Abstract

Legumes form symbiotic associations with either nitrogen-fixing bacteria or arbuscular mycorrhizal fungi. Formation of these two symbioses is regulated by a common set of signalling components that act downstream of recognition of rhizobia or mycorrhizae by host plants. Central to these pathways is the calcium and calmodulin-dependent protein kinase (CCaMK)–IPD3 complex which initiates nodule organogenesis following calcium oscillations in the host nucleus. However, downstream signalling events are not fully understood. Here we show that Medicago truncatula DELLA proteins, which are the central regulators of gibberellic acid signalling, positively regulate rhizobial symbiosis. Rhizobia colonization is impaired in della mutants and we provide evidence that DELLAs can promote CCaMK–IPD3 complex formation and increase the phosphorylation state of IPD3. DELLAs can also interact with NSP2–NSP1 and enhance the expression of Nod-factor-inducible genes in protoplasts. We show that DELLA is able to bridge a protein complex containing IPD3 and NSP2. Our results suggest a transcriptional framework for regulation of root nodule symbiosis.

Highlights

  • Legumes form symbiotic associations with either nitrogen-fixing bacteria or arbuscular mycorrhizal fungi

  • We show that DELLAs can form a protein complex with NSP2–Nodulation signaling pathway 1 (NSP1) and are able to bridge a protein complex containing IPD3/CYCLOPS and NSP2

  • The Pisum sativum gibberellic acid (GA) biosynthetic mutant, na-1, develops significantly fewer and underdeveloped nodules than its wild-type parent and GA application and grafting suggested that this nodulation phenotype in the na-1 mutant is due to reduced GA level in the roots[51], suggesting that GAs play a positive role in cortical cell divisions or nodule maturation

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Summary

Introduction

Legumes form symbiotic associations with either nitrogen-fixing bacteria or arbuscular mycorrhizal fungi Formation of these two symbioses is regulated by a common set of signalling components that act downstream of recognition of rhizobia or mycorrhizae by host plants. Rhizobia produce nodulation factors (Nod factors) that are recognized by the host plant to activate a common symbiotic signalling pathway required for mycorrhizal and rhizobial symbioses[2,4,5]. It has recently been shown that NSPs are not required for CYCLOPS-induced NIN expression, but they are required for CYCLOPLS-induced nodule organogenesis[27] It remains unknown how NSP1– NSP2 activity is associated with the action of CCaMK-CYCLOPS or whether the two protein complexes function independently in root nodule symbiosis. DELLA is an integrator of plant responses to hormones and environmental stresses, and DELLAdependent growth restraints are advantageous in adverse environment stresses[43]

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