Symbiosis signalling genes negatively regulate root responses to salt stress via the CCaMK-IPD3 module in <i>Medicago truncatula</i>

Legumes are important sources of dietary protein and are key crops for sustainable agriculture because they fix atmospheric nitrogen via symbiotic interactions with rhizobia bacteria. However, legume plants are particularly sensitive to salt stress, with salinity negatively affecting development of the root nodule symbiosis. Genes that control salt-symbiosis crosstalk or trade-offs are largely unknown and poorly characterised. To assess the role of symbiosis signalling genes in salt stress, we analysed wildtype and symbiosis signalling mutants of Medicago truncatula grown in the presence of NaCl, sorbitol and/or rhizobia bacteria. We assessed root growth, plant biomass, nodule number and gene expression responses in plants exposed to stress. Our findings demonstrate that several symbiosis signalling genes play a previously undescribed role in regulating root responses to salt stress, including a calcium- and calcium/calmodulin-dependent protein kinase (CCaMK) and its interacting partner and downstream transcription factor, IPD3. Our results also show that the identified responses to salt stress are due to sodium toxicity rather than osmotic stress. We conclude that symbiosis signalling genes, including the CCaMK-IPD3 signalling module, may mediate signalling crosstalk between salt stress and symbiosis. These findings open new research avenues to explore how the environment regulates the legume-Rhizobium symbiosis.