The leafy gall syndrome induced byRhodococcus fascians

Abstract

The Actinomycete Rhodococcus fascians causes the leafy gall syndrome, an infectious plant disease that affects a wide range of plants, primarily dicotyledonous herbs. The syndrome is associated with delayed senescence, loss of apical dominance, activation of dormant axillary meristems, and formation of multiple inflorescences, leading to a stunted and bushy plant appearance. A major breakthrough in the elucidation of the virulence strategy of this pathogen was the discovery of a linear virulence plasmid, pFiD188 for R. fascians strain D188. Upon perception of a compatible host plant, an autoregulatory mechanism mediated by the att operon directs a switch in the bacterial life style from a harmless epiphyte into a pathogenic endophyte and, concomitantly, activates gene expression of the fas operon that encodes a cytokinin biosynthesis pathway. A mixture of five cytokinins determines the cytokinin activity of R. fascians that directly affects plant responses and development. Moreover, the bacterial cytokinins stimulate the host to produce auxins and polyamines, that function as accessory signals to aid in symptom development. The plant reacts against the developmental hijacking by R. fascians by activating a set of counteracting measures that ultimately results in a delicate balance, allowing a long-lasting biotrophic interaction.

Methylated Cytokinins from the Phytopathogen Rhodococcus fascians Mimic Plant Hormone Activity

“…Nevertheless, whether bacterial MeCKs are transported or metabolized into corresponding conjugates requires further investigation. It is interesting to note that previous studies reported the accumulation of CK-methylthio derivatives in Arabidopsis after R. fascians infection as well as in R. fascians culture (Pertry et al, , 2010Stes et al, 2013). However, in our study, these CK derivatives were below the detection limit in the recombinant E. coli cultures expressing mts and fas4 as well as in the R. fascians-infected tobacco plants.…”

“…fas4 encodes IPT that catalyzes the rate-limiting step of CK biosynthesis and is vital for virulence (Stes et al, 2013). Interestingly, two methyltransferase-like genes are present upstream of the fas gene, whose functions have been unknown.…”

Taxonomy, Physiology, and Natural Products of Actinobacteria

“…Extensive epiphytic growth precedes intercellular invasion through stomata. R. fascians causes various effects in its hosts, including leaf deformation and formation of witches' broom, fasciations, and leafy galls (517,519). The symptoms are caused by the hyperinduction of shoots through activation of dormant axillary meristems and de novo meristem formation, probably as the result of elaborate manipulation of host hormone balances and pathogen-derived auxins and cytokinins (520,521).…”

Evolution and Ecology ofActinobacteriaand Their Bioenergy Applications

“…Other common plant pathogens fall within the Microbacteriaceae family, including Leifsonia xyli , which is the cause of ratoon stunting disease in sugarcane, and Clavibacter michiganensis , which infects xylem and causes plant wilting in alfalfa, corn, tomato, and potato (21, 47). Finally, leafy gall syndrome, which infects a wide range of dicotyledonous herbaceous plants, is caused by Rhodococcus fascians (128, 129). …”