Plant Molecular Biology 38: 1161–1168, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 1161 Jasmonic acid stimulates the expression of nod genes in Rhizobium Susana Rosas ∗ , Rosana Soria, N´ estor Correa and Guillermina Abdala Fisiolog´ ıa Vegetal, Departamento de Ciencias Naturales, Facultad de Ciencias Exactas, Universidad Nacional de R´ ıo Cuarto, 5800 R´ ıo Cuarto, Argentina ( ∗ author for correspondence) Received 26 February 1998; accepted in revised form 30 June 1998 Key words: nod genes, jasmonates, salicylic acid Abstract Jasmonates and salicylic acid are considered to be signal molecules that induce a variety of plant genes involved in wound or defence response, as well as affecting nos promoter activity. In this paper we examined whether these chemicals could also affect nod genes from isogenic rhizobia strains. Isogenic strains contain the Rhizobium leguminosarum nodA promoter fused to the lacZ gene of Escherichia coli and differ only in the source of the regulatory nodD gene. Naringenin, jasmonic acid and methyl jasmonate induced expression of nod genes in strain RBL1284 and salicylic acid showed no activity alone or when used in combination with other compounds; addition of naringenin + jasmonic acid produced a synergistic effect. Results obtained with strain RBL5284 were similar to those for RBL1284 albeit the combination of naringenin with the other compounds markedly inhibited nod gene expression. Whereas RBL5283 responded to naringenin with a strong induction, jasmonic acid, methyl jasmonate or salicylic acid showed no significant responses. The inhibitory effect of salicylic acid on nod gene expression indicates that the induction mechanism of jasmonic acid, methyl jasmonate, N-propyldihydrojasmonate and naringenin is probably different from that of salicylic acid. Abbreviations: CHS, chalcone synthase; JA, jasmonic acid; Me-JA, methyl jasmonate; NA, narin- genin; NI, naringin; NMT, naringenin methyltransferase; PAL, phenylalanine ammonia-lyase; PDJ, N- propyldihydrojasmonate; SA, salicylic acid Introduction Many studies have examined the role of several phe- nolic compounds in recognition processes in plant- microorganism interaction and as signal molecules. These compounds induce germination and haustoria development in plant parasites [24], expression of vir genes responsible for the transference of Agrobac- terium T-DNA to the plant [35], and the regulation of nod gene expression during rhizobia-legume symbio- sis [29]. A series of complex events unchain this associa- tion where bacteria and plants are mutually influenced in significant activities such as gene expression [36]. During the first events of the symbiosis, as a response to signals emitted by the exudates of the legume roots, flavonoids at low concentrations [38], some hormones and chalcones [32] may initiate the expres- sion of a series of genes carried in the rhizobia Sym plasmid or chromosome, giving place to the infec- tion and nodulation processes. These genes are named nod. The NodD gene is constitutively expressed and other genes such as A, B and C are inducible, highly conserved between rhizobia species and functionally interchangeable. Flavonoids, anthocyanins and be- tains affect nodulation genes [19, 30]. Flavonoids, flavonones and flavones are compounds derived from phenylpropanoid metabolism [13, 14, 15, 16]. Compared to the plant-pathogen interaction, the development of the symbiotic interaction is less contentious and apparently leads to a close plant- microorganism association. However, the plant shows a rapid reaction in the presence of rhizobia, being able to reject the symbiotic microorganism at any stage of