Reverse Ecology Of Local Adaptation To Saline Habitats In The Medicago truncatula-Ensifer Symbiosis

Legumes’ capacity for symbiotic nitrogen fixation makes them cornerstone species in agricultural and natural ecosystems but they are particularly vulnerable to salt since symbiosis is salt sensitive. Our studies of the model legume Medicago truncatula in natural field and controlled greenhouse settings demonstrate that natural Tunisian populations are locally adapted to saline soils. Taking a 'reverse ecology' approach, whole genome re-sequencing of 40 wild accessions reveals a small number of candidate genomic regions that assort non-randomly with saline source population. These candidate regions contain genes that regulate physiological acclimation to salt stress, such as abscisic acid and jasmonic acid signaling, including a novel salt-tolerance candidate orthologous to the uncharacterized gene AtCIPK21. Unexpectedly, these regions also contain biotic stress genes, including a NB-LRR gene, and flowering time pathway genes, including CONSTANS. We show that flowering time is differentiated between saline and non-saline populations and may allow salt stress escape as it is under significant natural selection in the greenhouse. Furthermore, we find significant epistatic variation between M. truncatula genotypes and co-occuring Ensifer strains for flowering time and stem length, suggesting that rhizobia contribute to salinity adaptation. Future challenges will also be discussed.