Does salinity affect lifestyle switching in the plant pathogen Fusarium solani?

Symbiotic microbes that live within plant hosts can exhibit a range in function from mutualistic to pathogenic, but the reason for this lifestyle switching remains largely unknown. Here we tested whether environmental stress, specifically salinity, is a factor that can trigger lifestyle switching in a fungus mainly known as a pathogen, Fusarium solani. F. solani was isolated from roots of Phragmites australis (common reed) in saline coastal marshes of Louisiana, USA, and we used Oryza sativa (rice) as a model organism from wetland environments to test the symbiont lifestyle. We plated rice seeds on control plates or plates with F. solani at three levels of salinity (0, 8 and 16 p.p.t.), then assessed germination and seedling growth after 20 days. Salinity strongly reduced percentage germination, slowed the timing of germination and reduced growth of rice. F. solani slowed germination, and it also caused a minor increase in root growth at medium salinity and a minor decrease in root growth at high salinity. Overall, despite being a common pathogen in other crop species (peas, beans, potatoes and many types of cucurbits), we found little evidence that F. solani has a strong pathogenic lifestyle in rice and we found weak evidence that pathogenicity may increase slightly with elevated salinity. These results have implications for both crops and native plant health in the future as soil salinization increases worldwide.