Modulation of phototactic behavior of first-instar larvae of rice leaffolder by plant factors

First-instar larvae of the rice leaffolder Cnaphalocrocis medinalis (Guene) oriented upwards on a vertically inclined filter paper and downwards on a rice leaf when the light source was from above. The direction of orientation was reversed when the light source was from below. The opposite directions of orientation on these substrates persisted when the substrates were held horizontal with light source from the sides. No difference was observed in the magnitude of reaction on susceptible and resistant rice varieties or when the rice leaf was held with its distal end pointing down. On non-host grasses, however, the directional orientation was lost. Dipping rice leaves in hexane decreased the magnitude of reaction. On dried rice leaf, the reaction was similar to that on filter paper but was of a lower magnitude. Application of rice plant volatile extracts on a filter paper decreased the intensity of upward orientation while 500 ppm of the volatile extracts applied on dried leaf caused a loss of oriented movement. Application of a mixture of seven volatile chemicals, identified from rice, decreased the magnitude of reaction significantly. It is concluded that rice plant volatiles along with some other hitherto unidentified rice plant factor cause a reversal of phototaxis, leading to downward orientation on the rice leaf.Based at the International Rice Research Institute     

Host finding behaviour of the coconut mite Aceria guerreronis

For the coconut mite, Aceria guerreronis Keifer, its host plant, the coconut palm, is not merely a source of food, but more generally a habitat to live in for several generations. For these minute organisms, finding a new plant is difficult and risky, especially because their main mode of dispersal is passive drifting with the wind and because they are highly specialized on their host plant. Consequently, the probability of landing on a suitable host is very low, let alone to land in their specific microhabitat within the host. How coconut mites manage to find their microhabitat within a host plant is still underexplored. We tested the hypothesis that they use volatile chemical information emanating from the plant to find a specific site within their host plants and/or use non-volatile plant chemicals to stay at a profitable site on the plant. This was investigated in a Y-tube olfactometer (i.e. under conditions of a directed wind flow) and on cross-shaped arenas (i.e. under conditions of turbulent air) that either allowed contact with odour sources or not. The mites had to choose between odours from specific parts (leaflet, spikelet or fruit) of a non-infested coconut plant and clean air as the alternative. In the olfactometer experiments, no mites were found to reach the upwind end of the Y-tube: <5 % of the mites were able to pass the bifurcation of the “Y”. On the cross-shaped arenas, however, a large number of coconut mites was found only when the arm of the arena contained discs of fruit epidermis and contact with these discs was allowed. The results suggest that coconut mites on palm trees are not attracted to specific sites on the plant by volatile plant chemicals, but that they arrested once they contact the substrate of specific sites. Possibly, they perceive non-volatile chemicals, but these remain to be identified.     

Orientation mechanisms and sensory organs involved in host location in a dipteran parasitoid larva

The robber fly Mallophora ruficauda is one of the principal pests of apiculture in the Pampas region of Argentina. Larvae are solitary ectoparasitoids of third-instar scarab beetle larvae. Females of M. ruficauda do not lay eggs on or near the hosts, but on tall grasses. After hatching, larvae are dispersed by the wind and drop to the ground, where they dig and search for potential hosts. It is known that second-instar larvae of M. ruficauda exhibit active host-searching behaviour towards their preferred hosts, i.e., third-instar larvae of Cyclocephala signaticollis. Although host-location seems to be mediated by chemical cues, the mechanism of orientation and the sensory organs involved in host location remain unknown. We carried out behavioural experiments in the laboratory to address these questions. We also tested whether the orientation behaviour is exclusively based on the use of chemical cues. We found that larvae of M. ruficauda detect the chemicals with chemosensilla on the maxillary palps. Only one maxillary palp suffices for orientation, but their bilateral ablation abolishes orientation. Besides, an hexane extract of the host body was as attractive as a live host. Our results support that M. ruficauda larvae find their hosts underground by means of chemoklinotaxis.     

Role of castor plant phenolics on performance of its two herbivores and their impact on egg parasitoid behaviour

Plants are reported to reconfigure their metabolism and generate secondary metabolites as a defence response towards herbivore attack. In this study we aimed to evaluate the anti-nutritive and defensive properties of phenolic compounds that were enhanced in Castor, Ricinus communis L. plant due to herbivore damage. Through HPLC studies it was observed that damage caused by two pests, Achaea janata L. and Spodoptera litura F. on the castor plant resulted in quantitative enhancement of syringic, coumaric, cinnamic and vanillic acids. These phenolic compounds showed considerable impact on insect’s feeding and their toxic nature towards the herbivores was determined by estimating three detoxification mid-gut enzymes, glutathione-s-transferase, carboxyl esterase and β-glucosidase. In the ovipositional and orientation bioassays with the egg parasitoid Trichogramma chilonis Ishii, syringic and coumaric acids had shown good kairomonal activity, thus defining a new role of these chemicals in indirect plant defence. Thus, this study demonstrates a quantitative association between plant phenolic accumulation and the elevated reproductive performance of the herbivore natural enemies, by illustrating the role of plant phenolics in both the direct and indirect induced plant defences.     

Attractiveness of CO2 released by root respiration fades on the background of root exudates

Plants are endangered at their roots by soil-dwelling rhizophagous insects. These below-ground living herbivores may orient to the source of carbon dioxide (CO₂), an ubiquitous volatile released by respiring plant roots. Here, we studied the interaction of CO₂ and other plant root-derived chemical stimuli with regard to the chemical orientation of the polyphagous larvae of Melolontha melolontha L. (Scarabaeidae). A soil arena was developed that enabled both determination of the actual soil CO₂ concentration and the behavioural response of an insect to (a) CO₂ gradients per se, (b) chemical stimuli released from respiring, undamaged roots of plants potted into vermiculite in this arena and (c) combinations of CO₂ gradients and root-derived stimuli. In a root-free arena, larvae of M. melolontha oriented to the source of synthetic CO₂. However, similar CO₂ gradients generated by host plant roots did not attract the larvae. Neither did a synthetic CO₂ gradient combined with aqueous extracts from rhizospheres with undamaged plant roots elicit an attractive effect. Our data suggest that orientation of cockchafer larvae within CO₂ gradients generated by respiring roots is ‘masked’ by an aqueous extract from a rhizosphere with undamaged roots. The results emphasise that effects of behaviour modifying plant-derived compounds need to be investigated against the background of naturally co-occurring chemicals. The significance of our results for orientation of soil living insects is discussed with respect to abiotic conditions in natural soil and the role of soil microorganisms for the attractiveness of plant roots.     

The biosynthesis of cutin and suberin as an alternative source of enzymes for the production of bio-based chemicals and materials

Oxygenated fatty acids such as ricinoleic acid and vernolic acid can serve in the industry as synthons for the synthesis of a wide range of chemicals and polymers traditionally produced by chemical conversion of petroleum derivatives. Oxygenated fatty acids can also be useful to synthesize specialty chemicals such as cosmetics and aromas. There is thus a strong interest in producing these fatty acids in seed oils (triacylglycerols) of crop species. In the last 15 years or so, much effort has been devoted to isolate key genes encoding proteins involved in the synthesis of oxygenated fatty acids and to express them in the seeds of the model plant Arabidopsis thaliana or crop species. An often overlooked but rich source of enzymes catalyzing the synthesis of oxygenated fatty acids and their esterification to glycerol is the biosynthetic pathways of the plant lipid polyesters cutin and suberin. These protective polymers found in specific tissues of all higher plants are composed of a wide variety of oxygenated fatty acids, many of which have not been reported in seed oils (e.g. saturated ω-hydroxy fatty acids and α,ω-diacids). The purpose of this mini-review is to give an overview of the recent advances in the biosynthesis of cutin and suberin and discuss their potential utility in producing specific oxygenated fatty acids for specialty chemicals. Special emphasis is given to the role played by specific acyltransferases and P450 fatty acid oxidases. The use of plant surfaces as possible sinks for the accumulation of high value-added lipids is also highlighted.     

Use of endophytes as biocontrol agents

Plant diseases, caused by various microorganisms, including viruses, bacteria, fungi, protozoa and nematodes, affect agricultural practices and result in significant crop losses. Fungal pathogens are the major cause of plant diseases and infect most plants. Agrochemicals play a significant role in plant disease management to ensure a sustainable and productive agricultural system. However, the intensive use of chemicals has adverse effects on humans and ecosystem functioning and also reduces agricultural sustainability. A sustainable agriculture is achieved through reduction or elimination of fertilizers and agrochemicals, resulting in minimal impact to the environment. Recently, the use of antagonistic endophytes as biocontrol agents is drawing special attention as an attractive option for management of some plant diseases, resulting in minimal impact to the environment. Endophytes that resides asymptomatically within a plant, have the potential to provide a source of candidate strains for potential biocontrol applications. This review addresses biocontrol methods using endophytic fungi such as Colletotrichum, Cladosporium, Fusarium, Pestalotiopsis and Trichoderma species as an attractive option for management of some plant diseases. Potential endophytes are screened in vitro and in vivo to test their antagonistic actions by different mechanisms, including mycoparasitism, production of lytic enzymes and/or antibiotics and induction of plant defenses. Currently, efforts are being made to commercialize these biocontrol agents. A continued research pipeline consisting of screening, in vitro and in vivo testing, biomass production and commercialization of endophytes as biocontrol agents may contribute to sustainable agriculture.     

Facilitation and inhibition: changes in plant nitrogen and secondary metabolites mediate interactions between above-ground and below-ground herbivores

To date, it remains unclear how herbivore-induced changes in plant primary and secondary metabolites impact above-ground and below-ground herbivore interactions. Here, we report effects of above-ground (adult) and below-ground (larval) feeding by Bikasha collaris on nitrogen and secondary chemicals in shoots and roots of Triadica sebifera to explain reciprocal above-ground and below-ground insect interactions. Plants increased root tannins with below-ground herbivory, but above-ground herbivory prevented this increase and larval survival doubled. Above-ground herbivory elevated root nitrogen, probably contributing to increased larval survival. However, plants increased foliar tannins with above-ground herbivory and below-ground herbivory amplified this increase, and adult survival decreased. As either foliar or root tannins increased, foliar flavonoids decreased, suggesting a trade-off between these chemicals. Together, these results show that plant chemicals mediate contrasting effects of conspecific larval and adult insects, whereas insects may take advantage of plant responses to facilitate their offspring performance, which may influence population dynamics.     

Can plants betray the presence of multiple herbivore species to predators and parasitoids? The role of learning in phytochemical information networks

In response to feeding by phytophagous arthropods, plants emit volatile chemicals. This is shown to be an active physiological response of the plant and the released chemicals are therefore called herbivore-induced plant volatiles (HIPV). One of the supposed functions of HIPV for the plant is to attract carnivorous natural enemies of herbivores. Depending on which plant and herbivore species interact, blends of HIPV show qualitative and quantitative variation. Hence, one may ask whether this allows the natural enemies to discriminate between volatiles from plants infested by herbivore species that are either suitable or unsuitable as a food source for the natural enemy. Another question is whether natural enemies can also recognise HIPV when two or more herbivore species that differ in suitability as a food source simultaneously attack the same plant species. By reviewing the literature we show that arthropod predators and parasitoids can tell different HIPV blends apart in several cases of single plant–single herbivore systems and even in single plant–multiple herbivore systems. Yet, there are also cases where predators and parasitoids do not discriminate or discriminate only after having learned the association between HIPV and herbivores that are either suitable or non-suitable as a source of food. In this case, suitable herbivores may profit from colonising plants that are already infested by another non-suitable herbivore. The resulting temporal or partial refuge may have important population dynamical consequences, as such refuges have been shown to stabilise otherwise unstable predator–prey models of the Lotka-Volterra or Nicholson-Bailey type.     

Folivory with leaf folding by giant flying squirrels: its patterns and possible function

Japanese giant flying squirrels, Petaurista leucogenys, are entirely arboreal folivores. From spring to summer, their principal food source consists of leaves of the deciduous oak Quercus acutissima in western Tokyo, Japan. Before consuming the central part of each leaf, the squirrels fold the leaf two or more times with their forelegs. In the wild, leaf folding is a very rare behavior, even in primates. In our study, this peculiar feeding behavior was not observed across all study sites or even in some local populations. Herbivores generally try to maximize their intake of nutritious foods (e.g., sugars) containing low levels of plant defensive chemicals (e.g., phenols). We found that total phenolic contents in individual leaves of Q. acutissima were lower in the center than at the margins, whereas sugar (glucose) was homogenously distributed. Consequently, consumption of only leaf centers appears to be an adaptation by Japanese giant flying squirrels to avoid leaf margin defensive chemicals and structures. These leaf margin characteristics, which may have developed as defenses against herbivorous insects that generally feed from the margins, have contributed to the skillful and complex feeding behavior of this generalist folivore.     

Exploitation of induced 2n-gametes for plant breeding

Key Message

Efficient induction of unreduced gamete in different crops and its genetic consequences will open new avenues for plant breeding.

Abstract

Unreduced gamete formation derived via abnormal meiotic cell division is an important approach to polyploidy breeding. This process is considered the main driving force in spontaneous polyploids formation in nature, but the potential application of these gametes to plant breeding has not been fully exploited. An effective mechanism for their artificial induction is needed to attain greater genetic variation and enable efficient use of unreduced gametes in breeding programs. Different approaches have been employed for 2n-pollen production including interspecific hybridization, manipulation of environmental factors and treatment with nitrous oxide, trifluralin, colchicine, oryzalin and other chemicals. These chemicals can act as a stimulus to produce viable 2n pollen; however, their exact mode of action, optimum concentration and developmental stages are still not known. Identification of efficient methods of inducing 2n-gamete formation will help increase pollen germination of sterile interspecific hybrids for inter-genomic recombination and introgression breeding to develop new polyploid cultivars and increase heterozygosity among plant populations. Additionally, the application of genomic tools and identification and isolation of genes and mechanisms involved in the induction of 2n-gamete will enable increased exploitation in different plant species, which will open new avenues for plant breeding.     

Soil microbial biomass C and symbiotic processes associated with soybean after sulfentrazone herbicide application

Resource competition and chemical interference are mechanisms of interaction among plants that may occur simultaneously. However, both mechanisms are rarely considered together when modelling plant growth. We propose a new empirical model that estimates biologically significant parameters on both plant competition and chemical interference. The model is tested with data sets from different density-dependent experiments done with two species (the grass Lolium rigidum Gaud. and the legume Glycine max soya L.) subjected to a noxious chemical environment when growing (allelochemicals and herbicides, respectively). Hypotheses on the effect of allelochemicals and its interaction with density are tested using maximum likelihood ratio tests in order to ask, for these species, whether chemical interference is playing a significant role in the interactions among plants or on the contrary, whether interactions among plants are sufficiently explained by the resource competition. In all cases a significant interaction between chemicals and density is observed. This interaction is inconsistent with the hypothesis of only resource competition having an influence of plant biomass and suggests a significant density-dependent effect of chemicals on plant growth.     

Antibacterial plant compounds,extracts and essential oils: An updated review on their effects and putative mechanisms of action

Background: Antibiotic-resistant bacteria pose a global health threat. Traditional antibiotics can lose their effectiveness, and the development of novel effective antimicrobials has become a priority in recent years. In this area, plants represent an invaluable source of antimicrobial compounds with vast therapeutic potential.Purpose: To review the full possible spectrum of plant antimicrobial agents (plant compounds, extracts and essential oils) discovered from 2016 to 2021 and their potential to decrease bacterial resistance. Their activities against bacteria, with special emphasis on multidrug resistant bacteria, mechanisms of action, possible combinations with traditional antibiotics, roles in current medicine and future perspectives are discussed.Methods: Studies focusing on the antimicrobial activity of compounds of plant origin and their mechanism of action against bacteria were identified and summarized, including contributions from January 2016 until January 2021. Articles were extracted from the Medline database using PubMed search engine with relevant keywords and operators.Results: The search yielded 11,689 articles from 149 countries, of which 101 articles were included in this review. Reports from 41 phytochemicals belonging to 20 families were included. Reports from plant extracts and essential oils from 39 plant species belonging to 17 families were also included. Polyphenols and terpenes were the most active phytochemicals studied, either alone or as a part of plant extracts or essential oils. Plasma membrane disruption was the most common mechanism of antimicrobial action. Number and position of phenolic hydroxyl groups, double bonds, delocalized electrons and conjugation with sugars in the case of flavonoids seemed to be crucial for antimicrobial capacity. Combinations of phytochemicals with beta-lactam antibiotics were the most studied, and the inhibition of efflux pumps was the most common synergistic mechanism.Conclusion: In recent years, terpenes, flavones, flavonols and some alkaloids and phenylpropanoids, either isolated or as a part of extracts, have shown promising antimicrobial activity, being membrane disruption their most common mechanism. However, their utilization as appropriate antimicrobials need to be boosted by means of new omics technologies and network pharmacology to find the most effective combinations among them or in combination with antibiotics.     

Influence of Urea,Hydroxyurea, and Thiourea on Meloidogyne javanica and Infected Excised Tomato Roots in Culture

Urea (U), hydroxyurea (HU), and thiourea (TU), in various concentrations, were added to chemically defined plant tissue culture medium on which Meloidogyne javanica was reared on excised tomato roots. Concentrations as low as 3 ppm HU or 12 ppm TU inhibited nematode maturation by 70-90% 4 weeks after inoculation, and the coenocytes in the parasitized tissue were poorly developed. Gall weight was also inhibited by 50% in cultures treated with 3 and 6 ppm HU. However, exposing juveniles of M. javanica and Tylenchulus semipenetrans or juveniles and adults of Pratylenchus thornei to increasing concentrations of HU or TU, up to 100 ppm, was not lethal. These two urea derivatives still inhibited nematode maturation when the infected region of the root was not in direct contact with the chemicals. Therefore, we suggest that these urea derivatives inhibit nematode development by affecting the plant metabolism essential to coenocyte formation, an occurrence similar to the hypersensitive reaction in a naturally resistant plant.     

Influence of carbohydrates on the induction of haustoria of the cowpea rust fungusin vitro

Rust fungi are obligate plant parasites whose successful parasitism appears to depend on the formation of haustoria within living plant cells. To test the hypothesis that the lack of haustorium formation in the absence of a living cell reflects a need for a simple “signal” released during attempted invasion, various carbohydrates were applied toin vitro-formed infection structures ofUromyces vignae just prior to haustorial mother cell (HMC) formation. Up to 6% of HMCs formed haustoria after treatment with arabinose, mannose, xylose, sucrose, or xylan. Other monosaccharides, oligosaccharides, and polysaccharides were less or not effective. Sugar mixtures induced haustorium frequencies of up to 10%. Timing of the application of inducing chemicals was critical. The data indicate that haustorium formation does not require the presence of a living plant cell and may be triggered by plant products at the time of penetration peg development.     

Thermoregulatory behavior and orientation preference in bearded dragons

The regulation of body temperature is a critical function for animals. Although reliant on ambient temperature as a heat source, reptiles, and especially lizards, make use of multiple voluntary and involuntary behaviors to thermoregulate, including postural changes in body orientation, either toward or away from solar sources of heat. This thermal orientation may also result from a thermoregulatory drive to maintain precise control over cranial temperatures or a rostrally-driven sensory bias. The purpose of this work was to examine thermal orientation behavior in adult and neonatal bearded dragons (Pogona vitticeps), to ascertain its prevalence across different life stages within a laboratory situation and its interaction with behavioral thermoregulation. Both adult and neonatal bearded dragons were placed in a thermal gradient and allowed to voluntarily select temperatures for up to 8 h to observe the presence and development of a thermoregulatory orientation preference. Both adult and neonatal dragons displayed a non-random orientation, preferring to face toward a heat source while achieving mean thermal preferences of ~ 33–34 °C. Specifically, adult dragons were more likely to face a heat source when at cooler ambient temperatures and less likely at warmer temperatures, suggesting that orientation behavior counter-balances local selected temperatures but contributes to their thermoregulatory response. Neonates were also more likely to select cooler temperatures when facing a heat source, but required more experience before this orientation behavior emerged. Combined, these results demonstrate the importance of orientation to behavioral thermoregulation in multiple life stages of bearded dragons.     

Active-site mobility revealed by the crystal structure of arylmalonate decarboxylase from Bordetella bronchiseptica

Arylmalonate decarboxylase (AMDase) from Bordetella bronchiseptica catalyzes the enantioselective decarboxylation of arylmethylmalonates without the need for an organic cofactor or metal ion. The decarboxylation reaction is of interest for the synthesis of fine chemicals. As basis for an analysis of the catalytic mechanism of AMDase and for a rational enzyme design, we determined the X-ray structure of the enzyme up to 1.9 Å resolution. Like the distantly related aspartate or glutamate racemases, AMDase has an aspartate transcarbamoylase fold consisting of two α/β domains related by a pseudo dyad. However, the domain orientation of AMDase differs by about 30° from that of the glutamate racemases, and also significant differences in active-site structures are observed. In the crystals, four independent subunits showing different conformations of active-site loops are present. This finding is likely to reflect the active-site mobility necessary for catalytic activity.     

Microtubule dynamics is required for root elongation growth under osmotic stress in Arabidopsis

Osmotic stress caused by drought and soil salinity is one of the factors that affect plant root system growth and development. Previous studies have shown that microtubule plays a critical role in plant roots response to osmotic stress, however, the underlying mechanism remains unclear. In the present study, the microtubule orientations in Arabidopsis roots growing under osmotic stress were determined using confocal fluorescence microscopy. The results showed that osmotic stress could significantly inhibit primary root elongation in Arabidopsis, and pharmacological tests confirmed that microtubules were involved in Arabidopsis roots response to osmotic stress. In vivo visualization of microtubule structures with the microtubule-binding domain–green fluorescent protein (GFP) reporter revealed altered microtubule orientation in rhizodermal cells under osmotic stress. These results above indicated that osmotic stress could inhibit the elongation growth of Arabidopsis primary root, and the inhibition effects might result from the changes in microtubule orientation.