Combined transcriptome and metabolome analysis of Nerium indicum L. elaborates the key pathways that are activated in response to witches' broom disease.

backgroundNerium indicum Mill. is an ornamental plant that is found in parks, riversides, lakesides, and scenic areas in China and other parts of the world. Our recent survey indicated the prevalence of witches' broom disease (WBD) in Guangdong, China. To find out the possible defense strategies against WBD, we performed a MiSeq based ITS sequencing to identify the possible casual organism, then did a de novo transcriptome sequencing and metabolome profiling in the phloem and stem tip of N. indicum plants suffering from WBD compared to healthy ones.

resultsThe survey showed that Wengyuen county and Zengcheng district had the highest disease incidence rates. The most prevalent microbial species in the diseased tissues was Cophinforma mamane. The transcriptome sequencing resulted in the identification of 191,224 unigenes of which 142,396 could be annotated. There were 19,031 and 13,284 differentially expressed genes (DEGs) between diseased phloem (NOWP) and healthy phloem (NOHP), and diseased stem (NOWS) and healthy stem (NOHS), respectively. The DEGs were enriched in MAPK-signaling (plant), plant-pathogen interaction, plant-hormone signal transduction, phenylpropanoid and flavonoid biosynthesis, linoleic acid and α-linoleic acid metabolism pathways. Particularly, we found that N. indicum plants activated the phytohormone signaling, MAPK-signaling cascade, defense related proteins, and the biosynthesis of phenylpropanoids and flavonoids as defense responses to the pathogenic infection. The metabolome profiling identified 586 metabolites of which 386 and 324 metabolites were differentially accumulated in NOHP vs NOWP and NOHS and NOWS, respectively. The differential accumulation of metabolites related to phytohormone signaling, linoleic acid metabolism, phenylpropanoid and flavonoid biosynthesis, nicotinate and nicotinamide metabolism, and citrate cycle was observed, indicating the role of these pathways in defense responses against the pathogenic infection.

conclusionOur results showed that Guangdong province has a high incidence of WBD in most of the surveyed areas. C. mamane is suspected to be the causing pathogen of WBD in N. indicum. N. indicum initiated the MAPK-signaling cascade and phytohormone signaling, leading to the activation of pathogen-associated molecular patterns and hypersensitive response. Furthermore, N. indicum accumulated high concentrations of phenolic acids, coumarins and lignans, and flavonoids under WBD. These results provide scientific tools for the formulation of control strategies of WBD in N. indicum.