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The Role of Auxins and Cytokinins in the Mutualistic Interaction Between Arabidopsis and Piriformospora indica

¹Friedrich-Schiller-Universität Jena, Institut für Allgemeine Botanik und Pflanzenphysiologie, Dornburger Str. 159, 07743 Jena, Germany; ²Amity Institute of Herbal and Microbial Studies, Sector 125, Noida 201303, UP, India; ³Laboratory of Growth Regulators, Palacky University & Institute of Experimental Botany AS CR, Slechtitelu 11, 783 71 Olomouc, Czech Republic; ⁴Institut für Botanik, Technische Universität Dresden, Zellescher Weg 20b, 01062 Dresden, Germany

Arabidopsis growth and reproduction are stimulated by the endophytic fungus Piriformospora indica. The fungus produces low amounts of auxins, but the auxin levels and the expression of auxin-regulated genes are not altered in colonized roots. Also, mutants with reduced auxin levels (ilr1-1, nit1-3, tfl2, cyp79 b2b3) respond to P. indica. However, the fungus rescues the dwarf phenotype of the auxin overproducer sur1-1 by converting free auxin into conjugates, which also results in the downregulation of the auxin-induced IAA6 and the upregulation of the P. indica--induced LRR1 gene. The fungus produces relatively high levels of cytokinins, and the cytokinin levels are higher in colonized roots compared with the uncolonized controls. trans-Zeatin cytokinin biosynthesis and the CRE1/AHK2 receptor combination are crucial for P. indica--mediated growth stimulation, while mutants lacking cis-zeatin, impaired in other cytokinin receptor combinations, or containing reduced cytokinin levels respond to the fungus. Since root colonization is not affected in the cytokinin mutants, we propose that cytokinins are required for P. indica--induced growth promotion. Finally, a comparative analysis of the phytohormone mutants allows the conclusion that the response to P. indica is independent of the architecture and size of the roots.

Additional keywords:plant-microbe interaction, Sebacinaceae.