http://www.noahwhiteman.org/uploads/7/1/3/1/7131048/trplsc952.pdf

Evolution of jasmonate and salicylate signal crosstalk.

Abiotic stresses (such as metals/metalloids, salinity, ozone, UV-B radiation, extreme temperatures and drought) are among the most challenging threats to agricultural system and economic yield of crop plants. These stresses (in isolation and/or combination) induce numerous adverse effects in plants, impair biochemical/physiological and molecular processes, and eventually cause severe reductions in plant growth, development and overall productivity. Phytohormones have been recognized as a strong tool for sustainably alleviating adverse effects of abiotic stresses in crop plants. The significance of salicylic acid (SA) has been increasingly recognized in improved plant abiotic stress-tolerance via SA-mediated control of major plant-metabolic processes. However, the basic biochemical/physiological and molecular mechanisms that potentially underpin SA-induced plant-tolerance to major abiotic stresses remain least discussed. Based on recent reports, this paper: (a) overviews historical background and biosynthesis of SA under both optimal and stressful environments in plants; (b) critically appraises the role of SA in plants exposed to major abiotic stresses; (c) cross-talks potential mechanisms those are expected to govern SA-induced plant abiotic stress-tolerance; and finally (d) briefly highlights major aspects so far unexplored in the current context.

/pdf/salicylic-acid-induced-abiotic-stress-tolerance-and-530gw94vkb.pdf

Salicylic acid-induced abiotic stress tolerance and underlying mechanisms in plants

Epigenetic information, which may affect an organism’s phenotype, can be stored and stably inherited in the form of cytosine DNA methylation. Changes in DNA methylation can produce meiotically stable epialleles that affect transcription and morphology, but the rates of spontaneous gain or loss of DNA methylation are unknown. We examined spontaneously occurring variation in DNA methylation in Arabidopsis thaliana plants propagated by single-seed descent for 30 generations. We identified 114,287 CG single methylation polymorphisms and 2485 CG differentially methylated regions (DMRs), both of which show patterns of divergence compared with the ancestral state. Thus, transgenerational epigenetic variation in DNA methylation may generate new allelic states that alter transcription, providing a mechanism for phenotypic diversity in the absence of genetic mutation.

/pdf/transgenerational-epigenetic-instability-is-a-source-of-33dqd89ojy.pdf

Transgenerational epigenetic instability is a source of novel methylation variants

Viral silencing suppressors: Tools forged to fine-tune host-pathogen coexistence

The health of the honeybee and, indirectly, global crop production are threatened by several biotic and abiotic factors, which play a poorly defined role in the induction of widespread colony losses. Recent descriptive studies suggest that colony losses are often related to the interaction between pathogens and other stress factors, including parasites. Through an integrated analysis of the population and molecular changes associated with the collapse of honeybee colonies infested by the parasitic mite Varroa destructor, we show that this parasite can de-stabilise the within-host dynamics of Deformed wing virus (DWV), transforming a cryptic and vertically transmitted virus into a rapidly replicating killer, which attains lethal levels late in the season. The de-stabilisation of DWV infection is associated with an immunosuppression syndrome, characterized by a strong down-regulation of the transcription factor NF-κB. The centrality of NF-κB in host responses to a range of environmental challenges suggests that this transcription factor can act as a common currency underlying colony collapse that may be triggered by different causes. Our results offer an integrated account for the multifactorial origin of honeybee losses and a new framework for assessing, and possibly mitigating, the impact of environmental challenges on honeybee health.

/pdf/synergistic-parasite-pathogen-interactions-mediated-by-host-1dy0dk7pf8.pdf

Synergistic parasite-pathogen interactions mediated by host immunity can drive the collapse of honeybee colonies.

The Cucumber mosaic virus (CMV) 2b counter-defense protein disrupts plant antiviral mechanisms mediated by RNA silencing and salicylic acid (SA). We used microarrays to investigate defensive gene expression in 2b-transgenic Arabidopsis thaliana plants. Surprisingly, 2b inhibited expression of few SA-regulated genes and, in some instances, enhanced the effect of SA on certain genes. Strikingly, the 2b protein inhibited changes in the expression of 90% of genes regulated by jasmonic acid (JA). Consistent with this, infection of plants with CMV, but not the 2b gene-deletion mutant CMVΔ2b, strongly inhibited JA-inducible gene expression. JA levels were unaffected by infection with either CMV or CMVΔ2b. Although the CMV–Arabidopsis interaction is a compatible one, SA accumulation, usually considered to be an indicator of plant resistance, was increased in CMV-infected plants but not in CMVΔ2b-infected plants. Thus, the 2b protein inhibits JA signaling at a step downstream of JA biosynthesis but it primes induc...

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-23-7-0835

Disruption of two defensive signaling pathways by a viral RNA silencing suppressor.

Background 
RNA-dependent RNA polymerases (RDRs) function in anti-viral silencing in Arabidopsis thaliana and other plants Salicylic acid (SA), an important defensive signal, increases RDR1 gene expression, suggesting that RDR1 contributes to SA-induced virus resistance In Nicotiana attenuata RDR1 also regulates plant-insect interactions and is induced by another important signal, jasmonic acid (JA) Despite its importance in defense RDR1 regulation has not been investigated in detail


Methodology/Principal Findings 
In Arabidopsis, SA-induced RDR1 expression was dependent on ‘NON-EXPRESSER OF PATHOGENESIS-RELATED GENES 1’, indicating regulation involves the same mechanism controlling many other SA- defense-related genes, including pathogenesis-related 1 (PR1) Isochorismate synthase 1 (ICS1) is required for SA biosynthesis In defensive signal transduction RDR1 lies downstream of ICS1 However, supplying exogenous SA to ics1-mutant plants did not induce RDR1 or PR1 expression to the same extent as seen in wild type plants Analysing ICS1 gene expression using transgenic plants expressing ICS1 promoter:reporter gene (β-glucuronidase) constructs and by measuring steady-state ICS1 transcript levels showed that SA positively regulates ICS1 In contrast, ICS2, which is expressed at lower levels than ICS1, is unaffected by SA The wound-response hormone JA affects expression of Arabidopsis RDR1 but jasmonate-induced expression is independent of CORONATINE-INSENSITIVE 1, which conditions expression of many other JA-responsive genes Transiently increased RDR1 expression following tobacco mosaic virus inoculation was due to wounding and was not a direct effect of infection RDR1 gene expression was induced by ethylene and by abscisic acid (an important regulator of drought resistance) However, rdr1-mutant plants showed normal responses to drought


Conclusions/Significance 
RDR1 is regulated by a much broader range of phytohormones than previously thought, indicating that it plays roles beyond those already suggested in virus resistance and plant-insect interactions SA positively regulates ICS1

/pdf/regulation-of-rna-dependent-rna-polymerase-1-and-3hn2988pyl.pdf

Regulation of RNA-dependent RNA polymerase 1 and isochorismate synthase gene expression in Arabidopsis.

The important plant hormone salicylic acid (SA; 2-hydroxybenzoic acid) regulates several key plant responses including, most notably, defence against pathogens. A key enzyme for SA biosynthesis is isochorismate synthase (ICS), which converts chorismate into isochorismate, and for which there are two genes in Arabidopsis thaliana . One ( AtICS1 ) has been shown to be required for increased SA biosynthesis in response to pathogens and its expression can be stimulated throughout the leaf by virus infection and exogenous SA. The other ( AtICS2 ) appears to be expressed constitutively, predominantly in the plant vasculature. Here, we characterise the enzymatic activity of both isozymes expressed as hexahistidine fusion proteins in Escherichia coli. We show for the first time that recombinant AtICS2 is enzymatically active. Both isozymes are Mg 2+ -dependent with similar temperature optima (ca. 33°C) and similar K m values for chorismate of 34.3 ± 3.7 and 28.8 ± 6.9 µM for ICS1 and ICS2, respectively, but reaction rates were greater for ICS1 than for ICS2, with respective values for V max of 63.5 ± 2.4 and 28.3 ± 2.0 nM s −1 and for k cat of 38.1 ± 1.5 and 17.0 ± 1.2 min −1 . However, neither enzyme displayed isochorismate pyruvate lyase (IPL) activity, which would enable these proteins to act as bifunctional SA synthases, i.e. to convert chorismate into SA. These results show that although Arabidopsis has two functional ICS enzymes, it must possess one or more IPL enzymes to complete biosynthesis of SA starting from chorismate.

/pdf/the-biochemical-properties-of-the-two-arabidopsis-thaliana-12luu8m929.pdf

Keith Macaulay

Papers

Disruption of two defensive signaling pathways by a viral RNA silencing suppressor.

Regulation of RNA-dependent RNA polymerase 1 and isochorismate synthase gene expression in Arabidopsis.

The biochemical properties of the two Arabidopsis thaliana isochorismate synthases.