R. fascians is a ubiquitous well-adapted soil bacterium, but it is an equally well-adapted epiphyte. A first crucial step to gain access into this new habitat is to reach the plant surface from its location in the soil. As a nonmotile organism, it is currently unknown how R. fascians accomplishes this aspect. Perhaps the stk locus enables the bacteria to exhibit directional (hyphal) growth toward the plant. Of course, it cannot be ruled out that the bacteria encounter the plant by chance via, e.g., plant growth, raindrops, insect vectors, or other soil-dwelling organisms. As soon as R. fascians contacts a host plant, it will form large epiphytic colonies embedded in a protective slime layer of bacterial origin. A constitutive level of secreted auxin may aid in the acquisition of nutrients or in the suppression of plant defense. The bacteria enter the plant via ingression sites that necessitate breaching through the cuticula and the epidermal cell layer. This local breakdown of the plant's outer barrier is possibly facilitated by a phytotoxin produced by the NRPS encoded by the nrp locus. Expression of the virulence genes is initially controlled by the production of the Att compound that gradually builds up under specific conditions favorable for infection. When a threshold concentration is reached, a positive autoregulatory loop is activated,
leading to full induction of att gene expression and activation of fas gene expression. Whereas the expression of the att genes is restricted to the epiphytic bacteria, the synthesis of the fas-dependent cytokinin-like signal molecules continues throughout the interaction. Expression of the fas genes in bacteria located in the epidermis coincides with the dedifferentiation of parenchyma cells to shoot meristems. The persistent delivery of morphogenic substances by the bacteria is imperative for the formation and maintenance of the leafy gall structure. In the leafy gall, physiological conditions are different from those of normal nondiseased plants. The spectrum of compounds that prevail in a leafy gall is believed to strongly differ from that of uninfected plants. Bacteria have most probably coevolved to utilize these molecules as specific nutrients via the chromosomal vic locus. Hence, the induction of leafy gall formation leads to a specific niche for R. fascians that will metabolically colonize this newly formed organ. Different aspects of the interaction are illustrated in Fig. 1, whereas a schematic overview is given in Fig. 4.
Acknowledgements The authors wish to thank Martine De Cock for help in preparing the manuscript. This work was supported by a research grant from the Research Foundation-Flanders (G0338.04). I.F. is indebted to the Institute for the Promotion of Innovation by Science and Technology in Flanders for a predoctoral fellowship. D.G. is a postdoctoral researcher of the Research Foundation-Flanders.
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