A cell wall extract from the endophytic fungus Piriformospora indica promotes growth of Arabidopsis seedlings and induces intracellular calcium elevation in roots

Abstract

Summary: Calcium (Ca 2+), as a second messenger, is crucial for signal transduction processes during many biotic interactions. We demonstrate that cellular [Ca 2+] elevations are early events in the interaction between the plant growth‐promoting fungus Piriformospora indica and Arabidopsis thaliana. A cell wall extract (CWE) from the fungus promotes the growth of wild‐type seedlings but not of seedlings from P. indica‐insensitive mutants. The extract and the fungus also induce a similar set of genes in Arabidopsis roots, among them genes with Ca 2+ signalling‐related functions. The CWE induces a transient cytosolic Ca 2+ ([Ca 2+]cyt) elevation in the roots of Arabidopsis and tobacco (Nicotiana tabacum) plants, as well as in BY‐2 suspension cultures expressing the Ca 2+ bioluminescent indicator aequorin. Nuclear Ca 2+ transients were also observed in tobacco BY‐2 cells. The Ca 2+ response was more pronounced in roots than in shoots and involved Ca 2+ uptake from the extracellular space as revealed by inhibitor studies. Inhibition of the Ca 2+ response by staurosporine and the refractory nature of the Ca 2+ elevation suggest that a receptor may be involved. The CWE does not stimulate H 2 O 2 production and the activation of defence gene expression, although it led to phosphorylation of mitogen‐activated protein kinases (MAPKs) in a Ca 2+‐dependent manner. The involvement of MAPK6 in the mutualistic interaction was shown for an mpk6 line, which did not respond to P. indica. Thus, Ca 2+ is likely to be an early signalling component in the mutualistic interaction between P. indica and Arabidopsis or tobacco.