CHEMOTROPIC SENSING RESPONSES OF FUNGAL BIOCONTROL AGENTS TO STRIGOLACTONES The three-player system: host-parasite-Fusarium

Host root-derived signals trigger the germination of seeds of obligate root parasitic weeds. Among these, strigolactones (SLs), a class of plant hormones, are the major natural germination stimulants in the rhizosphere. The early growth stages of parasitic plant development – such as seed germination, host attachment, and tubercle development – are key stages for establishing a successful parasitic interaction and are therefore ideal targets for controlling root parasitic weeds. Soil microorganisms and natural bioactive compounds interfering with these early stages of the parasite life cycle could be used as novel tools for management strategies. Our knowledge on the mechanism of SLs as signaling molecules during specific interactions in the rhizosphere is currently limited to arbuscular mycorrhizal fungi and parasitic plant seeds. By contrast, the role of plant secreted SLs in regulating the growth and development of plant-pathogenic-fungi remains controversial. Root-colonizing fungi can influence crop productivity either negatively by causing disease, or positively by enhancing plant growth. The production of plant hormones and growth regulators appears to be an important mechanism by which fungal endophytes improve plant growth and yield under stressful conditions. Fusarium oxysporum (F. o.) is a cosmopolitan species that survives in the soil as a saprophyte by thriving on dead or decaying organic matter. Some representatives also behave as plant pathogens causing vascular wilt disease in different crops, or as biocontrol agents, both as endophytes inducing systemic resistance and as enhancers of soil suppressiveness. F. o. has been suggested to be able to detect and degrade SLs, thereby preventing seed germination of parasitic weeds. Previous studies revealed that F. o. hyphae can sense and reorient their growth towards a variety of chemical signals including compounds from the host plant. To understand how F. o. senses and degrades SLs, we used a quantitative chemotropism assay on agar plates with different synthetic and natural SLs (e.g. GR24, strigol, 5-deoxystrigol and 4-deoxyorobanchol) as chemoattractant signals. We found that F. o. is able to chemotropically sense SLs, and that this process requires elements of a conserved fungal mitogen-activated protein kinase (MAPK) signaling cascade.