Ultrastructural and Cytological Studies on Mycosphaerella pinodes Infection of the Model Legume Medicago truncatula

Abstract

Ascochyta (Mycosphaerella) blight on cultivated peas is primarily caused by infection through asexual spores (pycnospores) of Mycosphaerella pinodes (Berk. et Blox.) Vestergren [recently renamed Peyronellaea pinodes (Berk. & A. Bloxam) Aveskamp, Gruyter & Verkley]. Using a model pathosystem involving Medicago truncatula and Mycosphaerella pinodes strain OMP-1, we examined the histology and ultrastructure of early infection events and fungal development including penetration by appressoria, vegetative growth of infection hyphae, and host responses. On the susceptible ecotype R108-1, pycnospores germinated and grew over the surface of the epidermis, then formed an appressoria and penetrated the cuticle. Beneath the cuticle, the infection peg expanded into a hyphae that grew within the outer wall of the epidermis. Subsequently, the hyphae penetrated down within mesophyll cells and proliferated vigorously, eventually, forming asexual fruiting bodies (pycnidia). In contrast, successful penetration and subsequent growth of infection hyphae were considerably restricted in the ecotype Caliph. Detected by its reaction with cerium chloride (CeCl3) to generate electron-dense cerium perhydroxides in transmission electron micrographs, hydrogen peroxide (H2O2) accumulated in epidermal and mesophyll cells of Caliph challenged with pycnospores of M. pinodes. This intracellular localization was confirmed by energy-dispersive X-ray spectroscopy. Our observations thus indicate that the oxidative burst reaction leading to the generation of reactive oxygen species is associated with a local host defense response in Caliph, since no clear H2O2 accumulation was detectable in susceptible R108-1. Indeed, aberrant hyphae such as intrahyphal hyphae and dead hyphae, probably due to a local defense elicited by the fungus, were abundant in Caliph but not in R108-1. Our results on the cellular interactions between the fungus and host cells provide additional insights to understand foliar infection by M. pinodes on cultivated peas.

Antifungal Properties of Bio-AgNPs against D. pinodes and F. avenaceum Infection of Pea (Pisum sativum L.) Seedlings

“…Plant responses to pathogen infection involve a series of reactions associated with the reinforcement of cell walls, oxidative burst, the activation of the antioxidative system, hormonal adjustment, the synthesis of stress-related biomolecules (e.g., phytoalexins), and proteins related to both secondary [70] and primary metabolism [71], which are common to various fungal infections [72][73][74]. Up to now, the changes in the transcriptome and proteome of pea plants infected with D. pinodes or F. oxysporum have revealed an increase in the levels of various proteins, i.e., those involved in energy and amino acid metabolism, redox response, the synthesis of pisatin or pathogenesis-related (PR) proteins, and proteins involved in changes in the structures of the cell walls (e.g., lignin biosynthesis, the modification of the degree of methyl esterification of pectins) [2,28,[75][76][77][78][79]. However, data on the metabolic responses of pea to fungal infections are scarce.…”

“…Comparative studies of cultivars more and less resistant to D. pinodes infection revealed common metabolomic responses such as increased contents of sugars (but not sucrose), sugar alcohols, and glycolysis/tricarboxylic acid (TCA) cycle intermediates and changes in amino acid content [87]. Moreover, it has been suggested that D. pinodes infection in pea plants is regulated by jasmonate (JA) and ethylene (ET) pathways [78,79] and can induce a hypersensitive response that leads to pathogen-induced cell death [28].…”

Evidence of recent increased pathogenicity within the AustralianAscochyta rabieipopulation

“…This suggested the hyphae were aberrated, which could be a structural defence response (Suzuki et al, 2017). During the penetration process, the host cell wall became degraded and/or swollen near the infection hyphae.…”

Genome and Transcriptome Analysis of Ascochyta pisi Provides Insights into the Pathogenesis of Ascochyta Blight of Pea

“…Plant-pathogenic fungi commonly form infection structures to penetrate the host plant cell wall during the early stage of the infection process. For example, A. pinodes forms infection cushions to penetrate the leaves of peas and Medicago truncatula ( 3 , 16 ). Various carbohydrate-active enzymes (CAZymes) secreted by fungi to degrade plant cell walls are critical for penetration and virulence.…”