Effect of biological control agents on grapevine rhizosphere microbiome and grapevine defenses

Plant diseases are a major obstacle to crop production. The main approaches to battle plant diseases, consist of synthetic chemicals to attack infecting pathogens. However, concerns are increasing about the effects of chemicals in the environment, leading to an increase in the use of biocontrol agents (BCAs), due to their assets, such as, antagonism, and competition. In this study, we tested the hypothesis that the introduction of Bacillus subtilis PTA-271 (Bs PTA-271) and Trichoderma atroviride SC1 (Ta SC1) produce distinctive modifications in the composition and network structure of the grapevine rhizosphere microbial community, as well as grapevine induced defenses. Therefore, we tested the effect of Ta SC1and Bs PTA-271, on grapevine rhizosphere bacterial and fungal microbiome, and plant gene expression, in two different soil types (clay and sandy soil). Additionally, we specifically quantified Ta SC1 and Bs PTA-271 in rhizosphere of both soil types. To do that, we used novel molecular approaches, such as high-throughput amplicon sequencing (HTAS), droplet digital PCR (ddPCR) and real-time PCR (qPCR). The results suggest that Bs PTA-271 established better in clay soil, where although its quantity was low at 30dpi, it was maintained at 90dpi. In contrast, Ta SC1 established better in sandy soil, with a lower quantity, that increased overtime. Regarding the BCAs impact on rhizosphere microbiome, bacterial diversity was not affected by any BCA application. However, inoculations with Ta SC1 significantly reduced fungal diversity. Ta SC1 applications affected the relationships between microorganisms, however this effect was clearer in clay soil. According to the selected plant defense markers, plants living in sandy soil appeared more impacted to BCA inoculation. Ta SC1 plants treated in sandy soil demonstrated a strong suppression defense genes 24hpi, that conversed in strong simulated defenses 4dpi, with a significant overexpression of PAL and STS. On the other hand, in clay soil, BCA-treated plants showed a slight increase in the expression of plant defense genes 24hpi, that intensifies 4dpi. In conclusion, the effect of Bs PTA-271 and Ta SC1 in grapevine rhizosphere appears to be soil-dependent, where a sandy soil favors the fungal BCA (Ta SC1) establishment, while a clay soil favors the bacterial BCA (Bs PTA-271) establishment.