Strategies utilized by trophically diverse fungal species for Pinus sylvestris root colonization

Abstract

Physiological changes in host plants in response to the broad spectrum of fungal modes of infection are still not well understood. The current study was conducted to better understand the infection of in vitro cultures of Pinus sylvestris L. seedlings by three trophically diverse fungal species, Fusarium oxysporum E. F. Sm. & Swingle, Trichoderma harzianum Rifai and Hebeloma crustuliniforme (Bull.) Quél. Biochemical methods and microscopy were utilized to determine (i) which factors (apoplastic and cellular pH, reactive oxygen species, glutathione and cell death) play a role in the establishment of pathogenic, saprotrophic and mycorrhizal fungi, and (ii) whether cell death is a common response of conifer seedling tissues when they are exposed to trophically diverse fungi. Establishment of the pathogen, F. oxysporum, was observed more frequently in the meristematic region of root tips than in the elongation zone, which was in contrast to T. harzianum and H. crustuliniforme. Ectomycorrhizal (ECM) hyphae, however, were occasionally observed in the studied root zone and caused small changes in the studied factors. Colonization of the meristematic zone occurred due to host cell death. Independently of the zone, changes in cellular pH resulting in an acidic cytoplasm conditioned the establishment of F. oxysporum. Additionally, cell death was negatively correlated with hydrogen peroxide (H2O2) in roots challenged by a pathogenic fungus. Cell death was the only factor uniquely associated with the colonization of host roots by a saprotrophic fungus. The mechanism may differ, however, between the zones since apoplastic pH was negatively correlated with cell death in the elongation zone, whereas in the meristematic zone, none of the studied factors explained cell death. Colonization by the ECM fungus, H. crustuliniforme, was associated with a decreasing number of cells with acidic apoplast and by production of H2O2 in the elongation zone resulting in cell death. Saprotrophic and ECM fungi had a greater effect on cell acidification in the meristematic zone than the pathogenic fungus.

The origin of reactive oxygen during interaction of Pinus sylvestris root and Heterobasidion annosum s.l. – the linkage with the iron

“…We showed that isolates of H. annosum s.s. and H. abietnum caused higher mortality in meristematic root zones as compared to elongating zones, with no difference observed for H. parviporum. Generally, nectrophic fungi preferably infect meristematic region to invade plant host root (Rodriguez-Galvez and Mendgen 1995;Mucha et al 2014), where cells are similar to parenchymatic cells, and are characterized by thin walls and little amount of storage compounds (Evert 2006). Thus, the higher mortality of P. sylvestris meristematic root cells could be associated with a preference for meristematic region as a point of pathogen entry (Asiegbu et al 1994).…”

“…In general, roots can be in various stages of development e.g., meristematic and elongation (Evert 2006). In a comparison between the meristematic and elongation zones of tap roots, Mucha et al (2014) showed that accumulation of ROS significantly differed in both zones when challenged by trophically diverse fungi, including a pathogen F. oxysporum and saprotrophic and ectomycorrhizal fungi. The authors also concluded that the mechanisms that promote establishment of pathogenic fungi operate in a different way in these different root zones.…”

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

“…Cell death is essential for necrotrophic fungi as they can only acquire nutrients from dead tissues (Govrin and Levine 2000). Cell death has been reported to facilitate fungal establishment in P. sylvestris roots during the initial stage of invasion (Adomas et al 2008;Mucha et al 2014). In contrast to necrotrophic pathogens, the observed correlation between lower levels of hydrogen peroxide and iron accumulation may facilitate the colonization of roots by ectomycorrhizal fungi, as they rely on living tissues to acquire carbon (Nehls et al 2010).…”

“…In contrast to necrotrophic pathogens, the observed correlation between lower levels of hydrogen peroxide and iron accumulation may facilitate the colonization of roots by ectomycorrhizal fungi, as they rely on living tissues to acquire carbon (Nehls et al 2010). The establishment of ectomycorrhizal fungi, however, does not always require or induce cell death (Baptista et al 2007;Ragnelli et al 2013;Mucha et al 2014); even though an accumulation of H 2 O 2 occurs. In this scenario, hydrogen peroxide restricts fungal growth rather than promoting fungal proliferation by killing plant tissues.…”

“…ROS accumulation in host tissues resulting from the colonization of fungi establishing a mutualistic association (e.g. mycorrhizal fungi), however, does not appear to result in the massive death of plant cells (Baptista et al 2007;Mucha et al 2014). Thus, specific redox levels associated with reactive oxygen species, as well as levels of iron, during plant-fungus interactions may either foster or prevent fungal establishment and disease.…”