive bioassay for spider mite (Tetranychus urticae) repellency: a double bond makes a difference. Experimental and Applied Acarology 55(3): 215-224.

Antonious GF, Dennis SO, Unrine JM, Snyder JC (2011) Ascorbic acid, B-carotene, sugars, phenols, and heavy metals in sweet potatoes grown in soil fertilized with municipal sewage sludge. Journal of Environmental Science and Health, Part B 46: 112-121