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Wasternack, C. A plant's balance of growth and defense - revisited New Phytol 215, 1291-1294, (2017) DOI: 10.1111/nph.14720

This article is a Commentary on Major et al., 215: 1533–1547.

Wasternack, C. The Trojan horse coronatine: the COI1–JAZ2–MYC2,3,4–ANAC019,055,072 module in stomata dynamics upon bacterial infection. New Phytol 213, 972-975, (2017) DOI: 10.1111/nph.14417

Coronatine (COR) is a phytotoxin produced by a plasmid-encoded operon of genes in several strains of Pseudomonas syringae (Bender et al., 1999). It is a mimic of the defense-associated phytohormone jasmonic acid isoleucine and delivered by the phytopathogenic bacterium to gain access to host plants through stomatal entry and to repress a specific sector of plant immunity. In this issue of New Phytologist (pp. 1378–1392) Gimenez-Ibanez et al. reveal exciting insights into the transcriptional regulation of COR/jasmonic acid isoleucine-governed transcriptional networks modulating stomatal aperture during bacterial invasion.

Farmer, E. E.; Gasperini, D.; Acosta, I. F. The squeeze cell hypothesis for the activation of jasmonate synthesis in response to wounding New Phytol 204, 282-288, (2014) DOI: 10.1111/nph.12897

Jasmonates are lipid mediators that control defence gene expression in response to wounding and other environmental stresses. These small molecules can accumulate at distances up to several cm from sites of damage and this is likely to involve cell‐to‐cell jasmonate transport. Also, and independently of jasmonate synthesis, transport and perception, different long‐distance wound signals that stimulate distal jasmonate synthesis are propagated at apparent speeds of several cm min–1 to tissues distal to wounds in a mechanism that involves clade 3 GLUTAMATE RECEPTOR‐LIKE (GLR) genes. A search for jasmonate synthesis enzymes that might decode these signals revealed LOX6, a lipoxygenase that is necessary for much of the rapid accumulation of jasmonic acid at sites distal to wounds. Intriguingly, the LOX6 promoter is expressed in a distinct niche of cells that are adjacent to mature xylem vessels, a location that would make these contact cells sensitive to the release of xylem water column tension upon wounding. We propose a model in which rapid axial changes in xylem hydrostatic pressure caused by wounding travel through the vasculature and lead to slower, radially dispersed pressure changes that act in a clade 3 GLR‐dependent mechanism to promote distal jasmonate synthesis.

Elleuch, A.; Chaâbene, Z.; Grubb, D.C.; Drira, N.; Mejdoub, H.; Khemakhem, B. Morphological and biochemical behavior of fenugreek (Trigonella foenum-graecum) under copper stress Ecotoxicol Environ Saf 98, 46-53, (2013) DOI: 10.1016/j.ecoenv.2013.09.028

The effects of copper on germination and growth of fenugreek (Trigonella foenum-graecum  ) was investigated separately using different concentrations of CuSO4. The germination percentage and radical length had different responses to cupric ions: the root growth increased with increasing copper concentration up to 1 mM Cu+2Cu2+ and was inhibited thereafter. In contrast, the germination percentage was largely unaffected by concentrations of copper below 10 mM.The reduction in root growth may have been due to inhibition of hydrolytic enzymes such as amylase. Indeed, the average total amylolytic activity decreased from the first day of treatment with [Cu+2Cu2+] greater than 1 mM. Furthermore, copper affected various plant growth parameters. Copper accumulation was markedly higher in roots as compared to shoots. While both showed a gradual decrease in growth, this was more pronounced in roots than in leaves and in stems. Excess copper induced an increase in the rate of hydrogen peroxide (H2O2) production and lipid peroxidation in all plant parts, indicating oxidative stress. This redox stress affected leaf chlorophyll and carotenoid content which decreased in response to augmented Cu levels. Additionally, the activities of proteins involved in reactive oxygen species (ROS) detoxification were affected. Cu stress elevated the ascorbate peroxidase (APX) activity more than two times at 10 mM CuSO4. In contrast, superoxide dismutase (SOD) and catalase (CAT) levels showed only minor variations, only at 1 mM Cu+2Cu2+. Likewise, total phenol and flavonoid contents were strongly induced by low concentrations of copper, consistent with the role of these potent antioxidants in scavenging ROS such as H2O2, but returned to control levels or below at high [Cu+2Cu2+]. Taken together, these results indicate a fundamental shift in the plant response to copper toxicity at low versus high concentrations.

Stumpe, M.; Göbel, C.; Faltin, B.; Beike, A. K.; Hause, B.; Himmelsbach, K.; Bode, J.; Kramell, R.; Wasternack, C.; Frank, W.; Reski, R.; Feussner, I. The moss Physcomitrella patens contains cyclopentenones but no jasmonates: mutations in allene oxide cyclase lead to reduced fertility and altered sporophyte morphology New Phytol 188 (3), 740-749, (2010) DOI: 10.1111/j.1469-8137.2010.03406.x

Two cDNAs encoding allene oxide cyclases (PpAOC1, PpAOC2), key enzymes in the formation of jasmonic acid (JA) and its precursor (9S,13S)‐12‐oxo‐phytodienoic acid (cis‐(+)‐OPDA), were isolated from the moss Physcomitrella patens.Recombinant PpAOC1 and PpAOC2 show substrate specificity against the allene oxide derived from 13‐hydroperoxy linolenic acid (13‐HPOTE); PpAOC2 also shows substrate specificity against the allene oxide derived from 12‐hydroperoxy arachidonic acid (12‐HPETE).In protonema and gametophores the occurrence of cis‐(+)‐OPDA, but neither JA nor the isoleucine conjugate of JA nor that of cis‐(+)‐OPDA was detected.Targeted knockout mutants for PpAOC1 and for PpAOC2 were generated, while double mutants could not be obtained. The ΔPpAOC1 and ΔPpAOC2 mutants showed reduced fertility, aberrant sporophyte morphology and interrupted sporogenesis.

Clarke, S.M.; Cristescu, S.M.; Miersch, O.; Harren, F.J.M.; Wasternack, C.; Mur, L.A.J. Jasmonates act with salicylic acid to confer basal thermotolerance in <i>Arabidopsis thaliana</i> New Phytol 182, 175-187, (2009) DOI: 10.1111/j.1469-8137.2008.02735.x

The cpr5-1 Arabidopsis thaliana mutant exhibits constitutive activation of salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) signalling pathways and displays enhanced tolerance of heat stress (HS). cpr5-1 crossed with jar1-1 (a JA-amino acid synthetase) was compromised in basal thermotolerance, as were the mutants opr3 (mutated in OPDA reductase3) and coi1-1 (affected in an E3 ubiquitin ligase F-box; a key JA-signalling component). In addition, heating wild-type Arabidopsis led to the accumulation of a range of jasmonates: JA, 12-oxophytodienoic acid (OPDA) and a JA-isoleucine (JA-Ile) conjugate. Exogenous application of methyl jasmonate protected wild-type Arabidopsis from HS. Ethylene was rapidly produced during HS, with levels being modulated by both JA and SA. By contrast, the ethylene mutant ein2-1 conferred greater thermotolerance. These data suggest that JA acts with SA, conferring basal thermotolerance while ET may act to promote cell death.

Abel, S.; Savchenko, T.; Levy, M. Genome-wide comparative analysis of the <em>IQD</em> gene families in <em>Arabidopsis thaliana</em> and Oryza sativa BMC Evolutionary Biology 5, 72 (1-25), (2005)

We identified and analyzed 33 and 29 IQD1-like genes in Arabidopsis thaliana and Oryza sativa, respectively. The encoded IQD proteins contain a plant-specific domain of 67 conserved amino acid residues, referred to as the IQ67 domain, which is characterized by a unique and repetitive arrangement of three different calmodulin recruitment motifs, known as the IQ, 1-5-10, and 1-8-14 motifs. We demonstrated calmodulin binding for IQD20, the smallest IQD protein in Arabidopsis, which consists of a C-terminal IQ67 domain and a short N-terminal extension. A striking feature of IQD proteins is the high isoelectric point (~10.3) and frequency of serine residues (~11%). We compared the Arabidopsis and rice IQD gene families in terms of gene structure, chromosome location, predicted protein properties and motifs, phylogenetic relationships, and evolutionary history. The existence of an IQD-like gene in bryophytes suggests that IQD proteins are an ancient family of calmodulin-binding proteins and arose during the early evolution of land plants. Comparative phylogenetic analyses indicate that the major IQD gene lineages originated before the monocot-eudicot divergence. The extant IQD loci in Arabidopsis primarily resulted from segmental duplication and reflect preferential retention of paralogous genes, which is characteristic for proteins with regulatory functions. Interaction of IQD1 and IQD20 with calmodulin and the presence of predicted calmodulin binding sites in all IQD family members suggest that IQD proteins are a new class of calmodulin targets. The basic isoelectric point of IQD proteins and their frequently predicted nuclear localization suggest that IQD proteins link calcium signaling pathways to the regulation of gene expression. Our comparative genomics analysis of IQD genes and encoded proteins in two model plant species provides the first step towards the functional dissection of this emerging family of putative calmodulin targets.
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