Background: Human tumors are still a major threat to human health and plant tumors negatively affect agricultural yields. Both areas of research are developing largely independent of each other. Treatment of both plant and human tumors remains unsatisfactory and novel therapy options are urgently needed. Hypothesis: The concept of this paper is to compare cellular and molecular mechanisms of tumor development in plants and human beings and to explore possibilities to develop novel treatment strategies based on bioactive secondary plant metabolites. The interdisciplinary discourse may unravel commonalities and differences in the biology of plant and human tumors as basis for rational drug development. Results: Plant tumors and galls develop upon infection by bacteria (e.g. Agrobacterium tumefaciens and A. vitis, which harbor oncogenic T-DNA) and by insects (e.g. gall wasps, aphids). Plant tumors are benign, i.e. they usually do not ultimately kill their host, but they can lead to considerable economic damage due to reduced crop yields of cultivated plants. Human tumors develop by biological carcinogenesis (i.e. viruses and other infectious agents), chemical carcinogenesis (anthropogenic and non-anthropogenic environmental toxic xenobiotics) and physical carcinogenesis (radioactivity, UV-radiation). The majority of human tumors are malignant with lethal outcome. Although treatments for both plant and human tumors are available (antibiotics and apathogenic bacterial strains for plant tumors, cytostatic drugs for human tumors), treatment successes are non-satisfactory, because of drug resistance and the severe adverse side effects. In human beings, attacks by microbes are repelled by cellular immunity (i.e. innate and acquired immune systems). Plants instead display chemical defense mechanisms, whereby constitutively expressed phytoanticipin compounds compare to the innate human immune system, the acquired human immune system compares to phytoalexins, which are induced by appropriate biotic or abiotic stressors. Some chemical weapons of this armory of secondary metabolites are also active against plant galls. There is a mutual co-evolution between plant defense and animals/human beings, which was sometimes referred to as animal plant warfare. As a consequence, hepatic phase I-III metabolization and excretion developed in animals and human beings to detoxify harmful phytochemicals. On the other hand, plants invented "pro-drugs" during evolution, which are activated and toxified in animals by this hepatic biotransformation system. Recent efforts focus on phytochemicals that specifically target tumor-related mechanisms and proteins, e.g. angiogenic or metastatic inhibitors, stimulators of the immune system to improve anti-tumor immunity, specific cell death or cancer stem cell inhibitors, inhibitors of DNA damage and epigenomic deregulation, specific inhibitors of driver genes of carcinogenesis (e.g. oncogenes), inhibitors of multidrug resistance (i.e. ABC transporter efflux inhibitors), secondary metabolites against plant tumors. Conclusion: The exploitation of bioactive secondary metabolites to treat plant or human tumors bears a tremendous therapeutic potential. Although there are fundamental differences between human and plant tumors, either isolated phytochemicals and their (semi)synthetic derivatives or chemically defined and standardized plant extracts may offer new therapy options to decrease human tumor incidence and mortality as well as to increase agricultural yields by fighting crown galls.

For biological control of plant pests, e.g. cockchafer grubs, in the rhizosphere of oak, apple or pine trees, ento­mopathogenic Beauveria spp. are increasingly applied. For successful use, it is important to monitor the spread and persistence of the inoculated fungi, both qualitati­vely and quantitatively. The determination of both para­meters by plating on selective nutrient media or by mole­cular methods such as PCR of soil samples are quite laborious and often do not yield satisfactory results. Therefore, the aim of the present study was to develop a spe­cific in situ method using immunofluorescence labelling of Beauveria spp. growing on young fine roots of three-year old oak saplings. All fine roots investigated were covered with a dense net of soil rhizosphere fungi, as visua­lized by staining with the nonspecific dye blankophor. On non-inoculated roots, polyclonal Beauveria anti­bodies did not label any of these naturally growing fungi. Only samples of roots inoculated with Beauveria brongniartii displayed specific labelling up to ten months after inocu­lation. Whereas the natural rhizosphere fungi were detected growing in the intercellular space of the root cortex in an ectomycorrhiza-like manner up to the endodermis, hyphae of the inoculated B. brongniartii were never seen within the root tissue but only growing on the surface of the rhizodermis. These observations indicate that B. brongniartii does not grow endophytically, and that the method used allows to discriminate B. brongniartii from the resident fungal flora in the oak tree rhizosphere. Detection by immunofluorescence labelling employed in the current study may be a useful tool to follow B. brongniartii in experiments aimed at establishing the entomopathogen in the rhizosphere and to monitor its fate in long-term control of entomopathogens.

The discovery of insecticidal activity in root-colonizing pseudomonads, best-known for their plant-beneficial effects, raised fundamental questions about the ecological relevance of insects as alternative hosts for these bacteria. Since soil bacteria are limited in their inherent abilities of dispersal, insects as vectors might be welcome vehicles to overcome large distances. Here, we report on the transmission of the root-colonizing, plant-beneficial and insecticidal bacterium Pseudomonas protegens CHA0 from root to root by the cabbage root fly, Delia radicum. Following ingestion by root-feeding D. radicum larvae, CHA0 persisted inside the insect until the pupal and adult stages. The emerging flies were then able to transmit CHA0 to a new plant host initiating bacterial colonization of the roots. CHA0 did not reduce root damages caused by D. radicum and had only small effects on Delia development suggesting a rather commensal than pathogenic relationship. Interestingly, when the bacterium was fed to two highly susceptible lepidopteran species, most of the insects died, but CHA0 could persist throughout different life stages in surviving individuals. In summary, this study investigated for the first time the interaction of P. protegens CHA0 and related strains with an insect present in their rhizosphere habitat. Our results suggest that plant-colonizing pseudomonads have different strategies for interaction with insects. They either cause lethal infections and use insects as food source or they live inside insect hosts without causing obvious damages and might use insects as vectors for dispersal, which implies a greater ecological versatility of these bacteria than previously thought.

The study involved six test fungi previously recorded in the literature as being endophytes (Beauveria bassiana, Metarhizium anisopliae, Isaria fumosorosea, Trichoderma harzianum, Fusarium proliferatum, Chaetomium globosum), two plant pathogenic fungi (Ascochyta fabae, Plenodomus lingam) and four host plants (Vicia faba, Bras­sica napus, Phaseolus vulgaris, Zea mays). Aerial conidia, blastospores, or ascospores, respectively were applied to leaf surfaces by spraying or by infiltrating spore suspensions through stomata directly into the leaves. Obser­vations using light microscopy showed that the test fungi germinated on the leaf surface but did not enter actively into the leaves. Within the leaves, germination of spores and growth of hyphae appeared to depend on the presence of damaged plant tissue. Various host reactions such as browning of epidermal cells and formation of papillae were observed. Colonization of healthy leaves by the test fungi in a manner similar to the pathogens A. fabae (on Faba bean) and P. lingam (on oilseed rape) was not observed. Spore germination and hyphal growth commenced when inoculated leaves were placed on agar medium. The results indicate that the test fungi possessed a saprotrophic rather than an endo­phytic life style when associated with leaf tissue of the studied hosts.

The presence and distribution of fungal endophytes in plants is commonly assessed by re-isolation on agar media or detection by PCR-techniques. Histological studies on the process of colonization of the host plant have only scarcely been performed. In the present study, the development of entomopathogenic fungi on the plant surface and inside the tissue was examined by light and fluorescence microscopy of leaf samples treated with vari­ous dyes or, to guarantee the specificity of injected endophytes, with primary polyclonal and secondary FITC-conjugated antibodies; diaminobenzidine-tetra­hy­dro­chloride (DAB) was applied as stress test for the detec­tion of hydrogen peroxide. Four species of entomopathogenic fungi were studied and compared with three phytopathogenic fungal species. The host plants were oilseed rape (Brassica napus), faba bean (Vicia faba), and cucumber (Cucumis sativus). When blastospores of selected four fungal species were infiltrated into B. napus leaves they appeared to germinate only on the leaf surface, but not within the mesophyll. Successful re-isolation from B. napus inoculated with B. bassiana, Isaria fumosorosea or Metarhizium anisopliae showed that these entomopathogens were able to persist in the tissue for at least two weeks. Formation of brown precipitates after leaf treatment with DAB in the presence of B. bassiana indicated the production of hydro­gen peroxide by B. napus but not by V. faba. Overall, the results indicate a lower endophytic colonization than could have been expected from the literature, suggesting nutrient availability in the plant intercellular space and absence of cell wall and cell membrane degrad­ing fungal enzymes as fungal growth-limiting factors. It is concluded that data on endophytic colonization should generally be supported by histological evidence of the kind and amount of fungal growth in the host tissue.

Biological control measures to prevent or reduce Agrobacterium vitis-caused losses in grapevine cultures are a worldwide increasing challenge. In the present study, tumour development in grapevine (Vitis vinifera L.) was induced in the sensitive cv. Kerner by infection with Agrobacterium vitis strain K306, carrying the p35Sgus-int plasmid with the gus gene as marker for transformation by the wild-type T-DNA. Pre-inoculation with the non-tumorigenic A. vitis strain F2/5 prevented tumour induction by K306(p35gus-int). Strain M1154, a Tn5 mutant of F2/5 in the luxR-like aviR gene, partially reduced the biocontrol efficiency compared to the wild-type F2/5. GUS-labelling by K306gus was poor in grapevine in contrast to A. tumefaciens 281(p35gusint)-induced tumours in Arabidopsis, indicating plant species-dependent variable gus expression. To use the more reliable direct mRNA expression assay by RTPCR, a new experimental plant/A. vitis system was established with Ricinus communis as model plant. Ricinus/A. vitis galls were available within one week after K306gus inoculation, reached diameters up to 5 cm, and contained more abundant GUS staining. An additional transformation marker, mRNA expression of the T-DNA-located iaaM oncogene, coding auxin synthesis, was apparent only in tumours induced by the wild-type A. vitis strain K306 in the absence of the gus construct, which is under the control of the strong 35S CaMV promoter. F2/5 pre-inoculation suppressed GUS staining and gus mRNA expression. DAPI staining revealed the loss of vital fluorescent cell nuclei in F2/5-inoculated grapevine tissue and thus inhibition of any successful T-DNA transfer into host cell nuclei. Differentiation of typical circular vessels in globular vascular bundles in M1154-pretreated galls suggests interference with plant auxin metabolism. In conclusion, together with successfully establishing a new experimental model system, Ricinus/A. vitis, pre-treatment of host tissue with the non-pathogenic strain F2/5 resulted in preventing the integration and expression of the oncogenic T-DNA of A. vitis strains by locally necrotizing host cell nuclei.

The antimalarial drug artesunate, a sesquiterpene trioxane lactone derivative of artemisinin from Artemisia annua L., is known for its extraordinary inhibitory effects on plasmodia and trematodes and also for suppressing the proliferation of human tumor cells. In the present study the effect of artesunate was investigated on rapidly dividing plant cells in Agrobacterium tumefaciens-induced crown gall tumor and wound callus cells at the model plant Ricinus communis. Low concentrations of artesunate (10 µM) were sufficient to completely suppress crown gall development upon permanent application. Within three weeks the shoots of artesunate-treated plants attained about double the size of the tumor-bearing plants and showed abundant, healthy and larger leaves. Moreover, artesunate retarded wound callus development and induced superficial necroses. However, artesunate did not prevent or inhibit infections of cucumber leaves by powdery (Podosphaera xanthii) or downy mildew (Pseudoperonospora cubensis). Young cucumber leaves showed symptoms of phytotoxicity upon treatment with very high artenusate concentrations of 100 µM and higher. Lower concentrations (50 µM or less) did not cause visible necrotic lesions. These novel findings suggest a general and conserved basic mode of action of artesunate in human, animal and plant cells, except of phytopathogenic fungi. A possible application of artesunate for biological control of crown gall development in grapevine and precious fruit trees is discussed.

Since 2002 / 2003 black rot caused by Guignardia bidwellii is regularly occurring in organic viticulture in the wine growing areas at the Mosel and Nahe River and in the Middle Rhine valley in Germany. Though the disease originates from the USA and is known in Europe already for more than 100 years, the knowledge about the biology of its causal organism is still scanty. In the present study the life cycle of G. bidwellii is analysed with histological microscopical methods (bright field, phase contrast, fluorescence and electron microscopy). The development of the fungus on a susceptible grapevine variety (Riesling) is followed from spore germination up to the development of pycnidia and pseudothecia. The study is focused on the phase after penetration of the fungus which is characterized by subcuticular spread. The aim is to broaden the knowledge of the biology of G. bidwellii, to provide the basis for efficient control measures and to enable the histological characterization of varietal resistance.

Metal ions essential for plant growth, such as Fe, Mn, Ni, Cu or Zn, are taken up by plants from the soil solution through metal transporters at the plasma membrane, mainly of the ZIP and Nramp families. These transport systems, however, can also give entry to other metals (Al, Cd, Hg, Pb). Non-nutritive elements, as well as the essential nutrients at higher than metabolic concentrations, can cause phytotoxicity. We have studied previously the effects of an essential (Ni) and a non essential (Cd) heavy metal on root cell plasma membranes, the first selective barrier encountered when entering the plant, using rice as model plant. Distinctive effects of Cd and Ni on membrane function (i.e., Em and membrane permeability) were observed in the short term. We have now confirmed the pattern of Em changes caused by Cd and Ni using barley roots and have also followed the effects of both metals in longer term in rice. Our data indicate that the distinct effects caused by Cd and Ni are due to differences in cellular responses, triggered when entering the cytoplasm (i.e., an efficient detoxifying mechanism for Cd), more than to different direct effects on membranes.

Since 2002 / 2003 black rot caused by Guignardia bidwellii is regularly occurring in organic viticulture in the wine growing areas at the Mosel and Nahe River and in the Middle Rhine valley in Germany. Though the disease originates from the USA and is known in Europe already for more than 100 years, the knowledge about the biology of its causal organism is still scanty. In the present study the life cycle of G. bidwellii is analysed with histological microscopical methods (bright field, phase contrast, fluorescence and electron microscopy). The development of the fungus on a susceptible grapevine variety (Riesling) is followed from spore germination up to the development of pycnidia and pseudothecia. The study is focused on the phase after penetration of the fungus which is characterized by subcuticular spread. The aim is to broaden the knowledge of the biology of G. bidwellii, to provide the basis for efficient control measures and to enable the histological characterization of varietal resistance.

The heavy metal nickel is an essential mineral trace nutrient found at low concentrations in most natural soils. However, it may reach toxic levels in certain areas and affect a number of biochemical and physiological processes in plants. Wilting and leaf necrosis have been described as typical visible symptoms of Ni2+ toxicity. The plasma membrane (PM) of root cells constitutes the first barrier for the entry of heavy metals but also a target of their toxic action. This work studies the relationship between disturbances of membrane functionality and the development of the typical symptoms of Ni2+ toxicity. Rice plants (Oryza sativa L. cv. Bahia) grown in nutrient medium containing 0.5 mM Ni2+ showed a significant decrease in water content as a consequence of the stress. Addition of Ni2+ to the solution bathing the roots induced a concentration-dependent PM depolarization but the activity of the PM-H+-ATPase was not inhibited by the presence of Ni2+ and the initial resting potential recovered in less than 1 h. In the short term (hours), membrane permeability of root cells was not significantly affected by Ni2+ treatments. However, in the long term (days) a drastic loss of K+ was measured in roots and shoots, which should be responsible for the changes in the water content measured, since stomatal conductance and the transpiration rate remained unaffected by Ni2+ treatment. The effects induced by Ni2+ were not permanent and could be reverted, at least in part, by transferring the plants to a medium without Ni2+.