The molecular bases of plant resistance and defense responses to aphid feeding: current status

Plant genes participating in the recognition of aphid herbivory in concert with plant genes involved in defense against herbivores mediate plant resistance to aphids. Several such genes involved in plant disease and nematode resistance have been characterized in detail, but their existence has only recently begun to be determined for arthropod resistance. Hundreds of different genes are typically involved and the disruption of plant cell wall tissues during aphid feeding has been shown to induce defense responses in Arabidopsis, Triticum, Sorghum, and Nicotiana species. Mi‐1.2, a tomato gene for resistance to the potato aphid, Macrosiphum euphorbiae (Thomas), is a member of the nucleotide‐binding site and leucine‐rich region Class II family of disease, nematode, and arthropod resistance genes. Recent studies into the differential expression of Pto‐ and Pti1‐like kinase genes in wheat plants resistant to the Russian wheat aphid, Diuraphis noxia (Mordvilko), provide evidence of the involvement of the Pto class of resistance genes in arthropod resistance. An analysis of available data suggests that aphid feeding may trigger multiple signaling pathways in plants. Early signaling includes gene‐for‐gene recognition and defense signaling in aphid‐resistant plants, and recognition of aphid‐inflicted cell damage in both resistant and susceptible plants. Furthermore, signaling is mediated by several compounds, including jasmonic acid, salicylic acid, ethylene, abscisic acid, giberellic acid, nitric oxide, and auxin. These signals lead to the development of direct chemical defenses against aphids and general stress‐related responses that are well characterized for a number of abiotic and biotic stresses. In spite of major plant taxonomic differences, similarities exist in the types of plant genes expressed in response to feeding by different species of aphids. However, numerous differences in plant signaling and defense responses unique to specific aphid–plant interactions have been identified and warrant further investigation.     

Jasmonate- and salicylate-induced defenses in wheat affect host preference and probing behavior but not performance of the grain aphid,Sitobion avenae

Jasmonate and salicylatemediated signaling pathways play significant roles in induced plant defenses, but there is no sufficient evidence for their roles in monocots against aphids. We exogenously applied methyl jasmonate （MeJA） and salicylic acid （SA） on wheat seedlings and examined biochemical responses in wheat and effects on the grain aphid, Sitobion avenae （Fab.）. Application of MeJA significantly increased levels of wheat＇s polyphenol oxidase, peroxidase and proteinase inhibitor 1, 2 and 6 days after treatment. In twochoice tests, adult aphids preferred control wheat leaves to MeJA or SA treated leaves. Electrical penetration graph （EPG） recordings of aphid probing behavior revealed that on MeJAtreated plants, the duration of aphid＇s first probe was significantly shorter and number of probes was significantly higher than those on control plants. Also total duration of probing on MeJAtreated plants was significantly shorter than on control plants. Total duration of salivation period on SAtreated plants was significantly longer, while mean phloem ingestion period was significantly shorter than on control plants. However, no significant difference in total duration of phloem sap ingestion period was observed among treatments. The EPG data suggest that MeJAdependent resistance factors might be due to feeding deterrents in mesophyll, whereas the SAmediated resistance may be phloembased. We did not observe any significant difference of MeJA and SA application on aphid development, daily fecundity, intrinsic growth rate and population growth. The results indicate that both MeJA and SAinduced defenses in wheat deterred S. avenae colonization processes and feeding behavior, but had no significant effects on its performance.     

Aphid induction of phytohormones in <Emphasis Type="Italic">Medicago truncatula</Emphasis> is dependent upon time post-infestation,aphid density and the genotypes of both plant and insect

This study examined the induction of the defence-related hormones jasmonic acid (JA), salicylic acid (SA) and abscisic acid (ABA) and the phytoalexin medicarpin in Medicago truncatula when challenged by the pea aphid Acyrthosiphon pisum. There was some induction of hormones in the compatible interaction between A. pisum clone N116 and M. truncatula cultivar DZA315, whereas JA, SA and medicarpin exhibited more significant increases in foliage concentration during the incompatible interaction between A. pisum clone PS01 and M. truncatula cultivar Jemalong A17. Foliar concentration of JA, SA and medicarpin exhibited a positive relationship with aphid density after 3-day feeding, whereas ABA was not affected by the presence of aphids. When aphids were restricted to a single leaf using plastic tubes, JA, SA and medicarpin displayed strong local induction, whereas there were no significant systemic increases in uninfested leaves. Medicarpin and SA appeared to increase with duration of aphid feeding, whereas JA showed a more transient increase in concentration 24 h after challenge commenced. Results suggest that increases in JA, SA and medicarpin are associated with M. truncatula resistance to particular clones of A. pisum. The variation in concentration of the defence-related compounds recorded with regard to aphid density, duration of challenge, genotypes of plant and aphids, and between locally challenged and distant leaves reinforces the need for consideration of these experimental factors when generalizing about the plant defence processes that occur during aphid–plant interactions.     

The Influence of Previous Experience and Starvation on Aphid Feeding Behavior

It was hypothesized that (1) previous experience of aphids on a host plant leads to differences in their feeding behavior relative to aphids without previous experience on it and that (2) a change in the physiological state of the aphid modifies their experience-induced behavior. Using electronic recording, the feeding behavior of the aphid Sitobion fragariae (Walker) on wheat Triticum aestivum L. and oat Avena sativa L. was examined, comparing aphids with or without previous experience on a given host and with or without a period of starvation before assessing probing behavior. All comparisons were performed within a single aphid clone to minimize the effect of genetic variation. Feeding behavior on wheat was significantly affected by previous experience and starvation. The effect of previous experience interacted with the host plant where feeding behavior was tested. Aphids feeding on wheat following previous experience on wheat showed a longer time and a higher number of pathway activities and less time in waveform F (i.e., mechanical stylet work and penetration difficulties) than did aphids feeding on wheat after a previous experience on oat. No differences in the time from the beginning of the recording until the first salivation into the sieve elements were found. When aphids were subjected to a period of starvation, the time devoted to xylem ingestion increased compared with that of nonconstrained aphids. These results are discussed in terms of factors affecting foraging decisions.     

The potential of hydroxamic acids in tetraploid and hexaploid wheat varieties as resistance factors against the bird‐cherry oat aphid,Rhopalosiphum padi

The potential for exploiting natural wheat resistance to control the cereal aphid Rhopalosiphum padi, the most important aphid pest of small grain cereals in the UK, was investigated as an alternative approach to the use of insecticides. The investigation focussed on a group of secondary metabolites, the hydroxamic acids or benzoxazinones, present naturally as glucosides, but which hydrolyse on tissue damage to give biologically active aglycones, e.g. 2,4‐dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one (DIMBOA) which are associated with natural plant defence. These can be important for resistance against insects, fungi, bacteria and nematodes for a range of cultivated monocotyledonous plants and could ultimately be combined with other defence mechanisms to provide a general approach to cereal aphid control. Levels of hydroxamic acids, particularly DIMBOA‐glucoside, were determined in hexaploid (Triticum aestivum) and tetraploid (Triticum durum) wheat varieties and differences were found between species and varieties. The effect of feeding by R. padi on the level of hydroxamic acids in the leaf tissue was also investigated. Thus, after 24 h of aphid feeding, as an apparently localised hydrolytic defence reaction in the leaf, levels of DIMBOA‐glucoside decreased noticeably. When aphids were fed on sucrose solution containing low doses of DIMBOA there was a significant mortality compared to the sucrose control. However, the levels of and variation in hydroxamic acids in the wheat varieties investigated were insufficient for significant differences in aphid behaviour and development.     

Community structure and abundance of insects in response to early‐season aphid infestation in wild cabbage populations

1. Changes in the arthropod community structure can be attributed to differences in constitutively expressed plant traits or those that change depending on environmental conditions such as herbivory. Early‐season herbivory may have community‐wide effects on successive insect colonisation of host plants and the identity of the initially inducing insect may determine the direction and strength of the effects on the dynamics and composition of the associated insect community. 2. Previous studies have addressed the effect of early infestation with a chewing herbivore. In the present study, the effect of early infestation was investigated with a phloem‐feeding aphid [Brevicoryne brassicae L. (Hemiptera, Aphididae)] on the insect community associated with three wild cabbage (Brassica oleracea L.) populations, which are known to differ in defence chemistry, throughout the season in field experiments. 3. Aphid infestation had asymmetric effects on the associated insect community and only influenced the abundance of the natural enemies of aphids, but not that of chewing herbivores and their natural enemies. The effect size of aphid infestation further depended on the cabbage population. 4. Aphid feeding has been previously reported to promote host‐plant quality for chewing herbivores, which has been attributed to antagonism between the two major defence signalling pathways controlled by the hormones salicylic acid (SA) and jasmonic acid (JA), respectively. Our results show no effects of early infestation by aphids on chewing herbivores, suggesting the absence of long‐term JA–SA antagonism. 5. Investigating the effects of the identity of an early‐season coloniser and genotypic variation among plant populations on insect community dynamics are important in understanding insect–plant community ecology.     

Salicylic acid-mediated reductions in yield in Nicotiana attenuata challenged by aphid herbivory

Aphid herbivory decreases primary production in natural ecosystems and reduces crop yields. The mechanism for how aphids reduce yield is poorly understood as some studies suggest aphid feeding directly impedes photosynthesis, whereas other studies suggest a change in allocation of resources from growth to defense compounds reduces yield. To determine the mechanisms underlying reduced plant growth by aphids, Nicotiana attenuata plants, native tobacco, were infested with Myzus persicae ssp. nicotianae, tobacco-adapted green peach aphids, at low and high densities, and plant performance including fitness was assessed. To test the direct defense capacity of salicylic acid (SA) on aphid performance, we fed aphids an artificial diet with varying levels of SA and measured their survivorship and fecundity. There was no detectable effect of aphid herbivory on net photosynthesis, yet herbivory reduced plant growth, final biomass (43 % at high aphid density), and seed set (18 % at high aphid density) at both low and high aphid infestation levels. High-density aphid attack during the rosette and flowering stage caused an increase in SA levels, but caused only a transient decrease in jasmonic acid concentration at low aphid density. SA concentrations similar to those found in infested flowering plants decreased aphid fecundity, suggesting that SA was an effective chemical defense response against aphids. These results suggest that as aphid densities increased the proximal cause of reduced growth and yield was not reduced photosynthesis, but instead resources may have been mobilized for defense via the SA pathway, decreasing the availability of resources for building plant biomass.     

麦长管蚜唾液中几种酶的鉴定、活力测定与功能分析

用Parafilm膜夹营养液法,以两种食料介质饲喂麦长管蚜Macrosiphum avenae 3龄若蚜并收集其唾液,对唾液中的酶类进行了鉴定、活力测定和功能分析。结果表明,在20%蔗糖介质提取液中,鉴定有果胶酶、多酚氧化酶和纤维素酶; 在水介质提取液中鉴定有纤维素酶; 两种介质提取液中都未鉴定出过氧化物酶。酶活力测定结果表明, 在20%蔗糖介质提取液中, 每30头蚜虫分泌的果胶酶、多酚氧化酶和纤维素酶的酶活力分别为2.59×10^-3 U/g、7×10^-3 U/g和7.89×10^-3 U/g; 在水介质提取液中,纤维素酶活力为3.68×10^-3 U/g。行为反应试验结果表明,果胶酶处理麦苗的挥发物组分能引起麦长管蚜寄生性天敌燕麦蚜茧蜂Aphidius avenae和捕食性天敌七星瓢虫Coccinella septempunctata 的嗅觉偏好反应,因此,果胶酶在麦长管蚜取食诱导小麦植株的间接防御反应中具有重要作用。     

蚜虫唾液蛋白研究进展

蚜虫属于半翅目蚜科,多为重要的农业害虫,通过刺吸式口器吸食植物汁液,传播病毒,其爆发常常造成重大经济损失。在漫长的协同进化历程中,植物建立了高效的防御系统以应对蚜虫威胁。为了克服植物的防御反应,蚜虫也发展了相应的反制手段,其中蚜虫在取食过程中分泌的唾液蛋白能调控植物防御反应,降解植物次生物质,从而在蚜虫与植物互作中发挥着至关重要的作用。本文综述了蚜虫唾液蛋白的组分鉴定方法和相关蛋白的功能,并对唾液蛋白在蚜虫防治的应用和今后的研究方向进行了展望。常见的蚜虫唾液蛋白组分的鉴定和分析方法包括唾液蛋白的酶活性分析、唾液蛋白组学分析、唾液腺转录组学和蛋白组学分析等。但这些方法各有利弊,仅采取一种分析方法不能客观全面地反映蚜虫唾液蛋白分泌谱,多种技术手段联合分析方可提供更为逼真详实的信息。蚜虫唾液蛋白种类繁多,可分为解毒酶、保护酶、水解酶、结合功能蛋白以及分类未知的效应蛋白等。蚜虫唾液蛋白功能多样,能参与唾液鞘的形成,诱导植物防御反应,促进蚜虫取食,提高蚜虫繁殖力等。通过RNAi干扰唾液蛋白编码基因会显著改变蚜虫取食行为,并降低蚜虫存活率、产蚜量和适合度。因此,唾液蛋白是防控蚜虫的理想靶标。目前,采用寄主诱导的基因沉默(host-induced gene silencing, HIGS)技术已培育了数种靶向唾液蛋白基因的高效抗蚜作物品系,展示出了良好的应用前景。从目前研究来看,各种蚜虫唾液蛋白谱急需采用多组学手段联合分析的方法来进行完整解析。各种唾液蛋白的具体功能方面的研究还严重缺乏,需从蚜虫、植物、两者之间的互作等多维度探究唾液蛋白的作用及相关的分子机制,为发展基于蚜虫唾液蛋白调控的蚜虫防治新策略打下基础。     

Insight into watery saliva proteomes of the grain aphid,Sitobion avenae

The grain aphid, Sitobion avenae, is an economically important cereal pest worldwide. Aphid saliva plays an essential role in the interaction between aphids and their host plants. However, limited information is available regarding the proteins found in the saliva of S. avenae. Here, the watery saliva proteins from S. avenae were collected in an artificial diet and identified using a liquid chromatography–mass spectrometry/mass spectrometry analysis. A total of 114 proteins were identified in S. avenae saliva, including several enzymes, binding proteins, and putative effectors, as well as other proteins with unknown functions. In comparison with salivary proteins from nine other aphid species, the most striking feature of the salivary protein from S. avenae was the different patterns of protein functions. Several orthologous proteins secreted by other aphid species such as glucose dehydrogenase, elongation factors, and effector C002 were also detected in S. avenae saliva and speculated to play a significant role in aphid–plant interactions. These results provide further insight into the molecular basis between aphids and cereal plant interactions.     

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.     

Aphids secrete watery saliva into plant tissues from the onset of stylet penetration

Aphid feeding requires the secretion of two types of saliva: gelling saliva (from the principal gland) that forms an intercellular sheath for the penetrating stylet, and watery saliva [from accessory salivary glands (ASGs)] that facilitates intracellular penetration and phloem feeding. Plant viruses can be used as salivary markers to investigate key steps in aphid feeding, and penetration can be monitored electrically using the electrical penetration graph (EPG) approach. We conducted a series of EPG‐controlled transmission experiments using Cucurbit aphid‐borne yellows virus [CABYV; Polerovirus spec. (Luteoviridae)], which is retained in the ASGs, as a marker for watery saliva secretions. The melon aphid, Aphis gossypii Glover (Hemiptera: Aphididae), was used as a vector and melon seedlings, Cucumis melo L. (Cucurbitaceae), as host plants. Viruliferous aphids were interrupted at various stages during stylet penetration, i.e., during intercellular penetration prior to intracellular puncture and following a potential drop within the first probe. Viruliferous aphids and leaf disc samples obtained from the stylet penetration site were used to detect CABYV by quantitative real‐time RT‐PCR. Approximately half of the inoculated leaf discs were found to be infected with CABYV after very brief (12.9 ± 1.9 s) intercellular stylet probes and before intracellular stylet puncture. The number of virus particles ejected during such probes was similar to the number ejected by aphids during longer probes including a single intracellular puncture. Our results therefore suggest that watery saliva is secreted by aphids from the onset of stylet penetration.     

Silencing of a lipase maturation factor 2‐like gene by wheat‐mediated RNAi reduces the survivability and reproductive capacity of the grain aphid,Sitobion avenae

Lipase maturation factor (LMF) family proteins are required for the maturation and transport of active lipoprotein lipases. However, the specific roles of LMF2 remain unknown. In this study, a grain aphid lmf2‐like gene fragment was cloned and was highly similar in sequence to a homologous gene in the pea aphid, Acyrthosiphon pisum. An RNAi vector was constructed with this fragment and used for wheat transformation. The expression of the lmf2‐like gene in aphid, as well as the growth and reproduction of the aphids, was analyzed after feeding on the transgenic wheat. There were no significant differences in the expression of the lmf2‐like gene over development. The expression of the lmf2‐like gene was significantly reduced by 27.6% on the fifth day, and 57.6% on the 10th day after feeding. The total number of aphids produced on the transgenic plants was less than the number produced on control plants, and the difference became significant or after 2 weeks. The molting numbers were also reduced in the aphids reared on the transgenic plants. Our findings indicate that lmf2‐like genes may have potential as a target gene for the control of grain aphids and show that feeding aphids with wheat expressing lmf2‐like RNAi resulted in significant reductions in survival and reproduction.     

Characterisation of bird cherry-oat aphid (Rhopalosiphum padi L.) behaviour and aphid host preference in relation to partially resistant and susceptible wheat landraces

The bird cherry-oat aphid (Rhopalosiphum padi L.) is a major pest of wheat (Triticum aestivum L.) and can cause up to 30% yield losses. Heritable plant resistance to aphids is both an economically and ecologically sound method for managing aphids. Here we report how the behaviour and performance of R. padi differs on two resistant, one susceptible wheat landrace and a susceptible elite wheat variety. Feeding behaviour differed among the genotypes, with aphids on resistant lines spending longer in the pathway phase and less time phloem feeding. These behaviours suggest that both inter- and intracellular factors encountered during pathway and phloem feeding phases could be linked to the observed aphid resistance. Locomotion and antennal positioning choice tests also revealed a clear preference for susceptible lines. Although feeding studies revealed differences in the first probe indicating that the resistance factors might also be located in the peripheral layers of the plant tissue, scanning electron microscopy revealed no difference in trichrome length and density on the surface of leaves. Aphids are phloem feeders and limiting the nutrient uptake by the aphids may negatively affect their growth and development as shown here in lower weight and survival of nymphs on resistant genotypes and decreased reproductive potential, with lowest mean numbers of nymphs produced by aphids on W064 (54.8) compared to Solstice (71.9). The results indicate that resistant lines markedly alter the behaviour, reproduction and development potential of R. padi and possess both antixenosis and antibiosis type of resistance.