Redistribution of iron and hydrogen peroxide in Pinus sylvestris roots in response to trophically diverse fungi

Abstract

The simultaneous over-accumulation of Fe, hydrogen peroxide and superoxide can result in the generation of hydroxyl radicals, which play a significant role in the interactions between plant hosts and invading fungi. The physiological response to infection by pathogenic or mycorrhizal fungi may, however, be determined by the root zone. The objective of the present study was to use light microscopy to determine whether iron and hydrogen peroxide are differentially distributed in P. sylvestris roots in response to colonization by necrotrophic pathogens (Fusarium oxysporum and Rhizoctonia solani) vs. mycorrhizal fungi (Hebeloma crustuliniforme and Laccaria bicolor) in in vitro culture. It was also of interest to determine how hydrogen peroxide, superoxide, and iron added to substrate in vitro impacted the ability of trophically different fungi to grow. Infection by necrotrophic fungi, especially by F. oxysporum, were associated with a significantly higher accumulation of both forms of iron (Fe2+ and Fe3+) in the meristematic root zone, relative to the accumulation of these forms of iron in response to ectomycorrhizal fungi. Iron supplementation of solid culture medium reduced the growth of pathogenic fungi to a greater extent than the growth of mycorrhizal fungi. In contrast to the distribution of iron, cells in the elongation zone of P. sylvestris roots generally exhibited the highest accumulation of hydrogen peroxide. The growth of mycorrhizal fungi was more inhibited by the presence of hydrogen peroxide in the solid culture medium than by any other supplemented compounds. This research indicate that the distribution of iron and hydrogen peroxide in roots of P. sylvestris is related to the trophic strategy of the colonizing fungi.