生长素信号转导途径与植物胁迫反应相互作用的证据(英)

生长素影响植物多种生理过程 ,有报道显示生长素可能影响植物对逆境胁迫的反应。我们利用cDNA阵列技术鉴定拟南芥 (Arabidopsisthaliana (L .)Heynh .)的生长素应答基因 ,发现多个胁迫应答基因受生长素抑制 ,包括ArabidopsishomologofMEKkinase1(ATMEKK1) ,RelA/SpoThomolog 3(At_RSH3) ,Catalase 1(Cat1)和Ferritin 1(Fer1) ,说明生长素可调节胁迫应答基因的表达。此外 ,我们还证明吲哚乙酸 (IAA)合成途径中的腈水解酶基因nitrilase 1(NIT1)和nitrilase 2 (NIT2 )受盐胁迫诱导 ,提示在逆境条件下IAA的合成可能随之增加。我们利用生长素不敏感突变体研究生长素与逆境反应相互作用的信号转导 ,发现胁迫应答基因在野生型和生长素不敏感突变体auxinresistant2 (axr2 )中可被盐胁迫诱导 ,而在auxinresistant1_3(axr1_3)中则不被诱导 ,说明生长素与逆境胁迫反应的相互作用可能发生在泛素途径。     

生长素信号转导途径与植物胁迫反应相互作用的证据

生长素影响植物多种生理过程,有报道显示生长素可能影响植物对逆境胁迫的反应.我们利用cDNA阵列技术鉴定拟南芥(Arabidopsis thaliana (L.) Heynh.)的生长素应答基因,发现多个胁迫应答基因受生长素抑制,包括Arabidopsis homolog of MEK kinase1 (ATMEKK1),RelA/SpoT homolog 3 (At-RSH3),Catalase 1 (Cat1) 和Ferritin 1 (Fer1),说明生长素可调节胁迫应答基因的表达.此外,我们还证明吲哚乙酸(IAA)合成途径中的腈水解酶基因nitrilase 1 (NIT1) 和nitrilase 2 (NIT2) 受盐胁迫诱导,提示在逆境条件下IAA的合成可能随之增加.我们利用生长素不敏感突变体研究生长素与逆境反应相互作用的信号转导,发现胁迫应答基因在野生型和生长素不敏感突变体auxin resistant 2 (axr2) 中可被盐胁迫诱导,而在auxin resistant 1-3 (axr1-3)中则不被诱导,说明生长素与逆境胁迫反应的相互作用可能发生在泛素途径.     

乙烯调控植物耐盐性的研究进展

乙烯具有复杂的生物学功能,它调节着植物生长发育和许多的生理生化过程。乙烯也被认为是一种胁迫应答激素,直到近几年关于乙烯生物合成及信号转导途径与植物盐胁迫的关系才逐渐被挖掘出来。乙烯在不同水平、层次参与盐胁迫反应,包括乙烯合成关键酶(ACS)和乙烯受体,细胞质中CTR1和EIN2以及细胞核中EIN3传导、响应盐信号。但是乙烯合成和信号转导途径在植物盐胁迫响应过程中仍然存在许多未解决的问题。主要介绍乙烯合成及信号转导途径的各组分与盐胁迫关系的最新研究进展,并讨论其存在的主要问题。     

毛竹对大气臭氧胁迫的生理响应变化

以毛竹1年生盆栽苗为材料,运用开顶式气室(OTCs)模拟环境背景大气O3浓度(AA,40～45 nL·L-1)和高O3浓度(EO,92～106 nL·L-1)情景,分析毛竹叶片光合生理、脂质过氧化及抗氧化酶等主要生理指标的变化,为气候变化背景下的竹林培育应对策略提供理论依据.结果显示:(1)EO较AA在同一处理时间的毛竹叶片O3通量均显著升高,且二处理的叶片O3通量均随着处理时间的延长呈升高趋势.(2) EO较AA的光合速率(Pn)、气孔导度(Gs)和可溶性糖含量均显著下降,且叶片叶绿素(ChD含量、胞间CO2浓度(Ci)显著下降的时间点分别出现在EO处理的60 d和92 d,可溶性蛋白在处理60 d后显著升高;随处理时间的延长,EO的叶片Pn、Ci、Chl含量均呈下降趋势,可溶性糖和可溶性蛋白含量呈先升高后降低的趋势;Pn下降由气孔限制因素引起.(3)超氧自由基(O2)含量、丙二醛(MDA)含量、相对电导率分别在处理29 d、60 d、60 d后均显著升高,且随着处理时间的延长呈升高趋势.(4)超氧化物歧化酶(SOD)活性在高浓度O3处理60 d时显著升高,后显著下降,而POD活性均显著升高,且SOD和POD活性均随着处理时间呈先升高后降低的趋势.研究表明,毛竹对大气高O3胁迫存在着短时间的主动生理生化适应,但长期高O3胁迫会对毛竹造成严重的过氧化伤害,从而影响毛竹的正常生长.     

Schizophrenia: Developmental Variability Interacts with Risk Factors to Cause the Disorder

A new etiological model is proposed for schizophrenia that combines variability-enhancing nonspecific factors acting during development with more specific risk factors. This model is better suited than the current etiological models of schizophrenia, based on the risk factors paradigm, for predicting and/or explaining several important findings about schizophrenia: high co-morbidity rates, low specificity of many risk factors, and persistence in the population of the associated genetic polymorphisms. Compared with similar models, e.g., de-canalization, common psychopathology factor, sexual-selection, or differential sensitivity to the environment, this proposal is more general and integrative. Recently developed research methods have proven the existence of genetic and environmental factors that enhance developmental variability. Applying such methods to newly collected or already available data can allow for testing the hypotheses upon which this model is built. If validated, this model may change the understanding of the etiology of schizophrenia, the research models, and preventionbrk paradigms.     

Focus on Ethylene: Role of Ethylene and Its Cross Talk with Other Signaling Molecules in Plant Responses to Heavy Metal Stress

Excessive heavy metals (HMs) in agricultural lands cause toxicities to plants, resulting in declines in crop productivity. Recent advances in ethylene biology research have established that ethylene is not only responsible for many important physiological activities in plants but also plays a pivotal role in HM stress tolerance. The manipulation of ethylene in plants to cope with HM stress through various approaches targeting either ethylene biosynthesis or the ethylene signaling pathway has brought promising outcomes. This review covers ethylene production and signal transduction in plant responses to HM stress, cross talk between ethylene and other signaling molecules under adverse HM stress conditions, and approaches to modify ethylene action to improve HM tolerance. From our current understanding about ethylene and its regulatory activities, it is believed that the optimization of endogenous ethylene levels in plants under HM stress would pave the way for developing transgenic crops with improved HM tolerance.In addition to common abiotic stresses seen in agricultural production, such as drought, submerging, and extreme temperatures (Thao and Tran, 2012; Xia et al., 2015), heavy metal (HM) stress has arisen as a new pervasive threat (Srivastava et al., 2014; Ahmad et al., 2015). This is mainly due to the unrestricted industrialization and urbanization carried out during the past few decades, which have led to the increase of HMs in soils. Plants naturally require more than 15 different types of HM as nutrients serving for biological activities in cells (Sharma and Chakraverty, 2013). However, when the nutritional/nonnutritional HMs are present in excess, plants have to either suffer or take these up from the soil in an unwilling manner (Nies, 1999; Sharma and Chakraverty, 2013). Upon HM stress exposure, plants induce oxidative stress due to the excessive production of reactive oxygen species (ROS) and methylglyoxal (Sharma and Chakraverty, 2013). High levels of these compounds have been shown to negatively affect cellular structure maintenance (e.g. induction of lipid peroxidation in the membrane, biological macromolecule deterioration, ion leakage, and DNA strand cleavage; Gill and Tuteja, 2010; Nagajyoti et al., 2010) as well as many other biochemical and physiological processes (Dugardeyn and Van Der Straeten, 2008). As a result, plant growth is retarded and, ultimately, economic yield is decreased (Yadav, 2010; Anjum et al., 2012; Hossain et al., 2012; Asgher et al., 2015). Moreover, the accumulation of metal residues in the major food chain has been shown to cause serious ecological and health problems (Malik, 2004; Verstraeten et al., 2008).Plants employ different strategies to detoxify the unwanted HMs. Among the common responses of plants to HM stress are increases in ethylene production due to the enhanced expression of ethylene-related biosynthetic genes (Asgher et al., 2014; Khan and Khan, 2014; Khan et al., 2015b) and/or changes in the expression of ethylene-responsive genes (Maksymiec, 2007). Conventionally, this hormone has been established to modulate a number of important plant physiological activities, including seed germination, root hair and root nodule formation, and maturation (fruit ripening in particular; Dugardeyn and Van Der Straeten, 2008). On the other hand, although ethylene has also been suggested to be a stress-related hormone responding to a number of biotic and abiotic triggers, little is known about the exact role of elevated HM stress-related ethylene in plants (Zapata et al., 2003). Enhanced production of ethylene in plants subjected to toxic levels of cadmium (Cd), copper (Cu), iron (Fe), nickel (Ni), and zinc (Zn) has been shown (Maksymiec, 2007). As an example, Cd- and Cu-mediated stimulation of ethylene synthesis has been reported as a result of the increase of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) activity, one of the enzymes involved in the ethylene synthesis pathway (Schlagnhaufer and Arteca, 1997; Khan et al., 2015b).Plants tend to adjust or induce adaptation or tolerance mechanisms to overcome stress conditions. To develop stress tolerance, plants trigger a network of hormonal cross talk and signaling, among which ethylene production and signaling are prominently involved in stress-induced symptoms in acclimation processes (Gazzarrini and McCourt, 2003). Therefore, the necessity of controlling ethylene homeostasis and signal transduction using biochemical and molecular tools remains open to combat stress situations. Stress-induced ethylene acts to trigger stress-related effects on plants because of the autocatalytic ethylene synthesis. Autocatalytic stress-related ethylene production is controlled by mitogen-activated protein kinase (MAPK) phosphorylation cascades (Takahashi et al., 2007) and through stabilizing ACS2/6 (Li et al., 2012). Strong lines of evidence have shown the multiple facets of ethylene in plant responses to different abiotic stresses, including excessive HM, depending upon endogenous ethylene concentration and ethylene sensitivities that differ in developmental stage, plant species, and culture systems (Pierik et al., 2006; Kim et al., 2008; Khan and Khan, 2014). Under HM stress conditions, plants show a rapid increase in ethylene production and reduced plant growth and development, suggesting a negative regulatory role of ethylene in plant responses to HM stress (Schellingen et al., 2014; Khan et al., 2015b). On the other hand, a potential involvement of ETHYLENE INSENSITIVE2 (EIN2), a central component of the ethylene signaling pathway, as a positive regulator in lead (Pb) resistance in Arabidopsis (Arabidopsis thaliana) has also been demonstrated (Cao et al., 2009). More recently, Khan and Khan (2014) showed that ethylene-regulated antioxidant metabolism maintained a higher level of reduced glutathione (GSH) and alleviated photosynthetic inhibition in mustard (Brassica juncea) plants exposed to Ni, Zn, or Cd through the optimization of ethylene homeostasis (Masood et al., 2012). Taken together, the purpose of this review is to update the research community with our current understanding of the roles of ethylene and its signaling in plant responses to HM stress. Moreover, the cross talk of ethylene with other phytohormones and signaling molecules upon HM stress will also be discussed.     

乙烯在植物应答水分胁迫和病原菌浸染中的作用

文章介绍乙烯在植物应答水分胁迫及病原菌侵染中作用的研究进展。     

The Role in Ozone Phytotoxicity of the Evolution of Ethylene upon Induction of 1-Aminocydopropane-1-carboxylic Acid Synthase by Ozone Fumigation in Tomato Plants

The rate of evolution of ethylene by tomato plants was rapidlyincreased by O₃ fumigation. The time course of the increasein 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activitywas the same as that in the rate of evolution of ethylene, suggestingthat ACC synthase activity might be a rate-limiting step inthe evolution of ethylene that is caused by O₃ fumigation. Therate of the O₃-induced evolution of ethylene was increased bythe application of ACC to tomato plants, suggesting the involvementof ACC oxidase in the O₃-induced evolution of ethylene. Treatmentof plants with tiron inhibited the evolution of ethane, butnot of ethylene. These results indicated that evolution of ethylenein O₃-treated tomato plants might result from enzymatic reactionscatalyzed by both ACC synthase and ACC oxidase, but not fromstimulation by O₃ of the peroxidation of lipids mediated byfree radicals. Pretreatment of leaves with aminoethoxyvinylglycine (AVG), aninhibitor of ACC synthase, significantly inhibited the evolutionof ethylene that was induced by O₃ and concomitantly reducedthe extent of O₃-induced visible damage to leaves. Treatmentwith 2,5-norbonadiene, an inhibitor of the action of ethylene,strongly reduced the extent of visible damage caused by O₃,even though it did not suppress the evloution of ethylene. Theseresults indicate that ethylene acts on certain metabolic processesto cause visible damage. (Received September 7, 1995; Accepted December 18, 1995)     

The Antipsychotic Olanzapine Interacts with the Gut Microbiome to Cause Weight Gain in Mouse

The second-generation antipsychotic olanzapine is effective in reducing psychotic symptoms but can cause extreme weight gain in human patients. We investigated the role of the gut microbiota in this adverse drug effect using a mouse model. First, we used germ-free C57BL/6J mice to demonstrate that gut bacteria are necessary and sufficient for weight gain caused by oral delivery of olanzapine. Second, we surveyed fecal microbiota before, during, and after treatment and found that olanzapine potentiated a shift towards an “obesogenic” bacterial profile. Finally, we demonstrated that olanzapine has antimicrobial activity in vitro against resident enteric bacterial strains. These results collectively provide strong evidence for a mechanism underlying olanzapine-induced weight gain in mouse and a hypothesis for clinical translation in human patients.     

Plant Growth Promoting Rhizobacteria Protect Wheat Plants Against Temperature Stress Through Antioxidant Signalling and Reducing Chloroplast and Membrane Injury

Journal of Plant Growth Regulation - In India and other tropics, wheat plants suffer from exposure to high temperature with heat increase above 35 °C causing stress-like conditions....     

Foxtail Millet CBL4 (SiCBL4) Interacts with SiCIPK24, Modulates Plant Salt Stress Tolerance

The calcineurin B-like (CBL) protein and the CBL-interacting protein kinase (CIPK) signaling pathway play important roles in plant abiotic stress tolerance. To investigate the molecular mechanism of salt stress tolerance of foxtail millet, SiCBL4 and SiCIPK24 were identified and functionally characterized. Both SiCBL4 and SiCIPK24 were induced by salt, abscisic acid (ABA), methyl viologen (MV), and heat shock stress in foxtail millet seedlings. Yeast two-hybrid and bimolecular fluorescence complementation assay showed that SiCBL4 interacted with SiCIPK24. The mutation of the N-myristoylation site of SiCBL4 changed the sub-cellular localization of SiCBL4 and directed the SiCBL4-SiCIPK24 protein complex from plasma membrane to cytoplasm, and disrupted its function in plant salt stress tolerance. Overexpression of SiCBL4 or SiCIPK24 in Arabidopsis sos3-1 or sos2-1 mutant plants rescued the mutant salt hypersensitivity phenotype. In addition, overexpression of SiCIPK24 also enhanced the salt stress tolerance of Arabidopsis wild-type plants. This work helps to understand the structure and function of the foxtail millet CBL and CIPK genes and confirmed that the foxtail millet CBL-CIPK pathway can be manipulated to enhance the plant salt stress tolerance.     

Ethylene Production by Fe-deficient Roots and its Involvement in the Regulation of Fe-deficiency Stress Responses by Strategy I Plants

Species that showed marked morphological and physiological responsesby their roots to Fe-deficiency (Strategy I plants) were comparedwith others that do not exhibit these responses (Strategy IIplants). Roots from Fe-deficient cucumber (Cucumis sativusL.‘Ashley’), tomato (Lycopersicon esculentumMill.T3238FER) and pea (Pisum sativumL. ‘Sparkle’) plantsproduced more ethylene than those of Fe-sufficient plants. Thehigher production of ethylene in Fe-deficient cucumber and peaplants occurred before Fe-deficient plants showed chlorosissymptoms and was parallel to the occurrence of Fe-deficiencystress responses. The addition of either the ethylene precursorACC, 1-aminocyclopropane-1-carboxylic acid, or the ethylenereleasing substance, Ethephon, to several Fe-sufficient StrategyI plants [cucumber, tomato, pea, sugar beet (Beta vulgarisL.),Arabidopsis(Arabidopsis thaliana(L.) Heynh ‘Columbia’), plantago(Plantago lanceolataL.)] promoted some of their Fe-deficiencystress responses: enhanced root ferric-reducing capacity andswollen root tips. By contrast, Fe-deficient roots from severalStrategy II plants [maize (Zea maysL. ‘Funo’), wheat(Triticum aestivumL. ‘Yécora’), barley (HordeumvulgareL. ‘Barbarrosa’)] did not produce more ethylenethan the Fe-sufficient ones. Furthermore, ACC had no effecton the reducing capacity of these Strategy II plants and, exceptin barley, did not promote swelling of root tips. In conclusion,results suggest that ethylene is involved in the regulationof Fe-deficiency stress responses by Strategy I plants.Copyright1999 Annals of Botany Company. Arabidopsis (Arabidopsis thaliana(L.) Heynch), barley (Hordeum vulgareL.), cucumber (Cucumis sativusL.), ethylene, iron deficiency, maize (Zea maysL.), pea (Pisum sativumL.), plantago (Plantago lanceolataL.), ferric-reducing capacity, sugar beet (Beta vulgarisL.), tomato (Lycopersicon esculentumMill.), wheat (Triticum aestivumL.).     

Contribution of Oxidative Stress to the Development of Cold-Induced Damage to Leaves of Chilling-Sensitive Plants: 1. Reactive Oxygen Species Formation during Plant Chilling

Changes in the levels of superoxide anion radical and total peroxides were studied immediately after the chilling of 7–11-day-old seedlings of maize (Zea mays L.), cucumber (Cucumis sativus L.), millet (Panicum miliaceum L.), and etiolated potato (Solanum tuberosum L.) shoots at 2°C for 1–24 h and one day after 24-h chilling. A short-term (1 h) chilling of chilling-sensitive plants resulted in the 2.4–7.5-fold acceleration of the O^– ₂ generation. A longer chilling period reduced somewhat the rate of O^– ₂ generation, but this rate did not achieve the control level. The highest level of H₂O₂ was observed after 2-h chilling with its subsequent lowering. In the cold-tolerant potato, the levels of O^– ₂ and peroxides reduced after chilling. The rate of lipid peroxidation (an index characterizing cold-induced membrane damage) increased gradually with the lengthening of the chilling period. Reactive oxygen species are supposed to be involved in the induction of the oxidative stress during chilling of chilling-sensitive plants and in the triggering of cold-induced damage.     

Actin-binding Protein-1 Interacts with WASp-interacting Protein to Regulate Growth Factor-induced Dorsal Ruffle Formation

Growth factor stimulation induces the formation of dynamic actin structures known as dorsal ruffles. Mammalian actin-binding protein-1 (mAbp1) is an actin-binding protein that has been implicated in regulating clathrin-mediated endocytosis; however, a role for mAbp1 in regulating the dynamics of growth factor–induced actin-based structures has not been defined. Here we show that mAbp1 localizes to dorsal ruffles and is necessary for platelet-derived growth factor (PDGF)-mediated dorsal ruffle formation. Despite their structural similarity, we find that mAbp1 and cortactin have nonredundant functions in the regulation of dorsal ruffle formation. mAbp1, like cortactin, is a calpain 2 substrate and the preferred cleavage site occurs between the actin-binding domain and the proline-rich region, generating a C-terminal mAbp1 fragment that inhibits dorsal ruffle formation. Furthermore, mAbp1 directly interacts with the actin regulatory protein WASp-interacting protein (WIP) through its SH3 domain. Finally, we demonstrate that the interaction between mAbp1 and WIP is important in regulating dorsal ruffle formation and that WIP-mediated effects on dorsal ruffle formation require mAbp1. Taken together, these findings identify a novel role for mAbp1 in growth factor–induced dorsal ruffle formation through its interaction with WIP.     

Plant Host Finding by Parasitic Plants: A New Perspective on Plant to Plant Communication

Plants release airborne chemicals that can convey ecologically relevant information to other organisms. These plant volatiles are known to mediate a large array of, often complex, interactions between plants and insects. It has been suggested that plant volatiles may have similar importance in mediating interactions among plant species, but there are few well-documented examples of plant-to-plant communication via volatiles, and the ecological significance of such interactions has been much debated. To date, nearly all studies of volatile-mediated interactions among plant species have focused on the reception of herbivore-induced volatiles by neighboring plants. We recently documented volatile effects in another system, demonstrating that the parasitic plant Cuscuta pentagona uses volatile cues to locate its hosts. This finding may broaden the discussion regarding plant-to-plant communication, and suggests that new classes of volatile-meditated interactions among plant species await discovery.Key Words: chemical communication, Cuscuta pentagona, host fiding, host selection, plant-plant communication, plant volatiles, parasitic plantsFor nearly 25 years, the ecological importance of plant-to-plant communication through volatiles has remained an open and much debated question. Plants exchange gases with the atmosphere and, in so doing, release plumes of volatile chemicals that can convey ecologically important information to other organisms. The potential ecological significance of these volatile cues is demonstrated by the large and growing array of interactions between plants and arthropods known to be mediated by plant volatiles. Volatiles serve as foraging cues both for insects that are beneficial to plants, such as pollinators,¹ and those that are harmful such as herbivores.^2,3 Because the volatile blends released by plants exhibit variation in response to environmental stimuli, volatiles can convey detailed information about the status of the emitting plant. Predatory and parasitic insects that feed on herbivorous insects respond preferentially to plant volatiles that are induced by insect feeding,⁴ while female herbivores use such cues to avoid laying their eggs on already-infested plants.^3,5 Moreover, the volatile blends released in response to herbivory can differ between individual herbivore species, providing highly specific cues to specialist parasitoids.⁶ Thus, plant volatiles are known to mediate complex interactions among plants and insects across multiple trophic levels.It has long been speculated that plant volatiles might have similar significance for interactions among plant species, yet there are few well-documented examples of communication between plants by way of volatile signals. Essentially all previous work on plant-to-plant communication has focused on the reception of herbivore-induced volatile signals by neighboring plants, which may use them as early warning signals to initiate their own direct and indirect defense responses. The first studies claiming to document such effects were published almost 25 years ago.^7,8 But issues with the experimental design of these early experiments and the availability of alternative explanations for their results led many ecologists to disregard the phenomenon.^9–11 Later, a number of studies demonstrated that direct and indirect plant defenses could be elicited by exposure to certain induced plant volatiles.^12–15 But many of these effects were demonstrated in airtight chambers with volatile concentrations far higher than those likely experienced in natural settings, again raising doubts about the ecological significance of plant-plant communication.^16–18 Still more recently, some researchers have provided evidence that more realistic volatile concentrations likely induce priming of the defenses of receiving plants, rather than the initiation of full scale responses,¹⁵ while others have documented volatile effects under natural conditions.^19–21 Thus, despite continuing caution about the interpretation of experiments in this area,^17,18 there is mounting evidence that plant herbivore-induced volatiles can serve as early warning signals to neighboring plants.We recently documented an entirely new class of volatile mediated interactions among plants: the role of plant volatiles in host location by parasitic plants that attach to above ground shoots of other plants. Plant parasites are important components of natural and agricultural ecosystems and play important roles in determining community structure and dynamics.^22,23 We are exploring the mechanisms of host-location and other interactions between parasitic plants in the genus Cuscuta (dodder) and their host plants. Dodder vines germinate from seeds containing limited energy reserves and, as the parasites have no roots and little photosynthetic ability, must quickly locate and attach to suitable hosts in order to survive (Fig. 1). Thus, there is presumably significant selection pressure for dodder vines to employ efficient strategies for host location, and host plant volatiles may be expected to provide relevant directional cues. Dodder seedlings exhibit a rotational growth habit (circumnutation) following germination and previous researchers have suggested that host-finding might involve random growth²⁴ or the exploitation of light cues.²⁵Open in a separate windowFigure 1Seedling of Cuscuta pentagona (A) foraging toward a 20-day-old tomato plant, (B) attaching to and beginning to grow from stems of tomato seedlings and (C) close up of C. pentagona attachment.Using a very simple experimental design, we explored the possibility that host-plant volatiles might mediate host-location by seedlings of C. pentagona. We placed a germinated seedling in a vial of water located at the center of a dry filter paper disk. A host plant (a 20-day old tomato seedling) was placed near the edge of the disk and the dodder seedling was allowed to forage for four days. By the end of the experiment the seedling would lay horizontally on the disk and we traced its position on the filter paper in order to assess the directionality of growth relative to the host plant. This experiment was replicated 30 times and our results clearly indicated directional growth toward the tomato plant (80% of the tested seedlings grew into the disk half nearest the host) demonstrating that C. pentagona seedlings were perceiving some host-derived cue.We did not observe directed growth when we tested dodder seedling response to alternative targets including pots of moist soil, artificial plants, and vials of colored water intended to mimic possible light cues. In order to confirm a role for plant volatiles in host location by C. pentagona, we tested seedling response to host plant volatiles extracted from filtered air in a volatile collection system and then released from rubber septa in the absence of any other host-derived cues. Here we observed a directed growth response similar to that exhibited toward an intact tomato seedling, confirming that host plant volatiles do provide a cue used for host location by C. pentagona. In subsequent experiments we found directed growth toward impatiens and alfalfa plants, which are attractive hosts for C. pentagona and also toward wheat plants which are poor hosts, suggesting that the host-location mechanisms operate over a wide range of host species.Since discriminating between more and less desirable host species is likely to be important in natural settings, we next explored whether dodder seedlings could distinguish volatile signals from host and nonhost plants. Cuscuta pentagona seedlings exhibited directional growth toward tomato plants in preference to wheat plants and also to extracted volatiles from tomato in preference to those from wheat, demonstrating an ability to distinguish and choose among volatiles from more and less preferred hosts.When we tested seedling responses to individual compounds from the wheat and tomato blends, we found that three compounds from tomato, α-pinene, β-myrcene, and β-phellandrene elicited directed growth. β-myrcene was also present in the wheat blend. Unexpectedly, we also found that one compound present in the wheat blend, (Z)-3-hexenyl acetate, was repellent, providing a plausible explanation for the lower attractiveness of the wheat blend. It is interesting to note that (Z)-3-hexenyl acetate is also released by tomato in response to feeding by herbivores, and we have some data suggesting that C. pentagona seedlings may find tomato seedlings infested by Heliothis virescens caterpillars less attractive than un-attacked plants (unpublished data).The discovery that some parasitic plants exploit host plant volatiles for host location provides a new perspective on volatile mediated interactions among plant species, demonstrating that plant volatiles play a role in mediating ecologically significant interactions in at least one system other than the transfer of herbivore-induced warning signals. We think it is quite likely that plant volatiles will be found to play a role in host location by other parasitic plants and perhaps even by vining plants generally. Moreover, we think it is more likely than not that more classes of volatile mediated interactions among plants remain to be discovered given the potential availability of volatile cues and the fitness benefits to be derived by plants using such cues to gather information about the identity and condition of their neighbors.     

Inhibition by Ethylene of Auxin-Promotion of Flower Bud Formation in Tobacco Explants Is Absent in Plants Transformed by Agrobacterium rhizogenes

The in vitro regeneration of flower buds was studied in pedicel explants from tobacco (Nicotiana tabacum L., cv Petit Havana) transformed with Agrobacterium rhizogenes, pRi 1855 (agropine type). At a low concentration (0.1 micromolar) of 1-naphthalene-acetic acid, pedicel strips from phenotypically aberrant plants regenerated two to three times more flower buds than explants from untransformed tobacco. Intermediate bud numbers were observed in transformants with a less extreme phenotype. The results can be explained by an increased sensitivity of the transformed explants to auxin with respect to flower bud regeneration. The effect of transformation on the auxin response is fully accounted for by the absence of a negative interaction of endogenous ethylene with 1-naphthaleneacetic acid, a phenomenon normally encountered in untransformed tissues. Three observations led to this conclusion. Application of 1 micromolar AgNO₃ to untransformed explants increased the number of flower buds to the level observed in transformed tissues but had no effect on transformed pedicel strips; exposure to 10 microliters per liter ethylene strongly reduced the response to auxin at all concentrations in untransformed explants but was almost ineffective in the transformed tissues; and endogenous ethylene synthesis occurred at the same rate in both types of explants.     

Plant Resistance Mechanisms to Air Pollutants: Rhythms in Ascorbic Acid Production During Growth Under Ozone Stress

Relationships between ozone (O₃) tolerance and leaf ascorbic acid concentrations in 0₃-susceptible (O₃S) 'Hark' and O₃-resistant (O₃R) 'Hood'soybean, Glycine max (L.) Merr., cultivars were examined with high-performance liquid chromatography (HPLC). Leaf samples were analyzed at 4 h intervals during a 24 h period. Soybean cultivars grown in the greenhouse with charcoal filtered (CF) and nonfiltered (NF) air showed daily oscillations in ascorbic acid production. Highest ascorbic acid levels in leaves during light coincided with highest concentrations of photochemical oxidants in the atmosphere at 2:00 p.m. The resistant genotype produced more ascorbic acid in its trifoliate leaves than did the corresponding susceptible genotype. Under CF air (an O₃-reduced environment) O₃-S and O₃-R cultivars showed rhythms in ascorbic acid production. In NF air (an O₃ stress environment) the O₃-R cultivar alone showed rhythms in ascorbic acid production. Results indicated that superior O₃ tolerance in the Hood soybean cultivar (compared with Hark) was associated with a greater increase in endogenous levels of ascorbic acid. Ascorbic acid may scavenge free radicals and thereby protect cells from injury by O₃ or other oxyradical products. Plants defend themselves against photochemical oxidant stress, such as O₃, by several mechanisms. Experimental evidence indicates that antioxidant defense systems existing in plant tissues may function to protect cellular components from deleterious effects of photochemical oxidants through endogenous and exogenous controls.