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PLANTS INTERACTING WITH OTHER ORGANISMS

Ectopic Expression of Constitutively Activated RACB in Barley Enhances Susceptibility to Powdery Mildew and Abiotic Stress¹

Institute of Phytopathology and Applied Zoology, University of Giessen, D–35392 Giessen, Germany (H.S., J.I., K.-H.K., R.H.); Institute of Plant Genetics and Crop Plant Research, D–06466 Gatersleben, Germany (G.H., S.B., J.K.); and Institute of Microbiology, Biochemistry and Genetics, University of Erlangen-Nürnberg, D–91058 Erlangen, Germany (U.S.)

Small RAC/ROP-family G proteins regulate development and stress responses in plants. Transient overexpression and RNA interference experiments suggested that the barley (Hordeum vulgare) RAC/ROP protein RACB is involved in susceptibility to the powdery mildew fungus Blumeria graminis f. sp. hordei. We created transgenic barley plants expressing the constitutively activated RACB mutant racb-G15V under control of the maize (Zea mays) ubiquitin 1 promoter. Individuals of the T1 generation expressing racb-G15V were significantly more susceptible to B. graminis when compared to segregating individuals that did not express racb-G15V. Additionally, racb-G15V-expressing plants showed delayed shoot development from the third leaf stage on, downward rolled leaves, and stunted roots. Expression of racb-G15V decreased photosynthetic CO₂-assimilation rates and transpiration of nonstressed leaves. In contrast, racb-G15V-expressing barley leaves, when detached from water supply, showed increased water loss and enhanced transpiration. Water loss was associated with reduced responsiveness to abscisic acid in regard to transpiration when compared to segregants not expressing racb-G15V. Hence, RACB might be a common signaling element in response to both biotic and abiotic stress.

² These authors contributed equally to the paper.

³ Present address: European Commission, JRC-IRMM-RM Unit, Retieseweg 111, 2440 Geel, Belgium.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.066613.

^* Corresponding author; e-mail ralph.hueckelhoven{at}agrar.uni-giessen.de; fax 49–641–9937499.

Received June 3, 2005; returned for revision June 24, 2005; accepted July 15, 2005.