Transcriptome Analysis of Resistant and Susceptible Pecan (Carya illinoinensis) Reveals the Mechanism of Resistance to Black Spot Disease (Colletotrichum fioriniae)

Pecan, Carya illinoinensis (Wangenh.) K. Koch, is an important dried fruit and woody oil tree species grown worldwide. With continuous expansion of pecan cultivation, the frequency and scope of diseases, especially black spot disease, are increasing, damaging trees and reducing yields. In this study, the key factors in resistance to black spot disease (Colletotrichum fioriniae) were investigated between the high-resistance pecan variety "Kanza" and the low-resistance variety "Mahan". Leaf anatomy and antioxidase activities confirmed much stronger resistance to black spot disease in "Kanza" than in "Mahan". Transcriptome analysis indicated that the increased expression of genes associated with defense response, oxidation-reduction, and catalytic activity was involved in disease resistance. A connection network identified a highly expressed hub gene CiFSD2 (CIL1242S0042), which might participate in redox reactions to affect disease resistance. Overexpression of CiFSD2 in tobacco inhibited enlargement of necrotic spots and increased disease resistance. Overall, the expression of differentially expressed genes differed in pecan varieties with different levels of resistance to C. fioriniae infection. In addition, the hub genes associated with black spot resistance were identified and the functions clarified. The in-depth understanding of resistance to black spot disease provides new insights for early screening of resistant varieties and molecular-assisted breeding in pecan.