OsHsfB4d Binds the Promoter and Regulates the Expression of OsHsp18.0-CI to Resistant Against Xanthomonas Oryzae

BACKGROUND

Bacterial leaf streak (BLS) and bacterial blight (BB) are two major prevalent and devastating rice bacterial diseases caused by the Gram-negative bacteria of Xanthomonas oryzae pv. oryzicola (Xoc) and Xanthomonas oryzae pv. oryzae (Xoo), respectively. Previously, we identified a defence-related (DR) gene encoding a small heat shock protein, OsHsp18.0-CI, that positively regulates BLS and BB resistance in rice.

RESULTS

To reveal the regulatory mechanism of the OsHsp18.0-CI response to Xoc and Xoo, we characterized the class B heat shock factor (Hsf), OsHsfB4d, through transcriptional analysis and a transgenic study. OsHsfB4d is upregulated post inoculation by either the Xoc strain RS105 or Xoo strain PXO99a in Zhonghua 11 (wild type, ZH11) as well as in OsHsp18.0-CI overexpressing rice plants. Transient expression of OsHsfB4d can activate the expression of green fluorescent protein (GFP) and luciferase (Luc) via the OsHsp18.0-CI promoter. Rice plants overexpressing OsHsfB4d exhibited enhanced resistance to RS105 and PXO99a as well as increased expression of OsHsp18.0-CI and pathogenesis-related genes. Furthermore, we found that OsHsfB4d directly binds to a DNA fragment carrying the only perfect heat shock element (HSE) in the promoter of OsHsp18.0-CI.

CONCLUSION

Overall, we reveal that OsHsfB4d, a class B Hsf, acts as a positive regulator of OsHsp18.0-CI to mediate BLS and BB resistance in rice.

OsPGIP1-Mediated Resistance to Bacterial Leaf Streak in Rice is Beyond Responsive to the Polygalacturonase of Xanthomonas oryzae pv. oryzicola

Polygalacturonase-inhibiting proteins (PGIPs) have been shown to recognize fungal polygalacturonases (PGs), which initiate innate immunity in various plant species. Notably, the connection between rice OsPGIPs and PGs in Xanthomonas oryzae pv. oryzicola (Xoc), which causes bacterial leaf streak (BLS), remains unclear. Here, we show that OsPGIP1 was strongly induced after inoculating rice with the Xoc strain RS105. Furthermore, OsPGIP1-overexpressing (OV) and RNA interference (RNAi) rice lines increased and decreased, respectively, the resistance of rice to RS105, indicating that OsPGIP1 contributes to BLS resistance. Subsequently, we generated the unique PG mutant RS105Δpg, the virulence of which is attenuated compared to that of RS105. Surprisingly, the lesion lengths caused by RS105Δpg were similar to those caused by RS105 in the OV lines compared with wild-type ZH11 with reduced Xoc susceptibility. However, the lesion lengths caused by RS105Δpg were still significantly shorter in the OV lines than in ZH11, implying that OsPGIP1-mediated BLS resistance could respond to other virulence factors in addition to PGs. To explore the OsPGIP1-mediated resistance, RNA-seq analysis were performed and showed that many plant cell wall-associated genes and several MYB transcription factor genes were specifically expressed or more highly induced in the OV lines compared to ZH11 postinoculation with RS105. Consistent with the expression of the differentially expressed genes, the OV plants accumulated a higher content of jasmonic acid (JA) than ZH11 postinoculation with RS105, suggesting that the OsPGIP1-mediated resistance to BLS is mainly dependent on the plant cell wall-associated immunity and the JA signaling pathway.

Functional analysis of the GRMZM2G174449 promoter to identify Rhizoctonia solani-inducible cis-elements in maize

BACKGROUND

Banded leaf and sheath blight (BLSB), caused by the necrotrophic fungus Rhizoctonia solani, is a highly devastating disease in most maize and rice growing areas of the world. However, the molecular mechanisms of perceiving pathogen signals are poorly understood in hosts.

RESULTS

Here, we identified a Rhizoctonia solani-inducible promoter pGRMZM2G174449 in maize. Deletion analysis showed that the -574 to -455 fragment was necessary for pGRMZM2G174449 in responding to R. solani and this fragment contained the unknown pathogen-inducible cis-elements according to the bioinformatics analysis. Furthermore, detailed quantitative assays showed that two cis-elements, GCTGA in the -567 to -563 region and TATAT in the -485 to -481 region, were specifically responsible for the R. solani-inducible activity. A series of point mutation analysis indicated that the two cis-elements have the conserved motifs of NHWGN and DWYWT, respectively.

CONCLUSION

Our results indicated that pGRMZM2G174449 is a good R. solani-inducible promoter suitable for genetic engineering of BLSB resistance. And NHWGN and DWYWT are two R. solani-inducible cis-elements that play important roles in pGRMZM2G174449 responding to R. solani.