Chronic Sublethal Aluminum Exposure and Avena fatua Caryopsis Colonization Influence Gene Expression of Fusarium avenaceum F.a.1

Abstract

Fusarium avenaceum F.a.1 is a novel strain of a fungal plant pathogen capable of preferentially decaying wild oat (Avena fatua) caryopses compared with those of wheat (Triticum aestivum). Understanding the molecular mechanisms governing weed seed-pathogen interactions is crucial to developing novel weed seed suppression technologies. Additionally, wild oat often competes with wheat in regions undergoing soil acidification, which leads to increases in soluble concentrations of many metals, including aluminum (Al). There is a dearth of information regarding the gene expression responses of Fusarium species to Al toxicity, or how metal toxicity might influence caryopsis colonization. To address this, a transcriptomic approach was used to investigate molecular responses of F.a.1 during wild oat caryopsis colonization in the presence and absence of chronic, sublethal concentrations of Al (400 μM). Caryopsis colonization was associated with induction of genes related to virulence, development, iron metabolism, oxidoreduction, stress, and detoxification, along with repression of genes associated with development, transport, cell-wall turnover, and virulence. Caryopsis colonization during Al exposure resulted in the induction of genes associated with virulence, detoxification, stress, iron metabolism, oxidoreduction, and cell wall turnover, along with repression of genes associated with cell wall metabolism, virulence, development, detoxification, stress, and transcriptional regulation. Aluminum exposure in the absence of caryopses was associated with induction of genes involved in siderophore biosynthesis, secretion, uptake, and utilization, along with several other iron metabolism-related and organic acid metabolism-related genes. The siderophore-related responses associated with Al toxicity occurred concurrently with differential regulation of genes indicating disruption of iron homeostasis. These findings suggest Al toxicity is attenuated by siderophore metabolism in F.a.1. In summary, both caryopsis colonization and Al toxicity uniquely influence transcriptomic responses of F.a.1.

Keywords: Fusarium; fungal siderophore; plant pathogens; soil acidification; soil microbiology; sublethal aluminum toxicity; weed seed decay; wild oat.

FIGURE 1

Fungal colony diameter of Fusarium avenaceum F.a.1 in cm. Days post inoculation (dpi) is days after transferring a 6 mm plug of colonized potato dextrose agar (PDA; 25 mL) to the center of fresh PDA (25 mL) amended with either 1 mL of water (diamond; dotted line) or 1 mL of 10 mM AlCl₃ (squares; solid lines; 400 μM is final aluminum concentration). Data represent samples used in transcriptomic studies. Because caryopses were not added until day eight, colony diameter analyses have 12 replicates for each treatment (water or Al). Asterisks indicate statistical significance at alpha = 0.05.

FIGURE 2

Polyphenol oxidase (PPO) activity in whole caryopses presented as average change (Δ) in absorbance at 475 nm divided by caryopsis grams fresh weight (gfwt). Bars are standard deviation and letters represent significance evaluated at alpha = 0.05. Caryopses in the absence of Fusarium avenaceum F.a.1 (F.a.1) were sampled from 25 mL water agar plates amended with 1 mL of water (H₂O) or 10 mM AlCl₃ (Al). Caryopses in the presence of F.a.1 were sampled from 25 mL PDA plates with the same amendments.

FIGURE 3

Influence of aluminum exposure and caryopsis colonization on global gene expression. FOW, fungus only exposed to water; FW, fungus exposed to water plus caryopses; FOA, fungus only exposed to aluminum; FA, fungus exposed to aluminum plus caryopses. Co-expression Venn diagrams are shown comparing (A) FOW vs. FW, (B) FOA vs. FA, (C) FW vs. FA (D) FOW vs. FOA), and (E) all treatments. (F) shows a clusters analysis of differentially expressed genes.