Root uptake of cereal benzoxazinoids grants resistance to root-knot nematode invasion in white clover

Plants synthesize a plethora of chemical defence compounds, which vary between evolutionary lineages. We hypothesize that plants evolved the ability to utilize defence compounds synthesized and released by neighbouring heterospecific plants. In two experiments, we incubated clover (Trifolium repens L.) seedlings with individual benzoxazinoid (BX) compounds (2,4-dihydroxy-1,4-benzoxazin-3-one, 2-hydroxy-1,4-benzoxazin-3-one, benzoxazolinone, and 6-methoxy- benzoxazolin-2-one), a group of bioactive compounds produced by cereals, to allow clover BX uptake. Subsequently, we transplanted the seedlings into soil and quantified BX root and shoot content and invasion of root-knot nematodes in clover roots up to 8 weeks after transplantation. We show that clover root uptake of BXs substantially enhanced clover's resistance against the root-knot nematode Meloidogyne incognita. This effect lasted up to 6 weeks after the clover roots were exposed to the BXs. BXs were absorbed by clover roots, and then translocated to the shoots. As a result of clover metabolization, we detected the parent BXs and a range of their transformation products in the roots and shoots. Based on these novel findings, we envisage that co-cultivation of crop species with complementary and transferable chemical defence systems can add to plant protection.

A neglected alliance in battles against parasitic plants: arbuscular mycorrhizal and rhizobial symbioses alleviate damage to a legume host by root hemiparasitic Pedicularis species

Despite their ubiquitous distribution and significant ecological roles, soil microorganisms have long been neglected in investigations addressing parasitic plant-host interactions. Because nutrient deprivation is a primary cause of host damage by parasitic plants, we hypothesized that beneficial soil microorganisms conferring nutrient benefits to parasitized hosts may play important roles in alleviating damage. We conducted a pot cultivation experiment to test the inoculation effect of an arbuscular mycorrhizal fungus (Glomus mosseae), a rhizobium (Rhizobium leguminosarum) and their interactive effects, on alleviation of damage to a legume host (Trifolium repens) by two root hemiparasitic plants with different nutrient requirements (N-demanding Pedicularis rex and P-demanding P. tricolor). Strong interactive effects between inoculation regimes and hemiparasite identity were observed. The relative benefits of microbial inoculation were related to hemiparasite nutrient requirements. Dual inoculation with the rhizobium strongly enhanced promotional arbuscular mycorrhizal effects on hosts parasitized by P. rex, but reduced the arbuscular mycorrhizal promotion on hosts parasitized by P. tricolor. Our results demonstrate substantial contribution of arbuscular mycorrhizal and rhizobial symbioses to alleviating damage to the legume host by root hemiparasites, and suggest that soil microorganisms are critical factors regulating host-parasite interactions and should be taken into account in future studies.

Transcription of Biotic Stress Associated Genes in White Clover (Trifolium repens L.) Differs in Response to Cyst and Root-Knot Nematode Infection

The transcription of four members of the Kunitz proteinase inhibitor (KPI) gene family of white clover (Trifolium repens L.), designated as Tr-KPI1, Tr-KPI2, Tr-KPI4 and Tr-KPI5, was investigated at both local infection (roots) and systemic (leaf tissue) sites in white clover in response to infection with the clover root knot nematode (CRKN) Meloidogyne trifoliophila and the clover cyst nematode (CCN) Heterodera trifolii. Invasion by the CRKN resulted in a significant decrease in transcript abundance of Tr-KPI4 locally at both 4 days post-infection (dpi) and at 8 dpi, and an increase in transcription of Tr-KPI1 systemically at 8 dpi. In contrast, an increase in transcript abundance of all four Tr-KPI genes locally at 4 and 8 dpi, and an increase of Tr-KPI1, Tr-KPI2, and Tr-KPI5 at 8 dpi systemically was observed in response to infection with the CCN. Challenge of a resistant (R) genotype and a susceptible (S) genotype of white clover with the CCN revealed a significant increase in transcript abundance of all four Tr-KPI genes locally in the R genotype, while an increase in abundance of only Tr-KPI1, Tr-KPI2, and Tr-KPI5 was observed in the S genotype, and only at 4 dpi. The transcript abundance of a member of the1-AMINOCYCLOPROPANE-1-CARBOXYLATE (ACC) SYNTHASE gene family from white clover (Tr-ACS1) was significantly down-regulated locally in response to CRKN infection at 4 and 8 dpi and at 4 dpi, systemically, while abundance increased locally and systemically at 8 dpi in response to CCN challenge. Conversely, the abundance of the jasmonic acid (JA) signalling gene, CORONATINE-INSENSITIVE PROTEIN 1 from white clover (Tr-COI1) increased significantly at 8 dpi locally in response to CRKN infection, but decreased at 8 dpi in response to CCN infection. The significance of this differential regulation of transcription is discussed with respect to differences in infection strategy of the two nematode species.

Host-specific races in the holoparasitic angiosperm Orobanche minor: implications for speciation in parasitic plants

BACKGROUND AND AIMS

Orobanche minor is a root-holoparasitic angiosperm that attacks a wide range of host species, including a number of commonly cultivated crops. The extent to which genetic divergence among natural populations of O. minor is influenced by host specificity has not been determined previously. Here, the host specificity of natural populations of O. minor is quantified for the first time, and evidence that this species may comprise distinct physiological races is provided.

METHODS

A tripartite approach was used to examine the physiological basis for the divergence of populations occurring on different hosts: (1) host-parasite interactions were cultivated in rhizotron bioassays in order to quantify the early stages of the infection and establishment processes; (2) using reciprocal-infection experiments, parasite races were cultivated on their natural and alien hosts, and their fitness determined in terms of biomass; and (3) the anatomy of the host-parasite interface was investigated using histochemical techniques, with a view to comparing the infection process on different hosts.

KEY RESULTS

Races occurring naturally on red clover (Trifolium pratense) and sea carrot (Daucus carota ssp. gummifer) showed distinct patterns of host specificity: parasites cultivated in cross-infection studies showed a higher fitness on their natural hosts, suggesting that races show local adaptation to specific hosts. In addition, histological evidence suggests that clover and carrot roots vary in their responses to infection. Different root anatomy and responses to infection may underpin a physiological basis for host specificity.

CONCLUSIONS

It is speculated that host specificity may isolate races of Orobanche on different hosts, accelerating divergence and ultimately speciation in this genus. The rapid life cycle and broad host range of O. minor make this species an ideal model with which to study the interactions of parasitic plants with their host associates.

Plant host effect on the development of Heliothis virescens F. (Lepidoptera: Noctuidae)

Heliothis virescens F. is an important polyphagous pest that can develop on >100 plant species, including 20 economic crops. Populations of this insect are believed to be locally maintained on a few crops and weed hosts in Washington County, MS. To find the intrinsic value of these plants for the development of H. virescens populations, we fed different laboratory and wild colonies with fresh and lyophilized plant tissue under a constant temperature. Development time of this insect under laboratory conditions varied up to 10 d between plant hosts and was dependent on the type of plant tissue provided: fresh or lyophilized. Life table parameters such as net reproductive rate, finite rate of increase, and generation time indicated that Trifolium repens, a wild host growing around agricultural fields year round, could be one of the most suitable local plant hosts for the development of H. virescens. Two species of Geranium, previously reported as the source of the first H. virescens generation in the region, had lower intrinsic value as a food source than did T. repens. Gossyipium hirsutum, perhaps the most important crop source of H. virescens in the region, produced low net reproductive rate and finite rate of increase parameters. Sampling conducted in agricultural fields during 2006 and 2007 found no larvae on the above mentioned wild hosts as it was previously reported. Results indicated that H. virescens populations in this region were not supported by the wild plant species growing around agricultural fields during the time when the survey took place.

Does host plant influence parasitism and parasitoid species composition in Lygus rugulipennis? A molecular approach

Lygus Hahn plant bugs (Hemiptera: Miridae) are serious pests of a wide variety of economically important crops in North America. European Peristenus digoneutis Loan and P. relictus Ruthe (Hymenoptera: Braconidae) are being considered for release in Canada as part of a classical biological control program for Lygus. The attractiveness of different host plants to European Peristenus has not been addressed, but may be an important consideration prior to parasitoid release. Lygus rugulipennis Poppius nymphs were collected in the Northern Temperate Atlantic (NTA) ecoregion on red clover (Trifolium pratense L.; Fabaceae) and chamomile (Matricaria recutita L.; Asteraceae), and in the Western European Broadleaf Forest (WEBF) ecoregion on red clover and alfalfa (Medicago sativa L.; Fabaceae). Parasitism levels and parasitoid species were determined using a multiplex PCR assay for P. digoneutis, P. relictus, and P. pallipes Curtis. Mean parasitism levels in L. rugulipennis were 45-49% in the NTA ecoregion and 25-32% in the WEBF ecoregion. However, in neither ecoregion were parasitism levels and parasitoid species compositions significantly different in nymphs from different host plant species. Furthermore, multiparasitism was low despite the fact that P. digoneutis and P. relictus share the same host species.

Resistance of red clover (Trifolium pratense) to the root parasitic plant Orobanche minor is activated by salicylate but not by jasmonate

BACKGROUND AND AIMS

Obligate root holoparasites of the genus Orobanche attack dicotyledonous crops and cause severe losses in many parts of the world. Chemical induction of plant defence systems such as systemic acquired resistance was proposed to be an available strategy to control the root parasite, but the detailed mechanisms involved have not been clarified. The aim of this study was to elucidate the effects of salicylic acid (SA), jasmonic acid (JA) and their analogues on resistance of red clover to Orobanche parasitism.

METHODS

Roots of red clover grown in plastic chambers were applied with SA, S-methyl benzo[1,2,3]thiadiazole-7-carbothioate (BTH), methyl jasmonate (MeJA) and n-propyl dihydrojasmonate (PDJ), and then were inoculated with O. minor seeds. Attachments of the parasite were observed after 5 weeks.

KEY RESULTS

SA and BTH, inducers of SA-mediated defences, significantly reduced the number of established parasites by more than 75 %. By contrast, MeJA and PDJ, inducers of JA-mediated defences, did not affect parasitism. The reduction in the number of established parasites by SA and BTH was due to the inhibited elongation of O. minor radicles and the activation of defence responses in the host root including lignification of the endodermis.

CONCLUSIONS

These results suggest that SA-induced resistance, but not JA-induced resistance, is effective in inhibiting Orobanche parasitism and that the resistance is expressed by the host root both externally and internally.

Genetic variation in the effect of a facultative symbiont on host-plant use by pea aphids

Ecological specialisation on different host plants occurs frequently among phytophagous insects and is normally assumed to have a genetic basis. However, insects often carry microbial symbionts, which may play a role in the evolution of specialisation. The bacterium Regiella insecticola is a facultative symbiont of pea aphids (Acyrthosiphon pisum) where it is found most frequently in aphid clones feeding on Trifolium giving rise to the hypothesis that it may improve aphid performance on this plant. A study in which R. insecticola was eliminated from a single naturally infected aphid clone supported the hypothesis, but a second involving two aphid clones did not find the same effect. We created a series of new pea aphid-R. insecticola associations by injecting different strains of bacteria into five aphid clones uninfected by symbionts. For all aphid clones, the bacteria decreased the rate at which aphids accepted Vicia faba as a food plant and reduced performance on this plant. Their effect on aphids given Trifolium pratense was more complex: R. insecticola negatively affected acceptance by all aphid clones, had no effect on the performance of four aphid clones, but increased performance of a fifth, thus demonstrating genetic variation in the effect of R. insecticola on pea aphid host use. We discuss how these results may explain the distribution and frequency of this symbiont across different aphid populations.

Phosphorus deficiency in red clover promotes exudation of orobanchol, the signal for mycorrhizal symbionts and germination stimulant for root parasites

Plant derived sesquiterpene strigolactones, which have previously been characterized as germination stimulants for root parasitic plants, have recently been identified as the branching factors which induce hyphal branching morphogenesis, a critical step in host recognition by arbuscular mycorrhizal (AM) fungi. We show here that, in red clover plants (Trifolium pratense L.), which is known as a host for both AM fungi and the root holoparasitic plant Orobanche minor Sm., reduced supply of phosphorus (P) but not of other elements examined (N, K, Mg, Ca) in the culture medium significantly promotes the release of a strigolactone, orobanchol, by the roots of this plant. In red clover plants, the level of orobanchol exudation appeared to be regulated by P availability and was in good agreement with germination stimulation activity of the root exudates. This implies that under P deficiency, plant roots attract not only symbiotic fungi but also root parasitic plants through the release of strigolactones. This is the first report demonstrating that nutrient availability influences both symbiotic and parasitic interactions in the rhizosphere.

Phosphorus deficiency in red clover promotes exudation of orobanchol, the signal for mycorrhizal symbionts and germination stimulant for root parasites

Plant derived sesquiterpene strigolactones, which have previously been characterized as germination stimulants for root parasitic plants, have recently been identified as the branching factors which induce hyphal branching morphogenesis, a critical step in host recognition by arbuscular mycorrhizal (AM) fungi. We show here that, in red clover plants (Trifolium pratense L.), which is known as a host for both AM fungi and the root holoparasitic plant Orobanche minor Sm., reduced supply of phosphorus (P) but not of other elements examined (N, K, Mg, Ca) in the culture medium significantly promotes the release of a strigolactone, orobanchol, by the roots of this plant. In red clover plants, the level of orobanchol exudation appeared to be regulated by P availability and was in good agreement with germination stimulation activity of the root exudates. This implies that under P deficiency, plant roots attract not only symbiotic fungi but also root parasitic plants through the release of strigolactones. This is the first report demonstrating that nutrient availability influences both symbiotic and parasitic interactions in the rhizosphere.

Effects of legume forages on ovine gastrointestinal parasite development, migration and survival

Lambs grazing certain legumes have reduced parasite intensities compared to lambs grazing ryegrass swards. Eighteen replicates of white clover (cv. AberHerald), lucerne (cv. Luzelle), red clover (cv. Merviot) and perennial ryegrass (cv. Abersilo) were sown at equivalent field rates in 25 cm diameter PVC pots and maintained outside for 6 months. On day 0, forage in each pot was cut to 50 mm from soil level and the pots were placed in a glasshouse (at 19-25 degrees C and 70% humidity) in a randomised block design. Ten grams sheep faeces containing 2,133 Haemonchus contortus eggs per gram were placed on the soil in each pot. Six replicates of each forage were destructively sampled on days 14, 21 and 29. Forage samples were cut at 50 mm from the soil surface and at the soil surface to give two samples per pot. The number of nematodes was determined by a modification of the Whitehead tray method. The ratio of free-living to infective-stage larvae was determined from at least 10% of the larvae. The number of H. contortus larvae kgdrymatter(-1) (DM) forage was calculated and the data rank transformed prior to analysis by ANOVA. There were fewer larvae on legumes compared with ryegrass on samples from forage above 50 mm (P<0.001) but there was no forage effect on larvae below this height. The sum of larvae present on all forage per kilogram DM showed fewer larvae on red clover compared with ryegrass on day 21 (P<0.05). There was an effect of day on the total number of larvae on forage (P<0.001) but there were no foragexday interactions. Analysis of the data according to the leaf area above 50 mm from the soil surface confirmed these results, that there were fewer larvae on legume forages than ryegrass above this height (P<0.01). Overall, red clover affected the development of H. contortus and all legumes affected larval migration above 50 mm compared with ryegrass but survival of larvae was similar on all forages. Further work is needed to determine if these effects of legume forages would reduce the number of parasitic larvae ingested by livestock under field conditions.

Host use of a hemiparasitic plant: no trade-offs in performance on different hosts

To examine putative specialization of a hemiparasitic plant to the most beneficial host species, we studied genetic variation in performance and trade-offs between performance on different host species in the generalist hemiparasite, Rhinanthus serotinus. We grew 25 maternal half-sib families of the parasite on Agrostis capillaris and Trifolium pratense and without a host in a greenhouse. Biomass and number of flowers of the parasite were the highest when grown on T. pratense. There were significant interactions between host species and R. serotinus seed-family indicating that the differences in performance on the two hosts and without a host varied among the families. However, we found no significant negative correlations between performance of R. serotinus on the host species or between performance on the two hosts and autotrophic performance. Thus, the genetic factors studied here are not likely to affect the evolution of specialization of R. serotinus to the most beneficial host.

The use of an alternative food source (red clover) as a means of reducing slug pest damage to winter wheat: towards field implementation

Slugs are major pests of many crops in the UK, including winter wheat, yet current methods of control are often unreliable. The aim of this study was to investigate three issues key to the successful field implementation of a control strategy that uses red clover as an alternative food source to reduce the amount of damage caused to winter wheat by the field slug, Deroceras reticulatum (Müller). A series of three experiments was designed to assess this aim. Firstly, under laboratory conditions, red clover was consumed in greater quantities than wheat, even when wheat was presented as a novel food. Secondly, red clover had no significant effects on the emergence and early growth of wheat in a polytunnel experiment. Both these results are crucial to the successful implementation of a strategy that uses red clover as an alternative food source. Lastly, the results of a field experiment were consistent with the results of the polytunnel experiment, in that red clover did not significantly affect wheat emergence. However, plots in which red clover was left to grow until the time of wheat harvest resulted in significantly lower (43%) wheat yields than plots without red clover. These results suggest that red clover must be removed from the field after the wheat has passed its vulnerable seedling stage. Recommendations for the potential use of red clover as an alternative food source for reducing damage to winter wheat in field conditions are discussed and opportunities for further work are suggested.

Attractive properties of an isoflavonoid found in white clover root nodules on the clover root weevil

The clover root weevil, Sitona lepidus, frequently feeds on N2 fixing rhizobial root nodules of white clover (Trifolium repens), which may contain isoflavonoids with defensive and plant regulatory properties. This study investigated the isoflavonoids present in N2 fixing (active) root nodules, root nodules that were not fixing N2 (inactive), and roots without nodules, and tested the behavioral responses of neonatal S. lepidus larvae to aglycones of the identified compounds. Formononetin concentrations were higher in the active nodules compared with inactive nodules and roots alone. Moreover, there was a statistically significant attraction to formononetin by S. lepidus in arena experiments, whereas the other isoflavonoids were unattractive. It is suggested that S. lepidus may have become tolerant to the toxic effects of formononetin with repeated exposure, and that it may play a role in root nodule location. Such coevolutionary relationships are widely reported for aboveground insects and plants, but the present study suggests they may also occur belowground.

Host plant recognition by the root feeding clover weevil, Sitona lepidus (Coleoptera: Curculionidae)

This study investigated the ability of neonatal larvae of the root-feeding weevil, Sitona lepidus Gyllenhal, to locate white clover Trifolium repens L. (Fabaceae) roots growing in soil and to distinguish them from the roots of other species of clover and a co-occurring grass species. Choice experiments used a combination of invasive techniques and the novel technique of high resolution X-ray microtomography to non-invasively track larval movement in the soil towards plant roots. Burrowing distances towards roots of different plant species were also examined. Newly hatched S. lepidus recognized T. repens roots and moved preferentially towards them when given a choice of roots of subterranean clover, Trifolium subterraneum L. (Fabaceae), strawberry clover Trifolium fragiferum L. (Fabaceae), or perennial ryegrass Lolium perenneL. (Poaceae). Larvae recognized T. repens roots, whether released in groups of five or singly, when released 25 mm (meso-scale recognition) or 60 mm (macro-scale recognition) away from plant roots. There was no statistically significant difference in movement rates of larvae.