Role of mycotoxin production of Fusarium culmorum and F. graminearum and interaction with inhibitory Streptomyces sp. during Fusarium crown and root rot of wheat
Research fellowship provided by the Deutsche Forschungsgemeinschaft (DFG) conducted at the Department of Plant Pathology, University of Minnesota, St. Paul, USA.
Fusarium crown and root rot (FCR) is a serious disease of increasing importance in wheat growing areas worldwide. Yield losses mainly occur under water limited conditions. The predominant causal agents along with F. pseudograminearum are F. culmorum and F. graminearum. Populations of these two species vary broadly in production of a variety of mycotoxins including deoxynivalenol (DON), which is considered a virulence factor for wheat ear and stem base infections. It is likely that DON is also necessary for successful wheat root infections by F. culmorum and F. graminearum, but there are no studies published documenting infection under controlled conditions, and whether trichothecenes like DON are virulence factors in that process. Members of the genus Streptomyces have been shown to inhibit various Fusarium species, and Fusarium is also able to inhibit Streptomyces in vitro. However, little is known about these complex interactions and the potential for inhibitory Streptomyces to reduce FCR in wheat.The aim of this study was to investigate the role of trichothecene mycotoxins such as deoxynivalenol (DON), its acetylated derivatives and nivalenol during root and stem base colonization of wheat during FCR caused by F. culmorum and F. graminearum. We therefore examined early root infestation under in vitro conditions in specific bioassays (Fig 1), and advanced root and stem base colonization at the milk ripeness stage in growth chamber experiments (Fig 2) following seedling stage inoculation with different chemotypes and a Tri5 deletion mutant of Fusarium spp. The main finding was that, in contrast to its role as aggressiveness factor in Fusarium head blight of wheat, trichothecene production by F. culmorum and F. graminearum is detrimental to early wheat root colonization during FCR in wheat.
To investigate the interaction between Fusarium species, DON and beneficial rhizobacteria, i.e. Streptomyces spp. we collected and screened a set of Streptomyces isolates from wheat rhizospheres obtained from the Upper Great Plains in Minnesota, USA for their in vitro inhibitory activity against Fusarium spp. and vice versa.
The inhibitory effect of Streptomyces was also tested in planta. We enriched sterilized potting soil with spore suspensions of Streptomyces isolates, inoculated the soil with F. culmorum-colonized wheat straw, and planted pre-germinated wheat seedlings. We could show that Streptomyces sp. can significantly reduce wheat stem base and root colonization by 75% compared to a treatment with F. culmorum inoculated alone. Interestingly, co-inoculation of F. culmorum with the non-inhibitory Streptomyces isolate led to higher Fusarium colonization and Streptomyces densities (CFU/g soil) in the rhizosphere. Carbon use phenotyping in Biolog assays showed differences in nutrient use among and between Streptomyces and Fusarium isolates. In vitro assays testing the inhibition of Fusarium spp. against Streptomyces sp. revealed that F. culmorum showed a strong inhibitory activity against the inhibitory Streptomyces isolate but not against the non-inhibitory isolate. The results of this study illustrate the inhibitory potential of Streptomyces for biocontrol of FCR in wheat, while suggesting that Streptomyces-Fusarium interactions appear to be highly isolate-specific. In conclusion, a broader understanding of the variation in susceptibility within Fusarium populations to Streptomyces inhibition, as well as the capacity of Fusarium spp. to inhibit antagonistic Streptomyces is needed, which are both keys to biocontrol advances.
Fig. 1: In vitro bioassay to investigate early root infection of wheat seedlings with Fusarium culmorum (left), non-inoculated control (right). Wheat seedlings were grown in a medium containing potato dextrose agar and Murashige and Skoog Basal Medium. The plate was inoculated by cutting out a 2-cm wide strip (between the 2 horizontal black lines) 48 hours after placing the seedlings and replacing it with the same medium inoculated with Fusarium macroconidia. Black arrows indicate zones of strong symptom development on roots after inoculation.
Fig. 2 Brownish discolorations on outer leaf sheaths at the stem base of wheat. Typical disease symptoms of Fusarium crown and root rot at the stem base of wheat after in inoculation with Fusarium spp. (left), and control plants without symptoms (right).