Dualism in symbiosis: Growth and defense enhancement of symptomless infection by the pathogen Fusarium circinatum in Pinus radiata seedlings
C. L. SWETT (1), T. R. Gordon (1). (1) University of California-Davis, Davis, CA, U.S.A.
Many fungal symbionts can inhabit their hosts both as endophytes that cause no symptoms and as pathogens, but their activity as pathogens generally receives the most attention. <i>Fusarium circinatum</i> is a pine symbiont that can cause a disease known as pitch canker but it can also inhabit pine tissue (typically roots) without inducing symptoms. Studies were undertaken to characterize physiological effects of this asymptomatic association in <i>Pinus radiata</i> seedlings. Seedlings sustaining symptomless infections had a total shoot and root fresh weight equal to or greater than un-infected plants, when measured at both 12 and 40 weeks after planting. This was accompanied by alterations in root system architecture, with greater branch density in infected plants. Symptomless infections enhanced seedling resistance to later above ground infections, reducing stem lesion size and increasing survival frequency. Field surveys documented symptomless infections of seedlings in native stands of <i>P. radiata</i>, with up to 80% of two-year old healthy-looking seedlings harboring root infections. These results suggest that in addition to being a cause of disease, <i>F. circinatum</i> can have a commensal relationship with <i>P. radiata</i>, and may also promote seedling survival in native stands, under some circumstances. As a consequence of these diverse effects on fitness, selection for genetic resistance to disease caused by <i>F. circinatum</i> may be less intense than would otherwise be expected.
View Presentation |
