| Summary: | The tobacco NtmybAS1 gene encodes a novel anther-specific member of the Myb family of transcription factors. Overexpression of NtmybAS1 driven by the CaMV35S promoter in tobacco resulted in dramatic alterations in plant architecture leading to a semi-dominant dwarf phenotype. Northern blot analysis demonstrated a direct relationship between the severity of dwarf phenotype and the level of NtmybAS1 transcripts, which clearly indicated that the NtmybAS1-induced dwarfing is gene dosage dependent. Analysis of cell morphology demonstrated that a severe reduction of cell elongation in hypocotyls was the major cause of NtmybAS1-induced dwarfing. In contrast, a dramatic increase in cell elongation and pronounced cell division was detected in palisade and mesophyll cells. Despite changes in cell morphology, the overall body organisation of NtmybAS1 plants remained unaffected. Furthermore, phenotypic alterations in NtmybAS1 overexpressing plants were not normalised by application of phytohormones or by grafting, indicating the involvement of non-hormonal factors and the cell autonomy of dwarfing. Transient expression analyses of the N-terminal Myb domain derivatives fused to sGFP revealed the presence of a nuclear localisation signal, which efficiently targeted sGFP to the pollen nucleus. Analysis of the cellular protein profiles of NtmybAS1 plants by one and two-dimensional gel electrophoresis revealed two proteins that were highly induced in NtmybAS1 plants. Tryptic peptides derived from these proteins by reversed-phase HPLC were sequenced and showed an exact match with two pathogenesis-related (PR) proteins, tobacco chitinase P (PR-P) and PR-1a.
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