PubMed · 9668127
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C B Taylor. 1998. Vampire plants?. https://doi.org/10.1105/tpc.10.7.1071
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Resistance to thyroid hormone (RTH) is usually inherited in a dominant fashion, and is characterized by elevated serum thyroid hormone levels and failure to suppress pituitary secretion of thyroid-stimulating hormone, with variable refractoriness to hormone action in peripheral tissues. Two major forms of the disorder are recognized: asymptomatic individuals with generalized resistance (GRTH) and patients with thyrotoxic features suggesting predominant pituitary resistance (PRTH). In over 100 families with GRTH or PRTH, we have identified heterozygous mutations in the thyroid hormone receptor beta isoform (TRbeta), which localize to three regions (amino acids 234-282, 310-353 and 429-461) of the hormone-binding domain of the receptor. The mutant receptors are transcriptionally impaired, due either to reduced ligand binding or to attenuated interaction with co-activators, and inhibit wild-type TR action in a dominant-negative manner. In the TRbeta crystal structure, most RTH mutations cluster around the hormone-binding pocket, with receptor regions that mediate functions (DNA binding, dimerization, co-repressor recruitment) required for dominant-negative activity being devoid of natural mutations. The pathogenesis of variable tissue resistance is not fully understood, but may be related to the differing tissue distributions of TRalpha and TRbeta, and to variable dominant-negative activity of mutant receptors on different target genes. The nuclear receptor peroxisome-proliferator-activated receptor gamma (PPARgamma) regulates adipogenesis and mediates the action of thiazolidinediones - novel anti-diabetic agents which enhance tissue insulin sensitivity. The PPARgamma gene was screened in 85 subjects with severe insulin resistance, and two different heterozygous receptor mutations (P467L and V290M) were identified in three affected individuals. The PPARgamma mutants are markedly transcriptionally impaired due to altered ligand binding and co-activator recruitment. Analogous to RTH, they inhibit the function of wild-type PPARgamma when co-expressed, and such dominant-negative inhibition is linked to their ability to silence basal gene transcription via aberrant interaction with co-repressors. In addition to insulin resistance, all three affected subjects developed Type II diabetes mellitus and hypertension at an unusually early age. Our findings provide compelling evidence that PPARgamma is important in the control of insulin sensitivity, glucose homoeostasis and blood pressure in humans. Future studies aim to elucidate the mechanism by which this receptor regulates insulin action and vascular tone.
The resistance of rice to its bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo) has both qualitative and quantitative components that were investigated using three near-isogenic line sets for four resistance (R) genes (Xa4, xa5, xa13, and Xa21) and 12 Xoo races. Our results indicate that these two resistance components of rice plants were associated with the properties of the R genes. The qualitative component of the R genes was reflected by their large effects against corresponding avirulent Xoo races. The quantitative component of the R genes was their residual effects against corresponding virulent races and their epistatic effects, which together could lead to high-level resistance in a race-specific manner. Our results revealed important differences between the different types of R genes. Two R genes, Xa4 and Xa21, showed complete dominance against the avirulent Xoo races and had large residual effects against virulent ones. They acted independently and cumulatively, suggesting they are involved in different pathways of the rice defensive system. The third R gene, xa5, showed partial dominance or additivity to the avirulent Xoo races and had relatively small but significant residual effects against the virulent races. In contrast, xa13 was completely recessive, had no residual effects against the virulent races, and showed more pronounced race specificity. There was a strong interaction leading to increased resistance between xa13 and xa5 and between either of them and Xa4 or Xa21, suggesting their regulatory roles in the rice defensive pathway(s). Our results indicated that high-level and durable resistance to Xoo should be more efficiently achieved by pyramiding different types of R genes.
Rsv1, a single dominant gene in soybean PI 96983, confers resistance to most strains of Soybean mosaic virus (SMV), including strain G2. The phenotypic response includes the lack of symptoms and virus recovery from mechanically inoculated leaves. To study the resistance mechanism, SMV-N (an isolate of strain G2) was introduced into PI 96983 by grafting. Hypersensitive response (HR)-like lesions occurred on the stems, petioles, and leaf veins, and virus was recovered from these lesions. The response demonstrated the cytological and histological characteristics of HR as well as elevated transcription of a soybean salicylic acid-inducible, pathogenesis-related (PR-1) protein gene. Mechanical inoculation of PI 96983 primary leaves with a high level of SMV-N virions caused no symptoms or up regulation of the PR-1 protein gene transcript. Furthermore, inoculation with infectious viral RNA did not alter the resistance phenotype. The data suggest that interaction of SMV-N with Rsv1 has the potential to induce an HR-like defense reaction. Rsv1-mediated resistance in the inoculated leaf, however, is HR-independent and operates after virion disassembly.