Gerbera Plants Research Articles

Identification of target genes for a MYB-type anthocyanin regulator in Gerbera hybrida

Genetic modification of the flavonoid pathway has been used to produce novel colours and colour patterns in ornamental plants as well as to modify the nutritional and pharmaceutical properties of food crops. It has been suggested that co-ordinate control of multiple steps of the pathway with the help of regulatory genes would lead to a more predictable control of metabolic flux. Regulation of anthocyanin biosynthesis has been studied in a common ornamental plant, Gerbera hybrida (Asteraceae). An R2R3-type MYB factor, GMYB10, shares high sequence similarity and is phylogenetically grouped together with previously characterized regulators of anthocyanin pigmentation. Ectopic expression of GMYB10 leads to strongly enhanced accumulation of anthocyanin pigments as well as to an altered pigmentation pattern in transgenic gerbera plants. Anthocyanin analysis indicates that GMYB10 specifically induces cyanidin biosynthesis in undifferentiated callus and in vegetative tissues. Furthermore, in floral tissues enhanced pelargonidin production is detected. Microarray analysis using the gerbera 9K cDNA array revealed a highly predicted set of putative target genes for GMYB10 including new gene family members of both early and late biosynthetic genes of the flavonoid pathway. However, completely new candidate targets, such as a serine carboxypeptidase-like gene as well, as two new MYB domain factors, GMYB11 and GMYB12, whose exact function in phenylpropanoid biosynthesis is not clear yet, were also identified.

Influence of pH, nutrient solution disinfestation and antagonists application in a closed soilless system on severity of fusarium wilt of gerbera

Three trials were carried out during the years 2002–2005 at the Agricultural Experimental Center of Albenga (northern Italy) on gerbera plants grown in a closed soilless system. The efficacy of slow sand filtration and UV treatment in eliminatingFusarium oxysporum f.sp.chrysanthemi (Foc) propagules, naturally present or artificially added to the recirculating nutrient solution, was evaluated. These techniques were tested alone and in combination with the application into the soilless system of different antagonistic strains ofFusarium spp. andTrichoderma spp., isolated from gerbera rhizosphere, and of a commercial formulation ofStreptomyces griseoviridis. The role of the nutrient solution pH in reducingFoc infections was also evaluated. Results showed that slow sand filtration, alone and in combination with the application of biocontrol agents, and a nutrient solution pH higher than 6.0, may induce a significant reduction inFoc infections on gerbera plants grown in closed soilless systems.

Fusarium Wilt of Gerbera Caused by a Fusarium sp. in Brazil.

In 2006, gerbera (Gerbera jamesonii) plants, cvs. Basic, Xena, and Olimpia grown for cut flower production in two greenhouse farms in the region of Guarapuava, Paraná, Brazil, exhibited symptoms of a wilt disease. Affected plants (approximately 20, 60, and 50% on cvs. Basic, Xena, and Olimpia, respectively) were stunted and developed yellow leaves unilaterally with initially brown and eventually black streaks in the vascular system. The first symptoms occurred 2 months after transplanting during the hottest period of the summer with an average air temperature of 27°C. Vascular streaks in the yellow leaves were continuous with a brown discoloration in the vascular system of the crown and upper taproot. Occasionally, the leaves of affected plants turned red. A Fusarium sp. was consistently and readily isolated onto a Fusarium-selective medium from symptomatic vascular tissue sampled from the crown of infected plants. Colonies were identified as Fusarium oxysporum on the basis of colony and conidia morphology (1) after subculturing on potato dextrose agar. Since F. oxysporum f. sp. chrysanthemi has been previously reported on Chrysanthemum morifolium, Argyranthemum frutescens, and gerbera (4), pathogenicity tests were carried out by using one monoconidial isolate obtained from wilted plants and one Italian isolate (F. oxysporum f. sp. chrysanthemi MASS 6). The isolates of F. oxysporum were grown in casein hydrolysate in shake culture (90 rpm) for 10 days at 25°C with 12 h of fluorescent light per day. Healthy rooted plants of 30-, 20-, and 45-day-old C. morifolium (cv. Captiva), A. frutescens (cv. Stella 2000), and gerbera (cvs. Jaska, Dalma, and Excellence), respectively, were inoculated by separately dipping roots into a conidial suspension (5 × 107 conidia/ml) of the two isolates of F. oxysporum. Plants were transplanted (one plant per pot) into pots (3.5 liter vol). Noninoculated plants served as control treatments. Plants (15 per treatment) were grown in a glasshouse at an average day temperature of 32°C and night temperature of 23°C (minimum 21°C and maximum 43°C). Wilt symptoms and discoloration of the vascular system in roots, crown, and petioles developed within 29 days on C. morifolium, 26 days on A. frutescens, and 14 days on gerbera. Noninoculated plants remained healthy. F. oxysporum was consistently reisolated from infected plants. The pathogenicity test was carried out twice. Gerbera wilt caused by F. oxysporum f. sp. chrysanthemi was recently reported in Italy (2) and Spain (3). Currently, the wilt of gerbera in the area of Paraná is limited to two farms. To our knowledge, this is the first report of the disease in Brazil as well as in South America.

Selective Enhancement of the Fluorescent Pseudomonad Population After Amending the Recirculating Nutrient Solution of Hydroponically Grown Plants with a Nitrogen Stabilizer

Fluorescent pseudomonads have been associated, via diverse mechanisms, with suppression of root disease caused by numerous fungal and fungal-like pathogens. However, inconsistent performance in disease abatement, after their employment, has been a problem. This has been attributed, in part, to the inability of the biocontrol bacterium to maintain a critical threshold population necessary for sustained biocontrol activity. Our results indicate that a nitrogen stabilizer (N-Serve, Dow Agrosciences) selectively and significantly enhanced, by two to three orders of magnitude, the resident population of fluorescent pseudomonads in the amended (i.e., 25 microg ml(-1) nitrapyrin, the active ingredient) and recycled nutrient solution used in the cultivation of hydroponically grown gerbera and pepper plants. Pseudomonas putida was confirmed as the predominant bacterium selectively enhanced. Terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rDNA suggested that N-Serve selectively increased P. putida and reduced bacterial diversity 72 h after application. In vitro tests revealed that the observed population increases of fluorescent pseudomonads were preceded by an early growth suppression of indigenous aerobic heterotrophic bacteria (AHB) population. Interestingly, the fluorescent pseudomonad population did not undergo this decrease, as shown in competition assays. Xylene and 1,2,4-trimethylbenzene (i.e., the inert ingredients in N-Serve) were responsible for a significant percentage of the fluorescent pseudomonad population increase. Furthermore, those increases were significantly higher when the active ingredient (i.e., nitrapyrin) and the inert ingredients were combined, which suggests a synergistic response. P. putida strains were screened for the ability to produce antifungal compounds and for the antifungal activity against Pythium aphanidermatum and Phytophthora capsici. The results of this study suggest the presence of diverse mechanisms with disease-suppressing potential. This study demonstrates the possibility of using a specific substrate to selectively enhance and maintain desired populations of a natural-occurring bacterium such as P. putida, a trait considered to have great potential in biocontrol applications for plant protection.

Conventional PCR and real time quantitative PCR detection of Phytophthora cryptogea on Gerbera jamesonii

A conventional PCR and a SYBR Green real-time PCR assays for the detection and quantification of Phytophthora cryptogea, an economically important pathogen, have been developed and tested. A conventional primer set (Cryp1 and Cryp2) was designed from the Ypt1 gene of P. cryptogea. A 369 bp product was amplified on DNA from 17 isolates of P. cryptogea. No product was amplified on DNA from 34 other Phytophthora spp., water moulds, true fungi and bacteria. In addition, Cryp1/Cryp2 primers were successfully adapted to real-time PCR. The conventional PCR and real-time PCR assays were compared. The PCR was able to detect the pathogen on naturally infected gerbera plants and on symptomatic artificially infected plants collected 21 days after pathogen inoculation. The detection limit was 5 × 103P. cryptogea zoospores and 16 fg of DNA. Real-time PCR showed a detection limit 100 times lower (50 zoospores, 160 ag of DNA) and the possibility of detecting the pathogen in symptomless artificially infected plants and in the re-circulating nutrient solution of closed soilless cultivation systems.

Comparison of transgenic Gerbera hybrida lines and traditional varieties shows no differences in cytotoxicity or metabolic fingerprints

Genetic modification using gene transfer (GM) is still controversial when applied to plant breeding at least in Europe. One major concern is how GM affects other genes and thus the metabolism of the plant. In this study, 225 genetically modified lines of the ornamental plant Gerbera hybrida and 42 non-GM gerbera varieties were used to investigate changes in secondary metabolism. The cytotoxicity of GM and non-GM gerbera extracts was evaluated on human cell lines derived from lung, liver, and intestinal tissues. The results indicate that the safety profile for GM gerbera lines is similar to the viability pattern for non-GM varieties-none of the extracts were toxic. In addition, metabolic fingerprints of gerbera extracts were identified using thin-layer chromatography and analysed by principal component analysis (PCA), the nearest neighbour classifier, and Fligner-Killeen test. No new compounds unique to GM lines were observed. With PCA, no separation between GM gerbera lines and varieties could be demonstrated. In the nearest neighbour classifier, 54% of the samples found the expected neighbour based on the gene constructs used for transformation. With Fligner-Killeen test, we studied if the amounts of compounds vary more in GM gerberas than in varieties. In most cases, there were no statistically significant differences between the varieties and GM lines or there was more variation among the non-GM varieties than in the GM lines. The variance of a single compound was significantly larger in transgenic gerbera lines than in varieties and of three compounds in non-GM varieties.

Whitefly control in cut gerbera: is it possible to control Trialeurodes vaporariorum with Encarsia formosa?

We investigated the impact of inundative releases of the parasitoid, Encarsia formosa Gahan (Hymenoptera: Aphelinidae), for control of greenhouse whitefly, Trialeurodes vaporariorum (Westwood), on cut gerbera (Gerbera jamesonii L.) under controlled greenhouse conditions. Experimental units consisted of ten plants covered and separated from other units by gauze tents. We assessed three release rates of the aphelinid parasitoid: a 7-week experiment with a standard release rate (10 m−2/14 days), and a subsequent 3-month trial with high (100 m−2/week) and very high (1,000 m−2/week) release rates. Experimental units without release of parasitoids served as control treatment. Gerbera plants were infested initially with 50–100 juvenile and 50–70 adult whiteflies in the first experiment, and in the second experiment with less than 50 juveniles per plant and 50–70 adults. Whitefly and parasitoid population density were assessed in weekly intervals using infestation and activity categories. Results show that parasitized whiteflies were present in all treatments within 2 weeks after initial release. Unfortunately, it was not possible to control whiteflies with standard release rates of E. formosa. Although parasitism rates slightly increased, the effect on whitefly populations was negligible. Large amounts of honeydew and growth of sooty mold fungi caused the termination of the first experiment. In a second experiment, E. formosa was tested at 10–100 times higher release densities. In contrast to the first experiment, whitefly densities increased steadily during the first 8 weeks, but remained constant until the end of the experiment in both treatments. Parasitism by E. formosa reached its maximum after 8 weeks. We discuss possible reasons for the low efficiency of E. formosa as a whitefly antagonist in greenhouse production of gerbera.

Fusarium Wilt of Gerbera in Spain in Soilless Crops.

In 2004, gerbera (Gerbera jamesonii cv. Excellence) plants, grown for cut flowers, were observed in a soilless cultivation system (coconut fiber substrate) in one farm in the Cadiz area (southwestern Spain) exhibiting symptoms of a wilt disease. Gerbera represents a relevant crop for the industry in the region, after rose and carnation. Affected plants were stunted and developed yellow leaves with initially brown and eventually black streaks in the vascular system. The vascular streaks in the yellow leaves were continuous with a brown discoloration in the vascular system of the crown and upper taproot. In some cases, the leaves of affected plants turned red. Fusarium spp. was consistently and readily isolated from symptomatic vascular tissue of infected plants onto a Fusarium-selective medium (3). Colonies were identified as F. oxysporum after subculturing on potato dextrose agar on the basis of morphological observations. Pathogenicity tests were carried out by using two monoconidial isolates, compared with an Italian one, obtained from wilted gerbera plants. Each isolate of F. oxysporum was grown in shake culture (90 rpm) for 10 days on casein hydrolysate at 25°C with 12 h of fluorescent light per day. Healthy rooted 30-day-old plants (cv. Jaska), were inoculated by dipping roots into a conidial suspension (5 × 107 conidia/ml) in one of the three test isolates of F. oxysporum. Plants were transplanted (1 plant per pot) into pots (3.5 liter vol.) containing rockwool-based substrate. Noninoculated plants served as control treatments. Plants (15 per treatment) were grown in a glasshouse at an average day temperature of 30°C and night temperature of 24°C (minimum of 22°C and maximum of 41°C). Wilt symptoms and vascular discoloration in the roots, crown, and veins developed within 30 days on each inoculated plant, while noninoculated plants remained healthy. F. oxysporum was consistently reisolated from infected plants. The pathogenicity test was conducted twice. A wilt of gerbera was described in the Netherlands in 1952 (1) but its presence was not confirmed in further observations (4). Gerbera wilt was recently reported in Italy (2) and identified as F. oxysporum f. sp. chrysanthemi (A. Garibaldi, personal communication). Currently, the wilt of gerbera in Spain is limited to a few farms and a very limited percent (2 to 3%) of plants.

Modelling the Transport of Metalaxyl in Gerbera Plants grown in a Closed-loop Hydroponic System

The transport and the kinetics of the fungicide metalaxyl were followed in various parts of gerbera plants ( Gerbera jamesonii ) grown in a closed hydroponic system. The pesticide was applied into the nutrient solution and supplied to the plants in the mode of a pulse at two concentrations, 112·5 and 225 mg l −1 . Its fate was then followed regularly for a period of 121 days, by collecting and analysing samples of irrigation solution, drainage water, roots, leaves and flowers of the plants by means of gas chromatography. The dimensionless parameter D/uL , where D is the diffusion coefficient, u is the linear velocity and L is the length of flow, for the movement of the tracer metalaxyl in the whole plant was found to be about 0·15–0·30 for both concentration treatments. These relatively high values support the statement that the plant operates as a well mixed reactor system. Next, the variation of analytical data in the solution, the roots, the leaves, and flowers were simulated using a model of four reactor vessels connected in series. From the results, it was possible to estimate the values for the apparent rate constant k app of elimination in each compartment. The values for k app appear similar for both treatments, but differ strongly in the various parts of the plants as follows: k leaves > k flowers > k roots . These variations of the apparent rate constants in the different parts of the plant are explained based on differences in the mode of long-distance transport to each of them.

Occurrence of Cucumber mosaic virus in Gerbera jamesonii in India

Gerbera jamesonii (family Asteraceae) is a popular perennial ornamental cut flower and potted plant with considerable economic importance. In a survey of gerbera grown in floriculture fields at the Institute of Himalayan Bioresource Technology (IHBT), Palampur and nearby nurseries, color break symptoms on the petals, asymmetrical ray florets, and deformed flowers were observed during 2003-2004. The virus evoked chlorotic local lesions on Chenopodium album, C. amaranticolor, and C. quinoa, while systemic mosaic was observed on Cucumis sativus, Nicotiana benthamiana, N. clevelandii, N. glutinosa, and N. tabacum cv. Samsun. The virus was transmitted nonpersistently by Myzus persicae and Aphis gossypii and was identified as Cucumber mosaic virus (CMV) using enzyme-linked immunosorbent assay (ELISA) with CMV-specific antibodies (Agdia, Elkhart, IN). Polyhedral particles approximately 29 nm were observed with electron microscopy of leaf dips from symptomatic gerbera leaves. Total RNA was isolated from the infected gerbera plants and N. glutinosa by using RNAqueous (Ambion, Austin, TX). CMV-specific primers (1) were used to detect the virus with reverse transcription-polymerase chain reaction that produced an amplicon predicted size of approximately 540 bp, but the virus was not detected in healthy controls. Sequence alignment of the amplicons (533 bp) utilizing BLAST resulted in 91 to 99% homology with the partial intercistronic region and partial coat protein gene (1042-1574 bp) (gene sequence submitted to EMBL database with Accession no. AJ634532) of CMV RNA3 in subgroup I. To our knowledge, this is the first report of CMV on gerbera in India. Reference: (1) C. De Blas et al. J. Phytopathol. 141:323, 1994.

ON-LINE ACQUISITION OF PLANT RELATED AND ENVIRONMENTAL PARAMETERS (PLANT MONITORING) IN GERBERA: DETERMINING PLANT RESPONSES

For on-line plant monitoring equipment to be functional in commercial glasshouse horticulture, relations between sensor readings and plant responses on both the short (days) and long term (weeks) are required. For this reason, systems were installed to monitor rockwool grown gerbera plants on a minute-to-minute basis from July 2002 until April 2003. Data collected included, amongst others, crop transpiration from lysimeter data (2 m2), canopy temperature using infrared sensors, rockwool water content, and greenhouse climatic parameters, such as global radiation and temperature. By combining data from lysimeter and water content, changes in crop fresh weight could be calculated on a daily basis. Both transpiration and daily fresh weight production were better related to light integral as measured inside the greenhouse, than outside the greenhouse. The contribution of heating pipes underneath the canopy to transpiration could be estimated with the system. From the relation between daily FW production and previous day light integral, a light use efficiency of ca. 10-g FW/MJ solar radiation as measured inside the greenhouse was calculated. Water use efficiency was on average 25 g FW/L. A 3-4 day drying cycle did not affect flower production. The results underline the importance of knowledge on the adaptation and tolerance of crops to stressful conditions, in order to be able to use plant monitoring under grower conditions effectively.

Application of selected antagonistic strains against Phytophthora cryptogea on gerbera in closed soilless systems with disinfection by slow sand filtration

In two separate trials during 2000–2002, the efficacy of slow sand filtration and UV treatment in eliminating Phytophthora cryptogea propagules, artificially added in the recirculating nutrient solution, was evaluated with gerbera plants grown in closed soilless systems. A slow sand filtration technique was tested both alone and in combination with different antagonistic strains belonging to Fusarium spp. and Trichoderma spp., isolated from gerbera rhizosphere and applied into the soilless system. The dynamics of these fungi in the recirculating nutrient solution and in the sand filter was also investigated by plate counts on selective media. Slow sand filtration and UV treatment were both effective in reducing P. cryptogea root rot. However, slow sand filtration may be a more feasible disinfection method than UV because of lower costs of installation and maintenance and for its adaptability to a wide range of production systems. Moreover, this disinfection technique can be successfully combine with the application of antagonistic microorganisms.

Selected microbial strains suppress Phytophthora cryptogena in gerbera crops produced in open and closed soilless systems

Several micro-organisms (Fusarium spp., Trichoderma spp., Oomycetes and bacteria) were isolated from the rhizosphere of gerbera plants grown in an open soilless system. Their capacity to suppress root rot caused by Phytophthora cryptogea was evaluated in 2000–2001 in three independent trials in open soilless systems. Different strains reduced disease incidence and/or increased flower production. The most effective strains (three of Fusarium and three of Trichoderma) were also tested in a closed soilless system in combination with slow sand filtration, in order to determine whether these two techniques together would be more effective in reducing P. cryptogea root rot. The results showed that slow sand filtration with antagonistic micro-organisms operated synergistically to significantly reduce disease incidence.

Application of centrifugal force to the extraction and separation of parasorboside and gerberin from Gerbera hybrida.

A method for the separation of parasorboside and gerberin from the ornamental plant Gerbera hybrida (Asteraceae) has been developed. The two closely related glucosides were extracted using an Extrachrom instrument, a prototype multi-functional separation tool equipped with an extraction chamber. The rotation planar extraction procedure was compared with that of a medium pressure solid-liquid extraction system. The resulting extracts were pre-purified using rotation planar chromatography and the results compared with those obtained using medium pressure liquid chromatography with silica gel as the stationary phase and a mobile phase of methanol:ethyl acetate:tetrahydrofuran at selectivity point Ps = 111 with 1% formic acid as modifier. The title compounds were isolated from the purified extracts by TLC and their structures confirmed by 1H- and 13C-NMR spectroscopy.

TRANSFORMATION OF GERBERA PLANTS WITH TOMATO SPOTTED WILT VIRUS (TSWV) NUCLEOPROTEIN GENE

Tomato spotted wilt virus (TSWV) is identified as a causal agent of an important disease in many vegetable and ornamental crops. In the Research Institute of Pomology and Floriculture, studies on the introduction of resistance-related, TSWV nucleoprotein gene to the gerbera genome (‘Alaska’, ‘Paul', ‘Prince’, ‘Zuzanna’) were undertaken. Gerbera explants (shoots, bases of shoot clumps and leaves with 2-3 mm lamina length) were transformed by Agrobacterium tumefaciens (strain LBA 4404). Construction of the expression vector containing the 35S CaMV promoter, the selectable nptII gene and the N-gene of the virus naturally infecting gerbera was based on pBin19 plasmid. After 10 months, seventy-four (5.4%) transgenic gerbera plants were selected on medium with the maximal concentration of kanamycin (7 mg l ). The presence and integration of the Ntransgene into the plant genome was confirmed by specific-PCR and Southern-blotting. The level of nucleoprotein estimated by ELISA was low in transformed tissues, but mechanically inoculated, transgenic plants demonstrated resistance to TSWV.

Spread of Phytophthora Root and Crown Rot in Saintpaulia, Gerbera and Spathiphyllum Pot Plants in ebb-and-flow-systems

Spread of Phytophthora root and crown rot in three pot plant species was studied on ebb-and-flow benches where the nutrient solution was recirculated. The plant species and their respective pathogens were: Saintpaulia ionantha – P. nicotianae, Gerbera jamesonii – P. cryptogea, and Spathiphyllum wallissii – Phytophthora spp. Ebb-and-flow benches were infested with the pathogen using different methods: 18–25% of the plants on a bench were inoculated or potted in soil infested with the pathogen or the nutrient solution was infested by either zoospores or mycelium fragments. More than 80% of the inoculated Saintpaulia plants and 22% of plants potted in infested soil developed disease but no spread of the disease was observed. Infestation of the nutrient solution did not result in any diseased Saintpaulia plant. More than 70% of the Gerbera plants developed disease as a result of spread of the pathogen irrespective of the infestation method used. No significant spread of the disease was observed with inoculated Spathiphyllum plants nor from plants potted in infested soil. A few Spathiphyllum plants developed disease symptoms after infestation of the nutrient solution with zoospores. In one experiment, nearly all Spathiphyllum plants were diseased after infestation of the nutrient solution with mycelium fragments. The presence of an irrigation mat significantly reduced the spread of the Phytophthora disease in Gerbera and Spathiphyllum. The possibility of an irrigation mat acting as a filter for zoospores is discussed.

Growth, flower production and mineral nutrition in gerbera (Gerbera jamesonii H. Bolus) plants grown in substrate culture with and without nutrient solution recycling

The aim of this work was to investigate if gerbera plants show significant differences in growth and flower production when grown in two different soilless growing systems, with and without nutrient solution recycling. Several parameters were measured and calculated: dry matter production and allocation, flower yield, quality and mineral nutrition. Growth indices did not show significant differences between the two systems. Nutrient concentration inside the substrate was lower in the closed system, possibly due to recycling of nutrient solution. Analysing mineral contents in dry matter luxury consumption of Nitrogen, Phosphorus and Potassium was found in the open growing system, but it did not have an effect on production and quality of flowers. Total flower production was 12.5 per plant, while diameter and stem length were, respectively, 10-12 and 40 cm in both systems. Moreover, the amount of nutrient lost per year was calculated measuring the nutrient concentrations in the drainage solution. Results showed that about 93 N, 6.8 P, 165 K and 107 Ca Kgha - 1 per year are released into the environment.

Organ identity genes and modified patterns of flower development in Gerbera hybrida (Asteraceae)

We have used Gerbera hybrida (the cultivated ornamental, gerera) to investigate the molecular basis of flower development in Asteraceae, a family of flowering plants that have heteromorphic flowers and specialized floral organs. Flowers of the same genotype may differ in a number of parameters, including sex expression, symmetry, sympetaly and pigmentation. In order to study the role of organ identity determination in these phenomena we isolated and functionally analysed six MADS box genes from gerbera; these were shown by phylogenetic analysis to be orthologous to well characterized regulatory genes described from Arabidopsis and Antirrhinum. Expression analysis suggests that the two gerbera agamous orthologues, the globosa orthologue and one of the deficiens orthologues may have functional equivalency to their counterparts, participating in the C and B functions, respectively. However, the function of a second deficiens orthologue appears unrelated to the B function, and that of a squamosa orthologue seems distinct from squamosa as well as from the A function. The induction patterns of gerbera MADS box genes conform spatiotemporally to the multi-flowered, head-like inflorescence typical of Asteraceae. Furthermore, gerbera plants transgenic for the newly isolated MADS box genes shed light onto the mechanistic basis for some floral characteristics that are typical for Asteraceae. We can conclude, therefore, that the pappus bristles are sepals highly modified for seed dispersal, and that organ abortion in the female marginal flowers is dependent upon organ identity and not organ position when position is homeotically altered.

Foraging behavior of the parasitoid Encarsia formosa on Gerbera jamesonii leaves

Insight into the foraging behavior of the parasitoid Encarsia formosa Gahan for whitefly hosts was gained by continuous observation of individual parasitoids on leaves of the ornamental plant Gerbera jamesonii, until females left the leaf. Comparison of the parasitoid behavior on three cultivars gave similar results. Mean searching time on uninfested G. jamesonii leaves of three cultivars was 1 h 30 min and the mean percentage of walking activity of the total observation time on those cultivars was 61%. Both parameters were not influenced by different leaf structures of Gerbera cultivars. Encounters with hosts arrested the parasitoids on the leaves. The walking activity and the percentage of host encounters that resulted in an oviposition decreased with decreasing egg load of the parasitoid. In comparison with tomato, where biological control of whiteflies is successful, only minor differences in the foraging behavior occur, except for the residence time of females, which was about three to four times longer on G. jamesonii leaves, but these leaves are about seven times larger than tomato leaves. The facts that (1) the foraging behavior of E. formosa on G. jamesonii is independant of the cultivar and (2) the foraging behavior is, in many aspects, similar to that on tomato suggest that biological control of whitefly on this ornamental plant is a potential option.

New pathway to polyketides in plants

The repertoire of secondary metabolism (involving the production of compounds not essential for growth) in the plant kingdom is enormous, but the genetic and functional basis for this diversity is hard to analyse as many of the biosynthetic enzymes are unknown. We have now identified a key enzyme in the ornamental plant Gerbera hybrida (Asteraceae) that participates in the biosynthesis of compounds that contribute to insect and pathogen resistance. Plants transformed with an antisense construct of gchs2, a complementary DNA encoding a previously unknown function1,2, completely lack the pyrone derivatives gerberin and parasorboside. The recombinant plant protein catalyses the principal reaction in the biosynthesis of these derivatives: GCHS2 is a polyketide synthase that uses acetyl-CoA and two condensation reactions with malonyl-CoA to form the pyrone backbone of thenatural products. The enzyme also accepts benzoyl-CoA to synthesize the backbone of substances that have become of interest as inhibitors of the HIV-1 protease3,4,5. GCHS2 is related to chalcone synthase (CHS) and its properties define a new class of function in the protein superfamily. It appears that CHS-related enzymes are involved in the biosynthesis of a much larger range of plant products than was previously realized.