Insights into Fusarium Basal Rot Resistance through Saponin Profiling of Basal Plate Tissue of Short-day Onion Cultivars

Fusarium basal rot (FBR), caused by the soil-borne fungus Fusarium oxysporum f. sp. cepae (FOC), can be a substantial threat to onion crops by affecting bulb yield adversely. With an objective to increase FBR resistance in onions, investigating the role of plant metabolites in defense mechanisms has the potential to make breeding efforts more accurate. Saponins, a type of secondary metabolite, have demonstrated in vitro antifungal properties in both onion and shallot against Fusarium pathogens. However, additional research is needed through in vivo studies to investigate the role of saponins in FBR resistance. Our study sought to identify saponins associated with FBR resistance and to quantify fluctuations in saponin levels in the onion basal plate tissue before and after inoculation with FOC. Basal plate tissue was sampled at 0, 5, 10, and 15 days postinoculation (dpi) from mature onion bulbs of three open-pollinated, short-day onion cultivar populations and two check cultivars (FBR partially resistant and FBR susceptible). Extracted saponins from the sampled basal plate tissues were analyzed using ultra-high-performance liquid chromatography coupled to an Orbitrap fusion mass spectrometer, which led to the identification of 19 putative saponins. The mean relative abundance of Allium cepa saponin 14 (ACS-14) differed between the two check cultivars and across cultivar populations over time after inoculation. An ∼10-fold increase in the levels of ACS-14 was detected in the partially resistant check compared with the susceptible check at 5 dpi. ACS-14 showed 8- to 24-fold increased levels in basal plates of the advanced selected populations at 5 dpi compared with 0 dpi (uninoculated bulbs). Additional research can be conducted to confirm the involvement of this and other putative saponins detected in FBR resistance through in vivo and in vitro studies, and to explore the biosynthetic pathway of these saponins, if demonstrated conclusively to be involved in resistance.