Regulation of lateral root development by ABA in Arabidopsis and Medicago. Key control points common to both species are initiation and meristem activation. Emergence from the primary root is an additional control point in Medicago. ABA stimulates initiation in both Arabidopsis and Medicago [55,56], but high concentrations of ABA inhibit it [56]. Salt stress inhibits lateral root emergence by stimulating ABA signaling, which induces expression of HB1, which in turn represses LBD1, required for emergence [56]. Lower levels of ABA stimulate lateral root emergence, but it is not known what functions downstream of ABA in this process [56]. Meristem activation is a key control point in both species, with systemic nitrate signaling via the classic ABA biosynthetic pathway in Arabidopsis and requiring ABI4 and ABI5, but not other ABA signaling genes [54]. ABA treatment in the absence of an environmental nitrate signal also regulates meristem activation, but does not require activity of any of the known ABA signaling genes [57]. In M. truncatula, this step requires activity of the MtLATD/NIP (MtNPF1.7) gene [17,58]. ABA treatment can bypass the requirement for MtLATD/NIP, inducing meristem activation in the absence of gene function [17]. Elongation of the lateral root subsequent to meristem activation is regulated by localized nitrate in Arabidopsis and can be repressed by ABA signaling [54].