Morphological variants of Moniliophthora roreri on artificial media and the biotroph/necrotroph shift

Area of Science:

• Plant Pathology
• Mycology
• Microbial Physiology

Background:

• Moniliophthora roreri (Mr) causes significant economic losses as the causal agent of frosty pod rot in Theobroma cacao.
• The hemibiotrophic lifestyle of Mr involves distinct phases, but its biotroph-like phase has remained largely uncharacterized in laboratory cultures.

Purpose of the Study:

• To investigate the in vitro culture conditions that promote the biotroph-like phase of Moniliophthora roreri.
• To characterize the morphological and cellular features of different growth phases of Mr in culture.
• To explore potential differences in gene expression between biotroph-like and necrotroph-like mycelial forms.

Main Methods:

• Germination of Mr spores (isolates Co12, Co52, B3) on nutrient-rich (V8) and nutrient-poor (BPMM) media with varying agarose concentrations (0.5%-3%).
• Cultivation on standard and low melting point (LMP) agarose to assess colony stability and morphology.
• Microscopic examination of cellular features, including nuclear content, in different colony types.
• Analysis of differential gene expression between slow-growing (biotroph-like) and rapidly expanding (necrotroph-like) mycelia.

Main Results:

• High spore germination occurred on V8 media, with most resulting colonies being slow-growing and non-sporulating, suggesting a potential biotrophic phase.
• Media hardness influenced germination on BPMM, and exaggerated, enlarged-cell mycelia formed on this medium.
• Rarely, rapidly expanding, sporulating necrotrophic colonies were observed on agarose media.
• Slow-growing colonies, particularly of isolate B3 on BPMM, showed instability on LMP agarose, sometimes forming hybrid growth patterns.
• Mr mycelial cells were predominantly uninucleate (haploid), with occasional binucleate cells in aged slow-growing mycelia; isolate Co52 showed higher multinucleation.

Conclusions:

• Specific culture conditions, particularly nutrient-poor media and controlled hardness, may favor the expression of the biotroph-like phase of Moniliophthora roreri in vitro.
• The observed slow-growing colonies are hypothesized to represent the biotroph-like phase, distinct from the rapidly expanding necrotrophic phase.
• Cellular characteristics, including nuclear behavior, differ between growth phases, warranting further investigation.
• Differential gene expression studies showed inconsistencies with in vivo findings, highlighting the need for refined experimental approaches to understand Mr's pathogenic mechanisms.