The symbiotic ion channel homolog DMI1 is localized in the nuclear membrane of Medicago truncatula roots

Legumes utilize a common signaling pathway to form symbiotic associations both with rhizobial bacteria and arbuscular mycorrhizal fungi. The perception of microbial signals is believed to take place at the plasma membrane, activating a cascade that converges on the nucleus where transcriptional reprogramming facilitates the symbioses. Forward genetic strategies have identified genes in this signaling pathway including Medicago truncatula DMI1 (Doesn't Make Infections 1) that encodes a putative ion channel. Although the DMI1 homologs from Lotus japonicus, CASTOR and POLLUX, were recently reported to be localized in plastids, we report here that a functional DMI1::GFP fusion is localized to the nuclear envelope in M. truncatula roots when expressed both from a constitutive 35S promoter and from a native DMI1 promoter. Localization may be mediated in part by sequences located within the amino-terminus of DMI1. This region of DMI1 is required for symbiotic signal transcluction, and its replacement with a bona fide plastid transit peptide from the glutamine synthetase 2 gene does not restore DMI1 function. These new data place DMI1 in the nuclear envelope in close proximity to the origin of Nod-factor-induced calcium spiking.