Red- and Blue-Light Sensing in the Plant Pathogen Alternaria alternata Depends on Phytochrome and the White-Collar Protein LreA

Abstract

The filamentous fungus Alternaria alternata is a common postharvest contaminant of food and feed, and some strains are plant pathogens. Many processes in A. alternata are triggered by light. Interestingly, blue light inhibits sporulation, and red light reverses the effect, suggesting interactions between light-sensing systems. The genome encodes a phytochrome (FphA), a white collar 1 (WC-1) orthologue (LreA), an opsin (NopA), and a cryptochrome (CryA) as putative photoreceptors. Here, we investigated the role of FphA and LreA and the interplay with the high-osmolarity glycerol (HOG) mitogen-activated protein (MAP) kinase pathway. We created loss-of function mutations for fphA, lreA, and hogA using CRISPR-Cas9 technology. Sporulation was reduced in all three mutant strains already in the dark, suggesting functions of the photoreceptors FphA and LreA independent of light perception. Germination of conidia was delayed in red, blue, green, and far-red light. We found that light induction of ccgA (clock-controlled gene in Neurospora crassa and light-induced gene in Aspergillus nidulans) and the catalase gene catA depended on FphA, LreA, and HogA. Light induction of ferA (a putative ferrochelatase gene) and bliC (bli-3, light regulated, unknown function) required LreA and HogA but not FphA. Blue- and green-light stimulation of alternariol formation depended on LreA. A lack of FphA or LreA led to enhanced resistance toward oxidative stress due to the upregulation of catalases and superoxide dismutases. Light activation of FphA resulted in increased phosphorylation and nuclear accumulation of HogA. Our results show that germination, sporulation, and secondary metabolism are light regulated in A. alternata with distinct and overlapping roles of blue- and red-light photosensors. IMPORTANCE Light controls many processes in filamentous fungi. The study of light regulation in a number of model organisms revealed an unexpected complexity. Although the molecular components for light sensing appear to be widely conserved in fungal genomes, the regulatory circuits and the sensitivity of certain species toward specific wavelengths seem different. In N. crassa, most light responses are triggered by blue light, whereas in A. nidulans, red light plays a dominant role. In Alternaria alternata, both blue and red light appear to be important. In A. alternata, photoreceptors control morphogenetic pathways, the homeostasis of reactive oxygen species, and the production of secondary metabolites. On the other hand, high-osmolarity sensing required FphA and LreA, indicating a sophisticated cross talk between light and stress signaling.

Alternaria alternata uses two siderophore systems for iron acquisition

“…RnA isolation and quantitative real time pcR. Conidia were inoculated with a loop on the surface of 20~25 ml of complete liquid medium mCDB in a Petri dish as described 21 . Mycelia were harvested, frozen in liquid nitrogen and stored at −80 °C until RNA isolation.…”

Light-regulated synthesis of extra- and intracellular enzymes related to wood degradation by the white rot fungus Cerrena unicolor during solid-state fermentation on ash sawdust-based medium

“…Therefore, the blue light may rather cause a cascade of several processes associated with reaction to stress than the synthesis of enzymes related to carbohydrate metabolism (Schmoll, 2018b). In fungi, exposure to light appears to prepare cells for oxidative stress by upregulating the expression of catalase and superoxide dismutase (Andries et al, 2016;Igbalajobi et al, 2019). Moreover, it is possible that both light and oxidative stress regulate the asexual-sexual switch via light-responsive and ROS (reactive oxygen species) pathways, as already proved in the model fungus N. crassa .…”

A fotobiologia de Metarhizium acridum: qualidade de luz, tolerância ao estresse e regulação gênica

“…This fungus possesses blue, red, and green light photoreceptors that were named exactly the same as their A. nidulans orthologues. Light induction of the conidiation-related gene ccgA is reduced in a ΔfphA background and lost in ΔlreA strains (Igbalajobi;Fischer, 2019). This is markedly different from N. crassa and A. nidulans.…”

“…There is also interplay between light and HOG signaling pathways in A. alternata. Both blue and red light cause HogA phosphorylation and this response is lost in both ΔfphA and ΔlreA strains together with ccgA photoinduction (Igbalajobi;Fischer, 2019). Interestingly, ΔfphA and ΔlreA strains display higher tolerance to oxidative stress (H2O2 and menadione) and increased expression of genes coding for catalases and superoxide dismutases (Igbalajobi;Fischer, 2019).…”

“…Both blue and red light cause HogA phosphorylation and this response is lost in both ΔfphA and ΔlreA strains together with ccgA photoinduction (Igbalajobi;Fischer, 2019). Interestingly, ΔfphA and ΔlreA strains display higher tolerance to oxidative stress (H2O2 and menadione) and increased expression of genes coding for catalases and superoxide dismutases (Igbalajobi;Fischer, 2019). This is in contrast to Botrytis cinerea in which deletion of WC-1 led to reduced tolerance to oxidative stress (Canessa et al, 2013) and highlights the variability between fungi in terms of light response and photoreceptor roles.…”