Computation of Surface Electrical Potentials of Plant Cell Membranes : Correspondence to Published Zeta Potentials from Diverse Plant Sources

A Gouy-Chapman-Stern model has been developed for the computation of surface electrical potential (ψ0) of plant cell membranes in response to ionic solutes. The present model is a modification of an earlier version developed to compute the sorption of ions by wheat (Triticum aestivum L. cv Scout 66) root plasma membranes. A single set of model parameters generates values for ψ0 that correlate highly with published ζ potentials of protoplasts and plasma membrane vesicles from diverse plant sources. The model assumes ion binding to a negatively charged site (R− = 0.3074 μmol m−2) and to a neutral site (P0 = 2.4 μmol m−2) according to the reactions R− + IΖ ⇌ RIΖ−1 and P0 + IΖ ⇌ PIΖ, where IΖ represents an ion of charge Ζ. Binding constants for the negative site are 21,500 m−1 for H+, 20,000 m−1 for Al3+, 2,200 m−1 for La3+, 30 m−1 for Ca2+ and Mg2+, and 1 m−1 for Na+ and K+. Binding constants for the neutral site are 1/180 the value for binding to the negative site. Ion activities at the membrane surface, computed on the basis of ψ0, appear to determine many aspects of plant-mineral interactions, including mineral nutrition and the induction and alleviation of mineral toxicities, according to previous and ongoing studies. A computer program with instructions for the computation of ψ0, ion binding, ion concentrations, and ion activities at membrane surfaces may be requested from the authors.

A Carboniferous insect gall: insight into early ecologic history of the Holometabola.

Although the prevalence or even occurrence of insect herbivory during the Late Carboniferous (Pennsylvanian) has been questioned, we present the earliest-known ecologic evidence showing that by Late Pennsylvanian times (302 million years ago) a larva of the Holometabola was galling the internal tissue of Psaronius tree-fern fronds. Several diagnostic cellular and histological features of these petiole galls have been preserved in exquisite detail, including an excavated axial lumen filled with fecal pellets and comminuted frass, plant-produced response tissue surrounding the lumen, and specificity by the larval herbivore for a particular host species and tissue type. Whereas most suggestions over-whelmingly support the evolution of such intimate and reciprocal plant-insect interactions 175 million years later, we provide documentation that before the demise of Pennsylvanian age coal-swamp forests, a highly stereotyped life cycle was already established between an insect that was consuming internal plant tissue and a vascular plant host responding to that herbivory. This and related discoveries of insect herbivore consumption of Psaronius tissues indicate that modern-style herbivores were established in Late Pennsylvanian coal-swamp forests.

The specific features of methionine biosynthesis and metabolism in plants

Plants, unlike other higher eukaryotes, possess all the necessary enzymatic equipment for de novo synthesis of methionine, an amino acid that supports additional roles than simply serving as a building block for protein synthesis. This is because methionine is the immediate precursor of S-adenosylmethionine (AdoMet), which plays numerous roles of being the major methyl-group donor in transmethylation reactions and an intermediate in the biosynthesis of polyamines and of the phytohormone ethylene. In addition, AdoMet has regulatory function in plants behaving as an allosteric activator of threonine synthase. Among the AdoMet-dependent reactions occurring in plants, methylation of cytosine residues in DNA has raised recent interest because impediment of this function alters plant morphology and induces homeotic alterations in flower organs. Also, AdoMet metabolism seems somehow implicated in plant growth via an as yet fully understood link with plant-growth hormones such as cytokinins and auxin and in plant pathogen interactions. Because of this central role in cellular metabolism, a precise knowledge of the biosynthetic pathways that are responsible for homeostatic regulation of methionine and AdoMet in plants has practical implications, particularly in herbicide design.

Molecular Characterization of the Oxalate Oxidase Involved in the Response of Barley to the Powdery Mildew Fungus1

Previously we reported that oxalate oxidase activity increases in extracts of barley (Hordeum vulgare) leaves in response to the powdery mildew fungus (Blumeria [syn. Erysiphe] graminis f.sp. hordei) and proposed this as a source of H2O2 during plant-pathogen interactions. In this paper we show that the N terminus of the major pathogen-response oxalate oxidase has a high degree of sequence identity to previously characterized germin-like oxalate oxidases. Two cDNAs were isolated, pHvOxOa, which represents this major enzyme, and pHvOxOb', representing a closely related enzyme. Our data suggest the presence of only two oxalate oxidase genes in the barley genome, i.e. a gene encoding HvOxOa, which possibly exists in several copies, and a single-copy gene encoding HvOxOb. The use of 3′ end gene-specific probes has allowed us to demonstrate that the HvOxOa transcript accumulates to 6 times the level of the HvOxOb transcript in response to the powdery mildew fungus. The transcripts were detected in both compatible and incompatible interactions with a similar accumulation pattern. The oxalate oxidase is found exclusively in the leaf mesophyll, where it is cell wall located. A model for a signal transduction pathway in which oxalate oxidase plays a central role is proposed for the regulation of the hypersensitive response.

Identifying functional domains within terpene cyclases using a domain-swapping strategy.

Cyclic terpenes and terpenoids are found throughout nature. They comprise an especially important class of compounds from plants that mediate plant- environment interactions, and they serve as pharmaceutical agents with antimicrobial and anti-tumor activities. Molecular comparisons of several terpene cyclases, the key enzymes responsible for the multistep cyclization of C10, C15, and C20 allylic diphosphate substrates, have revealed a striking level of sequence similarity and conservation of exon position and size within the genes. Functional domains responsible for a terminal enzymatic step were identified by swapping regions approximating exons between a Nicotiana tabacum 5-epi-aristolochene synthase (TEAS) gene and a Hyoscyamus muticus vetispiradiene synthase (HVS) gene and by characterization of the resulting chimeric enzymes expressed in bacteria. While exon 4 of the TEAS gene conferred specificity for the predominant reaction products of the tobacco enzyme, exon 6 of the HVS gene conferred specificity for the predominant reaction products of the Hyoscyamus enzyme. Combining these two functional domains of the TEAS and HVS genes resulted in a novel enzyme capable of synthesizing reaction products reflective of both parent enzymes. The relative ratio of the TEAS and HVS reaction products was also influenced by the source of exon 5 present in the new chimeric enzymes. The association of catalytic activities with conserved but separate exonic domains suggests a general means for generating additional novel terpene cyclases.

Chemical phenotype matching between a plant and its insect herbivore

Two potential outcomes of a coevolutionary interaction are an escalating arms race and stable cycling. The general expectation has been that arms races predominate in cases of polygenic inheritance of resistance traits and permanent cycling predominates in cases in which resistance is controlled by major genes. In the interaction between Depressaria pastinacella, the parsnip webworm, and Pastinaca sativa, the wild parsnip, traits for plant resistance to insect herbivory (production of defensive furanocoumarins) as well as traits for herbivore “virulence” (ability to metabolize furanocoumarins) are characterized by continuous heritable variation. Furanocoumarin production in plants and rates of metabolism in insects were compared among four midwestern populations; these traits then were classified into four clusters describing multitrait phenotypes occurring in all or most of the populations. When the frequency of plant phenotypes belonging to each of the clusters is compared with the frequency of the insect phenotypes in each of the clusters across populations, a remarkable degree of frequency matching is revealed in three of the populations. That frequencies of phenotypes vary among populations is consistent with the fact that spatial variation occurs in the temporal cycling of phenotypes; such processes contribute in generating a geographic mosaic in this coevolutionary interaction on the landscape scale. Comparisons of contemporary plant phenotype distributions with phenotypes of herbarium specimens collected 9–125 years ago from across a similar latitudinal gradient, however, suggest that for at least one resistance trait—sphondin concentration—interactions with webworms have led to escalatory change.

Agrobacterium VirD2 protein interacts with plant host cyclophilins

Agrobacterium tumefaciens induces crown gall tumors on plants by transferring a nucleoprotein complex, the T-complex, from the bacterium to the plant cell. The T-complex consists of T-DNA, a single-stranded DNA segment of the tumor-inducing plasmid, VirD2, an endonuclease covalently bound to the 5′ end of the T-DNA, and perhaps VirE2, a single-stranded DNA binding protein. The yeast two-hybrid system was used to screen for proteins interacting with VirD2 and VirE2 to identify components in Arabidopsis thaliana that interact with the T-complex. Three VirD2- and two VirE2-interacting proteins were identified. Here we characterize the interactions of VirD2 with two isoforms of Arabidopsis cyclophilins identified by using this analysis. The VirD2 domain interacting with the cyclophilins is distinct from the endonuclease, omega, and the nuclear localization signal domains. The VirD2–cyclophilin interaction is disrupted in vitro by cyclosporin A, which also inhibits Agrobacterium-mediated transformation of Arabidopsis and tobacco. These data strongly suggest that host cyclophilins play a role in T-DNA transfer.

Protein–protein interactions among the Aux/IAA proteins

The plant hormone indoleacetic acid (IAA) transcriptionally activates early genes in plants. The Aux/IAA family of early genes encodes proteins that are short-lived and nuclear-localized. They also contain a putative prokaryotic βαα DNA binding motif whose formation requires protein dimerization. Here, we show that the pea PS-IAA4 and Arabidopsis IAA1 and IAA2 proteins perform homo- and heterotypic interactions in yeast using the two-hybrid system. Gel-filtration chromatography and chemical cross-linking experiments demonstrate that the PS-IAA4 and IAA1 proteins interact to form homodimers in vitro. Deletion analysis of PS-IAA4 indicates that the βαα containing acidic C terminus of the protein is necessary for homotypic interactions in the yeast two-hybrid system. Screening an Arabidopsis λ-ACT cDNA library using IAA1 as a bait reveals heterotypic interactions of IAA1 with known and newly discovered members of the Arabidopsis Aux/IAA gene family. The new member IAA24 has similarity to ARF1, a transcription factor that binds to an auxin response element. Combinatorial interactions among the various members of the Aux/IAA gene family may regulate a variety of late genes as well as serve as autoregulators of early auxin-regulated gene expression. These interactions provide a molecular basis for the developmental and tissue-specific manner of auxin action.

Concerted biosynthesis of an insect elicitor of plant volatiles

A variety of agricultural plant species, including corn, respond to insect herbivore damage by releasing large quantities of volatile compounds and, as a result, become highly attractive to parasitic wasps that attack the herbivores. An elicitor of plant volatiles, N-(17-hydroxylinolenoyl)-l-glutamine, named volicitin and isolated from beet armyworm caterpillars, is a key component in plant recognition of damage from insect herbivory. Chemical analysis of the oral secretion from beet armyworms that have fed on 13C-labeled corn seedlings established that the fatty acid portion of volicitin is plant derived whereas the 17-hydroxylation reaction and the conjugation with glutamine are carried out by the caterpillar by using glutamine of insect origin. Ironically, these insect-catalyzed chemical modifications to linolenic acid are critical for the biological activity that triggers the release of plant volatiles, which in turn attract natural enemies of the caterpillar.

Rapid evolution in plant chitinases: Molecular targets of selection in plant-pathogen coevolution

Many pathogen recognition genes, such as plant R-genes, undergo rapid adaptive evolution, providing evidence that these genes play a critical role in plant-pathogen coevolution. Surprisingly, whether rapid adaptive evolution also occurs in genes encoding other kinds of plant defense proteins is unknown. Unlike recognition proteins, plant chitinases attack pathogens directly, conferring disease resistance by degrading chitin, a component of fungal cell walls. Here, we show that nonsynonymous substitution rates in plant class I chitinase often exceed synonymous rates in the plant genus Arabis (Cruciferae) and in other dicots, indicating a succession of adaptively driven amino acid replacements. We identify individual residues that are likely subject to positive selection by using codon substitution models and determine the location of these residues on the three-dimensional structure of class I chitinase. In contrast to primate lysozymes and plant class III chitinases, structural and functional relatives of class I chitinase, the adaptive replacements of class I chitinase occur disproportionately in the active site cleft. This highly unusual pattern of replacements suggests that fungi directly defend against chitinolytic activity through enzymatic inhibition or other forms of chemical resistance and identifies target residues for manipulating chitinolytic activity. These data also provide empirical evidence that plant defense proteins not involved in pathogen recognition also evolve in a manner consistent with rapid coevolutionary interactions.

Interactions between Senescence and Leaf Orientation Determine in Situ Patterns of Photosynthesis and Photoinhibition in Field-Grown Rice1

Photosynthesis and photoinhibition in field-grown rice (Oryza sativa L.) were examined in relation to leaf age and orientation. Two varieties (IR72 and IR65598-112-2 [BSI206]) were grown in the field in the Philippines during the dry season under highly irrigated, well-fertilized conditions. Flag leaves were examined 60 and 100 d after transplanting. Because of the upright nature of 60-d-old rice leaves, patterns of photosynthesis were determined by solar movements: light falling on the exposed surface in the morning, a low incident angle of irradiance at midday, and light striking the opposite side of the leaf blade in the afternoon. There was an early morning burst of CO2 assimilation and high levels of saturation of photosystem II electron transfer as incident irradiance reached a maximum level. However, by midday the photochemical efficiency increased again almost to maximum. Leaves that were 100 d old possessed a more horizontal orientation and were found to suffer greater levels of photoinhibition than younger leaves, and this was accompanied by increases in the de-epoxidation state of the xanthophyll cycle. Older leaves had significantly lower chlorophyll content but only slightly diminished photosynthesis capacity.

Human-caused environmental change: Impacts on plant diversity and evolution

Human-caused environmental changes are creating regional combinations of environmental conditions that, within the next 50 to 100 years, may fall outside the envelope within which many of the terrestrial plants of a region evolved. These environmental modifications might become a greater cause of global species extinction than direct habitat destruction. The environmental constraints undergoing human modification include levels of soil nitrogen, phosphorus, calcium and pH, atmospheric CO2, herbivore, pathogen, and predator densities, disturbance regimes, and climate. Extinction would occur because the physiologies, morphologies, and life histories of plants limit each species to being a superior competitor for a particular combination of environmental constraints. Changes in these constraints would favor a few species that would competitively displace many other species from a region. In the long-term, the “weedy” taxa that became the dominants of the novel conditions imposed by global change should become the progenitors of a series of new species that are progressively less weedy and better adapted to the new conditions. The relative importance of evolutionary versus community ecology responses to global environmental change would depend on the extent of regional and local recruitment limitation, and on whether the suite of human-imposed constraints were novel just regionally or on continental or global scales.

Physical stress and diversity-productivity relationships: The role of positive interactions

If environmental stress provides conditions under which positive relationships between plant species richness and productivity become apparent, then species that seem functionally redundant under constant conditions may add to community functioning under variable conditions. Using naturally co-occurring mosses and liverworts, we constructed bryophyte communities to test relationships between species diversity (1, 2, 4, 8, 16, 24, or 32 species) and productivity under constant conditions and when exposed to experimental drought. We found no relationship between species richness and biomass under constant conditions. However, when communities were exposed to experimental drought, biomass increased with species richness. Responses of individual species demonstrated that facilitative interactions rather than sampling effects or niche complementarity best explained results—survivorship increased for almost all species, and those species least resistant to drought in monoculture had the greatest increase in biomass. Positive interactions may be an important but previously underemphasized mechanism linking high diversity to high productivity under stressful environmental conditions.

Genetic Interactions between Phytochrome A, Phytochrome B, and Cryptochrome 1 during Arabidopsis Development1

Single, double, and triple null combinations of Arabidopsis mutants lacking the photoreceptors phytochrome (phy) A (phyA-201), phyB (phyB-5), and cryptochrome (cry) 1 (hy4-2.23n) were examined for de-etiolation responses in high-fluence red, far-red, blue, and broad-spectrum white light. Cotyledon unhooking, unfolding, and expansion, hypocotyl growth, and the accumulation of chlorophylls and anthocyanin in 5-d-old seedlings were measured under each light condition and in the dark. phyA was the major photoreceptor/effector for most far-red-light responses, although phyB and cry1 modulated anthocyanin accumulation in a phyA-dependent manner. phyB was the major photoreceptor in red light, although cry1 acted as a phyA/phyB-dependent modulator of chlorophyll accumulation under these conditions. All three photoreceptors contributed to most blue light deetiolation responses, either redundantly or additively; however, phyB acted as a modulator of cotyledon expansion dependent on the presence of cry1. As reported previously, flowering time in long days was promoted by phyA and inhibited by phyB, with each suppressing the other's effect. In addition to the effector/modulator relationships described above, measurements of hypocotyls from blue-light-grown seedlings demonstrated phytochrome activity in blue light and cry1 activity in a phyAphyB mutant background.

Polypeptide signaling for plant defensive genes exhibits analogies to defense signaling in animals.

The activation of plant defensive genes in leaves of tomato plants in response to herbivore damage or mechanical wounding is mediated by a mobile 18-amino acid polypeptide signal called systemin. Systemin is derived from a larger, 200-amino acid precursor called prosystemin, similar to polypeptide hormones and soluble growth factors in animals. Systemin activates a lipid-based signaling cascade, also analogous to signaling systems found in animals. In plants, linolenic acid is released from membranes and is converted to the oxylipins phytodienoic acid and jasmonic acid through the octadecanoid pathway. Plant oxylipins are structural analogs of animal prostaglandins which are derived from arachidonic acid in response to various signals, including polypeptide factors. Constitutive overexpression of the prosystemin gene in transgenic tomato plants resulted in the overproduction of prosystemin and the abnormal release of systemin, conferring a constitutive overproduction of several systemic wound-response proteins (SWRPs). The data indicate that systemin is a master signal for defense against attacking herbivores. The same defensive proteins induced by wounding are synthesized in response to oligosaccharide elicitors that are generated in leaf cells in response to pathogen attacks. Inhibitors of the octadecanoid pathway, and a mutation that interrupts this pathway, block the induction of SWRPs by wounding, systemin, and oligosaccharide elicitors, indicating that the octadecanoid pathway is essential for the activation of defense genes by all of these signals. The tomato mutant line that is functionally deficient in the octadecanoid pathway is highly susceptible to attacks by Manduca sexta larvae. The similarities between the defense signaling pathway in tomato leaves and those of the defense signaling pathways of macrophages and mast cells of animals suggests that both the plant and animal pathways may have evolved from a common ancestral origin.