PI230

Two Medicago truncatula nodulin genes putatively encoding proteins structurally 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 pathogenesis-related proteins. We show that neither MtN1 nor MtN13 is induced in leaves in response to pathogenic bacteria, and that both are exclusively expressed during nodulation. In situ hybridization experiments as well as Northern (RNA) studies of interactions between M. truncatula 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 nodule outer cortex. Possible roles for MtN1 and MtN13 are discussed. We also present the identification of another member of the PR10 family, designated as MtPR10-1, whose regulation is strikingly different from that observed for MtN13, being constitutively expressed in roots and pathogen-inducible in leaves.

Two pea genes, pI39 and pI230, which are specifically induced by two forma specials of Fusarium solani, encode closely related proteins with predicted molecular masses (Mr) of 8.2 and 8 kDa, respectively. Both proteins contain a signal sequence and are cleaved to mature proteins of Mr 5 kDa as indicated by an in vitro translation system. The mature proteins contain about 17% cysteine residues and have the potential to form four disulfide bonds. The two proteins share extensive homology in their signal sequences but much less homology as mature proteins. The cysteine residues of the mature proteins are highly conserved, suggesting functional importance. Southern hybridization suggests these genes belong to a multigene family. The relative accumulations of mRNA levels indicate that the two genes are expressed somewhat differentially. In both the compatible (susceptible) and incompatible reactions between F. solani and pea tissue, pI39 mRNA accumulates more slowly than pI230 mRNA and accumulates to relatively high levels after 24 hr of inoculation. The increase in accumulation of pI230 mRNA occurs within 6 hr and thus correlates with an initial suppression of the growth of both the compatible and incompatible pathogen, which is cytologically observable at 6 hr. pI39 and pI230 belong to a distinct class of pathogenesis-related proteins characterized previously, which are associated with and thus may contribute to nonhost resistance in plants.