Beneficial Microorganisms to Control the Gray Mold of Grapevine: From Screening to Mechanisms

In Vitro Evaluation of Some Endophytic Bacillus to Potentially Inhibit Grape and Grapevine Fungal Pathogens

Romania has a long history of grapevine culturing and winemaking. However, like any agricultural sector, viticulture faces devastating biological threats. Fungi responsible for grapevine trunk diseases (GTDs) and grape spoilage lead to considerable yield losses and a decline in grapevine quality. In the actual context, many countries, including Romania, have reoriented their approaches to minimize chemical inputs, which have been proven to be toxic and to have negative impacts on the environment, and to replace them with sustainable biocontrol strategies for the wine-growing sector. Within biocontrol strategies, Bacillus spp. is a well-known plant-protective bacteria with antifungal properties. Within this paper, six endophytic bacteria from various plant sources were studied. The bacterial strains were identified as B. pumilus, B. subtilis, and B. velezensis by sequencing their 16S rDNA region. Regardless of the in vitro test methods (using living bacterial cells, bacterial-cell-free supernatant (CFS), and volatile active compounds (VOCs)), B. velezensis strains revealed strong and broad antifungal activity against grape and grapevine fungal pathogens such as Aspergillus spp., Botrytis cinerea, Penicillium expansum, Diplodia seriata, Eutypa lata, Fusarium spp., Clonostachys rosea, Neofusicoccum parvum, and Stereum hirsutum. The functional antifungal genes encoding for difficidin, fengycin, iturins, macrolactin, and mycosubtilin were molecularly detected, which could support the proven antifungal activity of the endophytic strains. Lytic enzymes involved in fungal growth inhibition, such as chitinase, cellulase, and proteases, were also revealed to be produced by some of these bacterial strains. Various other in vitro tests, such as phosphate and phytate solubilization, phytohormone synthesis, the production of enzymes involved in the polyamine biosynthetic pathway, and pH as well as temperature tolerance tests were carried out to reveal the plant-beneficial potential of these bacterial strains. These results revealed that the B. velezensis strains, especially BAHs1, are the most suitable endophytes for grapevine biologic control, which could lead to the future development of sustainable management strategies.

Genome Mining Reveals High Biosynthetic Potential of Biocontrol Agent Bacillus velezensis B.BV10

The present study demonstrates the biocontrol potential of a plant growth-promoting bacterial strain using three different approaches: (i) an in vitro evaluation of antagonistic activity against important phytopathogenic fungi; (ii) an evaluation under greenhouse conditions with strawberry plants to assess the control of gray mold; and (iii) an in silico whole genome sequence mining to assign genetic features such as gene clusters or isolated genes to the strain activity. The in vitro assay showed that the B.BV10 strain presented antagonistic activity, inhibiting the mycelial growth in all the phytopathogenic fungi evaluated. The application of the Bacillus velezensis strain B.BV10 under greenhouse conditions reduced the presence of Botrytis cinerea and increased the mean fruit biomass. The genome of B.BV10 was estimated at 3,917,533 bp, with a GC content of 46.6% and 4088 coding DNA sequences, and was identified as B. velezensis. Biosynthetic gene clusters related to the synthesis of the molecules with antifungal activity were found in its genome. Genes related to the regulation/formation of biofilms, motility, and the important properties for the rhizospheric colonization were also found in the genome. The current study offers a comprehensive understanding of the genomic architecture and control activity of phytopathogenic fungi by the B. velezensis strain B.BV10 that may substantiate the industrialization of this strain in the future.

Paraburkholderia phytofirmans PsJN delays Botrytis cinerea development on grapevine inflorescences

Grapevine flowering is an important stage in the epidemiology of Botrytis cinerea, the causal agent of gray mold disease. To prevent infection and to minimize postharvest losses, the control of this necrotrophic fungus is mainly based on chemical fungicides application. However, there is a growing interest in other control alternatives. Among them, the use of beneficial microorganisms appears as an eco-friendly strategy. This study aims to investigate the effect of Paraburkholderia phytofirmans PsJN, root-inoculated or directly sprayed on fruiting cuttings inflorescences to control B. cinerea growth. For this purpose, quantification by real time PCR of Botrytis development, direct effect of PsJN on fungal spore germination and chemotaxis were assayed. Our results showed a significant protective effect of PsJN only by direct spraying on inflorescences. Moreover, we demonstrated an inhibition exerted by PsJN on Botrytis spore germination, effective when there was a direct contact between the two microorganisms. This study showed that PsJN is positively attracted by the pathogenic fungus B. cinerea and forms a biofilm around the fungal hyphae in liquid co-culture. Finally, microscopic observations on fruit cuttings revealed a co-localization of both beneficial and pathogenic microorganisms on grapevine receptacle and stigma that might be correlated with the protective effect induced by PsJN against B. cinerea via a direct antimicrobial effect. Taking together, our findings allowed us to propose PsJN as a biofungicide to control grapevine gray mold disease.