Indole derivative production by the root endophyte Piriformospora indica is not required for growth promotion but for biotrophic colonization of barley roots

摘要

Beneficial effects elicited by the root endophyte Piriformospora indica are widely known, but the mechanism by which these are achieved is still unclear. It is proposed that phytohormones produced by the fungal symbiont play a crucial role in the interaction with the plant roots. Biochemical analyses of the underlying biosynthetic pathways for auxin production have shown that, on tryptophan feeding, P. indica can produce the phytohormones indole-3-acetic acid (IAA) and indole-3-lactate (ILA) throughthe intermediate indole-3-pyruvicacid (IPA).Time course transcriptional analyses after exposure to tryptophan designated the piTaml gene as a key player. A green fluorescence protein (GFP) reporter study and transcriptional analysis of colonized barley roots showed that piTaml is induced during the biotrophic phase. Piriformospora indica strains in which the piTaml gene was silenced via an RNA interference (RNAi) approach were compromised in IAA and ILA production and displayed reduced colonization of barley (Hordeum vuigare) roots in the biotrophic phase, but the elicitation of growth promotion was not affected compared with the wild-type situation. Our results suggest that IAA is involved in the establishment of biotrophy in P. indica barley symbiosis and might represent a compatibility factor in this system.