Defense response of 'Cuimili' plum / 'Maotao' to Agrobacterium tumefaciens: A combined physiological and transcriptomic analysis

Crown gall, a tumor disease caused by Agrobacterium tumefaciens, results in considerable economic losses to orchards. Following pathogen invasion, plants trigger a series of defense responses. However, little is known about the physiological and molecular mechanisms of plum in response to A. tumefaciens infection. In this study, we characterized the changes in physiological and transcriptional levels of 'Cuimili' plum / 'Maotao' after infection with A. tumefaciens. The root vigor was decreased. The malondialdehyde and soluble sugar content, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities were elevated. Additionally, there was an increase in the lignin content and the activities of enzymes related to lignin biosynthesis, including phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), 4-coumarate CoA ligase (4CL), cinnamoylCoA reductase (CCR), and cinnamyl-alcohol dehydrogenase (CAD). Notably, the expression levels of the corresponding genes of the lignin biosynthesis pathway were also found to be up-regulated. Antioxidant system and lignin play a crucial role in the early defense response of 'Cuimili' plum / 'Maotao'. Transcriptome analysis revealed that 1,348 differentially expressed genes (DEGs) were involved in the response to A. tumefaciens infection. DEGs were enriched in the biosynthesis of secondary metabolites, monoterpenoid biosynthesis, ascorbate and aldarate metabolism, carbohydrate metabolism, and plant hormone signal transduction. Our findings provide new insights into the defense mechanisms and adaptive changes in plants against A. tumefaciens infection.