Stigma type and transcriptome analyses of mulberry revealed the key factors associated with Ciboria shiraiana resistance

Abstract

Ciboria shiraiana is a fungal pathogen and the causal agent of hypertrophy sorosis scleroteniosis (HSS) in mulberry, leading to substantial economic losses in the mulberry fruit-related industry. To obtain HSS resistant resources and investigate the resistance mechanism, the resistances of 14 mulberry varieties were assessed. Morus laevigata Wall. (MLW) varieties showed strong resistance to C. shiraiana, and the pathogen's infection was associated with mulberry fluorescence. Stigmas were identified as the infection site through cutting experiments. Susceptible varieties (S-varieties) displayed secretory droplets on their stigma papillar cell surfaces, while MLWs lacked these secretions. Correlation analysis between the secretion rate and the diseased fruit rate indicated that the differences between resistant varieties (R-varieties) and S-varieties were related to the stigma type. Furthermore, comparative transcriptome analysis was performed on stigma and ovary samples from R- and S-varieties. Compared with the stigma of R-varieties, the key differentially expressed genes (DEGs) with significantly higher expression in S-variety stigmas mainly participated in the fatty acid biosynthetic process. In R-variety stigmas and ovaries, the transcript levels of DEGs involved in defense response, including resistance (R) genes, were significantly higher than that of S-varieties. Overexpression of MlwRPM1-2 and MlwRGA3 enhances resistance to C. shiraiana and Sclerotinia sclerotiorum, but not Botrytis cinerea in tobacco. These findings help us explain the different resistance mechanisms of mulberry to C. shiraiana, and the critical defense genes in R-varieties can be applied to breeding antifungal plant varieties.

Keywords: Ciboria shiraiana; Fungus; Mulberry; Resistance mechanism; Stigma type.