Table of Contents

Volume 5, Issue 6

  June 2010

  Pages 623 - 772

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 Issue Catalogue (MARC XML)

SHORT COMMUNICATION

Open Access Article

Conserved thioredoxin fold is present in Pisum sativum L. sieve element occlusion-1 protein

Narendra Tuteja, Pavan Umate and Renu Tuteja
Pages 623 - 628
http://dx.doi.org/10.4161/psb.5.6.11422
Abstract | PDF

Homology-based three-dimensional model for Pisum sativum sieve element occlusion 1 (Ps.SEO1) (forisomes) protein was constructed. A stretch of amino acids (residues 320 to 456) which is well conserved in all known members of forisomes proteins was used to model the 3D structure of Ps.SEO1. The structural prediction was done using Protein Homology/analogY Recognition Engine (PHYRE) web server. Based on studies of local sequence alignment, the thioredoxin-fold containing protein [Structural Classification of Proteins (SCOP) code d1o73a_], a member of the glutathione peroxidase family was selected as a template for modeling the spatial structure of Ps.SEO1. Selection was based on comparison of primary sequence, higher match quality and alignment accuracy. Motif 1 (EVF) is conserved in Ps.SEO1, Vicia faba (Vf.For1) and Medicago truncatula (Mt.SEO3); motif 2 (KKED) is well conserved across all forisomes proteins and motif 3 (IGYIGNP) is conserved in Ps.SEO1 and Vf.For1.




MINI-REVIEW

Open Access Article

Dual-function transcription factors and their entourage: Unique and unifying themes governing two pathogenesis-related genes

Patrick Boyle and Charles Després
Pages 629 - 634
http://dx.doi.org/10.4161/psb.5.6.11570
Abstract | PDF

Much of what we, as plant molecular biologists studying gene regulation, know comes from paradigms characterized or developed in mammalian systems. Although plants, animals, and fungi have been diverging for a very long time, a great deal of the machineries and components discovered in yeast and mammals seem to have been maintained in plants. Nevertheless, despite this apparent conservation, evolutionary pressures on the mechanisms of gene regulation are likely to be different between these kingdoms, given their different environmental constraints. As such, it is imperative for plant molecular biologists to develop their own paradigms, even on seemingly conserved systems. It is with this intent that we compare and contrast the regulation of two pathogenesis-related genes, the arabidopsis PR-1 and potato PR-10a genes. The transcription factors regulating these genes present prime paradigms for the study of plant signal- and context-dependent dual-function transcription factors.




REVIEWS

Open Access Article

Humic substances biological activity at the plant-soil interface: From environmental aspects to molecular factors

Sara Trevisan, Ornella Francioso, Silvia Quaggiotti and Serenella Nardi
Pages 635 - 643
http://dx.doi.org/10.4161/psb.5.6.11211
Abstract | PDF

Humic substances (HS) represent the organic material mainly widespread in nature. HS have positive effects on plant physiology by improving soil structure and fertility and by influencing nutrient uptake and root architecture. The biochemical and molecular mechanisms underlying these events are only partially known. HS have been shown to contain auxin and an “auxin-like” activity of humic substances has been proposed, but support to this hypothesis is fragmentary. In this review article, we are giving an overview of available data concerning molecular structures and biological activities of humic substances, with special emphasis on their hormone-like activities.

Open Access Article

The carnivorous syndrome in Nepenthes pitcher plants: Current state of knowledge and potential future directions

Jonathan A. Moran and Charles M. Clarke
Pages 644 - 648
http://dx.doi.org/10.4161/psb.5.6.11238
Abstract | PDF

Nepenthes is the largest genus of pitcher plants, with its centre of diversity in SE Asia. The plants grow in substrates that are deficient in N and offset this deficiency by trapping animal prey, primarily arthropods. Recent research has provided new insights into the function of the pitchers, particularly with regard to prey tapping and retention. Species examined to date use combinations of wettable peristomes, wax layers and viscoelastic fluid to trap and retain prey. In many respects, this has redefined our understanding of the functioning of Nepenthes pitchers. In addition, recent research has shown that several Nepenthes species target specific groups of prey animals, or are even evolving away from a strictly carnivorous mode of operation. Future research into nutrient sequestration strategies and mechanisms of prey attraction would no doubt further enhance our knowledge of the ecology of this remarkable genus.

Open Access Article

Plant responses to drought and rewatering

Zhenzhu Xu, Guangsheng Zhou and Hideyuki Shimizu
Pages 649 - 654
http://dx.doi.org/10.4161/psb.5.6.11398
Abstract | PDF

Plants would be more vulnerable to water stress and thereafter rewatering or a cycled water environmental change, which occur more frequently under climatic change conditions in terms of the prediction scenarios. Effects of water stress on plants alone have been well-documented in many reports. However, the combined responses to drought and rewatering and its mechanism are relatively scant. As we known, plant growth, photosynthesis and stomatal aperture may be limited under water deficit, which would be regulated by physical and chemical signals. Under severe drought, while peroxidation may be provoked, the relevant antioxidant metabolism would be involved to annihilate the damage of reactive oxygen species. As rewatering, the recoveries of plant growth and photosynthesis would appear immediately through growing new plant parts, re-opening the stomata, and decreasing peroxidation; the recovery extents (reversely: pre-drought limitation) due to rewatering strongly depend on pre-drought intensity, duration and species. Understanding how plants response to episodic drought and watering pulse and the underlying mechanism is remarkably helpful to implement vegetation management practices in climatic changing.

Open Access Article

Physiological regulation and functional significance of shade avoidance responses to neighbors

Diederik H. Keuskamp, Rashmi Sasidharan and Ronald Pierik
Pages 655 - 662
http://dx.doi.org/10.4161/psb.5.6.11401
Abstract | PDF

Plants growing in dense vegetations compete with their neighbors for resources such as water, nutrients and light. The competition for light has been particularly well studied, both for its fitness consequences as well as the adaptive behaviors that plants display to win the battle for light interception. Aboveground, plants detect their competitors through photosensory cues, notably the red:far-red light ratio (R:FR). The R:FR is a very reliable indicator of future competition as it decreases in a plant-specific manner though red light absorption for photosynthesis and is sensed with the phytochrome photoreceptors. In addition, also blue light depletion is perceived for neighbor detection. As a response to these light signals plants display a suite of phenotypic traits defined as the shade avoidance syndrome (SAS). The SAS helps to position the photosynthesizing leaves in the higher zones of a canopy where light conditions are more favorable. In this review we will discuss the physiological control mechanisms through which the photosensory signals are transduced into the adaptive phenotypic responses that make up the SAS. Using this mechanistic knowledge as a starting point, we will discuss how the SAS functions in the context of the complex multi-facetted environments that plants usually grow in.

Open Access Article

Regulatory networks of cadmium stress in plants

Giovanni DalCorso, Silvia Farinati and Antonella Furini
Pages 663 - 667
http://dx.doi.org/10.4161/psb.5.6.11425
Abstract | PDF

During their life, plants have to cope with a variety of abiotic stresses. Cadmium is highly toxic to plants, water soluble and therefore promptly adsorbed in tissues and its presence greatly influences the entire plant metabolism. In this review, we focus on the signal pathways responsible for the sensing and transduction of the “metal signal” inside the cell, ultimately driving the activation of transcription factors and consequent expression of genes that enable plants to counteract the heavy metal stress.




RESEARCH PAPER

Open Access Article

Production of Nitric Oxide in host-virus interaction: A case study with a compatible Begomovirus-Kenaf host-pathosystem

Tuhin Subhra Sarkar, Uddalak Majumdar, Anirban Roy, Debasis Maiti, Achintya Mohan Goswamy, Arindam Bhattacharjee, Sanjay Ghosh and Subrata Kumar Ghosh
Pages 668 - 676
http://dx.doi.org/10.4161/psb.5.6.11282
Abstract | PDF

Nitric oxide (NO) plays a key role in plant diseases resistance. Here we have first time demonstrated that begomovirus infection in susceptible H. cannabinus plants, results in elevated NO and reactive nitrogen species production during early infection stage not only in infected leaf but also in root and shoot. Production of NO was further confirmed by oxyhemoglobin assay. Furthermore, we used Phenyl alanine ammonia lyase as marker of pathogenesis related enzyme. In addition evidence for protein tyrosine nitration during the early stage of viral infection clearly showed the involvement of nitrosative stress.




ARTICLE ADDENDA

Open Access Article

The Arabidopsis BET bromodomain factor GTE4 regulates the mitotic cell cycle

Federica Della Rovere, Chiara A. Airoldi, Giuseppina Falasca, Alessandra Ghiani, Laura Fattorini, Sandra Citterio, Martin Kater and Maria Maddalena Altamura
Pages 677 - 680
http://dx.doi.org/10.4161/psb.5.6.11571
Abstract | PDF

Proteins containing bromodomains are capable of binding to acetylated histone tails and have a role in recognizing and deciphering acetylated chromatin. Plant BET proteins contain one bromodomain. Twelve BET-encoding genes have been identified in the Arabidopsis genome. Two of these genes have been functionally characterized, one shows a role in seed germination, the other is involved in the establishment of leaf shape. Recently, we characterized a third AtBET gene, named GTE4. We demonstrated that GTE4 is involved in the activation and maintenance of cell division in the meristems and by this controls cell numbers in differentiated organs. Moreover, the quiescent center (QC) identity is partially lost in the apex of the primary root of gte4 mutant, and there is a premature switch from mitosis to endocycling. Genes involved in the retinoblastoma (RB)-E2F pathway, which is important for coupling cell division and cell differentiation in plants and animals, were either up- or down-regulated in the gte4 mutant. In this report we also show that the defect in germination observed in gte4 mutant seeds is not rescued by the action of GA3. Further the root pole of the mutant embryo shows irregular cytokinesis in the procambial stem cells, and the QC of the lateral root shows a partial, but not transient, loss of QC identity. These additional results reinforce the importance of GTE4 in the control of cell proliferation.

Open Access Article

Activation of the plasma membrane H⁺-ATPase by acid stress: antibodies as a tool to follow the phosphorylation status of the penultimate activating Thr

Krzysztof Bobik, Marc Boutry and Geoffrey Duby
Pages 681 - 683
http://dx.doi.org/10.4161/psb.5.6.11572
Abstract | PDF

Tight regulation of the plasma membrane proton pump ATPase (H⁺-ATPase) is necessary for controlling the membrane potential that energizes secondary transporters. This regulation relies on the phosphorylation of the H⁺-ATPase penultimate residue, a theonine, and the subsequent binding of regulatory 14-3-3 proteins, which results in enzyme activation. Using phospho-specific antibodies directed against the phosphorylable Thr of either PMA2 (Plasma membrane H⁺-ATPase from N. plumbaginifolia) or PMA4, we showed that the kinetics and extent of phosphorylation differ between both isoforms according to the growth or environmental conditions like cold stress.¹ Here, we used phospho-specific antibodies to follow PMA2 Thr phosphorylation upon acidification of the cytosol by incubating N. tabacum BY2 cells with four different weak organic acids. Increased PMA2 phosphorylation was observed for three of them, thus highlighting the role of the H⁺-ATPase in cell pH homeostasis.

Open Access Article

New insights into miR398 functions in Arabidopsis

Nicolas Bouché
Pages 684 - 686
http://dx.doi.org/10.4161/psb.5.6.11573
Abstract | PDF

We recently identified a new target of microRNA398 (miR398), a conserved miRNA in plants. In Arabidopsis, miR398 targets the mRNAs of two copper/zinc superoxide dismutases (Cu/Zn SODs) by triggering their cleavage or repressing their translation. We analysed the transcriptomes of mutants impaired in miR398 production, revealing that the mRNAs encoding the chaperone (CCS1), essential for copper delivering to the Cu/Zn SODs of Arabidopsis and to generate the mature proteins, were undiscovered targets of miR398. It is likely that CCS1 was not identified by previous bioinformatic predictions because of the number of mismatches between the mRNA and its target. Since CCS1 has four mismatches and one GU wobble, it would have been excluded by the majority of prediction algorithms. miR398 directs the post-transcriptional regulation of CCS1 mRNAs by cleavage and ARGONAUTE10 (AGO10)-mediated translational repression. Indeed, CCS1 protein accumulate in zwille (ago10) mutants while both miR398 and CCS1 mRNAs levels remain identical to the Landsberg erecta WT plants. Moreover, since AGO10 is a negative regulator of AGO1, the CCS1 protein is more abundant in a double ago1-27 ago10-3 Col mutant compared to the single hypomorphic ago1-27 mutant, as previously shown for CSD2.

Open Access Article

Maize SUT1 functions in phloem loading

Thomas L. Slewinski, Anshu Garg, Gurmukh S. Johal and David M. Braun
Pages 687 - 690
http://dx.doi.org/10.4161/psb.5.6.11575
Abstract | PDF

The functions of dicot sucrose transporters (SUTs) in apoplastic phloem loading of sucrose are well established; however, whether SUTs similarly function in monocots was unresolved. To address this question, we recently provided genetic evidence that ZmSUT1 from maize (Zea mays) is required for efficient phloem loading. sut1-m1 mutant plants hyperaccumulate carbohydrates in leaves, are defective in loading sucrose into the phloem, and have altered biomass partitioning. Presumably due to the hyperaccumulation of soluble sugars in leaves, mutations in ZmSUT1 lead to downregulation of chlorophyll accumulation, photosynthesis and stomatal conductance. However, because we had identified only a single mutant allele, we were not able to exclude the possibility that the mutant phenotypes were instead caused by a closely linked mutation. Based on a novel aspect of the sut1 mutant phenotype, secretion of a concentrated sugar solution from leaf hydathodes, we identified an additional mutant allele, sut1-m4. This confirms that the mutation of SUT1 is responsible for the impairment in phloem loading. In addition, the sut1-m4 mutant does not accumulate transcripts, supporting the findings reported previously that the original mutant allele is also a null mutation. Collectively, these data demonstrate that ZmSUT1 functions to phloem load sucrose in maize leaves.

Open Access Article

Small yet effective: The Ethylene responsive element binding factor-associated Amphiphilic Repression (EAR) motif

Sateesh Kagale and Kevin Rozwadowski
Pages 691 - 694
http://dx.doi.org/10.4161/psb.5.6.11576
Abstract | PDF

The Ethylene-responsive element binding factor-associated Amphiphilic Repression (EAR) motif is a small yet distinct regulatory motif that is conserved in many plant transcriptional regulator (TR) proteins associated with diverse biological functions. We have previously established a list of high-confidence Arabidopsis EAR repressors, the EAR repressome, comprising 219 TRs belonging to 21 different TR families. This class of proteins and the sequence context of the EAR motif exhibited a high degree of conservation across evolutionarily diverse plant species. Our comprehensive genome-wide analysis enabled refining EAR motifs as comprising either LxLxL or DLNxxP. Comparing the representation of these sequence signatures in TRs to that of other repressor motifs we show that the EAR motif is the one most frequently represented, detected in 10 to 25% of the TRs from diverse plant species. The mechanisms involved in regulation of EAR motif function and the cellular fates of EAR repressors are currently not well understood. Our earlier analysis had implicated amino acid residues flanking the EAR motifs in regulation of their functionality. Here, we present additional evidence supporting possible regulation of EAR motif function by phosphorylation of integral or adjacent Ser and/or Thr residues. Additionally, we discuss potential novel roles of EAR motifs in plant-pathogen interaction and processes other than transcriptional repression.

Open Access Article

Identification of recognition sequence of ANAC078 protein by the cyclic amplification and selection of targets technique

Yukinori Yabuta, Teruyuki Morishita, Yusuke Kojima, Takanori Maruta, Ayako Nishizawa-Yokoi and Shigeru Shigeoka
Pages 695 - 697
http://dx.doi.org/10.4161/psb.5.6.11577
Abstract | PDF

NAC (NAM, ATAF and CUC2) is one of the largest families of transcription factors in the plant genome, but the function and regulation of most NAC genes are still largely unknown. We recently isolated a gene encoding a NAC transcription factor designated ANAC078 from Arabidopsis plants and identified 166 genes up-regulated in ANAC078-overexpressing plants compared with the wild-type plants under high-light stress. The cyclic amplification and selection of targets (CASTing) technique showed that the ANAC078 recognition sequence contains T[A/T/C][A/T/G/C]C[T/G]TG[T/G]G as a DNA-binding site. The recognition sequence identified by this technique was detected in the promoter region of 52 up-regulated genes, including the gene for a transcription factor, proteasome subunits, peroxidase, and a protein kinase. The findings suggest these genes to be directly targeted by the ANAC078 protein.

Open Access Article

Extracellular ATP and nitric oxide signaling pathways regulate redox-dependent responses associated to root hair growth in etiolated Arabidopsis seedlings

María Cecilia Terrile, Claudia Virginia Tonón, María José Iglesias, Lorenzo Lamattina and Claudia Anahí Casalongué
Pages 698 - 701
http://dx.doi.org/10.4161/psb.5.6.11579
Abstract | PDF

Extracellular ATP (eATP) and nitric oxide (NO) have emerged as crucial players in plant development, stress responses and cell viability. Glutathione (GSH) is an abundant reducing agent with proposed roles in plant growth, development and stress physiology. In a recent publication, we demonstrated that eATP and NO restore hypocotyl elongation of etiolated Arabidopsis seedlings treated with GSH. Here it is reported that exogenous ATP also restore root hair growth suggesting a role for ATP and NO in the regulation of redox balance associated to specific processes of plant morphogenesis. A tentative model integrating redox-, eATP- and NO- signaling pathways during root hair growth in Arabidopsis seedlings is presented.

Open Access Article

Allelopathy of the moss Rhynchostegium pallidifolium and 3-hydroxy-β-ionone

Hisashi Kato-Noguchi and Takahiro Seki
Pages 702 - 704
http://dx.doi.org/10.4161/psb.5.6.11642
Abstract | PDF

The moss Rhynchostegium pallidifolium (Mitt.) A. Jaeger, which often forms large pure colonies on soils and rocks, inhibited the hypocotyls and root growth of cress (Lepidium sativum L.) seedlings when R. pallidifolium and cress were incubated to-gether on agar medium. The inhibition of cress was greater at the close position from the moss than at the far position from the moss. 3-Hydroxy-β-ionone was found in the medium and concentration of 3-hydroxy-β-ionone in the medium was greater at the close position than at the far position from R. pallidifolium, suggesting that R. pallidifolium may secrete 3-hydroxy-β-ionone into the medium. Exogenously ap-plied 3-hydroxy-β-ionone inhibited the growth of hypocotyls and roots of cress at con¬cen¬tra¬tions greater than 1 and 3 μM, respectively. Considering the growth inhibi-tory activity and concentrations found in the medium, 3-hydroxy-β-ionone was esti-mated to be able to cause 46 - 64 % of the observed growth inhibition of cress hypo-cotyls and roots by R. pallidifolium. Therefore, 3-hydroxy-β-ionone may play an important role in the allelopathic activity of R. pallidifolium and may help competi-tion with neighboring plants resulting in the formation of pure colonies.

Open Access Article

Symptom induction and RNA silencing suppression by the cucumber mosaic virus 2b protein

Mathew G. Lewsey, Inmaculada González, Natalia O. Kalinina, Peter Palukaitis, Tomas Canto and John P. Carr
Pages 705 - 708
http://dx.doi.org/10.4161/psb.5.6.11643
Abstract | PDF

The 2b protein encoded by Cucumber mosaic virus (CMV) and other cucumoviruses is multifunctional, having roles in local and systemic virus movement, symptom determination, evasion of defense mediated by salicylic acid, and in suppression of antiviral RNA silencing. It also perturbs silencing-mediated regulation of host transcripts, suggesting that another function of 2b protein is to manipulate host gene expression and physiology in a way that may aid the virus. The 2b proteins encoded by the various cucumoviruses (CMV strains, as well as Tomato aspermy virus and Peanut stunt virus) share conserved amino acid sequence domains, suggesting that these might determine specific functions of the protein. We analyzed the effect of mutations in these domains on functions of the 2b protein during viral infection. This revealed that binding of short RNAs, the key determinants of RNA silencing specificity, correlates with RNA silencing suppression activity. Two putative phosphorylation sites were found to be required for virus symptom induction, despite having no influence on RNA silencing suppression. This indicates that the ability to suppress silencing is not the only factor affecting symptom induction by the 2b protein. In accordance with this, our studies also revealed that the 2b protein acts synergistically with some other CMV product(s) to induce symptoms, and that the role of the 2b protein in symptom determination is host species specific.

Open Access Article

An auxin regulated positive feedback loop integrates Rho modulated cell polarity with pattern formation

Ora Hazak and Shaul Yalovsky
Pages 709 - 711
http://dx.doi.org/10.4161/psb.5.6.11644
Abstract | PDF

In plants, polar transport of the hormone auxin between cells connects cell polarity and pattern formation and is thus required for plant development. The direction of auxin transport is determined by polar localization of PIN auxin efflux transporters. The dynamic polar localization of PIN proteins depends on constitutive endocytic recycling to and from the plasma membrane. However, it was not well understood how PIN polarization is connected to regulators of cell polarity. In a paper that was published in the January issue of PLoS Biology, 2010 we described the involvement of the ROP GTPase associated scaffold Interactor of Constitutive active ROP 1 (ICR1) in recruitment of PIN proteins to polar domains in the plasma membrane. icr1 mutant plants and embryos have severe developmental aberrations that are caused by abnormal auxin distribution. ICR1 functions at or close to the plasma membrane where it is required for exocytosis. ICR1 transcription is quickly induced by auxin but is post-transcriptionally repressed at the site of stable auxin maximum in embryos and roots. Thus, ICR1 integrates auxin-regulated transcription with ROP modulated cell polarity.

Open Access Article

Does OsPHR2, central Pi-signaling regulator, regulate some unknown factors crucial for plant growth?

Ping Wu and Jiming Xu
Pages 712 - 714
http://dx.doi.org/10.4161/psb.5.6.11645
Abstract | PDF

OsPHR2, the homolog of AtPHR1, is a central Pi-signaling regulator. The Pi-signaling pathway downstream of AtPHR1, similarly of OsPHR2,1,2 involves a noncoding RNA which targets mimicry of miR399. miRNA399 mediates cleavage of PHO2.^3,4 The regulating pathway downstream of OsPHR2 is negatively regulated by the Pi-signaling responsive gene OsSPX1.^5,6 Overexpression of AtPHR1 and OsPHR2 leads to an increased concentration of Pi in the shoot tissues with leaf toxic symptom and growth retardation similar as the phenotype of pho2 mutant, especially under Pi abundant conditions.^7,2,6 It has been known that the low affinity Pi transporter OsPT2 mainly contributes to the shoot Pi accumulation mediated by OsPHR2, and overexpression of OsPT2 results in shoot Pi accumulation and leaf toxic symptom and growth retardation under Pi abundant conditions.⁶ Two curious questions are emerging from the reported results: How Os SPX1 functions on the negative regulation of the pathway and what mechanism of the growth retardation mediated by OsPHR2. For the second question, our favored hypothesis is that the growth inhibition mediated by overexpression of OsPHR2 is caused by toxic physiological effects due to excessive Pi accumulation in shoots (Pi toxicity). In fact, the toxic symptoms become diminished with decreased Pi levels in growth medium. However, the plant growth retardation mediated by overexpression of OsPHR2 may be caused by some unknown genetic factor(s) regulated by OsPHR2.

Open Access Article

The role of the transcript elongation factors FACT and HUB1 in leaf growth and the induction of flowering

Mieke Van Lijsebettens and Klaus D. Grasser
Pages 715 - 717
http://dx.doi.org/10.4161/psb.5.6.11646
Abstract | PDF

In the cell nucleus, the packaging of the DNA into chromatin represses transcription by restricting the access of transcriptional regulators to their binding sites and inhibiting the progression of RNA polymerases during transcript elongation. To efficiently transcribe genes in the context of chromatin, eukaryotes have a variety of transcript elongation factors promoting transcription in vivo. The facilitates chromatin transcription (FACT) complex consisting of the SSRP1 and SPT16 proteins, is a histone chaperone that assists transcription by destabilising nucleosomes in the path of RNA polymerases. In a recent study, we report that Arabidopsis FACT is critically involved in different aspects of development including leaf growth and the transition to flowering. Moreover, FACT was found to interact genetically with HUB1 that mono-ubiquitinates histone H2B. Depending on the underlying process that is regulated by the two complexes, there appear to be different levels of interaction.

Open Access Article

Plant mediator: Mediating the jasmonate response

Brendan N. Kidd, Elizabeth A. Aitken, Peer M. Schenk, John M. Manners and Kemal Kazan
Pages 718 - 720
http://dx.doi.org/10.4161/psb.5.6.11647
Abstract | PDF

Jasmonate (JA) signaling plays an important role in regulating both plant defense and development. We have recently reported that the PHYTOCHROME AND FLOWERING TIME1 (PFT1) gene, which encodes the MEDIATOR25 subunit of the plant Mediator complex, is a key regulator of JA regulated transcription. We showed that the pft1 mutant had attenuated expression of a wide range of JA responsive genes and altered resistance to fungal pathogens. Here we examine the position of PFT1/MED25 within the JA pathway and discuss its role in “mediating” the JA response.

Open Access Article

Keep the balloon deflated: The significance of protein maturation for thylakoid flattening

Joshua K. Endow, Nicholas J. Ruppel and Kentaro Inoue
Pages 721 - 723
http://dx.doi.org/10.4161/psb.5.6.11662
Abstract | PDF

Thylakoidal processing peptidase (TPP) catalyzes the removal of signal peptide which leads to maturation of a subset of proteins including photosynthetic electron transport components in thylakoids. The biochemical properties of TPP were highly defined during the 1980’s and 1990’s, but the physiological significance of the TPP activity had remained undefined. Completion of genome sequencing revealed the presence of three TPP isoforms in the model plant Arabidopsis thaliana. A recent genetic study demonstrated that one isoform, plastidic type I signal peptidase 1 (Plsp1), is necessary for proper thylakoid assembly. Interestingly, Plsp1 was found in both the chloroplast envelope and thylakoids, being responsible for maturation of an outer membrane protein Toc75 and a lumenal protein OE33. A more recent study has shown that Plsp1 is involved in maturation of two additional lumenal proteins, OE23 and plastocyanin, and that accumulation of unprocessed Toc75 does not disrupt normal chloroplast development. The study also revealed that plsp1-null plastids accumulate balloon-like vesicles that appear to be the remnants of thylakoids as they contain unprocessed OE33 in the peripheral regions. These findings suggest that proper maturation of lumenal proteins is required for correct assembly and/or maintenance of thylakoids, but may not be necessary for initiation of membrane development. The ballooned thylakoids in plsp1-null plastids may be a useful tool to elucidate the mechanism of thylakoid flattening, which correlates with the energized state of the membranes.

Open Access Article

Locus-specific dependency of endogenous silent loci on MOM1 and non-CG methylation in Arabidopsis thaliana

Yoshiki Habu and Manabu Yoshikawa
Pages 724 - 726
http://dx.doi.org/10.4161/psb.5.6.11663
Abstract | PDF

RNA-directed modification of histones is essential for maintenance of heterochromatin in higher eukaryotes. In plants, cytosine methylation, especially in non-CG sequence contexts, is tightly related to inactive chromatin, but the mechanisms regulating the coexistence of cytosine methylation and repressive histone modification remain obscure. We recently revealed that MORPHEUS' MOLECULE1 (MOM1) of Arabidopsis thaliana silences endogenous loci related to transposons and homologous to the 24-nt siRNAs accumulated in wild type plants, and suggested that MOM1 transduces RNA-directed DNA methylation (RdDM) signals to repressive histone modification. In this addendum, we focus on the involvement of MOM1 in multiple transcriptional gene silencing (TGS) pathways.

Open Access Article

The moss Physcomitrella patens, a model plant for the study of RNA editing in plant organelles

Eiji Tasaki and Mamoru Sugita
Pages 727 - 729
http://dx.doi.org/10.4161/psb.5.6.11664
Abstract | PDF

RNA editing is an enigmatic phenomenon in which specific cytidines (C) in the transcripts are changed to uridines (U). In flowering plants, over 500 editing sites have been identified in the mitochondria and plastids. By contrast, in a moss Physcomitrella patens, only 12 editing sites are found in both organelles. Recent extensive genetics studies have revealed involvement of the pentatricopeptide repeat (PPR) proteins with a C-terminal DYW domain (PPR-DYW) in site-specific RNA editing events. Flowering plants have ~100 PPR-DYW genes while P. patens has only 10 PPR-DYW genes. Thus, the number of PPR-DYW genes is somewhat related to the number of RNA editing sites. The P. patens gametophyte, the haploid phase of the life cycle, is dominant, making it possible to study the phenotype of knockouts directly after transformation for gene-targeting. This makes it easier to identify the PPR-DYW proteins required for RNA editing. Recently, we have shown that one of 10 PPR-DYW proteins, PpPPR_71, was responsible for RNA editing in the mitochondrial ccmFc mRNA. In this study, we propose a working model of PpPPR_71 function on RNA editing.

Open Access Article

Orientation of vascular cell divisions in Arabidopsis

J. Peter Etchells and Simon R. Turner
Pages 730 - 732
http://dx.doi.org/10.4161/psb.5.6.11665
Abstract | PDF

Orientation of cell division is essential for plant development as the direction of growth is determined by the direction of cell expansion and orientation of cell division. We have demonstrated that cell division orientation in vascular tissue is regulated by the interactions between a receptor kinase (PXY) expressed in dividing cells and its peptide ligand (CLE41) that is localized to adjacent phloem cells. Given that other receptor kinases have been identified as orienting the cell division plane in several developmental processes, we suggest that localized signaling from adjacent cells may be a general mechanism for defining the plane of cell division.

Open Access Article

Salt-triggered expression of the ANAC092-dependent senescence regulon in Arabidopsis thaliana

Salma Balazadeh, Anhui Wu and Bernd Mueller-Roeber
Pages 733 - 735
http://dx.doi.org/10.4161/psb.5.6.11694
Abstract | PDF

The NAC domain transcription factor ANAC092 plays a central role in leaf senescence in Arabidopsis thaliana. We recently identified 170 genes whose expression increases upon activation of ANAC092 in a chemically (estradiol) controlled experimental set-up, 78 of which are known senescence-associated genes (SAGs). In accordance with the well-known phenomenon that salt stress promotes early leaf senescence in many plant species, we previously observed salt stress-enhanced expression of many SAGs of the ANAC092 regulon. Global expression profiling now revealed that 36 genes, representing 46% of all ANAC092 downstream SAGs, are induced by long-term (4 days) salt stress in shoots of Arabidopsis, whereas short-term stress (6 hours) only slightly affects gene expression. Expression analysis also showed that 14 of the 36 genes are induced by hydrogen peroxide (H₂O₂) treatment. Additionally, 15 senescence-associated NAC genes (senNACs), including ANAC092, respond to H₂O₂ exposure. Our data support the model that salt-triggered senescence is at least partly mediated through the ANAC092 gene regulatory network. Other senNACs most likely contribute to the coordination of this process, potentially in concert with H2O2-mediated signaling.

Open Access Article

Adjusting ammonium uptake via phosphorylation

Viviane Lanquar and Wolf B. Frommer
Pages 736 - 738
http://dx.doi.org/10.4161/psb.5.6.11696
Abstract | PDF

In plants, AMT/MEP/Rh superfamily mediates high affinity ammonium uptake. AMT/MEP transporters form a trimeric complex, which requires a productive interaction between subunits in order to be functional. The AMT/MEP C-terminal domain is highly conserved in more than 700 AMT homologs from cyanobacteria to higher plants with no cases found to be lacking this domain. AMT1;1 exists in active and inactive states, probably controlled by the spatial positioning of the C-terminus. Ammonium triggers the phosphorylation of a conserved threonine residue (T460) in the C-terminus of AMT1;1 in a time- and concentration-dependent manner. The T460 phosphorylation level correlates with a decrease of root ammonium uptake. We propose that ammonium-induced phosphorylation modulates ammonium uptake as a general mechanism to protect against ammonium toxicity.

Open Access Article

Ethylene is crucial for cotyledon greening and seedling survival during de-etiolation

Shangwei Zhong, Hui Shi, Yanpeng Xi and Hongwei Guo
Pages 739 - 742
http://dx.doi.org/10.4161/psb.5.6.11698
Abstract | PDF

The most remarkable change of de-etiolation in seedling is chlorophyll synthesis and greening. This transition is achieved by photoreduction of dark-accumulated protochlorophyllide (Pchlide) in light. However, overaccumulation of Pchlide results in phototoxicity to plants, so appropriate accumulation and quick reduction of Pchlide are crucial for survival of seedlings during the transition from dark to light. We found that this vital process is tightly regulated by the plant gaseous hormone ethylene. Transgenic analysis using a promoter-GUS reporter system showed that the ethylene signaling was able to activate the expression of PORA (protochlorophyllide oxidoreductase A) gene in seedling cotyledons. We further found that application of ethylene rescued the greening defect of the flu mutant, which over-accumulated Pchlide in the dark. Additionally, genetic studies revealed that Ethylene Insensitive 3 (EIN3) and EIN3-like 1 (EIL1）regulate Pchlide accumulation and cotyledon greening largely independent of Phytochrome-Interacting Factor 1 (PIF1) but partly dependent on PIF3. Therefore, the ethylene signaling via EIN3/EIL1 presents a new pathway to constrain phototoxic Pchlide accumulation in darkness, and simultaneously facilitate Pchlide reduction to synthesize chlorophyll upon light exposure. Our results thus uncover an essential role of ethylene in protecting seedlings from photo-oxidative damage during the process of de-etiolation.

Open Access Article

Strengths of artificial neural networks in modeling complex plant processes

Jorge Gago, Mariana Landín and Pedro Gallego
Pages 743 - 745
http://dx.doi.org/10.4161/psb.5.6.11702
Abstract | PDF

Commonly, simple mathematical models can not be used to describe exactly the biological processes due to their higher complexity. In fact, most biological interactions cannot be elucidated by a simple stepwise algorithm or a precise formula, particularly when the data are complex or noisy. ANNs allows an accurate description of those kind of biological processes in plant science, offering new advantages over traditional treatments as the possibility of a model, prediction and optimize results. Different kind of data can be analyzed using a unique and “easy to use” technology. Researchers with a high specialized mathematical background are not required and ANNs offer the possibility of achieving the whole view of the experimental study with a limited number of experiments and costs. Additionally, it is possible to add new inputs and outputs to the database to reach a new understanding.

Open Access Article

Lessons learned from type III effector transgenic plants

Mike Wilton and Darrell Desveaux
Pages 746 - 748
http://dx.doi.org/10.4161/psb.5.6.11703
Abstract | PDF

The Gram negative bacterial phytopathogen Pseudomonas syringae employs a molecular syringe termed the Type III secretion system (TTSS) to deliver an array of Type III secreted effector (TTSE) proteins into plant cells. The major function ascribed to type III effectors of P. syringae is their ability to suppress plant immunity. Because individual pathovars of P. syringae can possess over 30 TTSEs, functional redundancy can provide a hurdle to ascribing functions by TTSE-deletion or -overexpression in such TTSE-rich backgrounds. Approaches to overcome functional redundancy have included the deletion of multiple TTSEs from individual pathovars as well as engineering the plant commensal P. fluorescens strain to express the P. syringae TTSS and deliver P. syringae TTSEs. As we describe here, transgenic Arabidopsis plants expressing individual TTSEs have also be used to overcome problems of functional redundancy and provide invaluable insights into TTSE virulence functions.

Open Access Article

Lavender inflorescence: A model to study regulation of terpenes synthesis

Yann Guitton, Florence Nicolè, Sandrine Moja, Tarek Benabdelkader, Nadine Valot, Sylvain Legrand, Frédéric Jullien and Laurent Legendre
Pages 749 - 751
http://dx.doi.org/10.4161/psb.5.6.11704
Abstract | PDF

We analysed VOC composition of complete inflorescences and single flowers of lavender during the flowering period. Our analyses, focused on the 20 most abundant terpenes, showed that three groups of components could be separated according to their patterns of variation during inflorescence ontogeny. These three groups were associated with three developmental stages: flower in bud, flower in bloom and faded flower. The expression of two terpene synthases (TPS) was followed using qPCR during inflorescence ontogeny. A comparison of these chemical and molecular analyses suggested that VOC production in lavender spike is mainly regulated at the transcriptional level. These results highlighted that lavender could be a model plant for future investigations on terpene biosynthesis and regulation, and could be used to explore the functions of terpene metabolites.

Open Access Article

Cortical microtubules are responsible for gravity resistance in plants

Takayuki Hoson, Shouhei Matsumoto, Kouichi Soga and Kazuyuki Wakabayashi
Pages 752 - 754
http://dx.doi.org/10.4161/psb.5.6.11706
Abstract | PDF

Mechanical resistance to the gravitational force is a principal gravity response in plants distinct from gravitropism. In the final step of gravity resistance, plants increase the rigidity of their cell walls. Here we discuss the role of cortical microtubules, which sustain the function of the cell wall, in gravity resistance. Hypocotyls of Arabidopsis tubulin mutants were shorter and thicker than the wild-type, and showed either left-handed or right-handed helical growth at 1 g. The degree of twisting phenotype was intensified under hypergravity conditions. Hypergravity also induces reorientation of cortical microtubules from transverse to longitudinal directions in epidermal cells. In tubulin mutants, the percentage of cells with longitudinal microtubules was high even at 1 g, and it was further increased by hypergravity. The left-handed helical growth mutants had right-handed microtubule arrays, whereas the right-handed mutant had left-handed arrays. Moreover, blockers of mechanoreceptors suppressed both the twisting phenotype and reorientation of microtubules in tubulin mutants. These results support the hypothesis that cortical microtubules play an essential role in maintenance of normal growth phenotype against the gravitational force, and suggest that mechanoreceptors are involved in signal perception in gravity resistance. Space experiments will confirm whether this view is applicable to plant resistance to 1 g gravity, as to the resistance to hypergravity.

Open Access Article

Nonhost resistance to Magnaporthe oryzae in Arabidopsis thaliana

Kana Maeda, Yasunari Houjyou, Takuma Komatsu, Hiroki Hori, Takahiro Kodaira and Atsushi Ishikawa
Pages 755 - 756
http://dx.doi.org/10.4161/psb.5.6.11770
Abstract | PDF

Rice blast, caused by Magnaporthe oryzae, is a devastating disease of rice (Oryza sativa). The mechanisms involved in resistance of rice to blast have been studied extensively and the rice–M. oryzae pathosystem has become a model for plant–microbe interaction studies. However, the mechanisms involved in nonhost resistance (NHR) of other plants to rice blast are still poorly understood. We have recently demonstrated that AGB1 and PMR5 contribute to PEN2-mediated preinvasion resistance to M. oryzae in Arabidopsis thaliana, suggesting a complex genetic network regulating the resistance. To determine whether other defense factors: RAR1, SGT1, and NHO1, affected the A. thaliana-M. oryzae interactions, double mutants were generated between pen2 and these defense-related mutants. All these double mutants exhibited a level of penetration resistance similar to that of the pen2 mutant, suggesting that none of these mutants significantly compromised resistance to M. oryzae in a pen2 background.

Open Access Article

Is acid-induced extension in seed plants only protein-mediated?

Dmitry Suslov, Jean-Pierre Verbelen and Kris Vissenberg
Pages 757 - 759
http://dx.doi.org/10.4161/psb.5.6.11771
Abstract | PDF

Cell wall extensibility controls the rate of plant cell growth. It is determined by intrinsic mechanical properties of wall polymers and by wall proteins modifying these polymers and their interactions. Heat-inactivation of endogenous cell wall proteins inhibited acid-induced extension of onion epidermis peels transverse to the net cellulose alignment in the cell wall but not parallel to it. In the former case the acid-induced extension could be controlled by expansins and in the latter case by pectins restricting shear between microfibrils. Heat-inactivated cell walls stretched transversely to the net cellulose orientation extended faster at pH 5.7 and slower at pH 4.5 compared to native walls. Expansins seem to be inactive at pH 5.7, so that faster extension may result from heat-induced viscous flow of pectins and conformational changes in the cuticle of the epidermis. This stimulation of wall extension is not seen at pH 4.5 as it is outweighed by the inhibitory effect of expansin heat-inactivation. Thus, cell wall extension in higher plants might be controlled by a complex interplay between protein-dependent and protein-independent mechanisms, the result of which depends on pH and preferential orientation of main wall polymers.

Open Access Article

Application of nanoscale secondary ion mass spectrometry to plant cell research

Matt R Kilburn, David L Jones, Peta L Clode, John B Cliff, Elizabeth A Stockdale, Anke M Herrmann and Daniel V Murphy
Pages 760 - 762
http://dx.doi.org/10.4161/psb.5.6.11775
Abstract | PDF

Imaging resource flow in soil-plant systems remains central to understanding plant development and interactions with the environment. Typically, subcellular resolution is required to fully elucidate the compartmentation, behavior, and mode of action of organic compounds and mineral elements within plants. For many situations this has been limited by the poor spatial resolution of imaging techniques and the inability to undertake studies in situ. Here we demonstrate the potential of Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), which is capable of the quantitative high-resolution spatial imaging of stable isotopes (e.g. ¹²C, ¹³C, ¹⁴N, ¹⁵N, ¹⁶O, ¹⁸O, ³¹P, ³⁴S) within intact plant-microbial-soil systems. We present examples showing how the approach can be used to investigate competition for ¹⁵N-labeled nitrogen compounds between plant roots and soil microorganisms living in the rhizosphere and the spatial imaging of ³¹P in roots. We conclude that NanoSIMS has great potential to elucidate the flow of isotopically-labeled compounds in complex media (e.g. soil) and opens up countless new opportunities for studying plant responses to abiotic stress (e.g. ¹⁸O₃, elevated ¹³CO₂), signal exchange, nutrient flow and plant-microbial interactions.

Open Access Article

When do arbuscular mycorrhizal fungi protect plant roots from pathogens?

Benjamin A. Sikes
Pages 763 - 765
http://dx.doi.org/10.4161/psb.5.6.11776
Abstract | PDF

Arbuscular mycorrhizal (AM) fungi are mainly thought to facilitate phosphorus uptake in plants, but they can also perform several other functions that are equally beneficial. Our recent study sheds light on the factors determining one such function, enhanced plant protection from root pathogens. Root infection by the fungal pathogen Fusarium oxysporum was determined by both plant susceptibility and the ability of an AM fungal partner to suppress the pathogen. The non-susceptible plant species (Allium cepa) had limited F. oxysporum infection even without AM fungi. In contrast, the susceptible plant species (Setaria glauca) was heavily infected and only AM fungi in the family Glomeraceae limited pathogen abundance. Plant susceptibility to pathogens was likely determined by contrasting root architectures between plants, with the simple rooted plant (A. cepa) presenting fewer sites for infection.AM fungal colonization, however, was not limited in the same way in part because plants with fewer, simple roots are more mycorrhizal dependent. Protection only by Glomus species also indicates that whatever the mechanism(s) of this function, it responds to AM fungal families differently. While poor at pathogen protection, AM fungal species in the family Gigasporaceae most benefited the growth of the simple rooted plant species. Our research indicates that plant trait differences, such as root architecture can determine how important each mycorrhizal function is to plant growth but the ability to provide these functions differs among AM fungi.

Open Access Article

Consequences of SOS1 deficiency: Intracellular physiology and transcription

Donga Ha Oh, Zahir Ali, Cheol Park Hyeong, Ray A Bressan, Jin Yun Dae and Hans J Bohnert
Pages 766 - 768
http://dx.doi.org/10.4161/psb.5.6.11777
Abstract | PDF

As much as there is known about the function of the sodium/proton antiporter SOS1 in plants, recent studies point towards a more general role for this protein. The crucial involvement in salt stress protection is clearly one of its functions –confined to the N-terminus, but the modular structure of the protein includes a segment with several domains that are functionally not studied but comprise more than half of the protein’s length. Additional functions of the protein appear to be an influence on vesicle trafficking, vacuolar pH and general ion homeostasis during salt stress. Eliminating SOS1 leads to the expression of genes that are not strictly salinity stress related. Functions that are regulated in sos1 mutants included pathogen responses, and effects on circadian rhythm.

Open Access Article

Mediation of the transition from biotrophy to necrotrophy in hemibiotrophic plant pathogens by secreted effector proteins

Sang-Jik Lee and Jocelyn K.C. Rose
Pages 769 - 772
http://dx.doi.org/10.4161/psb.5.6.11778
Abstract | PDF

Hemibiotrophs, such as Phytophthora infestans, exhibit distinct phases of their life cycle: an early asymptomatic biotrophic phase and a late necrotrophic stage that is characterized by tissue degradation and disease symptoms. To date, little is known of the molecular mechanisms that promote each distinct phase, nor those that mediate the transition between the two. We hypothesized that these phytopathogens might secrete distinct classes of effector proteins that first suppress plant defense responses and associated programmed cell death (PCD), and later induce large scale necrosis. To this end, we have identified proteins that are secreted by P. infestans early or late in the infection cycle. Recently we described the characterization of SNE1, which is specifically expressed during early biotrophic growth in the host plant tomato (Solanum lycopersicum). We found that SNE1 suppresses the action of necrosis-inducing effectors (Nep1-like proteins), including PiNPP1.1 and PsojNIP, which are secreted by Phytophthora during necrotrophic growth, as well as PCD mediated by a broad spectrum of Avr-R protein interactions. This suggests that SNE1 and PiNPP1.1 act antagonistically, thereby providing a highly regulated means to control the transition from biotrophy to necrotrophy.