TIR-only protein RBA1 recognizes a pathogen effector to regulate cell death in Arabidopsis.

Detection of pathogens by plants is mediated by intracellular nucleotide-binding site leucine-rich repeat (NLR) receptor proteins. NLR proteins are defined by their stereotypical multidomain structure: an N-terminal Toll-interleukin receptor (TIR) or coiled-coil (CC) domain, a central nucleotide-binding (NB) domain, and a C-terminal leucine-rich repeat (LRR). The plant innate immune system contains a limited NLR repertoire that functions to recognize all potential pathogens. We isolated Response to the bacterial type III effector protein HopBA1 (RBA1), a gene that encodes a TIR-only protein lacking all other canonical NLR domains. RBA1 is sufficient to trigger cell death in response to HopBA1. We generated a crystal structure for HopBA1 and found that it has similarity to a class of proteins that includes esterases, the heme-binding protein ChaN, and an uncharacterized domain of Pasteurella multocida toxin. Self-association, coimmunoprecipitation with HopBA1, and function of RBA1 require two previously identified TIR-TIR dimerization interfaces. Although previously described as distinct in other TIR proteins, in RBA1 neither of these interfaces is sufficient when the other is disrupted. These data suggest that oligomerization of RBA1 is required for function. Our identification of RBA1 demonstrates that "truncated" NLRs can function as pathogen sensors, expanding our understanding of both receptor architecture and the mechanism of activation in the plant immune system.

Silencing a candidate nematode effector gene corresponding to the tomato resistance gene Mi-1 leads to acquisition of virulence.

The Mi-1 gene in tomato confers effective resistance against several species of root-knot nematode, including Meloidogyne javanica. A strain of M. javanica that can reproduce on tomato with Mi-1 was obtained from a culture of an avirulent strain after greenhouse selection. DNA blots and amplified fragment length polymorphism (AFLP) analysis indicated that the two nematode strains are closely related. Expression patterns visualized as cDNA AFLPs were nearly identical except for a cDNA fragment, Cg-1, that was present in the avirulent strain but not in the virulent strain. DNA blots showed that Cg-1 corresponds to a member of a small gene family with one or more copies missing in the virulent strain compared with the avirulent strain. Except for the presence of a histone stem loop near the 3' end of the transcript, Cg-1 shows no similarity to other sequences in GenBank. The longest open reading frame is 32 amino acids and initiates at the fourth AUG in the predicted transcript. When nematode juveniles of the Mi-1-avirulent strain were soaked in dsRNA corresponding to part of the predicted Cg-1 transcript, they produced progeny that were virulent on tomato carrying the Mi-1 gene, strongly suggesting that Cg-1 is required in the nematode for Mi-1-mediated resistance.

Meiotic parthenogenesis in a root-knot nematode results in rapid genomic homozygosity.

Many isolates of the plant-parasitic nematode Meloidogyne hapla reproduce by facultative meiotic parthenogenesis. Sexual crosses can occur, but, in the absence of males, the diploid state appears to be restored by reuniting sister chromosomes of a single meiosis. We have crossed inbred strains of M. hapla that differ in DNA markers and produced hybrids and F(2) lines. Here we show that heterozygous M. hapla females, upon parthenogenetic reproduction, produce progeny that segregate 1:1 for the presence or absence of dominant DNA markers, as would be expected if sister chromosomes are rejoined, rather than the 3:1 ratio typical of a Mendelian cross. Codominant markers also segregate 1:1 and heterozygotes are present at low frequency (<3%). Segregation patterns and recombinant analysis indicate that a homozygous condition is prevalent for markers flanking recombination events, suggesting that recombination occurs preferentially as four-strand exchanges at similar locations between both pairs of non-sister chromatids. With this mechanism, meiotic parthenogenesis would be expected to result in rapid genomic homozygosity. This type of high negative crossover interference coupled with positive chromatid interference has not been observed in fungal or other animal systems in which it is possible to examine the sister products of a single meiosis and may indicate that meiotic recombination in this nematode has novel features.

Retinal oxalosis. A clinicopathologic report.

A 55-year-old woman with chronic renal failure treated with hemodialysis had severe bilateral visual loss develop due to retinal ischemia. Ophthalmoscopy showed crystals in the distribution of the retinal arteries, but not veins, and this led to a diagnosis of systemic oxalosis. Factors contributing to systemic oxalosis in addition to renal failure were ascorbic acid dietary supplementation, pyridoxine deficiency, and ileal resection. Histopathologic findings showed ocular calcium oxalate deposition limited nearly entirely to the walls of retinal blood vessels.