BYDV-infected Plants Research Articles

Evidence for additive effects of virus infection and water availability on phytohormone induction in a staple crop

Infection with phytoviruses influences plant responses to environmental stress, but the biochemical mechanisms underlying these interactions are unknown. Infection of wheat (Triticum aestivum) with a cereal virus (Barley yellow dwarf virus, BYDV) has context-dependent effects on plant productivity and survival conditional to water stress, and we hypothesized this was due to phythormone induction resulting from virus infection. We tested whether BYDV infection and water availability interact to influence hormone profiles in wheat across multiple time periods. Wheat plants were inoculated with BYDV by exposing them to infectious aphids (Rhopalosiphum padi). Concentrations of five hormones (abscisic acid, jasmonic acid, methyl jasmonate, methyl salicylate [MS], and salicylic acid [SA]) in leaf tissues were compared to concentrations in plants exposed to noninfectious aphids (sham treatment) and nondamaged control plants for five time-since-infection periods (0, 8, 16, 24, and 32 d) and two levels of water availability (0.2 and 0.8 g H20/g soil). Three important findings emerged: (1) total hormone concentrations in BYDV-infected plants exceeded concentrations in sham-treated and control plants up to 16 d following infection, after which nondamaged plants exhibited the highest concentrations of hormones; compared with nondamaged and BYDV-infected plants, hormone levels were reduced in sham-treated plants; (2) inoculation treatment affected concentrations of MS and SA: SA concentrations were increased in BYDV-infected plants, but control plants exhibited higher MS concentrations than either BYDV-infected or sham-treated plants irrespective of watering treatments and across all time periods; and (3) correlation analysis revealed no evidence of hormonal cross-inhibition. This study provides the first evidence that BYDV infection elevates both total phytohormone levels and SA in wheat in a time-sensitive manner, suggesting a potential biochemical basis for virus-induced hormonal responses that alter plant responses to environmental stress.

Comparison of the potential rate of population increase of brown and green color morphs of Sitobion avenae (Homoptera: Aphididae) on barley infected and uninfected with Barley yellow dwarf virus

Life tables of brown and green color morphs of the English grain aphid, Sitobion avenae (Fabricius) reared on barley under laboratory conditions at 20 ± 1°C, 65% ± 5% relative humidity and a photoperiod of 16 : 8 h (L : D) were compared. The plants were either: (i) infected with the Barley yellow dwarf virus (BYDV); (ii) not infected with virus but previously infested with aphids; or (iii) healthy barley plants, which were not previously infested with aphids. Generally, both color morphs of S. avenae performed significantly better when fed on BYDV-infected plants than on plants that were virus free but had either not been or had been previously infested with aphids. Furthermore, when fed on BYDV-infected plants, green S. avenae developed significantly faster and had a significantly shorter reproductive period than the brown color morph. There were no significant differences in this respect between the two color morphs of S. avenae when they were reared on virus-free plants that either had been or not been previously infested with aphids. These results indicate that barley infected with BYDV is a more favorable host plant than uninfected barley for both the color morphs of S. avenae tested, particularly the green color morph.

Plant viruses alter insect behavior to enhance their spread

Pathogens and parasites can induce changes in host or vector behavior that enhance their transmission. In plant systems, such effects are largely restricted to vectors, because they are mobile and may exhibit preferences dependent upon plant host infection status. Here we report the first evidence that acquisition of a plant virus directly alters host selection behavior by its insect vector. We show that the aphid Rhopalosiphum padi, after acquiring Barley yellow dwarf virus (BYDV) during in vitro feeding, prefers noninfected wheat plants, while noninfective aphids also fed in vitro prefer BYDV-infected plants. This behavioral change should promote pathogen spread since noninfective vector preference for infected plants will promote acquisition, while infective vector preference for noninfected hosts will promote transmission. We propose the “Vector Manipulation Hypothesis” to explain the evolution of strategies in plant pathogens to enhance their spread to new hosts. Our findings have implications for disease and vector management.

Rhopalosiphum padi(Hemiptera: Aphididae) Responses to Volatile Cues From Barley Yellow Dwarf Virus–Infected Wheat

In choice bioassays, Rhopalosiphum padi L. nonviruliferous apterae preferentially locate near volatile organic compounds (VOCs) emitted from Barley yellow dwarf virus (BYDV)-infected wheat plants compared with VOCs from noninfected plants. However, the specific VOCs responsible for R. padi responses are unknown. It is unclear also if R. padi responses to BYDV-infected wheat are caused by arrestment or attraction. Additionally, the responses of viruliferous apterae and nonviruliferous alate to BYDV-infected wheat have not been examined. R. padi responses were studied through emigration, immigration, and settling laboratory bioassays using BYDV-infected and noninfected wheat plants. Two wheat genotypes, virus-susceptible Lambert and virus-resistant Lambert-derived transgenic 103.1J expressing the BYDV-PAV coat protein gene, were evaluated. In a settling bioassay, alates preferentially settled on noninfected 103.1J. Responses of viruliferous and nonviruliferous R. padi to virus-infected, noninfected, and sham-inoculated (exposed to nonviruliferous aphids) Lambert and 103.1J were examined in separate bioassays. A paper leaf model served as a control. Immigration by viruliferous apterae was significantly lower toward the paper leaf model, but no significant differences were observed among plant treatments. Nonviruliferous apterae exhibited no significant differences in emigration among treatments, suggesting no arrestment occurred. Nonviruliferous apterae significantly preferred to immigrate toward BYDV-infected Lambert. Immigration toward the paper leaf model was significantly lower compared with plant treatments. Responses of R. padi to VOCs were tested by applying compounds to paper leaf models at concentrations designed to mimic those present in headspace of wheat plants. Nonviruliferous apterae immigrated in significantly greater numbers toward paper leaf models individually treated with nonanal, (Z)-3-hexenyl acetate, decanal, caryophyllene, and undecane than toward paper leaf models that served as controls and toward leaf models treated with synthetic blends made to mimic headspace of BYDV-infected compared with blends made to mimic headspace of noninfected wheat plants. Results suggest responses of R. padi to BYDV-infected plants are caused by attraction rather than arrestment.

Modelling influence of virus infection on leaf area and radiation use in oats under controlled climatic conditions

The effects of barley yellow dwarf virus (BYDV) infection on oat plants growing in pots in a greenhouse were incorporated into a plant growth model driven by incident solar radiation and air temperature. Changes in radiation use efficiency (RUE) and leaf area determining parameters were calibrated against observed reductions in leaf area, shoot biomass and grain yield. Plants were infected with BYDV-PAV at the two-leaf stage or left uninfected. Two nitrogen fertilization levels were used. BYDV-infected plants had smaller above-ground biomass and a smaller green plant area than uninfected plants. The decreased biomass could be explained by a decrease in leaf area, causing a decreased radiation interception and growth. Relatively higher RUE in the infected plants during a period of the growing season partially compensated for growth reductions due to the smaller leaf area. Increased RUE of the infected plants was related to higher nitrogen concentration per leaf area. The effects of an infection were similar for the two nitrogen fertilization treatments.

Effect of infections with barley yellow dwarf virus and Fusarium spp. on assimilation of 14CO2 by flag leaves and translocation of photosynthates in wheat

The effects of infections of Barley yellow dwarf virus (BYDV), Fusarium culmorum and F. graminearum alone as well as double infections of BYDV and Fusarium spp. on assimilation of 14CO2 by flag leaves and translocation of photosynthates in winter wheat cultivars ‘Agent’ (susceptible to Fusarium head blight (FHB)) and ‘Petrus’ (moderately resistant to FHB) were studied in outdoor pot experiments in 2001 and 2002. Wheat plants were inoculated with BYDV-PAV strain by Rhopalosiphum padi at growth stage (GS) 51 in 2001 and at GS 39 in 2002. Control plants were inoculated with virus free aphids. In the middle part of spikes single florets of wheat plants of both cvs were inoculated at GS 65 with conidia suspensions of F. culmorum in 2001 and F. graminearum in 2002. Exposure of flag leaves with 14CO2 revealed that photosynthetic fixation was most pronounced at GS 71 and declined from GS 75 to GS 85. F. culmorum infections diminished absorption of 14CO2 by flag leaves in cv. ‘Agent’ and very low percentages of photosynthates were transported into the middle and upper head parts, while absorption of 14CO2 in cv. ‘Petrus’ was only slightly reduced and high proportions of 14C assimilates were translocated also into the middle and upper spike sections. BYDV infections markedly reduced 14CO2 assimilation by flag leaves and export of photosynthates into wheat spikes of both cvs, especially in cv. ‘Petrus’. In double infected plants (BYDV and Fusarium spp.), photosynthetic fixation by flag leaves of both cvs at the first and second exposure times corresponded with that of BYDV-infected plants. However, export of assimilates in spikes was more effectively reduced in cv. ‘Agent’ than in cv. ‘Petrus’. Preinfestation with R. padi slightly reduced assimilation of 14CO2 in both cvs. In cv. ‘Agent’, translocation of 14C assimilates into the upper spike section was more impaired than in cv. ‘Petrus’. Assimilation of 14CO2 and distribution pattern of 14C assimilates in R. padi-preinfested plus Fusarium-infected plants of both cvs resembled that of fungus-infected plants at the first two exposure times.

Interactions Between Barley yellow dwarf virus and Fusarium spp. Affecting Development of Fusarium Head Blight of Wheat

Interactions between Barley yellow dwarf virus (BYDV) and Fusarium species causing Fusarium head blight (FHB) in winter wheat cvs Agent (susceptible to FHB) and Petrus (moderately resistant to FHB) were studied over three years (2001–2003) in outdoor pot experiments. FHB developed more rapidly in cv. Agent than in cv. Petrus. The spread of FHB was greater in BYDV-infected plants than in BYDV-free plants. Thousand grain weight (TGW) was reduced more in Fusarium-infected heads of cv. Agent than in cv. Petrus. A highly significant negative correlation was found between disease index and TGW in cv. Agent (r = −0.916), while in cv. Petrus the correlation was less significant (r = −0.765). Virus infection reduced TGW in cv. Petrus more than in cv. Agent. In plants with both infections, TGW reductions in cv. Petrus corresponded to those of BYDV infection, and in cv. Agent TGW was more diminished than in BYDV infection. Effects of different treatments determined over three years on ergosterol contents in grain were generally similar to effects on disease indices. Grain weight per ear and ear weight of the different treatments of both cultivars largely corresponded with the TGW results. Deoxynivalenol (DON) content in grain of cv. Agent infected with Fusarium spp. was 11–25 times higher compared to the corresponding treatments in cv. Petrus. The DON content in grain of plants of the two cultivars infected with both pathogens was higher than that of plants infected only with Fusarium over the three years.

Immunodetection of the replicative complex of Barley yellow dwarf virus-PAV in vivo

The genomic fragments of two open reading frames (ORFs) 1 and 2 of German and Canadian PAV isolates of Barley yellow dwarf virus (BYDV-PAV) were sequenced. Sequences only slightly differed from previously published sequences of this virus. Two polyclonal antisera against proteins encoded by ORFs 1 and 2 of a German ASL-1 isolate were developed using recombinant antigens expressed in E. coli as a fusion either to His6− or thioredoxin-tags. In Western blot analysis with total protein extracts from BYDV infected plants, antisera efficiently recognized the 99 kDa fusion protein expressed from ORF1 and ORF2 (P1–P2 protein). Later in infection the P1–P2 protein disappeared and two smaller proteins, revealing sizes of 39 and 60 kDa, could be detected.

Variation in Barley Yellow Dwarf Virus Transmission Efficiency by Rhopalosiphum padi (Homoptera: Aphididae) after Acquisition from Transgenic and Nontransformed Wheat Genotypes

The effects of different acquisition access periods (AAPs) and inoculation access periods (IAPs) on the transmission efficiency of barley yellow dwarf luteovirus (BYDV) by Rhopalosiphum padi (L.) (Homoptera: Aphididae) after feeding on transgenic or nontransformed wheat, Triticum aestivum L., genotypes were studied. Three wheat genotypes were tested as virus sources: virus-susceptible 'Lambert' and 'Lambert'-derived transgenic lines 103.1J and 126.02, which express the BYDV-PAV coat protein gene. Lower virus titers were measured in BYDV-infected transgenic plants compared with Lambert. No significant differences in transmission efficiency were detected for R. padi after varying IAPs, regardless of genotype. Transmission efficiency increased with an increase in AAP in all genotypes tested. However, AAPs ranging from 6 to 48 h on Lambert resulted in significantly greater transmission efficiency than similar periods on transgenic 103.1J. Maximum transmission efficiency (70%) was observed after a 48-h AAP on Lambert, whereas the same AAP on 103.1J and 126.02 resulted in a significantly lower transmission efficiency (57%). Contrasts were used to compare the rates of transmission and the theoretical maximum transmission percentage among the different wheat genotypes serving as virus sources. Both parameters were significantly different among genotypes, indicating that viral acquisition from each genotype resulted in a unique pattern of virus transmission by R. padi. The lowest rate of virus transmission after an AAP was observed on 103.1J compared with 126.02 or Lambert. This is likely associated with a lower virus titer in 103.1J. This is the first report of transgenic virus resistance in wheat affecting the transmission efficiency of a virus vector.

Volatile Cues Influence the Response ofRhopalosiphum padi(Homoptera: Aphididae) to Barley Yellow Dwarf Virus–Infected Transgenic and Untransformed Wheat

The attractiveness of Barley yellow dwarf luteovirus (BYDV)–infected wheat plants to Rhopalosiphum padi L. was evaluated under laboratory conditions. Two untransformed wheat varieties, virus-susceptible Lambert and virus-tolerant Caldwell, and one transgenic wheat genotype (103.1J) derived from Lambert and expressing the BYDV coat protein gene, were tested in three bioassays. First, R. padi responses to BYDV-infected or noninfected Lambert and Caldwell were evaluated. Significantly more aphids settled onto virus-infected than noninfected plants when aphids were able to contact the leaves. Second, aphid responses to headspace from virus-infected or noninfected Lambert and Caldwell were tested. Significantly more aphids congregated on screens above headspace of BYDV-infected plants than above headspace of noninfected plants of both varieties. Third, aphid responses to headspace from virus-infected or noninfected and sham-inoculated (exposed to nonviruliferous aphids) Lambert and 103.1J plants were examined. Significantly more aphids congregated on screens above BYDV-infected than above noninfected or sham-inoculated Lambert. No significant differences in R. padi preferences for headspace above BYDV-infected compared with noninfected or sham-inoculated 103.1J plants were observed. The concentration of volatiles extractable from whole plant headspace was greater on BYDV-infected Lambert than on BYDV-infected 103.1J, noninfected, or sham-inoculated plants of either genotype. This is the first report of volatile cues associated with BYDV infection in wheat plants influencing the behavior of the vector R. padi. Additionally, these findings show for the first time that transgenic virus resistance in wheat can indirectly influence the production of volatiles making virus-infected plants less attractive or arrestant to aphids than are infected untransformed plants.

Life history of the bird cherry-oat aphid, Rhopalosiphum padi (Homoptera: Aphididae), on transgenic and untransformed wheat challenged with barley yellow dwarf virus.

The life history of Rhopalosiphum padi (L.) was monitored on transgenic and untransformed (soft white winter wheat plants that were infected with Barley yellow dwarf virus (BLDV), noninfected, or challenged with virus-free aphids under laboratory conditions. Two transgenic soft white winter wheat genotypes (103.1J and 126.02) derived from the parental variety Lambert and expressing the barley yellow dwarf virus coat protein gene, and two untransformed varieties, virus-susceptible Lambert and virus-tolerant Caldwell, were tested. B. padi nymphal development was significantly longer on the transgenic genotypes infected with BYDV, compared with noninfected transgenic plants. In contrast, nymphal development on Lambert was significantly shorter on BYDV-infected than on noninfected plants. Nymphal development on noninfected Lambert was significantly longer than on noninfected transgenics. No significant difference in nymphal development period was detected between virus-infected and noninfected Caldwell. Aphid total fecundity, length of reproductive period, and intrinsic rate of increase were significantly reduced on BYDV-infected transgenic plants compared with BYDV-infected Lambert. In contrast, reproductive period, total adult fecundity, and intrinsic rate of increase on noninfected Lambert were significantly reduced compared with noninfected transgenics. Transgenic plants infected with BYDV were inferior hosts for R. padi compared with infected Lambert. However, noninfected transgenics were superior hosts for aphids than noninfected Lambert. Moderate resistance to BYDV, as indicated by a significantly lower virus titer, was detected in the transgenic genotypes compared with the untransformed ones. Results show for the first time that transgenic virus resistance in wheat can indirectly influence R. padi life history.

Biolistic-mediated inoculation of immature wheat embryos with Barley yellow dwarf virus-PAV

Successful mechanical inoculation of plant with viruses requires an efficient method to introduce the viral pathogen into the appropriate cells of the plant. Barley yellow dwarf virus-PAV (BYDV-PAV, Luteovirus), transmitted naturally by aphids, must be inoculated into the phloem tissue to infect systemically inoculated hosts. The particle bombardment method used widely for nucleic acid transfer into plant tissues was adapted to inoculate immature embryos of winter and spring wheat cultivars with either BYDV-PAV particles or viral full-length RNAs. DAS-ELISA and RT-PCR were carried out on extracts of developed leaves at 7 weeks post-bombardment and revealed that up to 14% of bombarded embryos produced BYDV-infected wheat plants. This is the first report of an aphid-free inoculation method for BYDV.

Russian Wheat Aphid (Homoptera: Aphididae) Survival and Fecundity on Barley Yellow Dwarf Virus-Infected Wheat Resistant and Susceptible to the Aphid

The aphid-resistant wheat lines PI137739 and PI262660, along with the susceptible wheat varieties ‘Bliss’ and ‘Stephens’, and the resistant oat variety ‘Border’, were infected with five barley yellow dwarf virus (BYDV) isolates (MAV, PAV, RMV, RPV, SGV) to test the influence of barley yellow dwarf virus (BYDV) infection on the performance (survival and fecundity) of the Russian wheat aphid, Diuraphis noxia (Mordvilko). Antibiosis in lines PIl37739 and PI262660 could not be detected statistically when the uninfected plants were analyzed separately. When data from only BYDV-infected plants were analyzed, Russian wheat aphid performance was significantly less on both PI137739 and PI262660 than on the susceptible varieties, indicating that plant stress influences expression of Russian wheat aphid antibiosis. Relative to uninfected controls, BYDV infection did not affect Russian wheat aphid performance on either susceptible wheat variety or the resistant line PI137739. In the resistant line PI262660, PAV, RPV, and SGV infection significantly reduced Russian wheat aphid performance. By contrast, PAV infection significantly increased aphid performance on ‘Border’ oats, but it was still far below performance on any other plant-virus combination. These results suggest that BYDV infection does influence Russian wheat aphid survival or fecundity, or both, on certain hosts even though the aphid is not a vector of the virus isolates used in this study.

Increased Production of Alatae by Aphids1 Reared on Oats Infected with Barley Yellow Dwarf Virus

Morphological development of parthenogenetic viviparous Sitobion avenae (F.) and Rhopalosiphum padi (L.) (Hemiptera: Aphidae) was influenced by the condition of a host plant, Avena byzantina (Koch). Aphids reared on oats infected with various isolates of barley yellow dwarf virus (BYDV) were much more likely to mature as winged adults (alatae), than were aphids reared on healthy oats. In 15 experiments during a 2-yr period, 75% of 2670 aphids reared on BYDV-infected oats developed as alatae; 36% of 2470 reared in parallel on healthy oats developed as alate forms. A greater percentage of winged progeny was consistently produced on infected oats in all experiments, regardless of aphid species, morphology of parent aphid, or the BYDV isolate used. Results of one experiment indicated that BYDV-infected plants could influence aphids during 1st instar development to mature as alatae. Alatae production was also favored by senescing oat tissues. Differences in nitrogen metabolism, resulting in increased amino acid concentration in diseased or senescing plants, could explain increased alatae production. The possible importance of these observations in spread of plant luteoviruses by aphids is discussed.