Characterization of Trichoderma harzianum T39-induced resistance of grapevine against downy mildew

Downy mildew, caused by the oomycete Plasmopara viticola, is one of the most destructive grapevine diseases. Its control is based on frequent chemical treatments, but concerns about health and environmental impact of pesticide overuse have made alternatives increasingly attractive. Enhancement of plant resistance by natural resistance inducers seems to be a promising strategy for controlling crop diseases, but scarce information is available on the molecular mechanisms and energy cost of induced resistance in non-model plants. Our aim is to characterize the resistance mechanisms activated in grapevine by the biocontrol agent Trichoderma harzianum T39 (T39) in order to identify genes and processes activated for defence against P. viticola infection. T39 treatments significantly reduced downy mildew symptoms without any direct toxic effect on P. viticola sporangia, by activating grapevine resistance, both locally and systemically. Local effect of T39 was comparable to the standard copper treatment under controlled greenhouse condition. Repeated T39 applications did not affect grapevine growth, shoot and root weight, leaf dimension and chlorophyll content, indicating absence of apparent energy cost for resistance activation. Expression analysis of marker genes suggested the involvement of jasmonic acid and ethylene signals in the T39-induced resistance. Trancriptomic analysis of T39-induced resistance was performed using the RNA-Seq protocol followed by next generation sequencing by Illumina. About 25 millions of 36 bp-long reads were obtained form control and T39-treated leaves, collected before or 24 h after P. viticola inoculation. Reads mapping to grapevine genome were used to estimate gene expression, and a complex transcriptional reprogramming during T39-induced resistance was detected. Particularly, T39 induced the expression of grapevine genes in the absence of pathogen infection and reinforces the expression of other genes after P. viticola inoculation. Interestingly, T39-treated plants showed a specific up-regulation of defence-related processes after P. viticola inoculation and attenuated modulation of genes repressed by downy mildew in control plants. Annotation of modulated genes revealed the activation of signal transduction, transcription and defence functional categories, indicating a potential key role of these genes in the activation of grapevine resistance. Further characterization of some regulatory genes will identify processed that could be enhanced to naturally increase the grapevine resistance.