Abstract
Penicillium digitatum is the main fungal postharvest pathogen of citrus fruit under Mediterranean climate conditions. The role of ethylene in the P. digitatum–citrus fruit interaction is unclear and controversial. We analyzed the involvement of the 2-oxoglutarate-dependent ethylene-forming enzyme (EFE)-encoding gene (efeA) of P. digitatum on the pathogenicity of the fungus. The expression of P. digitatum efeA parallels ethylene production during growth on PDA medium, with maximum levels reached during sporulation. We generated ΔefeA knockout mutants in P. digitatum strain Pd1. These mutants showed no significant defect on mycelial growth or sporulation compared to the parental strain. However, the knockout mutants did not produce ethylene in vitro. Citrus pathogenicity assays showed no differences in virulence between the parental and ΔefeA knockout mutant strains, despite a lack of ethylene production by the knockout mutant throughout the infection process. This result suggests that ethylene plays no role in P. digitatum pathogenicity. Our results clearly show that EFE-mediated ethylene synthesis is the major ethylene synthesis pathway in the citrus postharvest pathogen P. digitatum during both in vitro growth on PDA medium and the infection process, and that this hormone is not necessary for establishing P. digitatum infection in citrus fruit. However, our results also indicate that ethylene produced by P. digitatum during sporulation on the fruit surface may influence the development of secondary fungal infections.
Highlights
Ethylene (C2 H4 ) is a gaseous compound involved in the regulation of numerous biological processes in plants, such as senescence, germination, flowering, and fruit ripening [1]
Our results clearly show that ethylene-forming enzyme (EFE)-mediated ethylene synthesis is the major ethylene synthesis pathway in the citrus postharvest pathogen P. digitatum during both in vitro growth on potato dextrose agar (PDA) medium and the infection process, and that this hormone is not necessary for establishing P. digitatum infection in citrus fruit
Into 1-aminocyclopane-1-carboxylic acid (ACC) by ACC synthase (ACS), followed by the conversion of ACC into ethylene by ACC oxidase (ACO). This three-step reaction is commonly known as the ACC pathway. (ii) In most bacteria and fungi that produce ethylene, the major pathway for its synthesis involves the oxidation of 2-keto-4-methylthiobutyric acid (KMBA), a transaminated derivate of methionine produced in a NADH:Fe(III)EDTA oxidoreductase-mediated two-step reaction [7]
Summary
Ethylene (C2 H4 ) is a gaseous compound involved in the regulation of numerous biological processes in plants, such as senescence, germination, flowering, and fruit ripening [1] This hormone can be synthesized following three different pathways [2,3,4,5,6]: (i) in plants, methionine is transformed into S-adenosyl-L-methionine (SAM) by SAM synthetase. The subsequent reaction converts SAM into 1-aminocyclopane-1-carboxylic acid (ACC) by ACC synthase (ACS), followed by the conversion of ACC into ethylene by ACC oxidase (ACO). This three-step reaction is commonly known as the ACC pathway. KMBA can be used as an ethylene precursor by the fungus Penicillium digitatum [7,9], only trace amounts of ethylene are produced through this pathway. (iii) Ethylene can be synthesized
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