An atypical NLR gene confers bacterial wilt susceptibility in Arabidopsis

Quantitative disease resistance (QDR) remains the most prevalent form of plant resistance in crop fields and wild habitats. Genome-wide association studies (GWAS) have proved to be successful in deciphering the quantitative genetic basis of complex traits such as QDR. To unravel the genetics of QDR to the devastating worldwide bacterial pathogen Ralstonia solanacearum, we performed a GWAS by challenging a highly polymorphic local mapping population of Arabidopsis thaliana with four R. solanacearum type III effector (T3E) mutants, identified as key pathogenicity determinants after a first screen on an A. thaliana core collection of 25 accessions. Although most quantitative trait loci (QTLs) were highly specific to the identity of the T3E mutant (ripAC, ripAG, ripAQ, and ripU), we finely mapped a common QTL located on a cluster of nucleotide-binding domain and leucine-rich repeat (NLR) genes that exhibited structural variation. We functionally validated one of these NLRs as a susceptibility factor in response to R. solanacearum, named it Bacterial Wilt Susceptibility 1 (BWS1), and cloned two alleles that conferred contrasting levels of QDR. Further characterization indicated that expression of BWS1 leads to suppression of immunity triggered by different R. solanacearum effectors. In addition, we showed a direct interaction between BWS1 and RipAC T3E, and BWS1 and SUPPRESSOR OF G2 ALLELE OF skp1 (SGT1b), the latter interaction being suppressed by RipAC. Together, our results highlight a putative role for BWS1 as a quantitative susceptibility factor directly targeted by the T3E RipAC, mediating negative regulation of the SGT1-dependent immune response.

Genome-wide association studies in plant pathosystems: success or failure?

Harnessing natural genetic variation is an established alternative to artificial genetic variation for investigating the molecular dialog between partners in plant pathosystems. Herein, we review the successes of genome-wide association studies (GWAS) in both plants and pathogens. While GWAS in plants confirmed that the genetic architecture of disease resistance is polygenic, dynamic during the infection kinetics, and dependent on the environment, GWAS shortened the time of identification of quantitative trait loci (QTLs) and revealed both complex epistatic networks and a genetic architecture dependent upon the geographical scale. A similar picture emerges from the few GWAS in pathogens. In addition, the ever-increasing number of functionally validated QTLs has revealed new molecular plant defense mechanisms and pathogenicity determinants. Finally, we propose recommendations to better decode the disease triangle.

Study of natural diversity in response to a key pathogenicity regulator of Ralstonia solanacearum reveals new susceptibility genes in Arabidopsis thaliana

Ralstonia solanacearum gram-negative phytopathogenic bacterium exerts its virulence through a type III secretion system (T3SS) that translocates type III effectors (T3Es) directly into the host cells. T3E secretion is finely controlled at the posttranslational level by helper proteins, T3SS control proteins, and type III chaperones. The HpaP protein, one of the type III secretion substrate specificity switch (T3S4) proteins, was previously highlighted as a virulence factor on Arabidopsis thaliana Col-0 accession. In this study, we set up a genome-wide association analysis to explore the natural diversity of response to the hpaP mutant of two A. thaliana mapping populations: a worldwide collection and a local population. Quantitative genetic variation revealed different genetic architectures in both mapping populations, with a global delayed response to the hpaP mutant compared to the GMI1000 wild-type strain. We have identified several quantitative trait loci (QTLs) associated with the hpaP mutant inoculation. The genes underlying these QTLs are involved in different and specific biological processes, some of which were demonstrated important for R. solanacearum virulence. We focused our study on four candidate genes, RKL1, IRE3, RACK1B, and PEX3, identified using the worldwide collection, and validated three of them as susceptibility factors. Our findings demonstrate that the study of the natural diversity of plant response to a R. solanacearum mutant in a key regulator of virulence is an original and powerful strategy to identify genes directly or indirectly targeted by the pathogen.

Catecholamine-sensitive adenylate cyclase in frontal cortex of primate brain

Adenylate cyclase activity (AC) of homogenates of monkey frontal cortex was stimulated by catecholamines (dopamine, norepinephrine and isoproterenol), apomorphine, clonidine, NaF and GPP(NH)P. The increment in activity due to dopamine was enhanced in the presence of GPP(NH)P. The AC was also stimulated by 0.2-0.4 mM Ca2+ in the presence of 0.2 mM EGTA; at 0.8 mM Ca2+ had little or no influence on basal or NaF-stimulated activity. However, as Ca2+ concentration was increased from 0.2 to 0.8 mM stimulation by dopamine or GPP(NH)P was progressively inhibited. These results suggest a possible function of Ca2+ in modulating neurotransmitter stimulation of AC in the monkey frontal cortex. The AC exhibited higher sensitivity to dopamine than to norepinephrine or isoproterenol; however dopamine and norepinephrine caused the same maximum stimulation of the enzyme, a stimulation womewhat greater than that produced by isoproterenol. An additivity in stimulating AC was observed for dopamine and isoproterenol but not for dopamine and norepinephrine. Norepinephrine- or dopamine-stimulated AC was effectively blocked by fluphenazine and other dopamine-receptor blocking agents (relative potency for blockade; fluphenazine, haloperidol greater than clozapine, thioridazine greater than pimozide) but not by propranolol, a beta-receptor blocker. In contrast, isoproterenol-stimulated AC was antagonized by propranolol or alprenolol but not by fluphenazine. On the basis of these results, at least two distinct receptors appear to be associated with AC of monkey frontal cortex: (1) a beta-receptor stimulated by isoproterenol and (2) a new type of dopamine or dopamine-norepinephrine receptor, stimulated by either dopamine or norepinephrine. This latter system differs from more typical dopamine receptors found in caudate, retina and limbic cortex in that (a) it is not stimulated by 1-(3,4-dihydroxybenzyl)-4-(2-pyrimidinyl) piperazine (S584); (b) it is stimulated significantly by the putative alpha-receptor agonist, clonidine; (c) it is more sensitive to blockade by clozapine than primate retina or caudate; also the sensitivity to haloperidol is greater than has been reported for non-primate caudate; (d) it is very sensitive to stimulation by norepinephrine and to blockade by phentolamine (an alpha-receptor blocker).

The catecholamine pontine cellular groups locus coeruleus, A4, subcoeruleus in the primate Cebus apella

The distrubution of CA neurons of areas A6 and A4 was delineated in Cebus apella monkey using the fluorescent histochemical technique of Falck and Hillarp. Cytospectroscopy was utilized for CA differentiation. The noradrenergic cellular regions A6, A4, and subcoeruleus have extensively increased in size in the Cebus as compared to the rat and appear to be separate nuclear regions. Area A4 is made up of two cellular subgroups: a more abundant lateral magnocellular area with cells as large as 45 mum and a smaller medial parvocellular group where the neurons are spindle-shaped and lie within 10-100 mum of the ependyma. The neuronal processes of A4 tend to be directed towards the flocculus and paraflocculus of the cerebellum. Some processes seem to enter the ependyma and others end subependymally. The functional significance of the pontine CA neurons is discussed.