Depuydt S, Dolezal K, Van Lijsebettens M, Moritz T, Holsters M, Vereecke D
Modulation of the hormone setting by Rhodococcus fascians results in ectopic KNOX activation in Arabidopsis
Plant Physiology: 2008 146:1267-1281

Abstract
The biotrophic actinomycete Rhodococcus fascians has a profoundimpact on plant development and a common aspect of the symptomatologyis the deformation of infected leaves. In Arabidopsis (Arabidopsisthaliana), the serrated leaf margins formed upon infection resemblethe leaf phenotype of transgenic plants with ectopic expressionof KNOTTED-like homeobox (KNOX) genes. Through transcript profiling,we demonstrate that class-I KNOX genes are transcribed in symptomaticleaves. Functional analysis revealed that BREVIPEDICELLUS/KNOTTED-LIKE1and mainly SHOOT MERISTEMLESS were essential for the observedleaf dissection. However, these results also positioned theKNOX genes downstream in the signaling cascade triggered byR. fascians infection. The much faster activation of ARABIDOPSISRESPONSE REGULATOR5 and the establishment of homeostatic andfeedback mechanisms to control cytokinin (CK) levels supportthe overrepresentation of this hormone in infected plants dueto the secretion by the pathogen, thereby placing the CK responsehigh up in the cascade. Hormone measurements show a net decreaseof tested CKs, indicating either that secretion by the bacteriumand degradation by the plant are in balance, or, as suggestedby the strong reaction of 35S:CKX plants, that other CKs areat play. At early time points of the interaction, activationof gibberellin 2-oxidase presumably installs a local hormonalsetting favorable for meristematic activity that provokes leafserrations. The results are discussed in the context of symptomdevelopment, evasion of plant defense, and the establishmentof a specific niche by R. fascians.

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