Os06g0125800

OsBBI1, which encoding a RING finger protein with E3 ligase activity, mediates broad-spectrum disease resistance.

Annotated Information

Function

1.OsBBI1 modulates broad-spectrum resistance to blast fungus by modifying cell wall defence responses. Disease lesions on the osbbi1 leaves were significantly larger than on the WT leaves after inoculation with two races of M. oryzae. These results indicate that OsBBI1 is required for full immunity in rice against M. oryzae.

2.The functional characterization of OsBBI1 provides insight into the E3 ligase-mediated innate immunity, and a practical tool for constructing broad-spectrum resistance against the most destructive disease in rice. The level of protein ubiquitination increased with incubation time, indicating the E3 ligase activity of His-OsBBI1 (Figure 7B). In the presence of Ubi, E1, and E2 enzymes, His-OsBBI1 could carry out self-ubiquitination, while no clear protein ubiquitination was detected in the absence of E1, E2, or HisOsBBI1 (Figure 7C). The truncated mutant, OsBBI1 ∆RING ligase, and the RING domain in OsBBI1 is essential to its E3 Ubi ligase activity.

3.Enhancement of cell wall defence response in OsBBI1 overexpression plants The percentages of autofluorescent cell walls and cross-linking increased by ~10% in the OsBBI1-OE plants compared with that in the WT plants at 24 h.p.i. This increase was much more significant at 48 h.p.i. (Figure 5B and 5D). In contrast, the percentages of autofluorescent cell walls and cross-linking of wall-associated proteins at infection sites were lower in the osbbi1 plants than those in the WT plants at 24 and 48 h.p.i. (Figure 5E and 5F).

Expression

1. OsBBI1 is induced by BTH and M. oryzae The expression of OsBBI1 was induced by rice blast fungus Magnaporthe oryzae, as well as chemical inducers, benzothiadiazole and salicylic acid, and it encodes a RING protein with E3 Ubi ligase activity in vitro. Expression of OsBBI1 was induced in M. oryzae-infected plants within a period of 48 h, and reached the peak at 24 h after inoculation (Figure 1A). OsBBI1was upregulated by BTH and SA treatment and reached the peak at 12 h (BTH) or at 24 h (SA) after treatment, respectively; but was not inducible by JA and Xoo infection (Figure 1B)

2. OsBBI1 encodes a RING protein with E3 ligase activity The OsBBI1 gene encodes a 261-aa protein, which contains a conserved RING domain at the C-terminal end. To confirm whether the OsBBI1 protein has E3 Ubi ligase activity, we expressed full-length OsBBI1 as a six His-tagged fusion protein (His-OsBBI1) in Escherichia coli and purified the recombinant protein for enzyme activity assay.

Mutation

1.Mutation in OsBBI1 leads to increased susceptibility to M. oryzae Genetic analysis revealed that the loss of OsBBI1 function in a Tos17-insertion line increased susceptibility, while the overexpression of OsBBI1in transgenic plants conferred enhanced resistance to multiple races of M. oryzae. The level of OsBBI1 transcript in osbbi1 plants was similar to that in wild-type (WT) plants when creased susceptibility to races ZD1, ZD3, and ZG1, resulting in an average of one grade higher disease index than the WT Nipponbare plants (Figure 2C and 2D)

2.Overexpression of OsBBI1 leads to broad-spectrum resistance against M. oryzae The OsBBI1-overexpressing plants showed higher levels of H 2O2 accumulation in cells and higher levels of phenolic compounds and cross-linking of proteins in cell walls at infection sites byM. oryzaecompared with wild-type (WT) plants. The ell walls were thicker in the OsBBI1-overexpressing plants and thinner in the mutant plants than in the WT plants. We compared the disease lesion sizes on the OsBBI1-OE and WT plants after inoculation with traces of ZB1 (strain 2000-2) and ZB15 (strain 09-31-1). Lesion sizes on the OsBBI1-OE plants were significantly reduced compared with those on the WT plants, resulting in 50-80% and 45-50% reduction when infected by ZB1 and ZB15, respectively(figure3C).

Localization

The OsBBI1 gene is expressed in root, stem, sheath, and leaf tissues.

Labs working on this gene

1.National Key Laboratory for Rice Biology, Institute of Biotechnology, Zhejiang University, Hangzhou 310029, China;

2.National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, China

References

1.Wei Li, Sihui Zhong, Guojun et al.(2011) ,LiRice RING protein OsBBI1 with E3 ligase activity confers broad-spectrum resistance against Magnaporthe oryzae by modifying the cell wall defence,Cell Research (2011) 21:835-848.

2.Liu J, Wang X, Mitchell T, et al. Recent progress and understanding of the molecular mechanisms of the rice-Magnaporthe oryzae interaction. Mol Plant Pathol 2010; 11:419-427.