Citrus MAF1, a Repressor of RNA Polymerase III, Binds the Xanthomonas citri Canker Elicitor PthA4 and Suppresses Citrus Canker Development

Abstract

Transcription activator-like (TAL) effectors from Xanthomonas species pathogens act as transcription factors in plant cells; however, how TAL effectors activate host transcription is unknown. We found previously that TAL effectors of the citrus canker pathogen Xanthomonas citri, known as PthAs, bind the carboxyl-terminal domain of the sweet orange (Citrus sinensis) RNA polymerase II (Pol II) and inhibit the activity of CsCYP, a cyclophilin associated with the carboxyl-terminal domain of the citrus RNA Pol II that functions as a negative regulator of cell growth. Here, we show that PthA4 specifically interacted with the sweet orange MAF1 (CsMAF1) protein, an RNA polymerase III (Pol III) repressor that controls ribosome biogenesis and cell growth in yeast (Saccharomyces cerevisiae) and human. CsMAF1 bound the human RNA Pol III and rescued the yeast maf1 mutant by repressing tRNAHis transcription. The expression of PthA4 in the maf1 mutant slightly restored tRNA His synthesis, indicating that PthA4 counteracts CsMAF1 activity. In addition, we show that sweet orange RNA interference plants with reduced CsMAF1 levels displayed a dramatic increase in tRNA transcription and a marked phenotype of cell proliferation during canker formation. Conversely, CsMAF1 overexpression was detrimental to seedling growth, inhibited tRNA synthesis, and attenuated canker development. Furthermore, we found that PthA4 is required to elicit cankers in sweet orange leaves and that depletion of CsMAF1 in X. citri-infected tissues correlates with the development of hyperplastic lesions and the presence of PthA4. Considering that CsMAF1 and CsCYP function as canker suppressors in sweet orange, our data indicate that TAL effectors from X. citri target negative regulators of RNA Pol II and Pol III to coordinately increase the transcription of host genes involved in ribosome biogenesis and cell proliferation.

Identification of putative TAL effector targets of the citrus canker pathogens shows functional convergence underlying disease development and defense response

“…We selected PthA2 and PthA4 because they form heterodimers and interact with several citrus proteins implicated in transcriptional control [23,35-37]. By inspecting the RVD sequences of PthA2 and PthA4 (Figure  1B), we presumed that they would target common host genes.…”

“…This is noteworthy because PthA4 also physically interacts with the citrus MAF1 protein (CsMAF1), which is a negative regulator of RNA Pol III that controls ribosome biogenesis and suppresses canker symptoms [23,36]. Thus, the specific PthA4-induction of ribosomal protein genes appears to be a causal feedback mechanism of ribosome biogenesis that is required for cell division and growth.…”

Behind the lines–actions of bacterial type III effector proteins in plant cells

“…2012; Soprano et al. 2013) (Table 1). It is, therefore, assumed that TAL effectors do not only bind to DNA but also associate with components of the plant transcription machinery to activate gene expression.…”

Maf1 is a negative regulator of transcription in Trypanosoma brucei

“…Our results show that TbMaf1 is a negative regulator of transcription, a function that is conserved in multiple organisms including yeast, insects, worms, vertebrates and plants (Soprano et al, 2013;Khanna et al, 2015). Our results show that TbMaf1 is a negative regulator of transcription, a function that is conserved in multiple organisms including yeast, insects, worms, vertebrates and plants (Soprano et al, 2013;Khanna et al, 2015).…”