Fusarium oxysporumas an Opportunistic Fungal Pathogen onZinnia hybridaPlants Grown on board the International Space Station

Abstract

A plant production system called Veggie was launched to the International Space Station (ISS) in 2014. In late 2015, during the growth of Zinnia hybrida cv. 'Profusion' in the Veggie hardware, plants developed chlorosis, leaf curling, fungal growth that damaged leaves and stems, and eventually necrosis. The development of symptoms was correlated to reduced air flow leading to a significant buildup of water enveloping the leaves and stems in microgravity. Symptomatic tissues were returned to Earth on 18 May 2016 and were immediately processed to determine the primary causal agent of the disease. The presumptive pathogen was identified as Fusarium oxysporum by morphological features of microconidia and conidiophores on symptomatic tissues; that is, by epifluorescent microscopy (EFM), scanning electron microscopy (SEM), metabolic microarrays, and ITS sequencing. Both EFM and SEM imaging of infected tissues showed that germinating conidia were capable of stomatal penetration and thus acted as the primary method for infecting host tissues. A series of ground-based pathogenicity assays were conducted with healthy Z. hybrida plants that were exposed to reduced-airflow and high-water stress (i.e., encased in sealed bags) or were kept in an unstressed configuration. Koch's postulates were successfully completed with Z. hybrida plants in the lab, but symptoms only matched ISS-flown symptomatic tissues when the plants were stressed with high-water exposure. Unstressed plants grown under similar lab conditions failed to develop the symptoms observed with plants on board the ISS. The overall results of the pathogenicity tests imply that F. oxysporum acted as an opportunistic pathogen on severely high-water stressed plants. The source of the opportunistic pathogen is not known, but virulent strains of F. oxysporum were not recovered from unused materials in the Veggie plant pillow growth units assayed after the flight.

Spatial Characterization of Microbial Communities on Multi-Species Leafy Greens Grown Simultaneously in the Vegetable Production Systems on the International Space Station

“…Humidity is a factor that can influence the microbial density and community in the phyllosphere and the surface of the leaf. It has been demonstrated that higher humidity and leaf wetness increases fungal abundance on plants including fungal plant pathogens [29,52,53].…”

“…The number of shared genera among VEG-03E components was higher for mizuna mustard at 31 and VEG-03F had 30 shared genera between wick, substrate, and root tissue (Figure 9C). Root tissue had few unique genera, but in all crops the root had high numbers of shared genera with substrate and between root, substrate, and wick (22)(23)(24)(25)(26)(27)(28)(29)(30)(31) which included more than 40% of the genera present (Figure 9). Leaf tissue had a variable number of unique species with VEG-03D having the highest with 11 in red romaine lettuce, followed by 10 in mizuna in VEG-03E.…”

“…Without careful control and monitoring, the environmental conditions within Veggie can promote undesirable levels of microbial growth. Zinnia hybrida, a flowering plant grown in Veggie in 2015-2016 became infected with the plant opportunistic pathogen Fusarium oxysporum after failure of the ventilation system led to an increase in humidity inside the chamber [29]. The source of the fungus was not determined definitively, however F. oxysporum has been isolated on surfaces on the ISS [29][30][31][32][33].…”

“…Zinnia hybrida, a flowering plant grown in Veggie in 2015-2016 became infected with the plant opportunistic pathogen Fusarium oxysporum after failure of the ventilation system led to an increase in humidity inside the chamber [29]. The source of the fungus was not determined definitively, however F. oxysporum has been isolated on surfaces on the ISS [29][30][31][32][33]. Furthermore, F. oxysporum was among the fungal isolates identified from leaves and roots from two out of three Veggie demonstrations growing red romaine lettuce [28].…”

Supplemental Food Production With Plants: A Review of NASA Research

“…This began ground testing of porous membranes, tubes, or plates to contain the water and allow capillary movement to the roots (Wright et al, 1988;Dreschel and Sager 1989;Koontz et al, 1990), as well as flight testing of hybrid approaches that use porous tubes to sub-irrigate a solid rooting matrix (Morrow et al, 1992;Bula et al, 1991). ASC and PGBA also used porous thermo-electric plates to cool and dehumidify the air, while recycling the condensate back to the plants (Morrow et al, 1995;Hoehn et al, 1997;Conrad, 1968;Dreschel and Sager 1989;Johnson and Tibbitts, 1968;Khodadad et al, 2020;Koontz et al, 1990;Morrow et al, 1992;Schuerger et al, 2021;Stutte et al, 2005;Wright et al, 1998). By 2000, plant growth systems were launched to the ISS.…”