Genetic Analysis of Resistance to Tomato Gray Mold (Botrytis cinerea)

Qiang ZHANG Wenting DAI Hongbin WU Hui YANG Xinwen JIN

AbstractThe study used four tomato varieties, 03786 (susceptible), 05237 (susceptible), C2985 (resistant) and Wva536 (resistant) with different resistance, to formulate combinations according to Griffing (I) complete diallel cross method. And Botrytis cinerea was inoculated to investigate the incidence of gray mold. The results showed that: ① there were significant differences in disease resistance between different varieties and different combinations. ② There were significant differences in general combining ability and special combining ability between the tested varieties, and Wva536 and C2985 had a higher general combining ability effect, and were both relatively good parents when formulating resistant combinations. Combinations Wva536×03786 and Wva536× 05237 had the highest special combining ability, and thus have the value of further research and utilization. ③ The additive effect in resistance inheritance was located at the primary position, and partial dominance also existed as well as cytoplasmic effect. ④ The broadsense heritability and narrowsense heritability were, respectively, 89.73% and 82.15%, which were higher, indicating that the parents had a greater effect on offsprings. The resistance genes can be expressed in offsprings through gene accumulation, and should be selected in the early generation.

Key wordsTomato; Diallel cross; Tomato gray mold; Combining ability; Genetical analysis of resistance

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Received: September 9, 2018Accepted: November 19, 2018

Supported by Program of Science and Technology Support for Xinjiangs Development (2016AB028).

Qiang ZHANG (1977-), male, P. R. China, associate researcher, devoted to research about plant molecular biology.

*Corresponding author. Email: 821861127@qq.com.

Tomato gray mold (Botrytis cinerea) is a worldwide disease that outbreaks easily, causing huge losses to tomato production. Under the current situation of lacking effective control agents, breeding and planting resistant varieties is one of the most economical and effective measures to control tomato gray mold. For the resistance to gray mold, in addition to those who believe that it is ph vertically resistance, some scholars deem that there is horizontal resistance codetermined by major genes and modifying genes, or multiple genes determine the resistance. Gallegly［1-2］ believes that tomato gray mold trait is controlled by two kinds of different genes, one of which is single dominant gene ph, immune to physiological race 0, and the other is polygenes, related to multiple factors, belonging to quantitative inheritance. Palevitch［3］ reckons resistance is partially dominant, controlled by polygenes. It has a higher heritability ratio. The resistance of Solanum pimpinellifolium to pathogen 0 line is controlled by a dominant gene, and the resistance to the pathogen 1 line is controlled by polygenes［4］. Rebecca et al.［5］ reported that wild tomato MC723 was subjected to resistance identification, which showed that it had higher resistance, which was not completely dominant and controlled by two loci, and can be transformed easily. It has been found that the British variety "Atom" carries two incompletelydominant major genes of horizontal resistance, phf1, phf2, which are resistant to both 0 and 1 lines. Furthermore, there are vertical resistance genes phf1, phf2, and resistance controlled by polygenes also exist. Many reports indicate that S. peruvianum and S. habrochaites are resistant to gray mold.

Materials and Methods

Wild tomato resistant varieties Wva536 (Lycopersicon esculrnlentum) and C2985 (S. pimpinellifolium) provided by Asian Vegetable Center, and cultivated susceptible tomato varieties03786 and 05237 provided by Tomato Research Laboratory, Northeast Agricultural University, were selected to form 16 combinations according to Griffing (I) complete diallel cross, respectively. At the seedling stage with 7 to 8 leaves, T1 physiological race of B. cinerea (provided by Chinese Academy of Agricultural Sciences) was spray inoculated at a concentration of 5×104 sporangia/ml. Within the first 24 h after inoculation, the environment was darkness at the room temperature was (20±2) ℃ with air humidity of 100%. Afterwards, the daily air humidity was 75%-95%, and light ［70 μmol/(m2?s)］ was supplied for 14 h［6］. The disease grade of individual plants was investigated on the 7th day after inoculation.

Disease index (DI)=∑(Disease grade×Number of plants of this grade)/(The highest grade×Number of investigated plants)×100 (as shown in Table 1). Data analysis was performed by the Griffing (I) analysis method, and combining ability and genetic parameters were estimated.

Table 1Disease grades of individual plants of the four tomato varieties

Code of material0378605237L298505237

0378688.1687.1362.9855.22

0523782.9879.9553.9549.15

C298553.8550.2543.3138.897

Wva53646.3843.9840.6530.96

Results and Analysis

Significance test of difference in genotype between F1 generations

The variance analysis of disease resistance in each parent and F1 generation is shown in Table 2.

Table 2Variance analysis on observed disease index values of parents and F1 cross

Source of variationDegree of freedomSum of squaresMean squareFvalue Level of significance

Block 2115.986 557.672 3511.176 28

Treatment 16125.756 01 012.560 00179.832 750.0001

Error31165.689 35.583 61

Total variation4615 396.573 0

It could be seen from Table 2 that the resistance of each parent and combination to tomato gray mold was extremely significant (P=0.000 1), indicating that there were significant differences in the genetic basis between the tested materials. Therefore, further genetic analysis is necessary.

Variance analysis of combining ability of disease resistance

The results are shown in Table 3.

It could be seen from Table 3 that the general combining ability (GCA) variance and the special combining ability (SCA) variance of the F1 generations obtained from the four parents through diallel cross reached the very significant level (P=0.000 1), and the general combining ability variance was significantly larger than the special combining ability variance, GCA/SCA=26.9>4, indicating that the resistance to gray mold was mainly based on the additive effect of genes, and the dominant effect was in a secondary position. In the breeding of tomato for resistance to tomato gray mold, the general combining ability is more important.

Therefore, tomato breeding for resistance to tomato gray mold should mainly depend on cross breeding. The results of the variance analysis also showed that the four parents had significant reciprocal effects.

Table 3Variance analysis of combining ability of disease resistance in the four parents

Source of variationDegree of freedomSum of squaresMean squareFvalue Level of significance

General combining ability34 629.3761 511.802 0831.238 730.000 1

Specific combining ability6343.08256.107 729.987 350.000 1

Reciprocal cross6169.62431.127 316.237 720

Error 3155.8621.903 2

General combining ability effects between parents and their comparison

The general combining ability between various parents and its comparison results are shown in Table 4 and Table 5.

From the results of measured GCA effects between the parents, it could be seen that for the same trait, the GCA effects between different varieties had large differences. From the four parents, 05237 and C2985 showed a higher negative value of the general combining ability effect, indicating that the two were parents matching better when formulating a resistant combination. Parents 03786 and 05237 had a high positive value, indicating that these parents were relatively inferior parents when formulating a hybrid combination resistant to tomato gray mold, which led to weakened resistance, and therefore, they are not suitable to be used in breeding as a parent. Meanwhile, except the nonsignificant difference between 03786 and 05237, the differences in general combining ability between other parents all reached the very significant level. Although there were no obvious differences in the resistantsusceptible phenotype between the resistant parents and the susceptible parents, but the intrinsic genetic differences were obvious, i.e., there were significant differences in the additive effect between them. Therefore, in breeding, in order to obtain tomato materials with high resistance to tomato gray mold, it is not possible to simply select from the phenotype, and attention also should be paid to the selection of general combining ability. Only the resistant materials with good combining ability have use value.

Table 4Significance test of difference in general combining ability (GCA) effect between the various parents

Source of variationMean 1234

112.998 650.000 10.000 10.000 1

29.989 763.198 70.000 10.000 1

3-7.978 3921.432 617.975 30.000 1

4-14.855 6929.536 824.892 56.910 2

Down triangle is mean differences, and up triangle is significant level.

Table 5Multiple comparisons on general combining ability effect of disease resistance trait by LSD method

TreatmentMean5% significant level1% significant level

112.998 65aA

29.989 76bB

3-7.978 39cB

4-14.855 69dC

Significance test of difference in special combining ability (SCA) effect

The results are shown in Table 6.

Table 6Special combining ability effect of disease resistance trait

Parent234

14.7-3.913 1-5.099 8

2-3.162 3-5.298 7

36.132 5

It could be seen from Table 6 that combinations 03786×Wva536 and 05237×Wva536 had a higher negative value, and were better combinations in tomato breeding for resistance to tomato gray mold. Therefore, in the tomato breeding for resistance to tomato gray mold, the selection of general combining ability should be emphasized, and then the selection of special combining ability should be considered.

Estimation of genetic parameters

The estimated genetic parameters of complete diallel cross are shown in Table 7.

Table 7Estimation of genetic parameters

Trait Geneticvariance AdditivevarianceDominantvarianceEnvironmentalvariancePhenotypicvarianceBroadsenseheritabilityNarrowsenseheritability

Disease index396.118 6359.987 333.237 65.513 2405.336 789.73%82.15%

It could be seen from Table 7 that the additive variance was much larger than the dominant variance, indicating that the additive effect of resistance genes played a major role, and the environmental variance was larger, indicating that the expression of resistance was greatly affected by the environment. The broadsense heritability and narrowsense heritability were, respectively, 89.73% and 82.15%, which were both higher, indicating that the parents had a greater effect on offsprings. When formulating hybrid combinations, the traits of the parents have a greater impact on offsprings. In the early generations of hybrids, the effect is more obvious, and the space for genetic breeding using resistance genes is larger.

Discussion

There are significant differences in the genetic basis between the test materials. The F1 generation of resistant and susceptible parents or susceptible and resistant parents showed intermediate or slightly higher resistance, and the disease resistance was incompletely dominant with high heritability, which is consistent with previous studies. The general combining ability and special combining ability of the tested varieties reached the extremely significant level. The general combining ability variance was significantly larger than the special combining ability variance, GCA/SCA=26.9>4, indicating that tomato resistance to gray mold was genetically additive, the dominant effect was in a secondary position, and there was a cytoplasmic effect. Therefore, tomato breeding for resistant to gray mold should mainly depend on cross breeding.

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