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United States Department of Agriculture

Agricultural Research Service

Research Project: MOLECULAR AND GENETIC MECHANISMS OF FUNGAL DISEASE RESISTANCE IN GRAIN CROPS Title: A Circadian Rhythm Regulating Hyphal Melanization in Cercospora Kikuchii

Authors
item Bluhm, Burton - UNIV. OF ARKANSAS
item Burnham, A. Michele - COKER COLLEGE
item Dunkle, Larry

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 23, 2010
Publication Date: N/A

Interpretive Summary: Light is one of the most important environmental factors influencing plant disease development. Circadian rhythms, biological processes that cycle with an approximately 24-hour period, are typically triggered by light and have been thoroughly studied in plants and animals but not in plant pathogenic fungi. We identified a circadian rhythm governing colony morphogenesis in Cercospora kikuchii, an important foliar and seed-borne pathogen of soybean. Under a photocycle of 12h light/12h darkness, the fungus alternates between vegetative and reproductive growth, resulting in distinct concentric rings of hyaline conidia formed during the dark period. After entrainment and subsequent removal of external light stimuli, the rhythmic pattern of morphogenesis persists for at least seven days in complete darkness with a periodicity of 23-24 hours. Blue light is required to entrain the rhythm, and the period length is independent of temperature-induced variations in growth rates or nutritional status. The discovery of a circadian rhythm regulating morphogenesis in C. kikuchii may provide insight into the conservation of molecular clocks among fungi and facilitate future investigations that focus on the role of light-regulated biological rhythms in plant pathogenesis. Such information will be useful to scientists in devising innovative strategies for disease control by targeting expression of key plant defense genes and fungal virulence genes.

Technical Abstract: Circadian rhythms, biochemical or developmental processes with a period length of approximately 24 hours, are thoroughly documented in plants and animals. However, virtually all of what is currently known about circadian rhythms in fungi is derived from the model fungus, Neurospora crassa, including the identities of specific genes involved in molecular time keeping. Significantly, the sequenced genomes of many fungi contain orthologs of genes that are integral to the Neurospora biological clock, suggesting a widespread distribution of circadian rhythms in the kingdom Fungi. We identified a circadian rhythm governing colony morphogenesis in Cercospora kikuchii, an important foliar and seed-borne pathogen of soybean (Glycine max). Under a photocycle of 12h light/12h darkness, the fungus alternates between vegetative and reproductive growth, resulting in distinct concentric rings of hyaline conidia formed during the dark period and interband mycelium in the light. After entrainment and subsequent removal of external light stimuli, the rhythmic pattern of morphogenesis persists for at least seven days in complete darkness with a periodicity of 23.7 hours. As is the case for circadian rhythms in N. crassa, blue light is required to entrain the rhythm, and the period length is independent of temperature-induced variations in growth rates or nutritional status. The discovery of a circadian rhythm regulating morphogenesis in C. kikuchii may provide new insights into the conservation of molecular clocks among fungi and will facilitate future investigations that focus on the role of light-regulated biological rhythms in plant pathogenesis.

Last Modified: 11/23/2014
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