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Several complex phenotypic changes are induced when the transcription factor AmMYB308 is overexpressed in transgenic tobacco plants. We have previously shown that the primary effect of this transcription factor is to inhibit phenolic acid metabolism. In the plants that we produced, two morphological features were prominent: abnormal leaf palisade development and induction of premature cell death in mature leaves. Evidence from the analysis of these transgenic plants suggests that both changes resulted from the lack of phenolic intermediates. These results emphasize the importance of phenolic secondary metabolites in the normal growth and development of tobacco. We suggest that phenolic acid derivatives are important signaling molecules in the final stages of leaf palisade formation and that phenolic acid derivatives also play a prominent role in tissue senescence.

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A non-helical strain of Spiroplasma citri from little-leaf diseased oranges was subcultured over 40 times in different media without producing helical cells. It was non-motile and produced colonies of a characteristic morphology on soft agar plates. Serology, DNA hybridization and toxin production confirmed that it was a strain of S. citri but its membrane protein pattern after polyacrylamide gel electrophoresis differed from those of other S. citri strains in that one band was absent. The organism caused symptoms in plants identical to those produced by a pathogenic helical strain of S . citri. I N T R O D U C T I O NSpiroplasmas differ from other Mollicutes in shape and motility. All those isolated are typically helical during at least part of their growth phase and show rotary or 'screw'-like motility as well as flexing movements. The helical shape is apparently maintained without the assistance of a rigid cell wall or axial filaments (Cole et al., 1973).Spiroplasma citri was isolated and characterized by Saglio et al. (1973) and is the leafhopper-transmitted agent of little-leaf disease of citrus (Markham et al., I 974). We have isolated a pathogenic strain of S. citri, which lacks the characteristic helical morphology, from diseased citrus material. M E T H O D SIsolation and culture. A non-helical spiroplasma and six normal strains of S. citri were isolated from 'lop-sided' fruits of little-leaf diseased sweet orange trees by the method of Daniels et al. (1973). Isolates were subcultured twice in complete sorbitol medium (SMC; Saglio et al., 1973) without arginine and tryptone, before 0.1 ml samples were plated on SMC solidified with I % (w/v) agar (Oxoid no. I). After incubation for 7 days at 32 "C, colonies showing unusual morphology were picked off into SMC medium. One of these isolates was cloned three times and designated ASP-I. All cultures were maintained in SMC at 32 "C. Characterized strains of S. citri ~8 -~2 (type strain) and c189 were kindly supplied by Dr P. Saglio, and the pathogenic strain SP-A was isolated in this laboratory (Daniels et al., 1973).Media and incubation conditions. The colony and cell morphologies of strains SP-A and ASP-I were compared under different conditions. Plates of SMC solidified with 0.75 % and I % (wlv) agar were spread with 0-1 ml of diluted spiroplasma culture and incubated at 32 "C for 7 days in air or in N2/C02 (95 : 5, v/v). Cultures in SMC were incubated at 25, 28, 2 M I C 100

In the overtly differentiated colonies of Streptomyces coelicolor A3(2), discrete phases of glycogen synthesis are found at the vegetative/aerial mycelium boundary (phase I) and in the immature spore chains at aerial hyphal tips (phase II). We have characterized two S. coelicolor glgB genes encoding glycogen branching enzyme, which are well separated in the genome. Disruption of glgBl led to the formation of abnormal polyglucan deposits at phase I, with phase II remaining normal, whereas disruption of glgBII interfered specifically with phase II deposits, and not with those of phase I. Thus, each branching enzyme isoform is involved in a different phase of glycogen synthesis. This situation contrasts with that in simple bacteria, which typically have a single set of enzymes for glycogen metabolism, and more closely resembles that in plants.

Several complex phenotypic changes are induced when the transcription factor AmMYB308 is overexpressed in transgenic tobacco plants. We have previously shown that the primary effect of this transcription factor is to inhibit phenolic acid metabolism. In the plants that we produced, two morphological features were prominent: abnormal leaf palisade development and induction of premature cell death in mature leaves. Evidence from the analysis of these transgenic plants suggests that both changes resulted from the lack of phenolic intermediates. These results emphasize the importance of phenolic secondary metabolites in the normal growth and development of tobacco. We suggest that phenolic acid derivatives are important signaling molecules in the final stages of leaf palisade formation and that phenolic acid derivatives also play a prominent role in tissue senescence.

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