SYMBIOSIS-SPECIFIC EXPRESSION OF 2 MEDICAGO-TRUNCATULA NODULIN GENES,MTN1 AND MTN13, ENCODING PRODUCTS HOMOLOGOUS TO PLANT DEFENSE PROTEINS

Two Medicago truncatula nodulin genes putatively encoding proteins str ucturally related to two classes of proteins commonly associated with plant defense reactions have been characterized. MtN1 is homologous to two small, cysteine-rich, pathogen-inducible proteins from pea (pI39 and pI230), whereas MtN13 is closely related to the PR10 family of pat hogenesis-related proteins. We show that neither MtN1 nor MtN13 is ind uced in leaves in response to pathogenic bacteria, and that both are e xclusively expressed during nodulation, In situ hybridization experime nts as well as Northern (RNA) studies of interactions between M. trunc atula and either wild-type Rhizobium meliloti or mutants deficient in infection establish that MtN1 is associated with the infection process , while MtN13 represents the first specific marker described for the n odule outer cortex. Possible roles for MtN1 and MtN13 are discussed. W e also present the identification of another member of the PR10 family , designated as MtPR10-1, whose regulation is strikingly different fro m that observed for MtN13, being constitutively expressed in roots and pathogen-inducible in leaves.