Nodulation of leguminous plants by Rhizobium, Bradyrhizobium, and Azorhizobium spp. is dependent on the induction by the plant host of different bacterial modulation (nod) loci. The transcription of these nod loci is activated in the presence of plant-produced flavonoids upon binding of the

The role and metabolism of indole-3-acetic acid in gram-negative bacteria is well documented, but little is known about indole-3-acetic acid biosynthesis and regulation in gram-positive bacteria. The phytopathogen Rhodococcus fascians, a gram-positive organism, incites diverse developmental alterations, such as leafy galls, on a wide range of plants. Phenotypic analysis of a leafy gall suggests that auxin may play an important role in the development of the symptoms. We show here for the first time that R. fascians produces and secretes the auxin indole-3-acetic acid. Interestingly, whereas noninfected-tobacco extracts have no effect, indole-3-acetic acid synthesis is highly induced in the presence of infected-tobacco extracts when tryptophan is not limiting. Indole-3-acetic acid production by a plasmid-free strain shows that the biosynthetic genes are located on the bacterial chromosome, although plasmid-encoded genes contribute to the kinetics and regulation of indole-3-acetic acid biosynthesis. The indole-3-acetic acid intermediates present in bacterial cells and secreted into the growth media show that the main biosynthetic route used by R. fascians is the indole-3-pyruvic acid pathway with a possible rate-limiting role for indole-3-ethanol. The relationship between indole-3-acetic acid production and the symptoms induced by R. fascians is discussed.The phytopathogenic actinomycete Rhodococcus fascians induces the development of a wide range of malformations on its numerous monocotyledonous and dicotyledonous host plants: leaf deformation, growth inhibition, witches' brooms, fasciations, and leafy galls (reference 23 and references therein). The leafy-gall structure, considered an extreme form of apical dominance (59), is the most severe outcome of the R. fasciansplant interaction. Consisting of small misshapen leaves and numerous buds that are inhibited for further outgrowth, the leafy gall is believed to result from the alteration of the endogenous hormone balance of the host plant (23). Leafy-gall features, such as leaf wrinkling, shoot multiplication, and delayed senescence, are typical cytokinin effects. In this respect, R. fascians pathogenicity has been linked to a linear plasmid (pFiD188) that carries different virulence genes. On pFiD188 an isopentenyl transferase (ipt) homologue is located that is essential for symptom development and has been found in all virulent strains examined thus far (14,15,53). Moreover, it is known that R. fascians can produce at least 11 different cytokinins in vitro (2,18,24,28,48,51). In R. fascians, cytokinins probably originate from two sources, either release from tRNA (36, 38) and/or condensation of isopentenyl pyrophosphate and 5Ј-AMP (14). Nevertheless, hitherto no clear correlation between known cytokinins and symptom development could be established (16,18), suggesting that new cytokinin-like molecules could be involved in the formation of the leafy gall (23).Formation of a leafy gall cannot be attributed solely to the action of cytokinin. Some leafy-gall charac...

Rhodococcus fascians is a Gram-positive bacterium that infects dicotyledonous and monocotyledonous plants, leading to an alteration in the normal growth process of the host. The disease results from the modulation of the plant hormone balances, and cytokinins are thought to play an important role in the induction of symptoms. Generally, on the aerial parts of the plants, existing meristems were found to be most sensitive to the action of R. fascians, but, depending on the infection procedure, differentiated tissues as well gave rise to shoots. Similarly, in roots not only actively dividing cells, but also cells with a high competence to divide were strongly affected by R. fascians. The observed symptoms, together with the determined hormone levels in infected plant tissue, suggest that auxins and molecules of bacterial origin are also involved in leafy gall formation. The complexity of symptom development is furthermore illustrated by the necessary and continuous presence of the bacteria for symptom persistence. Indeed, elimination of the bacteria from a leafy gall results in the further development of the multiple embryonic buds of which it consists. This interesting characteristic offers novel biotechnological applications: a leafy gall can be used for germplasm storage and for plant propagation. The presented procedure proves to be routinely applicable to a very wide range of plants, encompassing several recalcitrant species.