A CCaMK-CYCLOPS-DELLA complex activates transcription of RAM1, a central regulator of arbuscule branching 
P. PIMPRIKAR (1), S. Carbonnel (1), M. Paries (2), K. Katzer (2), V. Klingl (2), L. Karl (2), D. Floss (3), M. Harrison (3), M. Parniske (2), C. Gutjahr (2) (1) University of Munich (LMU), Germany; (2) University of Munich (LMU), Germany; (3) Boyce Thompson Institute for Plant Research, USA, U.S.A.

Arbuscular mycorrhiza (AM) symbiosis is the most widespread strategy of plants to acquire mineral nutrients from soil. Root colonization by AM fungi culminates in the formation of highly branched structures called arbuscules that release the mineral nutrients to the plant. Although the number and branching status of arbuscules might influence symbiotic nutrient-transfer efficiency, its molecular basis is poorly understood. During colonization perception of fungal signals triggers nuclear calcium spiking, which is decoded by a nuclear localized calcium and calmodulin dependent kinase (CCaMK). CCaMK interacts with and phosphorylates the transcription factor CYCLOPS that activates downstream AM signaling.  Arbuscule development requires CCaMK and CYCLOPS as well as the GA-signaling repressor DELLA. Using a forward genetics strategy we found a novel Lotus japonicus allele of the GRAS protein encoding gene REDUCED ARBUSCULAR MYCORRHIZA 1 (RAM1). The ram1-3 mutant is perturbed in arbuscule branching.  To place RAM1 relative to the symbiosis and GA signaling we conducted an epistasis analysis. Ectopic expression of RAM1 restored arbuscule formation in cyclops mutants and in presence of suppressive gibberellin placing RAM1 downstream of CCaMK, CYCLOPS and DELLA. Furthermore, CCaMK and CYCLOPS form a complex with DELLA that activates RAM1 expression via binding of CYCLOPS to a cis-element in the RAM1 promoter. Thus, we reveal a transcriptional cascade that employs the RAM1 promoter as an integrator of symbiosis and GA signaling.

Abstract Number: P2-45
Session Type: Poster