Biochemical Characterization of the Fusarium graminearum Candidate ACC-Deaminases and Virulence Testing of Knockout Mutant Strains

Abstract

Fusarium graminearum is a plant pathogenic fungus which is able to infect wheat and other economically important cereal crop species. The role of ethylene in the interaction with host plants is unclear and controversial. We have analyzed the inventory of genes with a putative function in ethylene production or degradation of the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC). F. graminearum, in contrast to other species, does not contain a candidate gene encoding ethylene-forming enzyme. Three genes with similarity to ACC synthases exist; heterologous expression of these did not reveal enzymatic activity. The F. graminearum genome contains in addition two ACC deaminase candidate genes. We have expressed both genes in E. coli and characterized the enzymatic properties of the affinity-purified products. One of the proteins had indeed ACC deaminase activity, with kinetic properties similar to ethylene-stress reducing enzymes of plant growth promoting bacteria. The other candidate was inactive with ACC but turned out to be a d-cysteine desulfhydrase. Since it had been reported that ethylene insensitivity in transgenic wheat increased Fusarium resistance and reduced the content of the mycotoxin deoxynivalenol (DON) in infected wheat, we generated single and double knockout mutants of both genes in the F. graminearum strain PH-1. No statistically significant effect of the gene disruptions on fungal spread or mycotoxin content was detected, indicating that the ability of the fungus to manipulate the production of the gaseous plant hormones ethylene and H2S is dispensable for full virulence.

Different effects of phytohormones on Fusarium head blight and Fusarium root rot resistance in Brachypodium distachyon

“…Although Foroud et al (2018) found that ethephon and ACC had similar effects on FHB in most wheat varieties tested (Foroud et al 2018). The role of ethylene is further complicated given evidence that F. graminearum can produce ethylene in vitro (Svoboda et al 2019), however it is uncertain whether ethylene is produced in planta.…”

Wheat transcriptome profiling reveals abscisic and gibberellic acid treatments regulate early-stage phytohormone defense signaling, cell wall fortification, and metabolic switches following Fusarium graminearum-challenge

“…An encouraging flurry of recent publications emphasize roles for non-classical defense phytohormones in both resistance and susceptibility [ 7 – 11 ], suggesting new focused alternative mechanisms underlying the host-pathogen interaction. To date, these studies have highlighted variable responses between examined cultivars, an aspect which is further complicated by recent evidence suggesting that non-classical defense phytohormones are also biosynthesized by Fg itself [ 14 , 16 , 18 , 19 ]. In this light, the expression of a putative fungal ABA biosynthetic cytochrome P450 homolog under all conditions containing Fg examined herein is noted (Additional file 5 : Figure S2B), lending further support to the role for fungal-derived ABA as an effector of infection, as has been proposed previously [ 14 , 29 ].…”

“…It has been established that Fusarium spp. can themselves produce ABA [ 14 ], GA [ 16 ], auxin (indole acetic acid; IAA [ 17 ]), and cytokinins (CK [ 18 ];), while also encoding both 1-aminocyclopropane carboxylic acid (ACC) synthases and deaminases potentially involved in ET biosynthesis [ 19 ]. Therefore, although phytohormones may be traditionally thought to serve as plant host signaling and defense molecules, Fusarium spp .…”

Characterising candidate genes for roles in Fusarium pseudograminearum-host interactions in the model cereal Brachypodium distachyon

“…This leads to increased senescence-associated gene expression and disease susceptibility (Thatcher et al, 2009b). Relatedly, Fg can degrade 1-aminocyclopropane carboxylic acid (ACC), a precursor for production of ET (Svoboda et al, 2019), through an encoded ACC-Deaminase (ACD). As mentioned earlier, ET synergistically interacts with JA during defence signalling against necrotrophs , Van der Ent and Pieterse, 2018.…”

“…ET synergistically interacts with JA during defence signalling against necrotrophs , Van der Ent and Pieterse, 2018. Svoboda et al (2019) hypothesized that degradation of ACC may interfere with the JA/ET-mediated signalling during the necrotrophic phase of Fg's lifecycle; however, no differences in virulence were observed in ACD knockouts.…”