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

Abstract

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.

Enigmatic Sebacinales

“…Indole-3-acetic acid is produced by P. indica in liquid culture, and is assumed to affect plant growth as shown in sterile cultures of A. thaliana by Sirrenberg et al (2007). Vadassery et al (2008) suggested that P. indica-induced cell division and elongation in Arabidopsis requires a specific auxin to cytokinin ratio for root and shoot growth, and that growth promotion is independent of the root architecture. However, in Brassica campestris ssp.…”

Effect of inoculation with piriformospora indica and sebacina vermifera on growth of selected brassicaceae plants under greenhouse conditions

“…Sirrenberg et al (2007) suggested that auxin produced by P. indica affects root growth and it is responsible for, or at least contributes to, the beneficial effect of fungus on its host plants. Vadassery et al (2008) stated that P. indica produces low amounts of auxins and relatively high level of cytokinins, therefore cytokinin levels are higher in colonized roots of Arabidopsis thaliana compared with the uncolonized roots of controls. Our results are in agreement with those of Kumari et al (2003), who showed that B. oleracea var.…”

Functions of Novel Symbiotic Fungus - Piriformospora Indica

“…In A. thaliana, pronounced plant growth promotion and enhanced seed production were noticed in the presence of P. indica, involving synthesis of a leucine-rich protein repeat [17,18], increased expression of nitrate reductase and a starch-degrading enzyme [18] as well as expression of a ß-glucosidase in the endoplasmic reticulum [19]. In addition, changes in levels of auxins and cytokinins were noticed [20].…”