Table of Contents

Volume 3, Issue 1

  January 2008

  Pages 1 - 75

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 Issue Catalogue (MARC XML)

REVIEW

Open Access Article

Genetics, novel weapons and rhizospheric microcosmal signaling in the invasion of Phragmites australis

Thimmaraju Rudrappa and Harsh P. Bais
Pages 1 - 5
http://dx.doi.org/10.4161/psb.3.1.5279
Abstract | PDF

Chemical communication and perception strategies between plants are highly sophisticated but are only partly understood. Among the different interactions, the suppressive interaction of a class of chemicals released by one plant through root exudates against the neighboring plants (allelopathy) have been implicated in the invasiveness of many exotic weedy species. Phragmites australis (Common reed) is one of the dominant colonizers of the North American wetland marshes and exhibits invasive behavior by virtually replacing the entire native vegetation in its niche. Recently, by adopting a systemic bioassay driven approach we elucidated the role of root derived allelopathy as one of the important mechanisms by which P. australis exerts its invasive behavior. Additionally, our recent preliminary data indicates the involvement of rhizobacterial signaling in the invasive success of P. australis. A better understanding of biochemical weaponry used by P. australis will aid scientists and technologists in addressing the impact of root secretions in invasiveness of weedy species and thus promote a more informed environmental stewardship




RESEARCH PAPERS

Open Access Article

Surviving the passage: Non-canonical stromal targeting of an Arabidopsis mitogen-activated protein kinase kinase

Marcus A. Samuel, Balbir K. Chaal, Greg Lampard, Beverley R. Green and Brian E. Ellis
Pages 6 - 12
http://dx.doi.org/10.4161/psb.3.1.4856
Abstract | PDF

In plants, mitogen-activated protein kinases (MAPK) have been implicated in signalling associated with many processes, including cellular differentiation, organ development, cell death and stress/hormone signalling. While MAPK cascades are known to act in the cytosol and the nucleus, sequence analysis of the Arabidopsis MAPK cascade proteins predicts the presence of import signals that would target some of them to other organelles. In vitro uptake experiments confirm the predicted import of an oxidant-responsive MAPKK, AtMKK4, into the chloroplast. Unexpectedly, the imported MKK4 protein was not processed through stromal peptidase-dependent cleavage of the N-terminal signal peptide, thus leaving the pre-protein intact. Nevertheless, the N-terminal region was shown to be essential both for the import process and for the ability of MKK4 to activate its cognate MAPK targets in vivo. MKK4 import also occurred irrespective of the activation status of the kinase. The import of this primarily cytosolic oxidant-stimulated AtMKK4 into the chloroplasts, organelles with high redox fluxes, suggests that one of the functions of MKK4 might be to help coordinate intercompartment responses to cellular redox imbalances.

Open Access Article

Light- and dark-induced action potentials in Physcomitrella patens

Mateusz Koselski, Kazimierz Trebacz, Halina Dziubinska and Elzbieta Krol
Pages 13 - 18
http://dx.doi.org/10.4161/psb.3.1.4884
Abstract | PDF

Glass microelectrodes were inserted into Physcomitrella patens gametophyte leaves and action potentials (APs) were recorded in response to sudden illumination as well as after darkening, i.e. when the dark-induced membrane depolarization crossed a threshold. Application of 5 mM La³⁺ (a calcium channel inhibitor), 10 mM TEA⁺ (a potassium channel inhibitor), and increased free Ca²⁺ resulted in a loss of excitability. Lack of Ca²⁺ in the external medium did not prevent APs from occurring. It was concluded that during light- dark-induced excitation of Physcomitrella patens, APs might rely upon calcium influxes from the intracellular compartments. APs were not blocked by the proton pump inhibitors (DES, DCCD), although the resting potential (RP) diminished significantly.

Open Access Article

Darwin's bee-trap: The kinetics of Catasetum, a new world orchid

Charles C. Nicholson, James W. Bales, Joyce E. Palmer-Fortune and Robert G. Nicholson
Pages 19 - 23
http://dx.doi.org/10.4161/psb.3.1.4980
Abstract | PDF

The orchid genera Catasetum employs a hair-trigger activated, pollen release mechanism, which forcibly attaches pollen sacs onto foraging insects in the New World tropics. This remarkable adaptation was studied extensively by Charles Darwin and he termed this rapid response “sensitiveness”. Using high speed video cameras with a frame speed of 1000fps, this rapid release was filmed and from the subsequent footage, velocity, speed, acceleration, force and kinetic energy were computed.




ARTICLE ADDENDA

Open Access Article

Is Boron involved solely in structural roles in vascular plants?

Agustín González-Fontes, Jesús Rexach, María Teresa Navarro-Gochicoa, María Begoña Herrera-Rodríguez, Víctor Manuel Beato, José María Maldonado and Juan José Camacho-Cristóbal
Pages 24 - 26
http://dx.doi.org/10.4161/psb.3.1.4812
Abstract | PDF

It is very well proved that boron (B) plays a primary structural role in the plant cell wall. In addition, this micronutrient has been involved in a great variety of physiological processes in vascular plants. It has been reported that B deficiency induces stress-responsive genes and, in tobacco plants, it seems to decrease net nitrate uptake by repressing expression of root plasmalemma H⁺-ATPase gene. Moreover, root asparagine concentration is clearly increased under B deficiency, as also observed for other abiotic stresses. Accumulation of asparagine in response to abiotic stresses could be an ammonium detoxification mechanism when high amounts of ammonium are internally generated by deamination of soluble amino acids released from enhanced proteolysis under stress conditions. Nevertheless, the mechanisms underlying the several effects caused by B deficiency are unknown. Although a mechanism has been reported to explain B effects based on signals via the cell wall-plasma membrane-cytoskeleton continuum, we propose and discuss the possible role of B as a cellular signal through transcriptional factors. This hypothetical mechanism could explain not only its diverse effects on so many physiological processes, but also that a negligible amount of boron into the protoplast can be decisive for the normal development of such events.

Open Access Article

A role for symplastic gating in the control of the potato tuber life cycle

Robert D. Hancock, Alison G. Roberts and Roberto Viola
Pages 27 - 29
http://dx.doi.org/10.4161/psb.3.1.4813
Abstract | PDF

The control of the potato tuber life cycle has been the subject of significant interest over many years. A number of different approaches have been adopted and data is available regarding hormonal, metabolic and gene expression changes that occur over the tuber life cycle. Despite this intense effort, no unifying model for the control of the potato tuber life cycle has emerged. We have undertaken a detailed analysis of the tuber life cycle utilising physiological, biochemical and cell-biological techniques. It has emerged that a major factor contributing to both tuber induction and dormancy break is symplastic gating which controls the allocation of resources to meristematic or vegetative tissues. Future challenges include the determination of factors regulating symplastic gating at the molecular level and the extrapolation of these findings to other systems.

Open Access Article

Enhancement of post-transcriptional gene silencing by grafting

Yuanhuai Han and Donald Grierson
Pages 30 - 33
http://dx.doi.org/10.4161/psb.3.1.4814
Abstract | PDF

We showed previously that grafting transmitted silencing occurred when transgenic ACC oxidase 1 (ACO1) overexpressing tomato plants that also produced siRNAs were grafted onto transgenic stocks that already showed strong silencing. The presence of siRNAs in these overexpressing scions may indicate that silencing, though inefficient, may already occur at a low level before grafting. To test if a silencing state with a relatively high level of target mRNA can be shifted towards further more effective silencing, we grafted an ACO1 antisense (AS) line with a high level of antisense ACO1 transgene mRNA and low level of siRNAs to the ACO1 strong silencer stock. The AS mRNA level was reduced dramatically two weeks after grafting. More interestingly, self-grafting of ACO1 overexpressers and AS lines also induced strong silencing in the scions. We suggest that grafting transmitted silencing may involve the switching from an inefficient or weak silencing state to a stronger silencing by a systemic silencing signal, similar to the change of silencing states that sometimes occurs during development. Control experiments using non-transgenic stocks designed to test whether wounding alone is responsible for generating a signal that enhances silencing in transgenic scions gave negative results. We propose that the build-up of silencing signal and / or molecules at both sides of the grafting junction and their sudden release when the phloem is reconnected may be critical to grafting transmitted silencing.

Open Access Article

A role for auxin during actinorhizal symbioses formation?

Benjamin Péret, Sergio Svistoonoff, Benoit Lahouze, Florence Auguy, Carole Santi, Patrick Doumas and Laurent Laplaze
Pages 34 - 35
http://dx.doi.org/10.4161/psb.3.1.4816
Abstract | PDF

The symbiotic interaction between the soil bacteria Frankia and actinorhizal plants leads to the formation of nitrogen-fixing nodules resembling modified lateral roots. Little is known about the signals exchanged between the two partners during the establishment of these endosymbioses. However, a role for plant hormones has been suggested.

Recently, we studied the role of auxin influx activity during actinorhizal symbioses. An inhibitor of auxin influx was shown to perturb nodule formation. Moreover we identified a functional auxin influx carrier that is produced specifically in Frankia-infected cells. These results together with previous data showing auxin production by Frankia lead us to propose a model of auxin action during the symbiotic infection process.

Open Access Article

cGMP in ozone and NO dependent responses

Luisa Ederli, Stuart Meier, Andrea Borgogni, Lara Reale, Francesco Ferranti, Chris Gehring and Stefania Pasqualini
Pages 36 - 37
http://dx.doi.org/10.4161/psb.3.1.4818
Abstract | PDF

We have recently reported (Plant Physiology 2006; 142:595-608) that ozone (O₃) can inhibit mitochondrial respiration and induce activation of the alternative oxidase (AOX) pathway and in particular AOX1a in tobacco. While O₃ causes mitochondrial H₂O₂, early leaf nitric oxide (NO) as well as transient ethylene (ET) accumulation, the levels of jasmonic acid and 12-oxo-phytodienoic acid remained unchanged. It was shown that both, NO and ET dependent pathways can induce AOX1a transcription by O₃. AOX plays a role in reducing reactive oxygen species (ROS) which in turn are linked to biotic and abiotic plant stresses, much like the second messengers guanosine 3’, 5’-cyclic monophosphate (cGMP). The goal is to unravel specific cGMP signatures and induction pathways downstream from O₃ and NO, including transcription of AOX1a. Here we propose that some late (> 3 hours) responses to NO, e.g. the accumulation of phenylalanine lyase (PAL) transcripts, are critically cGMP dependent, while the early (< 2 hours) responses, including AOX1a induction are not.

Open Access Article

Signaling from soybean roots to Rhizobium: An ATP-binding cassette-type transporter mediates genistein secretion

Akifumi Sugiyama, Nobukazu Shitan and Kazufumi Yazaki
Pages 38 - 40
http://dx.doi.org/10.4161/psb.3.1.4819
Abstract | PDF

Legume plants have a unique ability to fix atmospheric nitrogen via symbiosis with rhizobia. For the establishment of symbiosis, legume plants secrete signaling molecules such as flavonoids from root tissues, leading to the attraction of rhizobia and the induction of rhizobial nod genes. Genistein and daidzein are found in soybean root exudates and function as signal molecules in soybean – Bradyrhizobium japonicum chemical communication. Although it is more than 20 years since these signal flavonoids were identified, almost nothing has been characterized concerning the membrane transport process of these molecules from soybean roots. To elucidate the transport mechanism we performed membrane transport assays with plasma membrane-enriched vesicles and various inhibitors. As a result, we concluded that an ATP-binding cassette-type transporter is involved in the secretion of genistein from soybean roots. The possible involvement of a pleiotropic drug resistance-type ABC transporter in this secretion is also discussed.

Open Access Article

A novel ROP/RAC GTPase effector integrates plant cell form and pattern formation

Daria Bloch, Ora Hazak, Meirav Lavy and Shaul Yalovsky
Pages 41 - 43
http://dx.doi.org/10.4161/psb.3.1.4838
Abstract | PDF

ROPs/RACs are the only known signaling Ras superfamily small GTPases in plants. As such they have been suggested to function as central regulators of diverse signaling cascades. The ROP/RAC signaling networks are largely unknown, however, because only few of their effector proteins have identified. In a paper that was published in the June 5, 2007 issue of Current Biology we described the identification of a novel ROP/RAC effector designated ICR1 (Interactor of Constitutive active ROPs 1). We demonstrated that ICR1 functions as a scaffold that interacts with diverse but specific group of proteins including SEC3 subunit of the exocyst vesicle tethering complex. ICR1-SEC3 complexes can interact with ROPs in vivo and are thereby recruited to the plasma membrane. ICR1 knockdown or silencing leads to cell deformation and loss of the root stem cells population and ectopic expression of ICR1 phenocopies activated ROPs/RACs. ICR1 presents a new paradigm in ROP/RAC signaling and integrates mechanisms regulating cell form and pattern formation at the whole plant level.

Open Access Article

Phototropin Receptor Kinase Activation by Blue Light

Matthew A. Jones and John M. Christie
Pages 44 - 46
http://dx.doi.org/10.4161/psb.3.1.4848
Abstract | PDF

Phototropins (phot1 and phot2) are blue light-activated serine/threonine protein kinases that function to mediate a variety of adaptive processes that serve to optimize the photosynthetic efficiency of plants and thereby promote their growth. Light sensing by the phototropins is mediated by a repeated motif located within the N-terminal region of the protein designated the LOV domain. Although phototropins possess two LOV photosensors (LOV1 and LOV2), recent biophysical and structure-function analyses clearly indicate that the LOV2 domain plays a predominant role in regulating phototropin kinase activity owing to specific protein changes that occur in response to LOV2 photoexcitation. In particular, the central β-sheet scaffold plays a role in propagating the photochemical signal generated from within LOV2 to protein changes at the surface that are necessary for kinase activation.

Open Access Article

Flowering regulation by tissue specific functions of photoreceptors

Motomu Endo and Akira Nagatani
Pages 47 - 48
http://dx.doi.org/10.4161/psb.3.1.4863
Abstract | PDF

Flowering is one of the most important steps in a plant life cycle. Plants utilize light as an informational source to determine the timing of flowering. In Arabidopsis, phytochrome A (phyA), phyB and cryptochrome2 (cry2) are major photoreceptors that regulate flowering. These photoreceptors perceive light stimuli by leaves for the regulation of flowering. A leaf is an organ consisting of different tissues such as epidermis, mesophyll and vascular bundles. In the present study, we examined in which tissue the light signals are perceived and how those signals are integrated within a leaf to regulate flowering. For this purpose, we established transgenic Arabidopsis lines that expressed a phyB–green fluorescent protein (GFP) fusion protein or a cry2-GFP fusion protein in organ/tissue-specific manners. Consequently, phyB was shown to perceive light stimuli in mesophyll. By contrast, cry2 functioned only in vascular bundles. We further confirmed that both phyB-GFP and cry2-GFP regulated flowering by altering the expression of a key flowering gene, FT, in vascular bundles. In summary, perception sites for different spectra of light are spatially separated within a leaf and the signals are integrated through the inter-tissue communication.

Open Access Article

The role of DORNROESCHEN (DRN) and DRN-LIKE (DRNL) in Arabidopsis embryonic patterning

John W. Chandler, Melanie Cole, Annegret Flier, Britta Grewe and Wolfgang Werr
Pages 49 - 51
http://dx.doi.org/10.4161/psb.3.1.4864
Abstract | PDF

Appropriate embryonic patterning is amongst the most fundamental processes in plant development, necessary for the correct specification of root and shoot apical meristems which generate all post-germination organs of a plant. Many mutations have been characterised which disrupt embryonic pattern formation and many recent studies have focussed on the role of auxin in establishing apical-basal polarity. Our recent work has demonstrated the role of two redundant AP2 transcription factors, DORNROESCHEN (DRN) and DORNROESCHEN-LIKE (DRNL) in the control of embryo patterning, upstream of auxin perception and/or response and that DRN in turn, is regulated by auxin. We also suggest both genes are involved in the change from radial to bilateral symmetry in the globular embryo and responsible for positional information of meristem-specific genes such as STM. The promiscuous interaction of DRN and DRNL proteins with the redundant family of class III HD-ZIP partners may represent a way by which embryonic cell specification can be controlled by combinations of transcription factor complexes, together with auxin.

Open Access Article

Regulation of cold signaling by sumoylation of ICE1

Kenji Miura and Paul M. Hasegawa
Pages 52 - 53
http://dx.doi.org/10.4161/psb.3.1.4865
Abstract | PDF

The small ubiquitin-related modifier (SUMO) E3 ligase SIZ1 is an ortholog of yeast and animal SIZ (SAP and Miz)/PIAS (protein inhibition of activated STAT) proteins, which function as transcriptional co-regulators either by facilitating SUMO conjugation to substrate proteins (sumoylation) or through other mechanisms that are sumoylation independent. SIZ/PIAS-type E3 ligases function in numerous eukaryotic biological processes, including regulation of organismal responses to environmental changes. This Addendum summarizes our recent paper in which it is established that the Arabidopsis E3 ligase SIZ1 mediates sumoylation of ICE1. SUMO conjugation to ICE facilitates ICE activity and stability that positively regulates CBF3/DREB1A-dependent cold signaling and freezing tolerance. Furthermore, sumoylated ICE1 represses MYB15, which is a negative regulator of CBF3/DREB1A and freezing tolerance.

Open Access Article

Tomato BRI1 and systemin wound signalling

Nicholas Holton, Kate Harrison, Takao Yokota and Gerard J. Bishop
Pages 54 - 55
http://dx.doi.org/10.4161/psb.3.1.4888
Abstract | PDF

Brassinosteroids (BRs) are perceived by Brassinosteroid Insensitive 1 (BRI1), that encodes a leucine-rich repeat receptor kinase. Tomato BRI1 has previously been implicated in both systemin and BR signalling. The role of tomato BRI1 in BR signalling was confirmed, however it was found not to be essential for systemin/wound signalling. Tomato roots were shown to respond to systemin but this response varied according to the species and growth conditions. Overall the data indicates that mutants defective in tomato BRI1 are not defective in systemin-induced wound signalling and that systemin perception can occur via a non-BRI1 mechanism.

Open Access Article

Salt stress signaling in Arabidopsis thaliana involves a membrane-bound transcription factor AtbZIP17 as a signal transducer

Jian-Xiang Liu, Renu Srivastava, Ping Che and Stephen H. Howell
Pages 56 - 57
http://dx.doi.org/10.4161/psb.3.1.4889
Abstract | PDF

Plants have developed robust signaling mechanisms to cope with environmental stresses. In a recent article in the Plant Journal,1 we reported on a salt stress signaling pathway in Arabidopsis, which is mechanically similar to endoplasmic reticulum (ER) stress responses described in mammalian systems. We found that a type II membrane-bound transcription factor (AtbZIP17) is a salt stress signal transducer and a subtilisin-like serine protease (AtS1P) is a key component of the protein processing machinery in the signaling pathway. AtbZIP17 is normally located in the ER, but in response to salt stress, the membrane-bound transcription factor is proteolytically processed in an AtS1P-dependent step, and the cytosolic component of AtbZIP17 is translocated to the nucleus. We have demonstrated in an in vitro assay that AtS1P directly cleaves AtbZIP17, an initial step in processing. Our study also identified several downstream target genes in the salt signaling pathway. In this addendum, we propose a testable model to describe the action of AtS1P and AtbZIP17 in salt stress signaling, and we also present an outlook for future work to test this model.

Open Access Article

Distinct RNA motifs mediate systemic RNA trafficking

Xuehua Zhong and Biao Ding
Pages 58 - 59
http://dx.doi.org/10.4161/psb.3.1.4890
Abstract | PDF

Systemic RNA trafficking is emerging as a new paradigm in gene regulation at the whole plant level. The mechanisms that ensure delivery of all sorts of RNAs to their respective cellular destinations remain poorly understood. Several lines of research suggest that specific sequence/structural motif(s) within an RNA directs its trafficking. The first motif was identified in Potato spindle tuber viroid (PSTVd), which mediates trafficking from bundle sheath to mesophyll in young tobacco leaves. Our recent work identified a tertiary structural motif in PSTVd that is required for trafficking from bundle sheath to phloem in Nicotiana benthamiana to initiate long-distance trafficking. This motif consists of a U/C cis Watson-Crick/Watson-Crick base pair with water insertion to widen the minor groove. Similar motifs that are structurally conserved in rRNAs serve as protein-binding sites. These data suggest that distinct RNA motifs interact with specific cellular factors for trafficking across various cellular boundaries to achieve systemic trafficking.

Open Access Article

The iron deficiency-induced phenolics secretion plays multiple important roles in plant iron acquisition underground

Chong Wei Jin, Guang Yi You and Shao Jian Zheng
Pages 60 - 61
http://dx.doi.org/10.4161/psb.3.1.4902
Abstract | PDF

In non-graminaceous monocots and dicots, phenolic compounds are frequently reported to be the main components of root exudates in response to Fe deficiency. We show that the phenolics secretion is an important part of a plant’s adaptive strategy to Fe deficiency stress that encourages a reutilization of the considerable amounts of Fe normally stored and unavailable in the root apoplast. Besides, we also found that the secreted phenolics can selectively alter the soil microbial community, and the altered soil microbial community may in turn favor plant Fe acquisition by producing siderophores and auxins.

Open Access Article

Peering through the pore: The role of AtTPR in nuclear transport and development

Yannick Jacob and Scott D. Michaels
Pages 62 - 64
http://dx.doi.org/10.4161/psb.3.1.4903
Abstract | PDF

In eukaryotes, most genetic material resides in a separate membrane-bound compartment known as the nucleus. Transport of cargo, such as RNA and protein, across this barrier is facilitated by the nuclear pore complex (NPC). In the July issue of Plant Physiology, we showed that a component of the NPC, Arabidopsis thaliana TPR (AtTPR), is required for normal development. Two striking phenotypes of attpr mutants are that they are early flowering and show an accumulation of polyadenylated RNA in the nucleus. In addition, the expression of several microRNAs (miRNAs) is reduced in attpr mutants. In this addendum, we have examined the effect of AtTPR on the expression of miRNA targets. Our results show that miRNA targets are more likely to be upregulated than other transcripts in attpr mutants. For example, when comparing the nuclear RNA pool between wild-type and attpr plants, we found that 75% of the miRNA targets showing a significant change in transcript level are upregulated in attpr mutants. Although the targets of some miRNAs were upregulated, other miRNA targets were relatively unaffected by attpr mutations. Thus it appears that AtTPR may be required for the proper expression or localization of a subset of miRNAs.

Open Access Article

Repression of transcription factors by microRNA during seed germination and postgerminaiton: Another level of molecular repression in seeds?

Hiroyuki Nonogaki
Pages 65 - 67
http://dx.doi.org/10.4161/psb.3.1.4918
Abstract | PDF

There are multiple layers of molecular repression during seed germination and seedling growth which are important to maintain normal developmental programs at the early stages of plant growth. Genes involved in hormone biosynthesis and deactivation which determine the dormant or germinative status of seeds are generally controlled at the transcriptional level. In contrast, regulation involving protein modification and degradation which occur at the posttranslational level seem to be more important for the removal of proteins suppressing seed germination and seedling growth. Recent work indicates that posttranscriptional repression, another level of molecular regulation, is also critical for seed germination and postgermination. This addendum highlights recent findings concerning the repression of transcription factors by microRNA (miRNA) in Arabidopsis seeds and seedlings. Characterization of the mutant expressing miR160-resistant AUXIN RESPONSE FACTOR10 suggested the possible involvement of auxin-ABA crosstalk in seed germination. The new findings are integrated into the current model of seed germination mechanisms depicting a balance between embryo growth potential and endosperm mechanical resistance.

Open Access Article

Efflux of hydraulically lifted water from mycorrhizal fungal hyphae during imposed drought

Louise M. Egerton-Warburton, José Ignacio Querejeta and Michael F. Allen
Pages 68 - 71
http://dx.doi.org/10.4161/psb.3.1.4924
Abstract | PDF

Apart from improving plant and soil water status during drought, it has been suggested that hydraulic lift (HL) could enhance plant nutrient capture through the flow of mineral nutrients directly from the soil to plant roots, or by maintaining the functioning of mycorrhizal fungi. We evaluated the extent to which the diel cycle of water availability created by HL covaries with the efflux of HL water from the tips of extramatrical (external) mycorrhizal hyphae, and the possible effects on biogeochemical processes. Phenotypic mycorrhizal fungal variables, such as total and live hyphal lengths, were positively correlated with HL efflux from hyphae, soil water potential (dawn), and plant response variables (foliar 15N). The efflux of HL water from hyphae was also correlated with bacterial abundance and soil enzyme activity (P), and the moistening of soil organic matter. Such findings indicate that the efflux of HL water from the external mycorrhizal mycelia may be a complementary explanation for plant nutrient acquisition and survival during drought.

Open Access Article

Does light taste salty?

Marta Rodriguez-Franco, Felipe Sarmiento, Katrin Marquardt, Ralf Markus and Gunther Neuhaus
Pages 72 - 73
http://dx.doi.org/10.4161/psb.3.1.4925
Abstract | PDF

As research advances acquisition of new data reveals novel aspects on already investigated issues. This is the case for SALT TOLERANCE (STO), an Arabidopsis protein that confers tolerance to high salt concentrations when ectopically expressed in yeast cells. For the last years, STO was considered to participate mainly in the response and tolerance of Arabidopsis to high salinity, as it does in yeast. However, recent investigations using gain- and loss-of-function mutants revealed a major role for STO as negative regulator of photomorphogenesis. Interestingly, and contrary to other negative regulators of light dependent inhibition of hypocotyl elongation, STO protein instability is controlled by COP1 activity in etiolated seedlings. Thus, light stabilizes STO protein levels during de-etiolation. Whether STO participates in other signaling cascades different from light signaling, as it has been shown in yeast and proposed in plants or not, is still an open question.

Open Access Article

Auxin, gibberellins, and the gravitropic response of grass leaf sheath pulvini

John J. Ross and Carla M. Wolbang
Pages 74 - 75
http://dx.doi.org/10.4161/psb.3.1.4929
Abstract | PDF

The role of hormones in mediating tropic responses has been a central question in plant biology. Another key issue concerns how interactions between hormones regulate plant responses. In the September 2007 issue of Physiologia Plantarum, we publish a paper relevant to both these questions.1 This paper focuses on gravitropism in the barley leaf sheath pulvinus. The results support the Cholodny-Went theory on hormones and tropic responses, and highlight how an environmental factor (gravity) appears to first affect auxin content and consequently that of bioactive gibberellins (GAs). It appears that while GAs do not actually trigger the gravitropic bending of barley pulvini, they do act to magnify the bending response.