The Coat Protein of Beet Western Yellows Luteovirus is Essential for Systemic Infection but the Viral Gene Products P29 and P19 are Dispensable for Systemic Infection and Aphid Transmission. Veronique Ziegler-Graff. Institut de Biologie Moleculaire des Plantes du CNRS et de I'Universite Louis Pasteur, 12 rue du General Zimmer, Strasbourg 67084 Cedex, France. Veronique Brault (1), J. D. Mutterer (1), Marie-Therese Simonis (1), E. Herrbach (2), H. Guilley (1), K. E. Richards (1), and G. Jonard (1). (1) Institut de Biologie Moleculaire des Plantes du CNRS et de I'Universite Louis Pasteur, 12 rue du General Zimmer, Strasbourg 67084 Cedex, France; (2) Station de Recherche 'Grandes Cultures', INRA, 68021 Colmar Cedex, France. MPMI 9:501-510. Accepted 1 May 1996. Copyright 1996 The American Phytopathological Society.

The beet western yellows luteovirus genes encoding the major coat protein (open reading frame [ORF] 3), the putative protein P29 (ORF 0), and the candidate movement protein P19 (ORF 4) have been mutated to study the role of the corresponding gene products in transcript-infected Chenopodium quinoa protoplasts and agro-infected Nicotiana clevelandii plants. Mutations blocking synthesis of the major coat protein inhibited systemic infection of agro-inoculatcd plants, indicating that coat protein is required for systemic movement. Mutations in ORF 0 partially inhibited viral RNA amplification in both protoplasts and agro-infected plants but the mutant virus could spread systemically in A'. clevelandii and be aphid-transmitted to other hosts. Thus, at present, no function can be assigned to the ORF 0 putative gene product. Viruses carrying mutations in ORF 4 accumulated to high levels in both protoplasts and agro-infected plants although symptom onset was delayed in plants and initial virus titers were somewhat lower. The ORF 4-mutant virus could be aphid-transmitted to other hosts. These findings rule out the possibility that P19 is an essential viral movement protein. A model in which beet western yellows virus can move in plants by two pathways is discussed.