Phytoplasma mediated transcriptional changes in poinsettia buds suggest MAF3 and bZIP67 transcription factors as potential suppressors of shoot branching
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Shoot branching is critical not only in breeding for yield but also for ornamentals’ architecture. In the ornamental plant poinsettia ( Euphorbia pulcherrima ), shoot branching has traditionally been induced by phytoplasma ( Candidatus Phytoplasma pruni) inoculation. This study aimed to identify regulatory genes that could be leveraged in future breeding-by–genetic engineering efforts to develop phytoplasma-free, branching poinsettia plants.
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To elucidate mechanisms of phytoplasma-induced shoot branching, we performed RNA-sequencing and assembled an axillary bud-specific transcriptome for expression analyses in phytoplasma-infected and -free poinsettia. Phenotyping and RNA-sequencing were also conducted on Arabidopsis mutants and wild-type lines to investigate the transcriptional regulatory effects of candidate genes.
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The transcription factors Ep MAF3 and Ep bZIP67 were highly de-regulated in phytoplasma-infected poinsettia. We also found a two-fold increase in primary-stem branching levels of the Arabidopsis maf 3 and bzip 67 mutants, suggesting the two transcription factors as potential shoot branching suppressors. AtTcp 1, a CYC-clade TCP transcription factor, was up-regulated (78x) in leaves of the maf 3 mutants. Analyzing previously reported protein-level interactions for the differentially expressed genes (e.g., AtClamt, AtGh 3.9/3.15, AtSaur 32/36, AtAbi 3, AtGamt 2, AtTcp 3, and AtDwf 4) in bzip 67 mutants shed light on other shoot branching regulators such as TCPs, PINs, ABIs, DWARF14, and BES1, highlighting two regulatory sub-networks including membrane transport and hormonal signaling.
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The results open the way to rational engineering of shoot branching in poinsettia by targeted mutagenesis of MAF3 and bZIP67. In that way, the tedious and viral-infection prone process of phytoplasma inoculation can be avoided and poinsettia plants would have more homogenous branching.
One-sentence summary
Phytoplasma infection in poinsettia induces bud-specific repression of the transcription factors EpMaf 3 and EpbZip 67, consistent with the enhanced stem branching observed in Arabidopsis maf 3 and bzip 67 mutant lines.
