Host Phloem Research Articles

Pest categorisation of Matsucoccus matsumurae.

The EFSA Panel on Plant Health performed a pest categorisation of Matsucoccus matsumurae (Hemiptera: Matsucoccidae), the Massonian pine bast scale, for the EU territory. This pest categorisation was initiated following the commodity risk assessment of artificially dwarfed plants from China consisting of Pinus parviflora (Japanese white pine) grafted on P. thunbergii (Japanese black pine) performed by EFSA, in which M. matsumurae was identified as a pest of possible concern. However, its identity is not firmly established due to uncertainty regarding its taxonomic relationship with Matsucoccus pini (Green), a species widespread in Europe. M. matsumurae occurs in western China and has been reported as a pest of P. massoniana (Chinese red pine) and P. thunbergii. These hosts occur in the EU as ornamental/amenity trees. Other scales in the Matsucoccus genus feed on a variety of Pinus species and the host range of M. matsumurae could be wider than is currently recorded. The scale has one or two generations per year. All stages occur on the branches and stems of hosts with developing nymphs and adult females feeding through the bark on host phloem vessels. Symptoms include the yellowing/browning of host needles, early needle drop, desiccation of shoots and bark necrosis. The most serious infestations occur in hosts that are 8-25 years old and there can be some host mortality. In principle, host plants for planting and plant products such as cut branches and wood with bark could provide entry pathways into the EU. However, prohibitions on the import of Pinus from non-European third countries regulate these pathways. In China, M. matsumurae occurs in regions with temperate humid conditions and hot summers. These conditions are also found in parts of southern EU. Were M. matsumurae to establish in the EU, it is conceivable that it could expand its host range; however, this remains uncertain. Some uncertainty exists over the magnitude of potential impacts. M. matsumurae satisfies the criteria that are within the remit of EFSA to assess for it to be regarded as a potential Union quarantine pest, assuming M. pini is not a synonym, which is a key uncertainty.

Plant‐mediated facilitation to conspecifics and heterospecifics by two phloem‐feeding maize pests: The influence of insect identity and order of arrival

Abstract Herbivory by insects leads to changes in both primary and secondary plant metabolism, ultimately impacting subsequent interactions. Some phloem‐feeding insects have developed mechanisms to suppress herbivore‐induced plant defences and manipulate the quality of host phloem sap, thereby facilitating their establishment on the host plant. Based on evidence that the specialized phloem‐feeding insects, Dalbulus maidis and Rhopalosiphum maidis, can suppress herbivore‐induced defences in maize, we hypothesized that infestation by either of these pests in maize would facilitate the colonization and establishment of subsequent conspecific and heterospecific individuals. We conducted a series of behavioural assays with these insects in both laboratory and greenhouse settings. The corn leafhopper D. maidis preferred previously infested plants as hosts, but it did not discriminate between conspecific‐ or heterospecific‐infested plants for colonization. However, when selecting a host for oviposition, it preferred depositing eggs on conspecific‐infested plants, which were the hosts that the offspring performed better compared to both non‐infested and heterospecific‐infested plants. The corn aphid R. maidis also preferred previously infested plants over non‐infested plants as hosts. However, the aphid preferred heterospecific‐infested over conspecific‐infested plants, even though heterospecific‐infested plants were hosts of lower quality to aphids. Aphids performed better on conspecific‐infested plants, followed by heterospecific‐infested plants, when compared to non‐infested plants. Our results confirm that previous colonization by the specialized phloem‐feeding insects facilitates subsequent infestations by conspecifics and heterospecifics. However, the facilitation for heterospecifics was asymmetric, as colonization by D. maidis facilitated infestations by R. maidis more than the reverse sequence of infestation.

'Candidatus Liberibacter' Pathosystems at the Forefront of Agricultural and Biological Research Challenges.

'Candidatus Liberibacter' Pathosystems at the Forefront of Agricultural and Biological Research Challenges.

Ultrastructure of phytoplasma-infected jujube leaves with witches’ broom disease

The subcellular characteristics of phytoplasma-infected jujube (Ziziphus jujuba) leaves were investigated using transmission electron microscopy. Midrib fragments of witches’ broom-diseased jujube leaves were collected from abnormally small leaves at an early stage of branch clustering. The diseased jujube leaves showed multivesicular bodies (MVBs) with vesicles and tubules in the phloem parenchyma cells and sieve elements. The MVBs were connected to the plasma membrane appressed to the cell wall. There were increased callose collars at the pore-plasmodesma unit ends of the sieve elements in the diseased leaves than in control leaves. The proliferation of MVBs in the diseased jujube leaves could be associated with endoplasmic reticulum stress-dependent exosome release. The phytoplasma produced pleomorphic cells in sieve elements. Several types of putative extracellular structures were observed on the phytoplasma cells: (i) fimbriae-like threads, (ii) pili-like projections, (iii) flagella-like appendages, and (iv) tube-like structures. This study provides novel insights into intracellular obligate cell wall-less prokaryotes and host phloem structures.

Imazapic Herbigation for Egyptian Broomrape (Phelipanche aegyptiaca) Control in Processing Tomatoes-Laboratory and Greenhouse Studies.

Parasitic plants belonging to the Orobanchaceae family include species that cause heavy damage to crops in Mediterranean climate regions. Phelipanche aegyptiaca is the most common of the Orobanchaceae species in Israel inflicting heavy damage to a wide range of broadleaf crops, including processing tomatoes. P. aegyptiaca is extremely difficult to control due to its minute and vast number of seeds and its underground association with host plant roots. The highly efficient attachment of the parasite haustoria into the host phloem and xylem enables the diversion of water, assimilates and minerals from the host into the parasite. Drip irrigation is the most common method of irrigation in processing tomatoes in Israel, but the delivery of herbicides via drip irrigation systems (herbigation) has not been thoroughly studied. The aim of these studies was to test, under laboratory and greenhouse conditions, the factors involved in the behavior of soil-herbigated imazapic, and the consequential influence of imazapic on P. aegyptiaca and tomato plants. Dose-response Petri dish studies showed that imazapic does not impede P. aegyptiaca seed germination and non-attached seedlings, even at the high rate of 5000 ppb. Imazapic applied to tomato roots inoculated with P. aegyptiaca seeds in a PE bag system revealed that the parasite is killed only after its attachment to the tomato roots, at concentrations as low as 2.5 ppb. Imazapic sorption curves and calculated Kd and Koc values indicated that the herbicide Kd is similar in all soils excluding a two-fold higher coefficient in the Gadash farm soil, while the Koc was similar in all soils except the Eden farm soil, in which it was more than twofold lower. In greenhouse studies, control of P. aegyptiaca was achieved at >2.5 ppb imazapic, but adequate control requires repeated applications due to the 7-day half-life (t1/2) of the herbicide in the soil. Tracking of imazapic in soil and tomato roots revealed that the herbicide accumulates in the tomato host plant roots, but its movement to newly formed roots is limited. The data obtained in the laboratory and greenhouse studies provide invaluable knowledge for devising field imazapic application strategies via drip irrigation systems for efficient and selective broomrape control.

Striking developmental convergence in angiosperm endoparasites.

A subset of parasitic plants bear extremely reduced features and grow nearly entirely within their hosts. Until recently, most of these endoparasites were thought to represent a single clade united by their reduced morphology. Current phylogenetic understanding contradicts this assumption and indicates these plants represent distantly related clades, thus offering an opportunity to examine convergence among plants with this life history. We sampled species from Apodanthaceae, Cytinaceae, Mitrastemonaceae, and Rafflesiaceae spanning a range of developmental stages. To provide a broader comparative framework, Santalaceae mistletoes with a similar lifestyle were also analyzed. Microtomography and microscopy were used to analyze growth patterns and the ontogeny of host-parasite vascular connections. Apodanthaceae, Cytinaceae, Mitrastemonaceae, and Rafflesiaceae species demonstrated a common development characterized by late cell differentiation. These species were also observed to form direct connections to host vessels and to cause severe alterations of host xylem development. Apodanthaceae and Rafflesiaceae species were additionally observed to form sieve elements, which connect with the host phloem. Endophytic Santalaceae species demonstrated a dramatically different developmental pattern, featuring early cell differentiation and tissue organization, and little effect on host anatomy and cambial activity. Our results illuminate two distinct developmental trajectories in endoparasites. One involves the retention of embryonic characteristics and late connection with host vessels, as demonstrated in species of Apodanthaceae, Cytinaceae, Mitrastemonaceae, and Rafflesiaceae. The second involves tissue specialization and early connection with host xylem, as exemplified by Santalaceae species. These differences are hypothesized to be related to the absence/presence of photosynthesis in these plants.

Organogenesis of Dodders, Parasitism, Infestation and Development Phases

Parasitic plants proliferation globally is daunting and a threat to our ecosystems. In this study we explore holoparasites with limitation to dodders (Cuscuta spp. & Cassytha filiformis). An experiment was performed to ascertain anatomical and morphological characteristics of dodder capsule and its stem. We present dodder infestation stages, development phases and close observable internal and external microscopic features. A distinct haustorium trait of dodders is shown by micrographs. The study finds that dodder seeds possess high ecological dispersal character with vast adaptability through morphological analysis. This ramifies their ecological phenotypic plasticity. Externally, dodder stems attack the host phloem through haustoria that suck nutrients from the sap weakening it.

Soil Available Water Holding Capacity Can Alter the Reproductive Performance of Mountain Pine Beetle (Coleoptera: Curculionidae) in Jack Pine (Pinales: Pinaceae) Through Phloem Nitrogen Concentration.

Mountain pine beetle (Dendroctonus ponderosae Hopkins, Coleoptera: Curculionidae, Scolytidae) has recently invaded novel jack pine (Pinus banksiana Lamb., Pinales: Pinaceae) forests in western Canada. Jack pine seems to be a suitable host for mountain pine beetle, but how growing conditions influence jack pine's quality as a host, and hence, its susceptibility for mountain pine beetle, is unknown. Specifically, how soil nutrient concentrations and available water holding capacity (AWHC) affect jack pine quality should be investigated. Host plant quality is an important determinant of mountain pine beetle host colonization and reproduction and is usually assessed by primary (nutrients) and secondary (defense chemistry) constituents of host phloem. We evaluated mountain pine beetle host acceptance and brood production by recording the percentage of female mountain pine beetle that entered the phloem and oviposited in 30 jack pine bolts from two sites that differed in soil nutrient concentrations and AWHC. We also compared the concentrations of phloem nutrients and defense monoterpenes among the selected trees and found that trees at the Low AWHC site had higher amounts of nitrogen, phosphorus, and potassium. Monoterpene concentrations did not differ among trees at the two sites. Host acceptance by and brood production of mountain pine beetle were greater in bolts from the Low AWHC site. We conclude that AWHC of the soil may influence mountain pine beetle host acceptance and brood production through altering host plant quality, particularly nitrogen in the phloem, and will potentially influence any further range expansion of the beetle in eastern North America.

Development of the endophytic parasite, Rafflesia patma Blume, among host plant (Tetrastigma leucostaphylum (Dennst.) Alston) vascular cambium tissue

The early life history of Rafflesia R.Br. is hidden by its cryptic nature. Being a holoparasitic plant, early growth and development of species in this genus have been suggested to occur inside the host plant. In this study, using microscopic analyses, we examined how Rafflesia patma Blume grows from a simple clump of cells or protocorm inside the cambium tissue of Tetrastigma leucostaphylum (Dennst.) Alston, its host plant. We further show how the protocorm expands in about 2 months into an early form of a flower knob or bud, just before the flower begins to emerge from the host plant vine. Our study suggests that the endophyte, R. patma, spreads within T. leucostaphylum vascular cambium tissue in a linear manner, but not as a continuous strand. We suspect that the parasitic endophyte spreads inside the host vascular cambium and is pushed farther away from its point of origin to another part of the host as the host vine cambium fusiform initial cells divide and enlarge over months. Moreover, the protocorm matures in the cambium region of the T. leucostaphylum vine which lies close to the xylem, and grow outwards, bursting through the host phloem, then the host cortex, and finally towards the host periderm as it begins to emerge. Although we believe this might be the closest prediction on how Rafflesia develops at an early stage and is distributed within its host body, a future analysis of endophytic tissue growth in a controlled environment, such as the use of tissue culture, is required to strengthen our findings.

New Insights into Phloem Unloading and Expression of Sucrose Transporters in Vegetative Sinks of the Parasitic Plant Phelipanche ramosa L. (Pomel).

The plant-parasitic plant interaction is a interesting model to study sink-source relationship and phloem unloading. The parasitic plants, such as the achlorophyllous plant Phelipanche ramosa, connect to the host phloem through the haustorium and act as supernumerary sinks for the host-derived photoassimilates, primarily sucrose. The application of the fluorescent symplastic tracer, carboxyfluorescein (CF) derived from carboxyfluorescein diacetate (CFDA), to the leaves of the host plant (Brassica napus) showed direct phloem connections at the host-parasite interface. These experiments also evidenced the dominant apoplastic pathway for phloem unloading in major vegetative sinks of the parasite, including tubercles and shoots, except the adventitious root apices. The CF experiments showed also the symplastic isolation of the phloem tissues from the sink tissues in tubercle and shoot of the parasite, then suggesting the pivotal role of sucrose transporters in sucrose unloading in P. ramosa sinks. Three cDNAs encoding sucrose transporters (PrSUT) were isolated from the parasitic plant. PrSUT1 transcripts accumulated at the same level in the tubercle throughout the parasite growth while a significant increase in transcript accumulation occurred after emergence in the flowering shoot, notably in the growing apical part. The in situ hybridization experiments revealed the PrSUT1 transcript accumulation in the mature phloem cells of both subterranean and flowering shoots, as well as in shoot terminal sinks corresponding to apical meristem, scale leaf primordia and immature vasculature. The transient expression experiments in Arabidopsis protoplasts showed that PrSUT1 was localized at the plasma membrane, suggesting its role in phloem functioning and sucrose uptake by the sink cells in P. ramosa. Conversely, the PrSUT2 transcript accumulation was constantly low in tubercles and shoots but PrSUT3 transcripts accumulated markedly in the subterranean and flowering shoots, in concordance with the PrSUT3 mRNA accumulation in multiple sink areas including apical meristem, scale-leaf primordia, immature vasculature and even storage parenchyma. However, the PrSUT3 transcripts did not accumulate in the mature phloem cells. The transient expression experiments in Arabidopsis protoplasts suggested a tonoplast localization of PrSUT3, for which nevertheless the involvement in intracellular sucrose transport needs clarification.

Impact of Aphids on Cereals and Phloem Based Defense Mechanism

A biological interaction is existed between host plant and phloem feeder. In this interaction, plant based defense responses are activated. These plant defense responses function in a variety of ways for hindering the invaders. In insect-plant interaction, recent researches focus on the interaction of aphids with their host plants and make them a model phloem feeder. Hence this review article is based on the consequences of aphid’s interaction to their host plants as well as phloem based defense mechanism that is activated in response of this interaction.

Pheromone Production by an Invasive Bark Beetle Varies with Monoterpene Composition of its Naïve Host.

The secondary chemistry of host plants can have cascading impacts on the establishment of new insect herbivore populations, their long-term population dynamics, and their invasion potential in novel habitats. Mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae) has recently expanded its range into forests of jack pine, Pinus banksiana Lamb., in western Canada. We investigated whether variations in jack pine monoterpenes affect beetle pheromone production, as the primary components of the beetle's aggregation pheromone, (-)-trans-verbenol and anti-aggregation pheromone (-)-verbenone, are biosynthesized from the host monoterpene α-pinene. Jack pine bolts were collected from five Canadian provinces east of the beetle's current range, live D. ponderosae were introduced into them, and their monoterpene compositions were characterized. Production of (-)-trans-verbenol and (-)-verbenone emitted by beetles was measured to determine whether pheromone production varies with monoterpene composition of jack pines. Depending on particular ratios of major monoterpenes in host phloem, jack pine could be classified into three monoterpenoid groups characterized by high amounts of (+)-α-pinene, 3-carene, or a more moderate blend of monoterpenes, and beetle pheromone production varied among these groups. Specifically, beetles reared in trees characterized by high (+)-α-pinene produced the most (-)-trans-verbenol and (-)-verbenone, while beetles in trees characterized by high 3-carene produced the least. Our results indicate that pheromone production by D. ponderosae will remain a significant aspect and important predictor of its survival and persistence in the boreal forest.

Dwarf mistletoe (Arceuthobium oxycedri) and damage caused by dwarf mistletoe to family Cupressaceae

Mistletoe is an angiospermic plant famous for its shoot hemi-parasitizes activity of higher plant species. It parasitize thousands of vascular plant species and depend almost entirely on their tree hosts for water and nutrients. Their damaging effects are growth reduction, branch distortions, and decreased longevity. All mistletoes take water and nutrients by tapping the host xylem but differ in their dependence on the host for carbon. Dwarf mistletoes (Arceuthobium spp.) are considered to be mainly heterotrophic, tapping the host phloem for carbon compounds. It is an obligate parasite with an endophytic 'root' system that infects trees and shrubs of the family Cupressaceae. The aim of this review is to cover the basic concepts of dwarf mistletoes and its

RAPID ON-SITE DETECTION OF THE HUANGLONGBING/CITRUS GREENING CAUSAL AGENT 'CANDIDATUS LIBERIBACTER ASIATICUS' BY AMPLIFYRP®, A NOVEL RAPID ISOTHERMAL NUCLEIC ACID AMPLIFICATION PLATFORM

Huanglongbing (HLB) has become the most serious threat to the citrus industry worldwide. Although the disease first emerged in the southeastern part of Asia, it is now well established in Brazil, Florida and California, which are the world’s most important citrus production areas. The disease is associated with three insect-vectored, obligate intracellular bacteria of the genus ‘Candidatus Liberibacter’. These bacteria reside in the host phloem cells and they are transmitted by psyllids. Early pathogen detection is important to protect free areas from pathogen introduction. Unfortunately, the disease has a long incubation period, during which the pathogen is at low concentration and non-systemically distributed. This makes diagnostic and eradication difficult. Agdia has recently developed a new, rapid, molecular detection technique, AmplifyRP ® , allowing PCR-level sensitivity detection within minutes, in the field. AmplifyRP ® uses a recombinase-polymerase methodology for DNA amplification at a single temperature. In contrast to conventional or real-time PCR, AmplifyRP ® has no DNA purification requirements, requires no thermocycling, and results can be read using small, user-friendly devices. A portable fluorescence reader or a lateral flow device (similar to Agdia’s ImmunoStrip ® ) can be used to visualize results in as little as 30 minutes, compared to several hours for conventional PCR. AmplifyRP ® eliminates the need for expensive PCR equipment, a large number of chemicals reagents, and the need for technically trained staff. Given its characteristics, AmplifyRP ® is the ideal tool to

Responses and adaptation by Nephotettix virescens to monogenic and pyramided rice lines with Grh‐resistance genes

AbstractThe green leafhopper, Nephotettix virescens (Distant) (Hemiptera: Cicadellidae), occasionally damages rice in Asia either directly, by feeding on the host phloem, or indirectly by transmitting tungro virus. We assessed the nature of resistance against the leafhopper in monogenic and pyramided near‐isogenic rice lines containing the resistance genes Grh2 and Grh4. Only the pyramided line was resistant to leafhopper damage. Leafhopper nymphs and adults had high mortality and low weight gain when feeding on the pyramided line and adults laid few eggs. In contrast, although there was some minor resistance in 45‐day‐old plants that possessed either Grh2 or Grh4 genes, the monogenic lines were generally as susceptible to the leafhopper as the recurrent parent line Taichung65 (T65). Resistance in the pyramided line was stable as the plant aged and under high nitrogen, and affected each of five Philippine leafhopper populations equally. Furthermore, in a selection study, leafhoppers failed to adapt fully to the pyramided resistant line: nymph and adult survival did improve during the first five generations of selection and attained similar levels as on T65, but egg‐laying failed to improve over 10 generations. Our preliminary results suggested that resistance was associated with physiological costs to the plants in some experiments. The results of this study demonstrate the success of pyramiding resistance genes through marker‐assisted breeding, to achieve a strong and potentially durable resistance. We discuss the utility of gene pyramiding and the development of near‐isogenic lines for leafhopper management.

Movement of protein and macromolecules between host plants and the parasitic weed Phelipanche aegyptiaca Pers.

Little is known about the translocation of proteins and other macromolecules from a host plant to the parasitic weed Phelipanche spp. Long-distance movement of proteins between host and parasite was explored using transgenic tomato plants expressing green fluorescent protein (GFP) in their companion cells. We further used fluorescent probes of differing molecular weights to trace vascular continuity between the host plant and the parasite. Accumulation of GFP was observed in the central vascular bundle of leaves and in the root phloem of transgenic tomato plants expressing GFP under the regulation of AtSUC2 promoter. When transgenic tomato plants expressing GFP were parasitized with P. aegyptiaca, extensive GFP was translocated from the host phloem to the parasite phloem and accumulated in both Phelipanche tubercles and shoots. No movement of GFP to the parasite was observed when tobacco plants expressing GFP targeted to the ER were parasitized with P. aegyptiaca. Experiments using fluorescent probes of differing molecular weights to trace vascular continuity between the host plant and the parasite demonstrated that Phelipanche absorbs dextrans up to 70kDa in size from the host and that this movement can be bi-directional. In the present study, we prove for the first time delivery of proteins from host to the parasitic weed P. aegyptiaca via phloem connections, providing information for developing parasite resistance strategies.

Host specificity, nutrient and water dynamics of the mistletoe Viscum rotundifolium and its potential host species in the Kalahari of South Africa

The mechanisms underpinning local host specificity in mistletoes remain elusive. We determined the degree of host specificity in the mistletoe Viscum rotundifolium at Pniel Estates, near Kimberley, South Africa. We found that V. rotundifolium parasitises only Ehretia rigida and Ziziphus mucronata at this site. Both commonly parasitised host species were not the most abundant trees, were not the tallest trees, and did not have the highest water or nutrient content of trees in the area, although these factors are good predictors for mistletoe parasitism. Mistletoe seeds deposited on branches of E. rigida and Z. mucronata have a greater chance of attachment and subsequent survival, compared with those seeds deposited on co-occurring Acacia and other potential host species. The mistletoes had higher water potential than their host trees and by doing so they can passively maintain the flow of nutrients. In addition, the mistletoes had a N:Ca ratio >1, indicating active uptake from host phloem. Thus, this mistletoe species uses both passive and active uptake, which may be a selective advantage in a nutrient-poor environment or on a nutrient-deficient host species.

Simulating the effectiveness of three potential management options to slow the spread of emerald ash borer (Agrilus planipennis) populations in localized outlier sites

The emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is a devastating, invasive insect pest of ash trees, Fraxinus spp., in North America. Using a simulation model, we evaluated three potential management options to slow the spread of A. planipennis in discrete outlier sites: (i) removing ash trees to reduce available host phloem resource, (ii) girdling ash trees to attract ovipositing female beetles and destroying the trees before larvae complete development, and (iii) applying a highly effective systemic insecticide. Simulations indicate that systemic insecticide applications provided the greatest reduction in the radial spread of A. planipennis. In simulations in which management options were applied only within a 300 m radius from the origin of the infestation, insecticide applications reduced the radial spread by 30% and larval consumption of ash phloem by 40% beyond the treated area. In contrast, girdling ash trees reduced the radial spread by 15% and larval consumption of ash phloem by 20% beyond the treated area. Both of these management options significantly reduced the spread of A. planipennis when treatments were applied 1 to 4 years after infestations were initiated. Reducing ash phloem by removing ash trees decreased population size within treated areas but did not reduce the radial spread, population size, or larval consumption of ash phloem beyond treated areas.

Structure and Development of the Endophyte in the Parasitic Angiosperm Cuscuta japonica

The endophyte, that is, the haustorial part within the tissues of the host plant Impatiens balsamina, of the parasitic angiosperm Cuscuta japonica was studied with light and electron microscopy. The endophyte consisted mainly of vacuolated parenchymatous axial cells and elongate, superficial (epidermal) cells. Then the elongate, epidermal cells separated from each other and transformed into filamentous cells, called searching hyphae. The hyphae grew independently either intercellularly or intracellularly in the host parenchyma. The apical end of the hyphal cells was characterized by conspicuous, large nuclei with enlarged nucleoli and very dense cytoplasm with abundant organelles, suggesting that the hyphal cells penetrating host tissue were metabolically very active. Numerous osmiophilic particles and chloroplasts were noted in the hyphae. The osmiophilic particles were assumed to be associated with elongation of the growing hyphe. Plasmodemata connections between the searching hyphal cells of the parasite and the host parenchyma cells were not detected. Hyphal cells that reached the host xylem differentiated into water-conducting xylic hyphae by thickening of the secondary walls. A xylem bridge connecting the parasite and the host was confirmed from serial sections. Some hyphal cells that reached the host phloem differentiated into nutrient-conducting phloic hyphae. Phloic hyphae had a thin layer of peripheral cytoplasm with typical features of sieve-tube members in autotrophic angiosperms, i.e., parallel arrays of smooth endoplasmic reticulum, mitochondria, and plastids with starch granules. Interspecific open connections via the sieve pores of the host sieve elements and plasmodesmata of the parasite phloic hyphae were very rarely observed, indicating that the symplastic translocation of assimilate to the parasite from the host occurred.

Two Regulatory Mechanisms of Monoterpenoid Pheromone Production in Ips spp. Of Bark Beetles

Bark beetles use aggregation pheromones to coordinate host colonization and mating. These monoterpenoid chemical signals are produced de novo in midgut cells via the mevalonate pathway, and pheromone production is induced when an adult beetle feeds on phloem of a host tree. In Ips pini, juvenile hormone (JH) III influences key regulatory enzymes along the mevalonate pathway that leads to pheromone production. In fact, topically applied JH III is sufficient to stimulate pheromone production in unfed males. In this study, we explore the influence of feeding and JH III treatment on pheromone production in male Ips confusus, the pinyon Ips. We also characterize the influence of feeding and JH III treatment on transcript levels and activity of three key enzymes involved in pheromone biosynthesis: 3-hydroxy-3-methylglutaryl-CoA (HMG) synthase (HMGS), HMG-CoA reductase (HMGR) and geranyl diphosphate synthase (GPPS). We also extend the current understanding of the regulation of pheromone biosynthesis in I. pini, by measuring the influence of feeding and JHIII treatment on enzymatic activity of HMGS and GPPS. Feeding on host phloem alone strongly induces pheromone production in male I. confusus, while JH III treatment has no effect. However, feeding and JH III both significantly up-regulate mRNA levels of key mevalonate pathway genes. Feeding up-regulates these genes to a maximum at 32 h, whereas with JH III-treatment, they are up-regulated at 4, 8, and 16 h, but return near to non-treatment levels at 32 h. Feeding, but not JH III treatment, also increases the activity of all three enzymes in I. confusus, while both feeding or treatment with JH III increase HMGS and GPPS activity in I. pini. Our data suggest that pheromone production in Ips is not uniformly controlled by JH III and feeding may stimulate the release of some other regulatory factor, perhaps a brain hormone, required for pheromone production.