The cost of surviving nitrogen excess: energy and protein demand in the lichen Cladonia portentosa as revealed by proteomic analysis

Abstract

Different nitrogen forms affect different metabolic pathways in lichens. In particular, the most relevant changes in protein expression were observed in the fungal partner, with NO mostly affecting the energetic metabolism and NH affecting transport and regulation of proteins and the energetic metabolism much more than NO did. Excess deposition of reactive nitrogen is a well-known agent of stress for lichens, but which symbiont is most affected and how, remains a mystery. Using proteomics can expand our understanding of stress effects on lichens. We investigated the effects of different doses and forms of reactive nitrogen, with and without supplementary phosphorus and potassium, on the proteome of the lichen Cladonia portentosa growing in a 'real-world' simulation of nitrogen deposition. Protein expression changed with the nitrogen treatments but mostly in the fungal partner, with NO mainly affecting the energetic metabolism and NH also affecting the protein synthesis machinery. The photobiont mainly responded overexpressing proteins involved in energy production. This suggests that in response to nitrogen stress, the photobiont mainly supports the defensive mechanisms initiated by the mycobiont with an increased energy production. Such surplus energy is then used by the cell to maintain functionality in the presence of NO, while a futile cycle of protein production can be hypothesized to be induced by NH excess. External supply of potassium and phosphorus influenced differently the responses of particular enzymes, likely reflecting the many processes in which potassium exerts a regulatory function.

More tolerant than expected: Taking into account the ability of Cladonia portentosa to cope with increased nitrogen availability in environmental policy

“…Although the importance of understanding the effects of prolonged exposures to N on the ecosystem and the extensive literature existing on the N-lichen relationship, most of these studies merely scratch the surface by describing symptoms of N excess. A deeper insight has been provided in a recent work investigating changes in the proteome of C. portentosa exposed to oxidized and reduced forms of N for 6 months (Munzi et al, 2017b). Results showed that different N forms affected different metabolic pathways in the lichen with the most relevant changes in protein expression observed in the fungal partner while in the photobiont only energy production was sensitive.…”

An Overview of Genomics, Phylogenomics and Proteomics Approaches in Ascomycota

“…Munzi et al [382] sought to expand the understanding of nitrogen stress effects on the proteome of Cladonia portentosa, reportedly able to cope with increased N availability, and found that different N forms affected metabolic pathways in the mycobiont and the photobiont differently. Changes in protein expression were mostly detected in the fungal partner in response to NO 3 − and NH 4 + , which impacted the energetic metabolism and protein synthesis, respectively.…”

Resilience of Epiphytic Lichens to Combined Effects of Increasing Nitrogen and Solar Radiation

“…Nitrogen tolerance in lichens seems to be provided by constitutive characteristics [ 63 , 64 ], but also by other inducible metabolic mechanisms [ 43 , 65 , 66 ]. The activation of such protective mechanisms comes at a cost, as shown by proteomic analysis in thalli of Cladonia portentosa : In thalli exposed to long-term N treatments, the ability to cope with increased N availability was related to an enhanced energetic metabolism [ 67 ]. In contrast, UV protection and tolerance to high solar radiation are provided by specific sunscreen compounds (e.g., usnic acid, phenolics, parietin [ 68 , 69 , 70 ]) and a general capacity to avoid and repair damages of oxidative stress [ 71 ].…”