Microplot Assay: In field-grown resistant (Mi-1.2+) and susceptible (Mi-1.2-) plants inoculated with 8,000 nematode eggs or mock-inoculated with water, the total yield of mature fruits was not significantly impacted by plant genotype or nematode inoculation, although when all immature fruits were collected after peak fruit production was over, resistant plants inoculated with nematodes yield significantly more green fruits than susceptible, inoculated plants (). Dry root weight (averages: Mi-1.2-, control = 0.92 ± 0.45g, Mi-1.2-, inoculated 1.40 = ± 0.48g, Mi-1.2+, control = 0.96 ± 0.83g, Mi-1.2+, inoculated = 1.01 ± 0.31g) was not significantly influenced by cultivar (F1,28= 0.8050, P=0.3772), inoculation (F1,28=1.8243, P=0.1876), or the interaction between the two (F1,28=1.1855, P=0.2855). Nematode reproduction, as measured by the number of eggs per gram of dry root weight, was significantly lower on the resistant cultivar Motelle (2,800 eggs/g) than on the susceptible cultivar Moneymaker (85,260 eggs/g) (F1,14=30.09, P < 0.0001).
Table 1 Fruit production from field-grown plants inoculated with 8,000 nematode eggs. Red-ripe fruits were harvested twice a week for 11 weeks, and then, as fruit production was waning, all remaining green fruits were collected at the end of the assay. Data on (more ...)
Greenhouse Bioassays: To follow up on the microplot assay, two greenhouse assays were performed to measure the impact of Mi-1.2 and of nematode infection at two inoculum levels (assay 1: 20,000 eggs/plant; assay 2: 200,000 eggs/plant) on fruit and seed production, nematode infection, and above and below-ground biomass.
Nematode infection: Nematode data was transformed for analysis using the formula ‘log +1’ to normalize the variances. Mi-mediated resistance significantly reduced nematode reproduction for both inoculum levels, as measured by the number of egg masses per root system ().
Table 2 Effects of Mi-mediated resistance on nematode reproduction in greenhouse bioassays. Plants were inoculated with nematode eggs at moderate (20,000 eggs/plant) and high (20,000 eggs/plant) inoculum levels under greenhouse conditions. After peak fruit production (more ...)
Foliar Biomass: When plants were inoculated with 20,000 nematode eggs, there was a significant interaction between the treatment (control or inoculated) and the plant genotype (P < 0.05), because nematode inoculation significantly reduced foliar dry weight of the susceptible (Mi-1.2-) genotype, but not the resistant (Mi-1.2+) cultivar (). At the higher (200,000 eggs/plant) inoculum level, foliar dry weight was significantly impacted by treatment, genotype, and the interaction between these factors (P < 0.001) (). Both genotypes suffered a reduction in foliar biomass, but this reduction was significantly greater in the susceptible cultivar (P < 0.05).
Fig. 1 Effects of Mi-mediated resistance and nematode inoculation on dry foliar weight (±S.D.) of tomato plants. Plants were inoculated with 20,000 (A) and 200,000 (B) nematode eggs, while control plants were mock-inoculated with water. Foliar dry weight (more ...)
Root biomass: At the lower inoculum level, nematode infestation did not have a significant effect on root weights, nor was there a significant interaction between the treatment and genotype (P > 0.10), although overall root weights were higher in the resistant genotype than in the susceptible cultivar (P = 0.02) (). At the higher inoculum level, there was a significant interaction between treatment and genotype (P = 0.007), because nematode infection increased the root biomass of the resistant genotype (Mi-1.2+) compared to the uninoculated resistant plants (). Nematode challenge may have stimulated the root growth, or the increase may have been due to modest gall formation.
Fig. 2 Effects of Mi-mediated resistance and nematode inoculation on root weight (±S.D.) of tomato plants. Plants were inoculated with 20,000 (A) and 200,000 (B) nematode eggs, while control plants were mock-inoculated with water. Nematode infection (more ...)
Fruit production: In assay 1, in which plants were inoculated with 20,000 eggs, neither nematode inoculation nor plant genotype had a significant effect on the number or total yield (in grams) of mature fruits collected (). However, in the presence of nematodes, resistant plants produced significantly larger fruits than susceptible plants. There was no significant difference among treatments in the number, total weight, or average weight of green fruits collected from the plants at the termination of the experiment, when fruit production was waning. In assay 2, in which plants were inoculated with 200,000 nematode eggs, nematode infection dramatically reduced the number and total weight of mature fruits produced by susceptible plants, but had no effect on the fruit production of resistant plants, which was comparable to that of uninoculated controls (). Compared to susceptible plants challenged with nematodes, inoculated resistant plants also bore more green fruits at the end of the experiment. These dramatic differences in yield resulted from the fact that all but one of the susceptible inoculated plants died before the end of the experiment.
Table 3 Fruit production from greenhouse-grown plants inoculated with 20,000 nematode eggs. Red-ripe fruits were harvested twice a week for 10 weeks, and all remaining green fruits were harvested as fruit production was waning, at the end of the assay. Data on (more ...)
Table 4 Fruit production of greenhouse-grown plants inoculated with 200,000 nematode eggs. Red-ripe fruits were harvested twice a week for 18 weeks, and all remaining green fruits were harvested as fruit production was waning, at the end of the assay. Data on (more ...)
Seed production: At the lower inoculum level (20,000 eggs), nematode challenge caused a decrease in the average number of seeds per mature fruit from both cultivars, but did not significantly reduce the estimated lifetime seed production of either genotype (). Because nematode infection did not have a marked impact on seed production in this assay, we did not discern a fitness benefit associated with Mi-mediated resistance at this inoculum level. At the higher inoculum level (200,000 eggs), however, nematode infection dramatically reduced seed production in the susceptible line (), and the majority of fruits produced by infected susceptible plants bore no seeds. In contrast, the seed production of inoculated resistant plants was comparable to that of uninoculated controls, indicating that resistance provided a dramatic fitness benefit. The average weights of individual seeds as well as their germination rates were also measured, but nematode infection and Mi-mediated resistance did not impact these parameters (data not shown).
Table 5 Seed production of plants inoculated with 20,000 nematode eggs. Seeds were harvested from three representative fruits per plant. The lifetime seed production was estimated by multiplying the total number of grams of ripe fruit produced by the average (more ...)
Table 6 Seed production of plants inoculated with 200,000 nematode eggs. Seeds were harvested from a cross-sample of fruits§, and the estimated lifetime seed production was estimated by multiplying the total number of grams of ripe fruit produced by the (more ...)