Gene Expression Changes Of Peach In Response To LASIODIPLODIA THEOBROMAE Infection And Infection Characteristics Of Botryosphaeria Spp

Gummosis is a commonly occurring disease of peach in the south peach production area of the Yangtze River of China. A serious case of peach gummosis can cause tree death, seriously affecting peach cultivation and production. Fusicoccum aesculi, Diplodia seriata, and Lasiodiplodia theobromae, belonging to Botryosphaeria spp. were the predominant causal agents of peach tree gummosis. Polysaccharides are the main components of peach gum, hence, glycometabolism of peach trees was thought to be related to the gum formation. RNA-Seq was performed to investigate gene expression in peach shoots inoculated or mock-inoculated with L.theobromae. The role of ethylene and jasmonic acid in development of peach gummosis was also explored. In addition, the gummosis on lenticels of peach trees was investigated based on the isolation and identification of pathogens and histopathology. The infection characteristics of F. aesculi and L. theobromae were discussed, and one isolate of Neofusicoccum parvum was identified, which is a new causal agent of peach tree gummosis. The main results are summarized as follows:1. The results of RNA-Seq revealed that the senescence and defense response of peach shoots were induced by L.theobromae, the biosynthesis of new polysaccharides was promoted and the degradation of some polysaccharides was suppressed. A total of 20772 genes were detected in eight samples; 4231, 3750, 3453, and 3612 differentially expressed genes were identified at 12, 24, 48, and 60 h after inoculation, respectively. Furthermore, 920 differentially co-expressed genes(515 up-regulated and 405 down-regulated) were found. Gene ontology annotation revealed that genes related to biosynthesis and metabolism of phenylpropanoid, and uridine diphosphate-glucosyltransferase were significantly up-regulated during gummosis development. Genes associated with photosynthesis were obviously down-regulated. The degradation of cell wall was promoted according to the analysis of the differentially expressed genes involved in glycometabolism. The new biosynthesis of cellulose and pectin can be accompanied by degradation. Genes related to the decomposition of starch and sucrose were up-regulated. The expression levels of glucose 6-phosphate translocator genes increased in the inoculated peach shoots. Analysis of gene expression patterns indicated that the biosynthesis of UDP-galactose, UDP-xylose, and UDP-mannose was increased during peach gummosis. Genes related to the degradation of glycosaminoglycan and other glycans were down-regulated. The production rate of ethylene was enhanced in peach shoots after inoculated with L.theobromae. The expression of genes related to the biosynthesis of ethylene and signaling pathway were also significantly promoted. At 12 h post-inoculation, genes associated with jasmonic acid biosynthesis were significantly up-regulated. Methyl jasmonate with a concentration of 2%(Methyl jasmonate/lanolin, m/m) could induce peach shoots gummosis, enhance the production of ethylene, and reduce the content of soluble sugar. The expression of genes related to the biosynthesis of jasmonic acid were suppressed at early phase but promoted at the later stage by exogenous methyl jasmonate.2. F. aesculi, with a latent infection characteristic on field, could be isolated from the lenticel tissues of peach trees with a stable and high probability. Neofusicoccum parvum was identified to be a new causal agent of peach tree gummosis. F. aesculi could also be isolated in healthy tissues of lenticels. Pycnidia of F. aesculi produced on peach shoots located on the phloem, while pycnidia of L.theobromae colonized on the outer bark. Conidia of N. parvum are fusiform or ellipsoid, hyaline and sometimes become two-septate and with a darker middle cell. Distinct brown lesion and typical gummosis can be appeared on peach shoots inoculated with N. parvuma because of its highly pathogenic.