Whole-plant Response Research Articles

Salt tolerance in Populus: Significance of stress signaling networks, mycorrhization, and soil amendments for cellular and whole-plant nutrition

Abiotic stress tolerance is important for trees that have to withstand unfavorable environmental condi- tions for longer periods of time than crop plants with short life cycles. Salinity (excess NaCl) is a common abiotic stress factor that limits tree growth by interfering with major physiological functions, disrupting ion homeostasis and diminishing nutrient uptake in plant cells. Here we review the salt signaling cascades that control cellular K+ and Ca2+ homeostasis, which are also affected by reactive oxygen species signaling, extracellular ATP signaling, and crosstalk among pathways in the salt-resistant model tree Populus euphratica. We discuss the uptake and transport of essential nutrients, especially N (NH4+, NO3−), P, S, K+, Ca2+, and Mg2+, that constitute the whole-plant response to salinity and its impact on tree physiology. To date, transgenic approaches have achieved only limited enhancements of the salinity tolerance of salt-sensitive Populus. Therefore, we recommend the use of alternative biotechnological tools such as mycorrhization and polymer amendment. Ectomycorrhizal fungi have beneficial effects for their hosts under salt stress because they exclude Na+ and improve nutrient conditions, e.g. by increasing N, P, Ca2+, and K+ levels. Applying hydrogel to the soil improves poplar growth under salinity, an effect attributed to both increased K+ and Ca2+ uptake as well as the reduced accumulation of salt ions.

Synergistic, additive and antagonistic impacts of drought and herbivory on Pinus sylvestris: leaf, tissue and whole-plant responses and recovery

Forests typically experience a mix of anthropogenic, natural and climate-induced stressors of different intensities, creating a mosaic of stressor combinations across the landscape. When multiple stressors co-occur, their combined impact on plant growth is often greater than expected based on single-factor studies (i.e., synergistic), potentially causing catastrophic dysfunction of physiological processes from an otherwise recoverable situation. Drought and herbivory are two stressors that commonly co-occur in forested ecosystems, and have the potential to 'overlap' in their impacts on various plant traits and processes. However, the combined impacts from these two stressors may not be predictable based on additive models from single-stressor studies. Moreover, the impacts and subsequent recovery may be strongly influenced by the relative intensities of each stressor. Here, we applied drought stress and simulated bark-feeding herbivory at three levels of intensity (control, moderate and severe) in a full factorial design on young Pinus sylvestris L. seedlings. We assessed if the combined effects from two stressors were additive (responses were equal to the sum of the single-factor effects), synergistic (greater than expected) or antagonistic (less than expected) on a suite of morphological and physiological traits at the leaf-, tissue- and whole-plant level. We additionally investigated whether recovery from herbivory was dependent on relief from drought. The two stressors had synergistic impacts on specific leaf area and water-use efficiency, additive effects on height and root-to-shoot ratios, but antagonistic effects on photosynthesis, conductance and, most notably, on root, shoot and whole-plant biomass. Nevertheless, the magnitude and direction of the combined impacts were often dependent on the relative intensities of each stressor, leading to many additive or synergistic responses from specific stressor combinations. Also, seedling recovery was far more dependent on the previous year's drought compared with the previous year's herbivory, demonstrating the influence of one stressor over another during recovery. Our study reveals for the first time, the importance of not only the presence or absence of drought and herbivory stressors, but also shows that their relative intensities are critical in determining the direction and magnitude of their impacts on establishing seedlings.

Red rice (Oryza sativa) cross-resistance to imidazolinone herbicides used in resistant rice cultivars grown in northern Greece

A putative resistant red rice (Oryza sativa) accession, an imidazolinone-resistant rice cultivar (Clearfield), a susceptible red rice accession and a susceptible rice cultivar were evaluated for cross-resistance to imazamox and imazethapyr in a whole-plant response experiment and seed bioassay. Additionally, a 210-bp fragment of the ALS gene was sequenced to identify mutations responsible for resistance. Also, a 574bp of the ALS gene was sequenced and PCR for detection of the ‘Clearfield allele’ was conducted by the Andalusian Institute of Agricultural Research and Training (IFAPA, Spain). In the whole-plant response experiment, the putative resistant red rice was >23 and >21 times more resistant to imazamox and imazethapyr than the susceptible accession, respectively, whereas the respective resistance factor values based on seed bioassay were 86.4 and 141.7. Also, the respective resistance factor values for the Clearfield rice cultivar were similar with those calculated for putative resistant red rice. Additionally, the sequence of the 210bp ALS gene fragment from the putative resistant red rice and Clearfield rice cultivar revealed the same amino acid substitution of Ser653Asn in both alleles (homozygous). Furthermore, the sequence of a 574bp ALS gene fragment and the PCR for detection of the ‘Clearfield allele’ confirmed that the putative resistant red rice is homozygous mutant for the Ser653Asn mutation and provided additional evidence that its genetic background matches that of Clearfield rice. These findings indicate clearly that the insufficient control of the putative resistant red rice with imazamox was due to target-site resistance and particularly due to a point mutation at the Ser653 codon, which is positively identified as having been derived from the Clearfield rice cultivar.

Association of candidate genes with drought tolerance traits in diverse perennial ryegrass accessions.

Drought is a major environmental stress limiting growth of perennial grasses in temperate regions. Plant drought tolerance is a complex trait that is controlled by multiple genes. Candidate gene association mapping provides a powerful tool for dissection of complex traits. Candidate gene association mapping of drought tolerance traits was conducted in 192 diverse perennial ryegrass (Lolium perenne L.) accessions from 43 countries. The panel showed significant variations in leaf wilting, leaf water content, canopy and air temperature difference, and chlorophyll fluorescence under well-watered and drought conditions across six environments. Analysis of 109 simple sequence repeat markers revealed five population structures in the mapping panel. A total of 2520 expression-based sequence readings were obtained for a set of candidate genes involved in antioxidant metabolism, dehydration, water movement across membranes, and signal transduction, from which 346 single nucleotide polymorphisms were identified. Significant associations were identified between a putative LpLEA3 encoding late embryogenesis abundant group 3 protein and a putative LpFeSOD encoding iron superoxide dismutase and leaf water content, as well as between a putative LpCyt Cu-ZnSOD encoding cytosolic copper-zinc superoxide dismutase and chlorophyll fluorescence under drought conditions. Four of these identified significantly associated single nucleotide polymorphisms from these three genes were also translated to amino acid substitutions in different genotypes. These results indicate that allelic variation in these genes may affect whole-plant response to drought stress in perennial ryegrass.

Multiple Pro-197 substitutions in the acetolactate synthase of rigid ryegrass (Lolium rigidum) and their impact on chlorsulfuron activity and plant growth

The evaluation of 21 Lolium rigidum accessions for chlorsulfuron resistance in a whole-plant response experiment indicated that 10 accessions were highly resistant (R) to chlorsulfuron, three partially resistant (r) and seven susceptible (S) to this herbicide. The excellent control of the r accessions in the presence of terbufos provided strong evidence for the existence of herbicide metabolism resistance due to cytochrome P450 enzymes. The sequence of 34 L. rigidum plants from the 10 R accessions revealed that all mutant ALS alleles encoded an amino acid replacement at codon 197, which resulted in the substitution of Pro-197 by Ala, Arg, Gln, Leu, or Ser and probably His or Val. Most of the R accessions revealed more than one mutation and among them the Pro-197 by Ala was the most common, whereas twelve out of the 34 R sequenced individual plants were homozygous. The sequence of 12 and 9 L. rigidum plants from the respective four S and three r accessions did not indicate any ALS mutation at the codon 197. ALS activity assays confirmed the whole-plant response experiments and the ALS gene sequence results. The resistance ratios (R/S) for the R accessions ranged from 36 to 620 showing that ALS gene mutations resulted in less susceptible to chlorsulfuron ALS enzyme. The R/S values for the r accessions were similar to the S-control accession. The growth rate evaluation of four R and four S L. rigidum accessions in the absence of competition indicated that the R accessions had similar growth pattern to that of S accessions, suggesting that the resistance-endowing ALS mutations did not result in detectable resistance fitness cost.

Herbivory of wild Manduca sexta causes fast down-regulation of photosynthetic efficiency in Datura wrightii: an early signaling cascade visualized by chlorophyll fluorescence

Plants experiencing herbivory suffer indirect costs beyond direct loss of leaf area, but differentially so based on the herbivore involved. We used a combination of chlorophyll fluorescence imaging and gas exchange techniques to quantify photosynthetic performance, the efficiency of photochemistry, and heat dissipation to examine immediate and longer-term physiological responses in the desert perennial Datura wrightii to herbivory by tobacco hornworm, Manduca sexta. Herbivory by colony-reared larvae yielded no significant reduction in carbon assimilation, whereas herbivory by wild larvae induced a fast and spreading down-regulation of photosynthetic efficiency, resulting in significant losses in carbon assimilation in eaten and uneaten leaves. We found both an 89% reduction in net photosynthetic rates in herbivore-damaged leaves and a whole-plant response (79% decrease in undamaged leaves from adjacent branches). Consequently, herbivory costs are higher than previously estimated in this well-studied plant-insect interaction. We used chlorophyll fluorescence imaging to elucidate the mechanisms of this down-regulation. Quantum yield decreased up to 70% in a small concentric band surrounding the feeding area within minutes of the onset of herbivory. Non-photochemical energy dissipation by the plant to avoid permanent damage was elevated near the wound, and increased systematically in distant areas of the leaf away from the wound over subsequent hours. Together, the results underscore not only potential differences between colony-reared and wild-caught herbivores in experimental studies of herbivory but also the benefits of quantifying physiological responses of plants in unattacked leaves.

Resistance mechanism to acetyl coenzyme A carboxylase inhibiting herbicides in Phalaris paradoxa collected in Mexican wheat fields

In this study, we describe the molecular, physiological and agronomic aspects involved in the resistance to acetyl coenzyme A carboxylase inhibiting herbicides (ACCase) observed in one biotype of Phalaris paradoxa from Mexico. Dose–response Assays: The herbicide rate inhibiting plant growth of each biotype by 50% with respect to the untreated control, ED50. Enzyme purification and ACCase assays to determine herbicide rate inhibiting the enzyme of each biotype by 50% with respect to the untreated control, I50. Absorption and Translocation Assays with [14C]diclofop-methyl. Metabolism of diclofop-methyl and its metabolites were identified by thin-layer chromatography. Study of target site resistance mechanism at enzyme and molecular levels. In this work, it has been studied the whole-plant response of Phalaris paradoxa biotypes from Mexico resistant (R) and susceptible (S) to ACCase-inhibiting herbicides: aryloxyphenoxypropionate (APP), cyclohexanedione (CHD) and phenylpyrazoline (PPZ), and the mechanism behind their resistance were studied. To analyse the resistance mechanism, the enzyme ACCase activity was investigated. Results from biochemical assays indicated a target-site resistance as the cause of reduced susceptibility to ACCase inhibitors. The absorption, translocation and metabolism were similar between R and S biotypes. A point mutation never described before was detected within the triplet of glycine at the amino acid position 2096 (referring to EMBL accession no. AJ310767) and resulted in the triplet of serine. This new mutation could explain the loss of affinity for the ACCase-inhibiting herbicides. We found a new mutation, which had never been described before. This mutation was detected within the triplet of glycine at the amino acid position 2096. This new mutation confers cross-resistance to three different chemical groups of ACCase-inhibiting herbicides.

Herbivore-Induced Changes in Tomato (Solanum lycopersicum) Primary Metabolism: A Whole Plant Perspective

Induced changes in primary metabolism are important plant responses to herbivory, providing energy and metabolic precursors for defense compounds. Metabolic shifts also can lead to reallocation of leaf resources to storage tissues, thus increasing a plant's tolerance. We characterized whole-plant metabolic responses of tomato (Solanum lycopersicum) 24h after leaf herbivory by two caterpillars (the generalist Helicoverpa zea and the specialist Manduca sexta) by using GC-MS. We measured 56 primary metabolites across the leaves, stems, roots, and apex, comparing herbivore-attacked plants to undamaged plants and mechanically damaged plants. Induced metabolic change, in terms of magnitude and number of individual concentration changes, was stronger in the apex and root tissues than in undamaged leaflets of damaged leaves, indicating rapid and significant whole-plant responses to damage. Helicoverpa zea altered many more metabolites than M. sexta across most tissues, suggesting an enhanced plant response to H. zea herbivory. Helicoverpa zea herbivory strongly affected concentrations of defense-related metabolites (simple phenolics and precursor amino acids), while M. sexta altered metabolites associated with carbon and nitrogen transport. We conclude that herbivory induces many systemic primary metabolic changes in tomato, and that changes often are specific to a single tissue or type of herbivore. The potential implications of primary metabolic changes are discussed in relation to resistance and tolerance.

A non-invasive observation parameter to complement sediment bioassays using Myriophyllum aquaticum

Purpose The objective of this study was to modify a sediment contact protocol to a multiwell plate exposure system which supplements measurement of the fresh weight change (FWC) using the non-invasive effective quantum yield of energy conversion at photosystem II (PS II) reaction centres (Y(II)) in parallel. Since Y(II) is a functional parameter and FWC represents a whole-plant structural response, the determination of a more pronounced response in one of these parameters may hint at the mode of action of contaminants. By the observation of Y(II) at different time points, extrapolation of effect development over time may be gained from modelling.

Joint Action Analysis Utilizing Concentration Addition and Independent Action Models

In weed science literature, models such as concentration addition, independent action, effect summation, and the parallel line assay technique have been used to predict and analyze whole-plant response to herbicide mixtures. Although a joint action reference model is necessary for determining whether the herbicide mixture provides less than (antagonistic), equal to (zero-interaction or additive), or greater than (synergistic) expected control, model selection often occurs with little regard to the model's underlying biological assumptions. The joint action models of concentration addition (CA) and independent action (IA) are the appropriate models to consider for analysis of herbicide mixtures of two active components. CA assumes additivity of dose, with herbicides differing only in potency, whereas IA assumes multiplicativity of effects, in which herbicides behave independently and sequentially within the plant. CA and IA predicted mixture responses were computed for a sample mixture data set of mesotrione plus atrazine. IA predicted lower mixture responses than CA; for example, mesotrione at 17.5 g ha−1+ atrazine at 140 g ha−1was predicted to provide 45% (IA) compared with 53% (CA) control of Palmer amaranth. Joint action claims of synergism and antagonism were shown to be dependent on the reference model selected. Although mesotrione plus atrazine combinations were synergistic under IA assumptions, analysis under CA assumptions indicated mesotrione plus atrazine to be synergistic, additive, and antagonistic according to the selected effective concentration (ECx) level and fixed-ratio mixture. Because it is not possible to determine the appropriate joint action model on the basis of fit of predicted to observed mixture data, the appropriateness of underlying biological assumptions was considered for the sample mixture data set. Additionally, we provide decision criteria to aid researchers in their selection of an appropriate joint action reference model.

Mechanism of Resistance to ACCase-Inhibiting Herbicides in Wild Oat (Avena fatua) from Latin America

Whole-plant response of two suspected resistant Avena fatua biotypes from Chile and Mexico to ACCase-inhibiting herbicides [aryloxyphenoxypropionate (APP), cyclohexanedione (CHD), and pinoxaden (PPZ)] and the mechanism behind their resistance were studied. Both dose-response and ACCase enzyme activity assays revealed cross-resistance to the three herbicide families in the biotype from Chile. On the other hand, the wild oat biotype from Mexico exhibited resistance to the APP herbicides and cross-resistance to the CHD herbicides, but no resistance to PPZ. Differences in susceptibility between the two biotypes were unrelated to absorption, translocation, and metabolism of the herbicides. PCR generated fragments of the ACCase CT domain spanning the potential mutations sited in the resistant and susceptible biotypes were sequenced and compared. A point mutation was detected in the aspartic acid triplet at the amino acid position 2078 in the Chilean biotype and in isoleucine at the amino acid position 2041 in the Mexican wild oat biotype, which resulted in a glycine triplet and an asparagine triplet, respectively. On the basis of in vitro assays, the target enzyme (ACCase) in these resistant biotypes contains a herbicide-insensitive form. This is the first reported evidence of resistance to pinoxaden in A. fatua.

Living with Limited Water: I. Shoot Hydraulic Resistance as Water-Saving Strategy and its Significance in Varietal Improvement of Rice

Knowledge of the location and magnitude of resistance to water flow in a plant is fundamental for describing whole-plant response to water deficit as the water after absorption by roots has to pass through various magnitudes of resistance offered by the stem and leaf sheath tissues before reaching the transpiring surfaces, i.e., laminae. Therefore, the hydraulic resistance to water flux was studied in aboveground portions of excised rice plant (Oryza sativa L.) by incubating them with their cut ends under water. The findings suggest that the hydraulic resistance occurs in every organ of the plant, though in varying degrees, the order of its magnitude being greatest in the stem followed by leaf sheath and lamina in that order, i.e., stem > leaf sheath > lamina. Pressurization of water to force entry through the cut end into the detached plant was used to elucidate the temporal dynamics of shoot hydraulic properties in rice cultivars. For this, the time taken for unrolling of leaves that roll back following...

Leaf hydraulic vulnerability is related to conduit dimensions and drought resistance across a diverse range of woody angiosperms

Hydraulic dysfunction in leaves determines key aspects of whole-plant responses to water stress; however, our understanding of the physiology of hydraulic dysfunction and its relationships to leaf structure and ecological strategy remains incomplete. Here, we studied a morphologically and ecologically diverse sample of angiosperms to test whether the water potential inducing a 50% loss in leaf hydraulic conductance (P50(leaf)) is predicted by properties of leaf xylem relating to water tension-induced conduit collapse. We also assessed the relationships between P50(leaf) and other traits considered to reflect drought resistance and ecological strategy. Across species, P50(leaf) was strongly correlated with a theoretical predictor of vulnerability to cell collapse in minor veins (the cubed ratio of the conduit wall thickness to the conduit lumen breadth). P50(leaf) was also correlated with mesophyll traits known to be related to drought resistance, but unrelated to traits associated with carbon economy. Our data indicate a link between the structural mechanics of leaf xylem and hydraulic function under water stress. Although it is possible that collapse may contribute directly to dysfunction, this relationship may also be a secondary product of vascular economics, suggesting that leaf xylem is dimensioned to avoid wall collapse.

Is osmotic potential a more appropriate property than electrical conductivity for evaluating whole-plant response to salinity?

Studies of whole-plant or crop responses to salinity often focus on yield or growth reduction in terms of solution ion concentration or electrical conductivity. The response functions describing salt stress may be better presented in terms of solution osmotic potential. We looked at the effect of increasing concentrations of NaCl and CaCl 2, either alone or in equinormal combination, on three different plant species: bean ( Phaseolus vulgaris L.), corn ( Zea mays L.) and melon ( Cucumis melo L.). Corn and melon were found to be relatively tolerant and beans more sensitive to salinity. When yield response was related to the electrical charge concentration of the salts, i.e. salinity was expressed in units of mequiv. L −1 or electrical conductivity, the stress effects of Na and Ca appeared to be of different magnitudes: plant growth was more sensitive to excess Na than to excess Ca and the effect of combined Na and Ca was intermediate. The effects of the two salts were, however, indistinguishable when salinity was expressed in terms of osmotic potential of the water. For all three species, the response curves of yield as a function of level of equipotential solutions of NaCl, CaCl 2 or combinations of the two salts practically overlapped. Presentation and interpretation of the whole-plant salinity response in terms of osmotic potential would be beneficial in attempts to differentiate between the osmotic and toxic effects of salinity, in normalizing data sets and in increasing their relevance in practical applications.

Coping mechanisms for crop plants in drought-prone environments.

Drought is a major limitation to plant productivity. Various options are available for increasing water availability and sustaining growth of crop plants in drought-prone environments. After a general introduction to the problems of water availability, this review focuses on a critical evaluation of recent progress in unravelling mechanisms for modifying plant growth responses to drought. Investigations of key regulatory mechanisms integrating plant growth responses to water deficits at the whole-organism, cellular and genomic levels continue to provide novel and exiting research findings. For example, recent reports contradict the widespread conception that root-derived abscisic acid is necessarily involved in signalling for stomatal and shoot-growth responses to soil water deficits. The findings bring into question the theoretical basis for alternate-side root-irrigation techniques. Similarly, recent reports indicate that increased ABA production or increased aquaporin expression did not lead to improved drought resistance. Other reports have concerned key genes and proteins involved in regulation of flowering (FT), vegetative growth (DELLA), leaf senescence (IPT) and desiccation tolerance (LEA). Introgression of such genes, with suitable promoters, can greatly impact on whole-plant responses to drought. Further developments could facilitate the introduction by breeders of new crop varieties with growth physiologies tailored to improved field performance under drought. Parallel efforts to encourage the introduction of supplementary irrigation with water made available by improved conservation measures and by sea- or brackish-water desalination, will probably provide comprehensive solutions to coping with drought-prone environments.

Transient accident analysis of a supercritical carbon dioxide Brayton cycle energy converter coupled to an autonomous lead-cooled fast reactor

The supercritical carbon dioxide (S-CO 2) Brayton cycle is a promising advanced alternative to the Rankine steam cycle and recuperated gas Brayton cycle for the energy converters of specific reactor concepts belonging to the U.S. Department of Energy Generation IV Nuclear Energy Systems Initiative. A new plant dynamics analysis computer code has been developed for simulation of the S-CO 2 Brayton cycle coupled to an autonomous, natural circulation lead-cooled fast reactor (LFR). The plant dynamics code was used to simulate the whole-plant response to accident conditions. The specific design features of the reactor concept influencing passive safety are discussed and accident scenarios are identified for analysis. Results of calculations of the whole-plant response to loss-of-heat sink, loss-of-load, and pipe break accidents are demonstrated. The passive safety performance of the reactor concept is confirmed by the results of the plant dynamics code calculations for the selected accident scenarios.

Natural Tolerance to Imazethapyr in Red Rice (Oryza sativa)

Red rice is a major weed problem in rice production of the southern United States and other rice-producing countries. One hundred thirty red rice accessions from 26 rice-growing counties in Arkansas were tested for tolerance to imazethapyr in seed- and whole-plant response bioassays. The red rice accessions were compared with imazethapyr-resistant (ClearfieldTM) rice cultivars (‘CL121’, ‘CL161’, and ‘CL-XL8’) and conventional rice cultivars (‘Bengal’, ‘Dongjin’, ‘Drew’, and ‘Wells’). Red rice accessions 79, 84, and 118 showed 17-, 48-, and 37-fold more tolerance to imazethapyr, respectively, than the standard susceptible red rice accession (82) in whole-plant bioassays. The imazethapyr-resistant rice cultivars, CL121, CL161, and CL-XL8 were 41-, >177-, and 48-fold more resistant to imazethapyr, respectively than the susceptible standard. The imazethapyr-tolerant red rice and ClearfieldTM cultivars were generally cross tolerant to other acetolactate synthase (ALS; EC 4.1.3.18) inhibiting herbicides such as imazapyr, imazaquin, imazamox, and pyrithiobac. The tolerance level of red rice or rice to imidazolinone herbicides was highest with imazaquin and lowest with imazapyr. The imazethapyr-tolerant red rice accessions and ClearfieldTM rice were susceptible to glufosinate and glyphosate. The ALS enzyme of tolerant red rice accessions was less sensitive to imazethapyr than the susceptible standard, but tolerance at the enzyme level was less than at the whole-plant level. Therefore, tolerance of red rice to imazethapyr may involve other mechanisms besides an insensitive target site. We learned that a few imazethapyr-tolerant red rice populations existed probably before ClearfieldTM rice was introduced, supporting the hypothesis that evolution of herbicide-resistant red rice populations can happen with intensive herbicide selection pressure.

A New Mutation in PlantALSConfers Resistance to Five Classes of ALS-Inhibiting Herbicides

Experiments were conducted to evaluate a biotype of smooth pigweed that had survived applications of sulfonylurea (SU) and imidazolinone (IMI) herbicides in a single season. The source field had a history of repeated acetolactate synthase (ALS)-inhibiting herbicide use over several years. Whole-plant response experiments evaluated the resistant (R11) biotype and an ALS-inhibitor susceptible (S) smooth pigweed biotype to herbicides from the SU, IMI, pyrimidinylthiobenzoate (PTB), and triazolopyrimidine sulfonanilide (TP) chemical families. The R11 biotype exhibited 60- to 3,200-fold resistance to all four ALS-Inhibiting herbicide chemistries compared with the S biotype. Nucleotide sequence comparison ofALSgenes from R11 and S biotypes revealed a single nucleotide difference that resulted in R11 having an amino acid substitution of aspartate to glutamate at position 376, as numbered relative to the protein sequence of mouseearcress. This is the first report of an amino acid substitution at this position of anALSgene isolated from a field-selected weed biotype. To verify the role of this mutation in herbicide resistance, theALSgene was cloned and expressed inArabidopsis. TransgenicArabidopsisexpressing thisALSgene exhibited resistance to SU, IMI, PTB, TP, and sulfonylaminocarbonyltriazolinone ALS-Inhibiting herbicide classes.

Mechanisms of response to ultraviolet-b radiation — A whole plant perspective

Plant responses to ultraviolet (UV) radiation are numerous, resulting in rapid and permanent alteration to numerous aspects of plant form, physiology and biochemistry. Such responses to UV and UV-B (290–320 nm) radiation in particular have been previously studied largely due to concerns over ozone depletion, but a recent refocus towards the effects of environmentally relevant UV-B doses has provided an opportunity to investigate the mechanistic basis for several well defined plant responses. We have characterised some of these underlying mechanisms using a combination of crop species and Arabidopsis mutants, in an effort to build up a model of whole-plant response. (1) The role of UV in regulating leaf growth has been investigated using Lactuca sativa as a model species in a top-down approach, whereby leaf expansion rate, leaf biophysical properties, cell growth and cell wall peroxidase content have been subject to UV-B mediated change. (2) Upregulation of secondary metabolite biosynthesis has also been characterised in L. sativa, with UV absorbing compounds increased according to UV-B, plus leaf anthocyanin (pigment compound) concentration elevated by 60% in plants exposed to the highest UV dose compared to control. (3) The importance of various key pathways to the UV whole plant response are also currently under investigation with a selection of A. thaliana mutants. Such findings will not only add to the understanding of fundamental photobiological responses, but are also currently being successfully integrated into sustainable agronomic practice.

REVIEW OF PHENOTYPIC PLASTICITY AND HIERARCHICAL SELECTION IN CLONAL PLANTS

Phenotypic plasticity is the ability of a genotype to produce distinct phenotypes when exposed to different environments during its ontogeny. There is new strong evidence for the heritable nature of phenotypic plasticity, and variation in plasticity should thus be recognized as an integral component of evolution, but not as a factor that buffers and thereby constrains the action of selection. Clonal plants have both clonal modularity and organismic modularity. As a result, phenotypic selection in clonal plants is hierarchical. Modular hierarchy of clonal plants consists of three levels, the genet, ramet fragment and ramet levels. Modes of hierarchical selection and genotypic selection have been considered in studying natural selection in clonal plants. The hierarchical selection model treats each hierarchical level as a single level of phenotypic selection, and selection effects are determined by both the selective pressure on each level and ioteractions among levels. Net effects of multi-level phenotypic selection are transmitted finally to the succeeding generations through the ramet level. The genotypic selection model puts forward that clonal growth can lead to genetic variances between ramets within a genet. Then allele frequencies will change because of non-random mating within or among genets, leading to microevolution. In clonal plants, ramets may have the greatest phenotypic variation because it is the fundamental unit of sexual and asexual reproduction, whereas genets may have the lowest phenotypic variation because it is a relative stable unit. It is termed as the mode of hierarchical responses of phenotypic plasticity. The module of a plant has the greatest phenotypic plasticity, and phenotypic plasticity is not a whole-plant response but a property of module triggered by local environmental conditions. Therefore, the mode of hierarchical responses of phenotypic plasticity in clonal plants may be universal. If the mode indicates a response mode of adaptive plasticity of different `individuals' in modular hierarchy, we can predict that: 1) the effects of natural selection on clonal plants should be shown first at ramet level and will ultimately result in microevolution as predicted by both hierarchical selection and genotypic selection models and 2) the conservativeness in evolutionary changes may be greater in clonal plants than in non-clonal plants because of lower ability of sexual reproduction and high degree of physiological integration. The most promising directions for future research include areas such as those demonstrating the universality of hierarchical responses to natural selection in clonal plants and revealing the mechanisms of hierarchical responses.