Testifying the rice bacterial blight resistance gene xa5 by genetic complementation and further analyzing xa5 (Xa5) in comparison with its homolog TFIIAγ1

Abstract

The recessive gene xa5 for resistance to bacterial blight resistance of rice is located on chromosome 5, and evidence based on genetic recombination has been shown to encode a small subunit of the basal transcription factor IIA (Iyer and McCouch in MPMI 17(12):1348-1354, 2004). However, xa5 has not been demonstrated by a complementation test. In this study, we introduced the dominant allele Xa5 into a homozygous xa5-line, which was developed from a cross between IRBB5 (an indica variety with xa5) and Nipponbare (a japonica variety with Xa5). Transformation of Xa5 and subsequent segregation analysis confirmed that xa5 is a V39E substitution variant of the gene for TFIIAgamma on chromosome 5 (TFIIAgamma5 or Xa5). The rice has an addition gene for TFIIAgamma exists on chromosome 1 (TFIIAgamma1). Analysis of the expression patterns of Xa5 (TFIIAgamma5)/xa5 and TFIIAgamma1 revealed that both the genes are constitutively expressed in different rice organs. However, no expression of TFIIAgamma1 could be detected in the panicle by reverse transcriptase-polymerase chain reaction. To compare the structural difference between the Xa5/xa5 and TFIIAgamma1 proteins, 3-D structures were predicted using computer-aided modeling techniques. The modeled structures of Xa5 (xa5) and TFIIAgamma1 fit well with the structure of TFIIA small subunit from human, suggesting that they may all act as a small subunit of TFIIA. The E39V substitution in the xa5 protein occurs in the alpha-helix domain, a supposed conservative substitutable site, which should not affect the basal transcription function of TFIIAgamma. The structural analysis indicates that xa5 and Xa5 potentially retain their basic transcription factor function, which, in turn, may mediate the novel pathway for bacterial blight resistance and susceptibility, respectively.

Host and Pathogen Factors Controlling the Rice-Xanthomonas oryzae Interaction

“…Two recessive R genes have been characterized from rice. The R gene xa5 confers broad resistance and, oddly, is an allele of the gene for the g or small subunit of the transcription factor TFIIA (Iyer and McCouch, 2004;Jiang et al, 2006). The recessive allele is not a null allele but differs from the susceptible form by a single codon substitution of Val at position 39 to Glu.…”

“…The proteins are closely related but not identical. The fact that the TFIIAg5 message is present at greater levels based on hybridization analyses of leaf tissue indicates that TFIIAg5 is likely the predominant form of the proteins in rice (Iyer and McCouch, 2004;Jiang et al, 2006;Sugio et al, 2007). The presence of two genes raises the question of whether or not rice or an ancestral relative adapted to bacterial infection by duplication of an ancestral gene for TFIIAg.…”

Fertility versus disease resistance, a hard choice: Figure 1.

“…RNAi plants have increased resistance but also increased sterility due to a failure to progress from the unicellular stage of pollen. susceptible allele, Xa5, and neither allele is induced or suppressed in response to Xoo (Iyer and McCouch 2004;Jiang et al 2006). xa5 was identified as a natural mutation conserved among different varieties of rice and encodes the ␥ subunit of transcription factor IIA (TFIIA␥).…”

Molecular Phylogeny, Homology Modeling, and Molecular Dynamics Simulation of Race-Specific Bacterial Blight Disease Resistance Protein (xa5) of Rice: A Comparative Agriproteomics Approach

“…Among the members of the recessive resistance gene family, xa5 is a race-specific resistance gene that provides immunity to races of Xoo expressing avirulence gene avrxa5 (IyerPascuzzi et al, 2008). It is an allele of a gene on chromosome 5 that encodes the gamma subunit of the general transcription factor (TFIIAc) ( Jiang et al 2006;Iyer and McCouch, 2004). Among the members of recessive genes, xa13 of rice (Yuan et al, 2009), mlo of barley (Buschges et al, 1997) and RRS1-R of Arabidopsis (Deslandes et al, 2005) provide a wide spectrum of immunity to their respective pathogens in contrast to the xa5 gene.…”

“…Until now, more than 35 BB resistance genes (Xa) have been identified and categorized into dominant and recessive genes. Out of them, xa5 (Iyer and McCouch, 2004;Jiang et al, 2006), xa8 (Iyer-Pascuzzi and McCouch, 2007), xa13 (Yuan et al, 2009), xa24 (Wu et al, 2008), xa26 (Cao et al, 2007), and xa28 occur naturally and confer race-specific resistance. Although several resistance genes from crop plants and their corresponding avirulence genes from pathogens have been identified and cloned, only the interaction between Xa27 and avrXa27 (Tian and Yin, 2009) has been fully characterized till date.…”