The root endophytic fungus Piriformospora indica requires host cell death for proliferation during mutualistic symbiosis with barley

doi:10.1073/pnas.0605697103

The root endophytic fungus Piriformospora indica requires host cell death for proliferation during mutualistic symbiosis with barley

*Research Centre for Biosystems, Land Use, and Nutrition, Institute of Phytopathology and Applied Zoology, Justus Liebig Universität, D-35392 Giessen, Germany;
^†Institute of Phytopathology, Technische Universität München, D-85350 Freising, Germany; and
^‡Institute of Systematic Botany and Mycology, Eberhard Karls Universität, D-72076 Tübingen, Germany

Edited by Diter von Wettstein, Washington State University, Pullman, WA, and approved September 14, 2006 (received for review July 10, 2006)

Abstract

Fungi of the recently defined order Sebacinales (Basidiomycota) are involved in a wide spectrum of mutualistic symbioses (including mycorrhizae) with various plants, thereby exhibiting a unique potential for biocontrol strategies. The axenically cultivable root endophyte Piriformospora indica is a model organism of this fungal order. It is able to increase biomass and grain yield of crop plants. In barley, the endophyte induces local and systemic resistance to fungal diseases and to abiotic stress. To elucidate the lifestyle of P. indica, we analyzed its symbiotic interaction and endophytic development in barley roots. We found that fungal colonization increases with root tissue maturation. The root tip meristem showed no colonization, and the elongation zone showed mainly intercellular colonization. In contrast, the differentiation zone was heavily infested by inter- and intracellular hyphae and intracellular chlamydospores. The majority of hyphae were present in dead rhizodermal and cortical cells that became completely filled with chlamydospores. In some cases, hyphae penetrated cells and built a meshwork around plasmolyzed protoplasts, suggesting that the fungus either actively kills cells or senses cells undergoing endogenous programmed cell death. Seven days after inoculation, expression of barley BAX inhibitor-1 (HvBI-1), a gene capable of inhibiting plant cell death, was attenuated. Consistently, fungal proliferation was strongly inhibited in transgenic barley overexpressing GFP-tagged HvBI-1, which shows that P. indica requires host cell death for proliferation in differentiated barley roots. We suggest that the endophyte interferes with the host cell death program to form a mutualistic interaction with plants.

Footnotes

^§To whom correspondence should be addressed. E-mail: karl-heinz.kogel{at}agrar.uni-giessen.de
Author contributions: K.-H.K. designed research; S.D., P.S., J.I., M.S., and M.W. performed research; S.D., R.H., P.S., M.W., and F.W. analyzed data; and R.H., F.W., and K.-H.K. wrote the paper.
This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, “From Functional Genomics of Model Organisms to Crop Plants for Global Health,” held April 3–5, 2006, at The National Academy of Sciences in Washington, DC. Papers from this Colloquium will be available as a collection on the PNAS web site. The complete program is available on the NAS web site at www.nasonline.org/functional_genomics.
The authors declare no conflict of interest.
This article is a PNAS direct submission.
Abbreviations:

AMF,

arbuscular mycorrhiza fungi;

Ct,

cycle threshold;

dai,

days after inoculation;

nucLSU,

nuclear gene coding for the large ribosomal subunit;

PCD,

programmed cell death;

WGA-AF 488,

wheat germ agglutinin-Alexa Fluor 488;

WGA-TMR,

wheat germ agglutinin-tetramethylrhodamine.