Ciaran Griffin


A circadian clock is present in some capacity in almost all forms of life, and is useful for a wide array of traits, but crucially allows organisms to predict future conditions and adapt their behaviour to synchronise with, and thrive under, their dynamic environment. Accordingly, plant environmental stress responses are gated in a circadian manner, including that for pathogenic defence and immunity. Comparatively less work has been carried out on the plant pathogens, but there are increasing reports of pathogens capable of rhythmically altering their behaviour and virulence-related traits. Magnaporthe oryzae, the fungal pathogen responsible for the destruction of enough rice to feed at least 60 M people annually, has been shown to possess some circadian clock components, and based on bioinformatic analyses, likely contains all the core, accessory, and circadian-associated genes. M. oryzae displays a conidial banding pattern, reminiscent of the model clock species, N. crassa, and (after sufficient entrainment) this pattern can continue to occur under free running conditions for a number of days, with a period of approximately 24 h. This rhythm is also presented on a range of nutrient-rich and poor media, suggesting a nutritionally-compensated circadian rhythm in M. oryzae. This onset of conidial banding is partially determined by the presence of secreted metabolites, the sensation of which is facilitated by the circadian clock, predominantly via WC2. The entraining light conditions that M. oryzae is exposed to can significantly alter its vegetative growth, conidiation and conidial development, and even pathogenicity. Further, inoculation timing (dawn or dusk) plays a role in both the virulence of M. oryzae, and in the susceptibility of the plant host, seemingly in a species-by-species manner, where rice is most susceptible at dawn, and barley most susceptible at dusk. For M. oryzae, pre-inoculation entrainment to darkness predominantly favours dawn inoculations, and those exposed to prolonged periods of light prefer dusk inoculation. Upon mutation of the core clock genes, WC2 and FRQ, vegetative growth, conidiation and conidial development, photoadaptation, and pathogenicity were all significantly altered compared to the wild type, suggesting an important role of the clock in the general fitness of M. oryzae. This work discusses how entraining light cycles and the circadian clock impacts the growth, development, conidiation, virulence, and ultimate severity in the economically important rice blast disease.

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Creative Commons Attribution-Share Alike 4.0 International License
This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.