XA21 is a receptor-like kinase protein in rice (Oryza sativa) that confers gene-for-gene resistance to specific races of the causal agent of bacterial blight disease, Xanthomonas oryzae pv oryzae. We identified XA21 binding protein 3 (XB3), an E3 ubiquitin ligase, as a substrate for the XA21 Ser and Thr kinase. The interaction between XB3 and the kinase domain of XA21 has been shown in yeast and in vitro, and the physical association between XB3 and XA21 in vivo has also been confirmed by coimmunoprecipitation assays. XB3 contains an ankyrin repeat domain and a RING finger motif that is sufficient for its interaction with the kinase domain of XA21 and for its E3 ubiquitin ligase activity, respectively. Transgenic plants with reduced expression of the Xb3 gene are compromised in resistance to the avirulent race of X. oryzae pv oryzae. Furthermore, reduced levels of Xb3 lead to decreased levels of the XA21 protein. These results indicate that Xb3 is necessary for full accumulation of the XA21 protein and for Xa21-mediated resistance.

No abstract

Gene transfer techniques have been developed previously for certain model rice (Oryza sativa L.) cultivars, but problems persist in U.S. lines for low transformation rates and in vitro callus culture. Moreover, few studies have evaluated traits such as herbicide resistance in transgenic U.S. rice lines under field conditions. A rapid and efficient method was developed for production and field evaluation of transgenic herbicide‐resistant elite U.S. rice lines and cultivars. Six elite U.S. rice lines were transformed for glufosinate herbicide resistance by particle bombardment of mature seed‐derived embryogenic calli; resistance was confered by either the pat or bar gene. By utilizing optimized media for embryogenic callus induction and bialaphos or hygromycin B as a selection agent, an average transformation efficiency of 5% (258 independent events/5201 calli) was obtained across six lines. Southern blot analysis of genomic DNA isolated from primary R0 and R1 progeny plants demonstrated that the pat and hygromycin phosphotransferase (hph) genes were stably integrated into the rice genome. Glufosinate resistance in R0, R1, and R2 progeny was confirmed in the greenhouse and under field conditions. All R1 and a majority (79%) of R2 progeny exhibited one to two gene segregation patterns for glufosinate resistance. The high efficiency and reproducibility of the improved transformation system should make it possible to routinely introduce genes of interest into any elite U.S. rice breeding line.

No abstract