Function And Chromosomal Localization Of Differentially Expressed Genes Induced By Marssonina Brunnea F.sp. Multigermtubi In Populus Deltoides

The Poplar black spot disease, caused by fungal pathogen Marssonina brunnea f. sp. Multigermtubi, is one of the main diseases in genus populus. The genes involved in resistance and transcriptional regulation were investigated for Populus deltoides cv. 'Lux' (Ⅰ-69/55) with time-series cDNA Microarray including 2,952 cDNAs from two cDNA libraries constructed with 72 h inoculated poplar leaves. The pathogen Marssonina brunnea f. sp. Multigermtubi was inoculated at 25℃, total RNA of leaves were extracted at 0 h (control), 4 h, 24 h, 72 h and 7 d respectively after inoculation and fluorescent labeled in the presence of Cy5-dCTP or Cy3-dCTP in a reverse transcription reaction before hybridization. In total, 1,160 genes were differentially expressed during infection. The strongest response observed was at 72 h, 475 genes were up-regulated and 326 genes down-regulated, followed by 4 h, 235 genes up-regulated and 290 genes down-regulated, the weakest was at 7 d, 34 genes up-regulated and 43 genes down-regulated, which indicated the complex response of plant to pathogen.Functional analysis showed that 1,160 genes were classified into 11 functional categories that are involved in metabolism (15.9%), signal transduction (9.5%), transcription and replication (8.7%), and cell rescue and defense (7.8%). Among them, 926 genes were sporadically localized on 19 linkage groups. Chromosome 2 contained 102 (11%) differentially expressed genes, followed by chromosome 1 which contains 93 genes (10%), and chromosome 17 had the least number of differentially expressed genes. Clustering of expressed sequence tags (ESTs) in poplar genome was observed at the terminal regions of several chromosomes. The relationship between cluster of genes and plant defense response would be further studied.WRKY transcription factors are unique to plant. It was reported that WRKY proteins could be induced by many pathogens and played a key role in regulating the pathogen induced defense response by binding to the W box and involved in several signaling pathways. Microarray results indicated four WRKY transcription factors were induced by M. f. sp. Multigermtubi. Among them, expression profiles of two WRKY transcription factors were evaluated by using real-time RT-PCR. The results indicated that the two WRKY transcription factors were both up-regulated by M. brunnea f. sp. Multigermtubi in resistant and susceptible clones, but the reaction in resistant clone was more rapid and the abundance was lower than susceptible clone. Also, the expression levels of them were evaluated by RT-PCR after exposure to methyl jasmonate (MeJA) and salicylic acid (SA). The two WRKY transcription factors were both up-regulated by SA, but MeJA suppressed the expression of them. Thus, the observed change in expression level indicated that the two WRKY transcription factors might involve in the response and act as important roles in signal transductions of poplar to M. f. sp. Multigermtubi.