Evaluation of disease resistance and research of resistance mechanism of kiwifruit hybrid population derived from Actinidia rufa and Actinidia chinensis var. chinensis

Abstract: 【Objective】Kiwifruit is highly appreciated by consumers because of its delicious taste and high nutritional value. Although the global kiwifruit industry has grown rapidly in recent years, it is still facing the great challenge of bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa). The disease can cause large scale death of kiwifruit because of its fast transmission and strong pathogenicity, leading to serious yield and economic losses in many countries and limiting the development of the kiwifruit industry. Utilization of resistant kiwifruit cultivars has always been recognized as the most cost-effective and environment- friendly strategy for disease control, but there still is a lack of knowledge about the disease resistance of different cultivars. The analysis of the resistance of different kiwi-fruit germplasms to bacterial canker and the correlation between different evaluation indexes are of great significance to breeding new kiwifruit varieties resistant to the disease.【Methods】The kiwifruit germplasm resources used in this study are the hybrid populations of Actinidia rufa × A. chinensis var. chinensis in the National Kiwifruit Resource Nursery, with consistent ploidy and tree age. Psa M228 was provided by the laboratory of Pathogen Biology and the Research Team of Integrated Control of Fruit Tree Diseases, Northwest A & F University, China. Psa was diluted to 1.0×109 CFU·mL-1 before inoculation. The one- year old detached branches, approximately 0.8 cm in diameter, were sterilized with 75% alcohol and then cut into 12-14 cm, the ends of the branches were dipped in candle wax to reduce dehydration. A wound of about 3 mm was made and Psa was added to the wound. Subsequently, all of the branches were put on a draining board on which two layers of sterile absorbent paper had previously been placed. The lower tray was filled with sterile water close to the bottom of the draining board, and another two layers of sterile absorbent paper were placed over the cane pieces. The germplasms with differences in disease resistance were selected, and the leaf tissue structure and stomatal characteristics were observed by paraffin section method and scanning electron microscope technique, and the total phenol, soluble sugar and lignin content of leaves were determined to screen out the indicators significantly related to canker disease resistance. After 42 d of incubation, the outer cortex of the branches was peeled off with a sterile knife for observing and measuring the lesion; the germplasm resistance was classified according to the length of the lesion: Resistant (R): lesion length ≤7.0 mm; Tolerant (T): 7.0 mm≤9.0 mm; Susceptible (S): 9.0 mm≤11.0 mm; and High susceptible (HS): lesion length >11.0 mm. The data were analyzed by one-way ANOVA (one-way ANOVA) Duncan’s New Compound Extreme Variance method and Pearson correlation analysis using SPSS 21.0 software.【Results】Significant difference in the level of resistance of different kiwifruit germplasms were found. There were 67 accessions of disease-resistant germplasms in 84 accessions of germplasms, accounting for 79.76%, 13 accessions of disease-tolerant germplasms, accounting for 15.48%, 3 accessions of susceptible germplasms, accounting for 3.57% , and 1 accession of highly susceptible germplasm, accounting for 1.19%. There were significant differences in the thickness of leaf spongy tissue of different kiwifruit germplasms, and the thickness of the susceptible varieties were generally higher than the that of the resistant varieties, with a maximum stomatal density of 855.2 stomata · mm- 2 on the susceptible varieties. The distribution of stomatal apparatus length ranged from 16.78 to 7.68 μm. The total phenol content of the most resistant germplasm was highest at 52.53 mg·g-1 . The soluble sugar content varied significantly among the germplasms, from 33.05 mg · g-1 to 51.05 mg · g-1 . The higher the lignin content, the higher the resistant to the disease. The thickness of the upper epidermis was greater than that of the lower epidermis in all the germplasms, but it was not related to disease resistance, and the width of the fenestrated tissues and stomata were also not related to disease resistance of the branch. The leaf spongy tissue thickness, stomatal density, and stomatal length were significantly and negatively correlated with resistance, while total phenolic content, soluble sugar content, and lignin content were significantly and positively correlated with resistance.【Conclusion】In this study, 67 accessions of resistant germplasm were screened out from the 84 accessions of germplasms of he hybrid populations of A. rufa and A. chinensis var. chinensis. The spongy tissue thickness, stomatal density, stomatal length, lignin content, soluble sugar content, and total phenol content could be used as disease resistance indicators.