Histone-like nucleoid-structuring (H-NS) proteins, which are conserved in Gram-negative bacteria, bind DNA and act as the global transcriptional repressors. In this study, we identified and characterized the xrvC gene encoding a H-NS protein in Xathomonas oryzae pv. oryzae (Xoo) Philippines strain PXO99(A) Compared with the wild type, the xrvC-deficient mutant of PXO99(A) (named PXO99ΔxrvC) showed a reduced growth rate in both nutrient-rich and nutrient-limited media. Interestingly, PXO99ΔxrvC exhibited significantly reduced virulence on rice cultivar IRBB214, but its virulence on 31 other rice cultivars was not affected. Transcriptional analysis revealed that the expression of hrpG, hrpX and hpa1 and of 15 out of 18 tested non-TAL (transcription activator-like) effector genes was decreased significantly in the xrvC mutant compared with that in the wild type. In addition, loss of xrvC also impaired the induction of the rice susceptibility gene Os8N3 in IRBB214 by PXO99(A) Our results suggest that the xrvC gene is involved in bacterial growth, and it plays a vital role in virulence by positively regulating the expression of hrp genes and non-TAL effector genes in PXO99(A) and the susceptibility gene Os8N3 in rice.
Transcription activator-like (TAL) effectors encoded by tal genes were recognized as a key virulence strategy used by Xanthomonas oryzae pv. oryzae (Xoo) to cause bacterial leaf blight of rice. TAL effector PthXo3 is a major virulence factor identified in a Philippine Xoo strain PXO61, and it can induce the expression of susceptibility gene OsSWEET14 by binding to the effector-binding element (EBE) in the promoter region. In this study, pthXo3 homologous genes were also identified and isolated from Xoo Chinese strain OS198 and Japanese strain JXOV, which were named as pthXo3OS198 and pthXo3JXOV, respectively. When pthXo3JXOV was delivered into PXO99A, the resulting strain PXO99A/pthXo3JXOV significantly increased virulence in 18 out of 23 rice varieties tested, with the most prominent increase in lesion length and bacteria propagation in rice IRBB13. PthXo3JXOV suppresses the plant's innate immunity by inhibiting hypersensitive response (HR) and callose deposition. The Agrobacterium tumefaciens-mediated transient expression assays showed that, besides OsSWEET14, PthXo3JXOV also interacts with other targets by binding to the EBEs in their promoter regions. Our results suggest that PthXo3JXOV may interact with multiple targets to execute its virulence functions.
Ambient light and the endogenous circadian clock play key roles in regulating Arabidopsis (Arabidopsis thaliana) seedling photomorphogenesis. PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) acts downstream of both light and the circadian clock to promote hypocotyl elongation. Several members of the R2R3-MYB transcription factor (TF) family, the most common type of MYB TF family in Arabidopsis, have been shown to be involved in regulating photomorphogenesis. Nonetheless, whether R2R3-MYB TFs are involved in connecting the light and clock signaling pathways during seedling photomorphogenesis remains unknown. Here, we report that MYB112, a member of the R2R3-MYB family, acts as a negative regulator of seedling photomorphogenesis in Arabidopsis. The light signal promotes the transcription and protein accumulation of MYB112. myb112 mutants exhibit short hypocotyls in both constant light and diurnal cycles. MYB112 physically interacts with PIF4 to enhance the transcription of PIF4 target genes involved in the auxin pathway, including YUCCA8 (YUC8), INDOLE-3-ACETIC ACID INDUCIBLE 19 (IAA19) and IAA29. Furthermore, MYB112 directly binds to the promoter of LUX ARRHYTHMO (LUX), the central component of clock oscillators, to repress its expression mainly in the afternoon and relieve LUX-inhibited expression of PIF4. Genetic evidence confirms that LUX acts downstream of MYB112 in regulating hypocotyl elongation. Thus, the enhanced transcript accumulation and transcriptional activation activity of PIF4 by MYB112 additively promotes the expression of auxin-related genes, thereby increasing auxin synthesis and signaling and fine-tuning hypocotyl growth under diurnal cycles.
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