Biosynthesis of Auxin by the Gram-Positive PhytopathogenRhodococcus fasciansIs Controlled by Compounds Specific to Infected Plant Tissues

Abstract

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...

Bacterial and Plant Signal Integration via D3-Type Cyclins Enhances Symptom Development in the Arabidopsis-Rhodococcus fasciansInteraction

“…Asterisks and hashes indicate statistical differences (P , 0.05) between DFMA-treated and untreated plants and between DFMAtreated and DFMA + Put-treated plants, respectively. chromosome, the kinetics of IAA secretion are linear plasmid dependent (Vandeputte et al, 2005). Thus, to identify the bacterial signal that triggers the accumulation of Put during the interaction, Arabidopsis plants were placed on medium supplemented with 10 mM IAA, 10 mM of a single cytokinin (trans-zeatin [tZ], isopentenyladenine [iP], cis-zeatin [cZ], 2-methylthiotrans-zeatin [2MeStZ], 2-methylthio-cis-zeatin [2MeScZ]), or a mix of the six cytokinin bases produced by R. fascians (1 mM each of tZ, cZ, iP, and their 2MeS derivatives).…”

“…The bacteria disturb the plant's hormone balance by producing morphogenic signals, such as auxins and cytokinins, that provoke the outgrowth of existing and the development of new shoot meristems de O Manes et al, 2001;Vandeputte et al, 2005;Pertry et al, 2009), ultimately leading to the establishment of a leafy gall. In strain D188, virulence is conferred by a linear plasmid, pFiD188, that harbors the fasciation (fas) genes essential for cytokinin biosynthesis and symptom development (Crespi et al, 1992;Stange et al, 1996;Pertry et al, 2010).…”

Eternal Youth, the Fate of Developing Arabidopsis Leaves uponRhodococcus fasciansInfection

“…Whereas the role of auxin production in the symptomatology remains to be determined (Vandeputte et al, 2005), the central position of cytokinin secretion has been clearly established (Crespi et al, 1992(Crespi et al, , 1994Temmerman et al, 2000;Pertry et al, 2009). The proliferation of shooty tissue in tobacco (Nicotiana tabacum) not only relies on activation of existing (dormant) meristems but also on de novo meristem formation in cortical tissues by reactivation of the cell cycle, a process that is accompanied by expression of the cyclins CYCB1;1 and CYCD3;2 (de O Manes et al, 2001).…”

Endophytic and rhizospheric bacteria from Opuntia ficus-indica mill and their ability to promote plant growth in cowpea, Vigna unguiculata (L.) Walp

“…This value increased when media was supplemented with tryptophan to 13.9 and 20.7 mg/L per batch on RPO-free media (Vassilev et al, 2007). Rhodococcus, especially virulent strains, are able to produce IAA (Vandeputte et al, 2005). It is interesting to note that except for the species C. freundii, all bacteria with the potential to fix N 2 were isolated from the Alagoas state near the São Francisco river, with predominantly high temperatures and low rainfall.…”