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Applied and Environmental Microbiology, December 2001, p. 5377-5383, Vol. 67, No. 12
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.12.5377-5383.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

In Planta Regulation of Extension of an Endophytic Fungus and Maintenance of High Metabolic Rates in Its Mycelium in the Absence of Apical Extension

Yong Y. Tan,1 Martin J. Spiering,1,dagger Vicki Scott,1 Geoffrey A. Lane,2 Michael J. Christensen,2 and Jan Schmid1,*

Institute of Molecular BioSciences, Massey University,1 and AgResearch Grasslands Research Centre,2 Palmerston North, New Zealand

Received 15 May 2001/Accepted 12 September 2001

The fungus Neotyphodium lolii is an endophytic symbiont. It grows in the intercellular spaces of the perennial ryegrass Lolium perenne, producing secondary metabolites which enhance the fitness of the association over that of uninfected L. perenne. We report that the average number of hyphal strands in a given section of a leaf remains constant during the life of a leaf, indicating synchrony of leaf and hyphal extension, including cessation of hyphal extension when leaf extension ceases. We used a constitutively expressed reporter gene as an indicator of the mycelium's metabolic activity during and after hyphal extension. Reporter gene activity decreased when the mycelium stopped extending in liquid culture but not in planta. This indicates that in planta endophyte hyphae remain metabolically highly active when extension has ceased and throughout the life of the leaf they are colonizing. The behavior of the fungus in planta indicates the existence of signaling pathways which (i) synchronize the extension of leaf and hypha by regulating hyphal extension, (ii) suppress hyphal branching, and (iii) stop apical extension of fungal hyphae, without reducing the mycelium's metabolic activity. These signals may be crucial for the symbiosis, by allowing the endophyte to switch the focus of its metabolic activity from extension to the production of secondary metabolites.

* Corresponding author. Mailing address: Institute of Molecular BioSciences, Massey University, Private Bag 11222, Palmerston North, New Zealand. Phone: 64-6-350-4018. Fax: 64-6-350-5688. E-mail: J.Schmid{at}massey.ac.nz.

dagger Present address: Department of Plant Pathology, University of Kentucky, Lexington, Ky.

Applied and Environmental Microbiology, December 2001, p. 5377-5383, Vol. 67, No. 12
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.12.5377-5383.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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