Abstract
Piriformospora indica is an endophytic fungus that colonizes roots of many plant species and promotes growth and resistance to certain plant pathogens. Despite its potential use in agriculture, little is known on the molecular basis of this beneficial plant-fungal interaction. In a genetic screen for plants, which do not show a P. indica- induced growth response, we isolated an Arabidopsis mutant in the OXI1 (Oxidative Signal Inducible1) gene. OXI1 has been characterized as a protein kinase which plays a role in pathogen response and is regulated by H2O2 and PDK1 (3-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE1). A genetic analysis showed that double mutants of the two closely related PDK1.1 and PDK1.2 genes are defective in the growth response to P. indica. While OXI1 and PDK1 gene expression is upregulated in P. indica-colonized roots, defense genes are downregulated, indicating that the fungus suppresses plant defense reactions. PDK1 is activated by phosphatidic acid (PA) and P. indica triggers PA synthesis in Arabidopsis plants. Under beneficial co-cultivation conditions, H2O2 formation is even reduced by the fungus. Importantly, phospholipase D (PLD)α1 or PLDδ mutants, which are impaired in PA synthesis do not show growth promotion in response to fungal infection. These data establish that the P. indica-stimulated growth response is mediated by a pathway consisting of the PLD-PDK1-OXI1 cascade.
Highlights
The endophytic fungus Piriformospora indica, a cultivable basidiomycete of Sebacinales, colonizes the roots of many plant species including Arabidopsis [1,2]
In a genetic screen for Arabidopsis plants, which do not show a P. indica-induced growth response, we isolated a mutant in the OXIDATIVE SIGNAL INDUCIBLE1 (OXI1) (Oxidative Signal Inducible1) gene
This protein kinase has been shown to play a role in pathogen response and is regulated by H2O2 and PDK1 (3PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE1)
Summary
The endophytic fungus Piriformospora indica, a cultivable basidiomycete of Sebacinales, colonizes the roots of many plant species including Arabidopsis [1,2]. The broad host range of P. indica indicates that the beneficial interaction may be based on general recognition and signalling pathways. Plant growth can be induced by a fungal exudate component [9], suggesting the involvement of specific receptors at the plant cell surface. In support of this hypothesis, an atypical receptor kinase with leucine-rich repeats was identified to be required for the growth response in Arabidopsis [5]. A rapid increase in the intracellular calcium concentration in the root cells indicates that an intracellular signalling cascade is triggered early upon plantfungal interaction [9]. No further components of the signalling pathway have been identified
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