Metabolic profile analysis of sweet orange in response to X. citri infection and genetically modified plants that silence or overexpress the maf1 gene

Abstract

Transcription activator-like (TAL) effectors from Xanthomonas pathogens act as transcription factors in plant cells; however, how TAL effectors activate host transcription is unknown. We found previously that PthA4, the main TAL effector of the citrus canker pathogen Xanthomonas citri, specifically binds to citrus MAF1 (CsMAF1), an RNA Pol III repressor that controls ribosome biogenesis and cell growth in yeast and humans and is mainly regulated by via TOR pathway. CsMAF1 interacted with the human RNA Pol III and rescued the yeast MAF1 mutant by repressing the tRNAHis transcription. Expression of PthA4 in the MAF1 mutant slightly restored the tRNAHis synthesis, indicating that PthA4 counteracts CsMAF1 activity. In addition, we show that sweet orange RNAi plants with reduced CsMAF1 levels displayed a dramatic increase in tRNA transcription and a marked phenotype of cell proliferation during canker formation. In the other hand, CsMAF1 overexpression was detrimental to seedling growth, inhibited tRNA synthesis and attenuated canker development. In conclusion, we have identified CsMAF1 as a novel target of the TAL effector PthA that functions as a citrus canker suppressor. Nonetheless, despite the advances in the characterization of the first plant MAF1 protein, there are still important questions that need to be addressed. What are the citrus kinases and protein phosphatases that regulate CsMAF1 activity? Is CsMAF1 a component of the TOR pathway in plants? What is the relationship between the phenotype of enhanced cell proliferation observed in RNAi CsMAF1 plants infected with X. citri and TOR function in the regulation of cell growth and development? Thus, this proposal aims to answer these questions and uncover not only how plant cells regulate their growth in physiological/ metabolic terms under stress conditions, but also how TAL effectors manipulate critical host targets for pathogen's benefit. (AU)