Hydroxamic acids affecting barley yellow dwarf virus transmission by the aphid Rhopalosiphum padi

2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), a hydroxamic acid (Hx) occurring in wheat, was shown to deter feeding by the aphid Rhopalosiphum padi (L.), and to reduce BYDV transmission to the plant. Dual choice tests with wheat leaves showed the preferential settlement of aphids on leaves with lower levels of DIMBOA. Electric monitoring of aphid feeding behaviour showed that in seedlings with higher DIMBOA levels fewer aphids reached the phloem and they needed longer times to contact a phloem vessel than in those with lower levels. When aphids carrying BYDV were allowed to feed on wheat cultivars with different DIMBOA levels, fewer plants were infected with BYDV in the higher DIMBOA cultivars than in the lower ones. Preliminary field experiments showed a tendency for wheat cultivars with higher Hx levels to be more tolerant to infection by BYDV than lower Hx level ones.     

Possible roles of esterase, glutathione S-transferase, and superoxide dismutase activities in understanding aphid–cereal interactions

Activities of the detoxification enzymes esterase, glutathione S‐transferase, and of superoxide dismutase in aphids and aphid‐infested cereal leaves were assayed using polyacrylamide gel electrophoresis and a spectrophotometer to elucidate the enzymatic mechanisms of aphid resistance in cereal plants. A chlorosis‐eliciting Russian wheat aphid, Diuraphis noxia (Mordvilko), and non‐chlorosis‐eliciting bird cherry‐oat aphid, Rhopalosiphum padi (L.), and four cereals were used in this study. The four cereal genotypes were ‘Arapahoe’ (susceptible) and ‘Halt’ (resistant) wheat (Triticum aestivum L.), ‘Morex’ (susceptible) barley (Hordeum vulgare L.), and ‘Border’ (resistant) oat (Avena sativa L.). Esterase isozymes differed between the two aphid species, although glutathione S‐transferase and superoxide dismutase did not. Esterase, glutathione S‐transferase, and superoxide dismutase activities in either aphid species were not affected by the level of resistance of a cereal to D. noxia. The assays of cereal leaf samples showed that D. noxia feeding elicited an increase in esterase activity in all four cereal genotypes, although R. padi feeding did not. The increase of esterase activity in cereals, however, was not correlated to aphid resistance in the cereals. The time‐series assays of aphid‐infested cereal leaves showed that D. noxia‐infested Morex barley had a significant increase in esterase activity on all sampling dates (3, 6, and 9 days) in comparison with either uninfested or R. padi‐infested barley. No difference in glutathione S‐transferase activity was detected among either aphid infestations or sampling dates. The electrophoretic assays, however, revealed that aphid feeding elicited a significant increase in superoxide dismutase activity, which served as the control of glutathione S‐transferase activity assays. The increase in esterase and superoxide dismutase activities suggested that D. noxia feeding imposes not only toxic, but also oxidative stresses on the cereals. The ramification of using these enzyme activity data to understand the etiology of D. noxia‐elicited chlorosis is discussed.     

Tolerance of antibiotic and susceptible cereal seedlings to the aphids Metopolophium dirhodum and Rhopalosiphum padi

Some cereal seedlings exhibit antibiotic and antixenotic resistance to the aphids Metopolophium dirhodum (Walker) and Rhopalosiphum padi (L.), because the seedlings contain hydroxamic acids or gramine. The association between tolerance to aphids and aphid antibiosis was investigated for three cereals, Dollarbird wheat Vulcan wheat and Yagan barley. The dry biomass gained by the aphids and the simultaneous reduction in the biomass of the plants (biomass conversion ratio) quantified tolerance. Biomass production and the density dependence of biomass production by the aphids quantified antibiosis more effectively than fecundity. Vulcan wheat, which has more hydroxamic acid than Dollarbird wheat showed the highest level of antibiosis, and the barley was not antibiotic for either aphid. The biomass conversion ratio was a constant; the biomass of an infested plant was reduced by 3 mg for each mg of aphid biomass gained, regardless of aphid species, plant cultivar, or aphid density. The three plants showed no differential tolerance to the aphids, and therefore tolerance is not associated with antibiosis in this case.     

Searching for wheat resistance to aphids and wheat bulb fly in the historical Watkins and Gediflux wheat collections

Insect pests can reduce wheat yield by direct feeding and transmission of plant viruses. Here we report results from laboratory and field phenotyping studies on a wide range of wheat, including landraces from the Watkins collection deriving from before the green revolution, more modern cultivars from the Gediflux collection (north‐western Europe) and modern UK Elite varieties, for resistance to the bird cherry‐oat aphid, Rhopalosiphum padi (Homoptera: Aphididae) and the English grain aphid, Sitobion avenae (Homoptera: Aphididae). A total of 338 lines were screened for R. padi and 340 lines for S. avenae. Field trials were also conducted on 122 Watkins lines to identify wheat bulb fly, Delia coarctata, preference on these landraces. Considerable variation was shown in insect performance among and within different wheat collections, with reduced susceptibility in a number of varieties, but phenotyping did not identify strong resistance to aphids or wheat bulb fly. Field trials showed within collection differences in aphid performance, with fewer aphids populating lines from the Watkins collection. This differs from development data in laboratory bioassays and suggests that there is a pre‐alighting cue deterring aphid settlement and demonstrates differences in aphid preference and performance on older plants in the field compared with seedlings in the laboratory, highlighting the need for phenotyping for aphid resistance at different plant growth stages. No association was identified between performance of the different insect species on individual varieties, potentially suggesting different nutritional requirements or resistance mechanisms.     

Partial antibiosis to Rhopalosiphum padi in wheat and some phytochemical correlations

The fecundity of the aphid Rhopalosiphum padi during the first ten days of reproduction was measured on five wheats at three growth stages. The wheats were of UK and Iranian origin, but also involved both spring and winter wheats as well as hexaploid and tetraploid types. Aphids which had already been reared on the respective variety for at least one generation were transferred to the experimental plants. The tetraploid Emmer showed some resistance in comparison with the hexaploid UK varieties at later growth stages, whereas the Iranian variety Moghan 2 appeared relatively resistant only at the tillering stage. The other Iranian variety, Ommid, was resistant at all growth stages, and appears to be the most resistant hexaploid wheat variety to R. padi so far identified. Correlations of aphid fecundity with plant chemistry were attempted with total phenolic compounds, hydroxamic acids and amino acids. All three of these chemical groups have previously been reported as correlated with plant resistance to cereal aphids. Even just for the results at the tillering stage, no correlation could be found for phenolics or hydroxamic acids, but a multiple regression based on the levels of alanine, histidine and threonine accounted for over 95% of the variation in aphid fecundity at all 15 data points (5 wheats × 3 growth stages). No cause or effect relationship is necessarily implied.     

Probing behaviour of Diuraphis noxia on five cereal species with different hydroxamic acid levels

The feeding behaviour of Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae) was electronically monitored on five cereal species (Triticum aestivum L., T. turgidum L., Secale cereale L. and x Triticosecale Wittmack) containing different hydroxamic acids levels and on Hordeum vulgare L., lacking these compounds, by means of a DC-System. With increasing DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) levels D. noxia showed less probing particularly before reaching the phloem phase and a lower percentage of aphids achieved sustained phloem ingestion. However, no significant correlation was found between DIMBOA levels and the total time of phloem feeding. These findings indicate that factors from the mesophyll and vascular tissues appear to be involved in the effects of resistance. The underlying mechanism, however, remains unclear. Thus, DIMBOA seems to be only part of the chemical defenses of the cereal plants that might be implicated in D. noxia resistance.     

Genetic mapping shows intraspecific variation and transgressive segregation for caterpillar‐induced aphid resistance in maize

Plants in nature have inducible defences that sometimes lead to targeted resistance against particular herbivores, but susceptibility to others. The metabolic diversity and genetic resources available for maize (Zea mays) make this a suitable system for a mechanistic study of within‐species variation in such plant‐mediated interactions between herbivores. Beet armyworms (Spodoptera exigua) and corn leaf aphids (Rhopalosiphum maidis) are two naturally occurring maize herbivores with different feeding habits. Whereas chewing herbivore‐induced methylation of 2,4‐dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one glucoside (DIMBOA‐Glc) to form 2‐hydroxy‐4,7‐dimethoxy‐1,4‐benzoxazin‐3‐one glucoside (HDMBOA‐Glc) promotes caterpillar resistance, lower DIMBOA‐Glc levels favour aphid reproduction. Thus, caterpillar‐induced DIMBOA‐Glc methyltransferase activity in maize is predicted to promote aphid growth. To test this hypothesis, the impact of S. exigua feeding on R. maidis progeny production was assessed using seventeen genetically diverse maize inbred lines. Whereas aphid progeny production was increased by prior caterpillar feeding on lines B73, Ki11, Ki3 and Tx303, it decreased on lines Ky21, CML103, Mo18W and W22. Genetic mapping of this trait in a population of B73 × Ky21 recombinant inbred lines identified significant quantitative trait loci on maize chromosomes 1, 7 and 10. There is a transgressive segregation for aphid resistance, with the Ky21 alleles on chromosomes 1 and 7 and the B73 allele on chromosome 10 increasing aphid progeny production. The chromosome 1 QTL coincides with a cluster of three maize genes encoding benzoxazinoid O‐methyltransferases that convert DIMBOA‐Glc to HDMBOA‐Glc. Gene expression studies and benzoxazinoid measurements indicate that S. exigua ‐induced responses in this pathway differentially affect R. maidis resistance in B73 and Ky21.     

Costs and benefits of hydroxamic acids-related resistance in winter wheat against the bird cherry-oat aphid, Rhopalosiphum padi L.

Estimations of infestation by the bird cherry-oat aphid (Rhopalosiphum padi) as well as measurements of grain yield in 26 Hungarian winter wheat cultivars under field conditions were correlated with the concentration of hydroxamic acids (Hx) in seedlings of those cultivars. The significant inverse relationship between infestation ratings and Hx levels in wheat showed that Hx, despite their decreased accumulation at later plant phenological stages, may be able to confer resistance against aphid infestation in the field. Since no significant relationship was found between grain yield and Hx levels in plants it is suggested that Hx accumulation does not impose a cost to the plant in terms of yield. These findings support earlier claims stressing the potential of Hx as breeding targets for aphid resistance in wheat.     

In vitro enzymatic chlorophyll catabolism in wheat elicited by cereal aphid feeding

Chlorophyll degradation is a complex phenomenon that often accompanies insect feeding damage to plants. Loss of chlorophyll can be initiated by several reactions, including oxidative bleaching, chlorophyllase activity, and Mg-dechelatase activity. Extracts from the Russian wheat aphid [Diuraphis noxia (Mordvilko)], the bird cherry-oat aphid [Rhopalosiphum padi (L.)], and aphid-infested and uninfested wheat plants were assayed in vitro for activities involved in chlorophyll degradation. Although the initial infestation was the same (10 apterous adults) for both aphid species, D. noxia weight was significantly higher than R. padi after feeding for 12 days. Consequently, D. noxia feeding caused greater fresh leaf weight reduction than R. padi feeding. Chlorophyll degradation assays showed no activity from either D. noxia or R. padi extracts. Plant extract assays showed a significant difference in Mg-dechelatase activity, while no difference was detected in either the chlorophyllase or oxidative bleaching pathways among the aphid-infested or uninfested plant extracts. Diuraphis noxia-infested leaf extracts showed a greater increase of Mg-dechelatase activity than either R. padi-infested or the uninfested plants. The findings suggest that leaf chlorosis elicited by D. noxia feeding is different from the chlorophyll degradation that occurs in natural plant senescence. Aphid-elicited chlorosis might be the result of a Mg-dechelatase-driven catabolism of chlorophyll in challenged wheat seedlings, however, the factor(s) from D. noxia that elicited the increase of Mg-dechelatase activity still remain to be determined.     

Effect of light on DIMBOA-glucoside concentration in wheat {Triticum aestivum L.)

DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) glucoside in wheat has been suggested to play a role in plant resistance to cereal aphids. Thus there is considerable interest in trying to breed modern wheat varieties with increased concentrations of this compound. To further such work we assessed the effects of light intensity on DIMBOA glucoside production in wheat seedlings under both indoor and outdoor conditions. Light had a marked effect on DIMBOA-glucoside concentrations, with levels in plants being negatively related to light intensity. Our findings suggest that breeding aimed at increasing levels of DIMBOA-glucoside would be most effective if test plants for selection were to be grown under weak light conditions.     

A screen of worldwide wheat cultivars for hydroxamic acid levels and aphid antixenosis

In a screen of seedlings of a worldwide range of 47 cultivars of Triticum (mainly T. aestivum) the concentration of the hydroxamic acid DIMBOA ranged between 1 and 8 mmol/kg fresh wt. In a bioassay in which alatae of the aphid Sitobion avenae were released among replicated test seedlings, there were highly significant correlations between aphid ‘preference’ and DIMBOA levels in the seedlings. The value of these results in work leading to the production of aphid-resistant cultivars is discussed.     

Effect of wheat resistance, the parasitoid Aphidius rhopalosiphi, and the entomopathogenic fungus Pandora neoaphidis, on population dynamics of the cereal aphid Sitobion avenae

The influence of wheat (Triticum aestivumL.) resistance, the parasitoid Aphidius rhopalosiphiDe Stephani-Perez (Hymenoptera: Braconidae) and the entomopathogenic fungus Pandora neoaphidis(Remaudière et Hennebert) Humber (Zygomycetes: Entomophthorales) on the density and population growth rate of the cereal aphid Sitobion avenae(F.) (Hemiptera: Aphididae) was studied under laboratory conditions. Partial wheat resistance was based on hydroxamic acids, a family of secondary metabolites characteristic of several cultivated cereals. The partial resistance of wheat cultivar Naofén, the action of the parasitoid and the joint action of the parasitoid and fungus, reduced aphid density. The lowest aphid densities were obtained with the combination of the parasitoid and the fungus, but wheat resistance under these circumstances did not improve aphid control. Significant reductions of population growth rate (PGR) of aphids were obtained with the joint action of wheat resistance and natural enemies. In particular, the combined effects of parasitoids and fungi showed significantly lower PGR than the control without natural enemies in both wheat cultivars. Our results support the hypothesis that wheat resistance and the utilization of biological control agents could be complementary strategies in an integrated pest management program against cereal aphids.     

The effect of hydoroxamic acid concentration at late growth stages of wheat on the performance of the aphid Sitobion avenae

From a previous worldwide screen of wheat cultivars as seedings, five were selected to encompass a wide concentraiton range of the hydroxaminc acid DIMBOA (0.99-8.07 mmol Kg ^-1 fr. wt). Compared with those in seedings, the concentrations of DIMBOA in flag leaves of the same cultivars were relatively low (0.06-0.87 mmol kg^-1 fr. wt). Concentraions of hydroxamic acids in whole ears at anthesis and the early milk stage were even lower, ranging from undetectable to 0.05 mmol kg^-1 fr. wt. The three-day mean relaive growth rate (MRGR) was determined for nymphs of Sitobion avenace on the flag leaf (GS 39) and on the ear (GS 60-60) of the five cultivars. No significant differences in MRGR were found between cultivatrs at the same growth stage, although MRGR was higher on ears on than on flag leaces. The results are discussed in realtion to the potential of hydroxamic acids in programmses aimed at breeding mature-plant aphid resistance into wheat cultivars.     

Comparison of DIMBOA concentrations among wheat isolines and corresponding plant introduction lines

The concentration of a hydroxamic acid, also known as DIMBOA (2,4-dihydroxy-7-methoxy-1, 4-benzoxazin-3-one), in 6-d old wheat seedlings was examined using reverse-phase high performance liquid chromatography (HPLC). Wheat plant introduction (PI) lines PI 137739 (Dn1 gene), PI 262660 (Dn2 gene), and PI 294994 (Dn5 gene), the corresponding near-isogenic lines`Betta'-Dn1, Betta-Dn2 and Betta-Dn5, and susceptible Betta wheat were used in the study. The Dn2 gene conferring Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae), tolerance was not related to DIMBOA concentration in wheat. Of the lines with Dn1 and Dn5 genes that confer antibiosis to D. noxia, only lines with the Dn5 gene showed increased DIMBOA accumulation. However, the Dn5 and the DIMBOA biosynthesis genes are not located in the same chromosome group. Possible relationship between the Dn5 gene and DIMBOA accumulation was discussed. This study indicates that DIMBOA concentration does not completely explain D. noxia resistance in the wheat lines examined and a comprehensive examination of other allelochemicals (e.g., phenolics) is necessary.     

The effect of banker plant species on the fitness of Aphidius colemani Viereck and its aphid host (Rhopalosiphum padi L.)

Banker plants, a type of open-rearing unit, are increasingly used in greenhouse crops to sustain natural enemy populations at times of low pest abundance. The most common banker plant system is a non-crop, cereal plant which supports Rhopalosiphum padi L. as an alternative host for Aphidius colemani Viereck. Although bottom-up effects of plants are known to affect natural enemies, this aspect has generally been ignored in previous investigations of banker plant efficacy. Here, we tested four cereal plant species with three varieties each to investigate host plant effects on R. padi and A. colemani. Though limited differences were observed in laboratory experiments spanning one aphid or parasitoid generation, longer greenhouse experiments spanning several generations revealed significant plant effects on both insects. R. padi performed poorly on oats (Avena sativa L.), resulting in wasps with the longest female development time, lowest emergence rates, and the lowest number of wasps produced per unit. Rye (Secale cereal L.) – intermediate in terms of aphid performance – produced a significantly male-biased wasp population with the smallest males. Conversely, R. padi placed onto either wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) performed consistently well in terms of aphid and parasitoid fitness and abundance, though neither species was obviously superior over the other. Overall, cultivars within each plant species did not significantly affect outcomes. As each plant species tested had different positive effects on aphid and parasitoid phenotypes, the potential benefits of mixing of cereal species is an area for future investigation.     

Plant water stress intensity mediates aphid host choice and feeding behaviour

1. The effects of drought-induced changes in plant quality on aphid performance and population growth is well-studied. The response of aphid behaviour to plant water limitation has received less attention. Water limitation may affect host-plant colonization by altering the attractiveness of plants. Additionally, plant water limitation may inhibit feeding site establishment and phloem ingestion.
2. Our goal was to examine bird cherry-oat aphid (Rhopalosiphum padi L.) host selection and feeding behaviour under water limitation. We assessed aphid response to well-watered, mildly-stressed, and highly-stressed wheat (Triticum aestivum L.) by evaluating (i) host-plant selection through two-choice assays, (ii) feeding behaviour using the electrical penetration graph technique, and (iii) phloem ingestion by quantifying honeydew production.
3. Aphids were less likely to select highly stressed plants than a mildly stressed or well-watered alternative. Aphids did not distinguish between mildly stressed and well-watered plants. Aphid feeding behaviours, including duration of phloem ingestion, were not affected by water availability. However, honeydew production was reduced under both levels of water limitation. These results suggest that the volume of phloem ingested by aphids per unit time declined on stressed plants. The combination of lower colonization and diminished access to food on stressed plants may lead to a reduction in aphid abundance, independent of the direct effects of nutrition on individual aphid performance.
4. This study highlights the potential contribution of herbivore behaviour to documented changes in aphid abundance on stressed plants and underscores the important role of plant water stress intensity in mediating plant-herbivore interactions.

     

Allelopathy in wheat (Triticum aestivum)

Wheat (Triticum aestivum) allelopathy has potential for the management of weeds, pests and diseases. Both wheat residue allelopathy and wheat seedling allelopathy can be exploited for managing weeds, including resistant biotypes. Wheat varieties differ in allelopathic potential against weeds, indicating that selection of allelopathic varieties might be a useful strategy in integrated weed management. Several categories of allelochemicals for wheat allelopathy have been identified, namely, phenolic acids, hydroxamic acids and short‐chain fatty acids. Wheat allelopathic activity is genetically controlled and a multigenic model has been proposed. Research is underway to identify genetic markers associated with wheat allelopathy. Once allelopathic genes have been located, a breeding programme could be initiated to transfer the genes into modern varieties for weed suppression. The negative impacts of wheat autotoxicity on agricultural production systems have also been identified when wheat straws are retained on the soil surface for conservation farming purposes. A management package to avoid such deleterious effects is discussed. Wheat allelopathy requires further study in order to maximise its allelopathic potential for the control of weeds, pests and diseases, and to minimise its detrimental effects on the growth of wheat and other crops.     

Proteomic profile of aphids exposed to wheat with different contents of benzoxazinoids

Benzoxazinoids are key defence chemicals in cereals that are known to affect several aspects of aphid biology. However, little is known about how they affect aphid physiology. In the present study, we report changes in the whole‐body proteomic profiles of a 2,4‐dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one (DIMBOA)‐susceptible genotype of the grain aphid Sitobion avenae (F.) after being exposed to wheat cultivars containing contrasting levels of DIMBOA. The proteome is analyzed after 14 days (short term) and 28 days (long term) of rearing on these cultivars. Seventy‐two proteins are differentially regulated among the treatments and 49 are identified. Exposure to high‐DIMBOA plants results in a higher number of proteins regulated long term. DIMBOA exposure in S. avenae initially generates greater cellular activities, mostly involving cytoskeleton function and possibly related to detoxification. This function appears to be unimportant at long term and is eventually replaced by effects on metabolism function and homeostasis. Taken together, the results of the present study show that the responses of aphids to the secondary plant compounds, such as DIMBOA, exhibit a temporal dynamic in the proteome, possibly helping aphids to overcome the effect of these toxic compounds.     

Molecular interactions between wheat and cereal aphid (Sitobion avenae): analysis of changes to the wheat proteome

Aphids are major insect pests of cereal crops, acting as virus vectors as well as causing direct damage. The responses of wheat to infestation by cereal aphid (Sitobion avenae) were investigated in a proteomic analysis. Approximately, 500 protein spots were reproducibly detected in the extracts from leaves of wheat seedlings after extraction and 2‐DE. Sixty‐seven spots differed significantly between control and infested plants following 24 h of aphid feeding, with 27 and 11 up‐regulated, and 8 and 21 down‐regulated, in local or systemic tissues, respectively. After 8 days, 80 protein spots differed significantly between control and aphid treatments with 13 and 18 up‐regulated and 27 and 22 down‐regulated in local or systemic tissues, respectively. As positive controls, plants were treated with salicylic acid or methyl jasmonate; 81 and 37 differentially expressed protein spots, respectively, were identified for these treatments. Approximately, 50% of differentially expressed protein spots were identified by PMF, revealing that the majority of proteins altered by aphid infestation were involved in metabolic processes and photosynthesis. Other proteins identified were involved in signal transduction, stress and defence, antioxidant activity, regulatory processes, and hormone responses. Responses to aphid attack at the proteome level were broadly similar to basal non‐specific defence and stress responses in wheat, with evidence of down‐regulation of insect‐specific defence mechanisms, in agreement with the observed lack of aphid resistance in commercial wheat lines.