Nodulationi s tightly regulatedin legumesto ensurea ppropriatele vels of nitrogen fixation without excessived epletiono f carbonr eservesT. his balancei s maintainedb y intimately linking nodulationa ndi ts regulationw ith plant hormonesI.t hasp reviously beens hownt hat ethylenea ndj asmonica cid (JA) are ablet o regulaten odulationa nd Nod factor signalt ransductionI.n this thesis,t he natureo f abscisica cid (ABA) regulationo f nodulationi s characterizedT. he applicationo f ABA inhibits nodulation, bacteriali nfection, andn odulin genee xpressionin Medicagot runcatula.A BA actsi n a similar manner as JA and ethylene, regulating Nod factor signaling and affecting the natureo f Nod factori nducedc alcium spiking. However,t his actioni s independenot f the ethylenes ignalt ransductionp athway.G enetici nhibition of ABA signalingt hrough the useo f a dominant-negativea lleleo f ABI1 (ABSCISICA CID INSENSITIVE1 ) leads to a hypernodulationp henotypeI.n addition,a novel locuso f M truncatula is identified STA (SENSITIVITY TO ABA), that dictates the sensitivity of the plant to ABA and, as such, impacts the regulation of nodulation. A known nodulation gene Nflv is shown to be involved in ABA signalt ransductionI.t is shownt hat ABA not only suppresseNs od factor signal transduction in the epidermis but can also regulate cytokinin induction of the nodule primordium in the root cortex. The dynamic distribution of ABA (a negative regulator)a nd cytokinin (a positive regulator)i s assesseddu ring rhizobiac olonization. This reveals a possible positive function for ABA during nodule formation. Therefore, ABA is capableo f coordinatelyr egulatingt he diversed evelopmentapl athways associatedw ith nodulef ormationa ndc ani ntimately dictatet he natureo f the plants' responseto the symbiotic bacteria.