GUEST EDITORIAL

Bob Schippers, born 1934. Prof. of Plant Pathology, U. of Utrecht (UU), the Netherlands; M.Sc. in Animal Physiology and Plant Pathology (1959); Ph.D. in Plant Virology (1963), both at U. of Amsterdam (UA). 1964-65, Guest scientist at Dept. of Plant Pathology, U. of California, Berkeley; 1963-68, Assistant Prof.; 1968-73, Associate Prof.; 1973-80, Senior Lecturer; 1980-92, Prof. of Plant Pathology, all at UU; 1987-91, Chairman, Dept. of Plant Pathology UU & UA, Director of Willie Commelin Scholten Phytopathological Laboratory at Baarn, the Netherlands. Author or co-author of more than 100 refereed joumal articles and book chapters. Editor of 'Soilborne Plant Pathogens', Academic Press, 1979. Numerous national and international professional activities and responsibilities. Editor in Chief, European Journal of PIant Pathology.

Exploitation of Microbial Mechanisms to Promote Plant Health and Plant Growth The worldwide concern about environmental pollution with pesticides results increasingly in new restrictions on their use in agriculture and horticulture, including glasshouse management. In the Netherlands, the government is aiming at reducing the application of pesticides by 50% by the year 2000 and is encouraging the rapid development of alternative approaches, in particular biological control (1). In their second (1983) book on biological control of plant pathogens, Cook and Baker (5) presented an optimistic view of the potentials of biological control, setting their hope on the new tools of gene manipulation. Ten years later, the recently obtained knowledge seems to justify their optimism on the biotechnological exploitation of mechanisms of microbial promotion of plant health, plant growth and plant resistance. With regard to resistance, of g~eat interest is the recenOy developed two-component model for the induction of a hypersensitive response by various plant pathogens using the

Phytoparasitica 21:4, 1993

275

Cladosporium fulvum - tomato avirulence gene 9 (avr9) and corresponding virulence gene (Cf9) combination (7). These two genes under control of selected pathogen-induced promoters could be introduced as a cassette into any plant. This opens up the possibility of broad-spectrum resistance against many pathogens, including fungi, viruses, bacteria and nematodes. Even if this approach should prove to be successful, it may not be easy to accomplish for each plant cultivar/pathogen combination. Moreover, the continuous stress imposed on most plant tissues by abiota and nonplant parasitic biota (minor pathogens and microfauna interacting with plant cells) may keep the hypersensitive response turned on, all over the plant (2,10). This will tap the plant's energy, and eventually may lead to generalized necrosis. Therefore, the exploitation of microbial mechanisms to suppress plant pathogens constitutes an important alternative or complementary method, and an essential part of integrated crop protection. Our knowledge on pathogen-suppressing micro-organisms, especially over the last few years, increased dramatically with the unraveling of the microbial mechanisms of field soils suppressive to particular soilborne fungal pathogens, using gene manipulation. The three types of natural disease-suppressive soils studied most intensively are: (i) the take-all disease in wheat caused by Gaeumannomyces graminis var. tritici (Ggt); (ii) Fusarium wilt diseases in a variety of crops caused by Fusarium oxysporum (Fox); and (iii) root rot of tobacco caused by Thielaviopsis basicola (Tbas) (6,8,11). In all three types of disease-suppressive soils, strains of fluorescent Pseudomonas spp. were shown to play a dominant role in disease suppression. It is remarkable that the most effective Pseudomonas strains have a variety of mechanisms suppressing pre-infection activities of the fungal pathogen, such as propagule germination stimulated by seed or root exudates, but also infection and post-infection development. One of the best take-all disease-suppressing Pseudomonas strains isolated from a take-all-suppressive soil, Pseudomonas fluorescens 2-79, can produce antibiotic phenazins, anthranilic acid and siderophores; the last mentioned compete for iron with the pathogen, to an extent which depends on the environment. After exclusion of all three mechanisms by gene manipulation, this strain provided significantly less, but still some, suppression of the disease. This points to even additional mechanisms involved, e.g. competition for nutrients other than iron and/or induction of host resistance against Ggt (11). One of the good Pseudomonas strains suppressing root rot in tobacco caused by Tbas, P. fluorescens strain CHA0, has the potential to produce HCN, the antibiotics 2,4diacetylphloroglucinol and pyoluteorin, and also salicylic acid. HCN is considered a major factor in suppressing Tbas in tobacco. HCN and/or salicylic acid may enhance plant resistance but this has not yet been demonstrated experimentally (6). Finally, the suppression of Fusarium.~vAlt in some soils in France is ascribed to an association of particular fluorescent Pseudomonas populations with saprophytic strains of F. oxysporum. The pathogenic Fusarium strain suffers much more from the competition with the pseudomonads for ferric iron than do the saprophytic strains, and is thereby hampered in the competition for carbon with the saprophytic Fusarium strains (8). Similarly, the significant and consistent suppression of Fusarium wilt in commercially grown radish by application of strain P. fluorescens WCS374, seems to be partly due to

276

B. Schippers

its association with endogenous antagonistic soil fungi including non-pathogenic F. oxysporum strains (M. Leeman, P.A.H.M. Bakker and B. Schippers, unpublished). For some Pseudomonas strains, induction of resistance was recently shown for the first time to be a mechanism of suppression of soilborne pathogens. It is involved in the suppression of Fusarium wilt in carnation and in radish by P. fluorescens strains WCS417 and WCS374, respectively. A determinant for this induced resistance seems to be located on the lipopolysaccharide o-antigenic side chains of the cell wall of these bacteria (13; M. Leeman, P.A.H.M. Bakker and B. Schippers, unpublished). The rapid progress in unraveling mechanisms of microbial suppression of soilborne fungal pathogens, thanks to gene manipulation techniques, is by no means restricted to studies of natural disease-suppressive soils. It also applies to several other pathogensuppressing micro-organisms (12). The disease-suppressive soils, however, display most clearly the importance of the associations of many different microbial mechanisms and of the cooperation of several pathogen-suppressing micro-organisms (9). This may also explain the high efficacy and consistency of pathogen suppressiveness of these soils and could be the key to overcoming the inconsistency generally encountered with the application of a single disease-suppressing microbial strain. Co-inoculation of two or more pathogen-suppressing micro-organisms, based on detailed knowledge of the mechanisms involved, is still in its infancy and should be explored further. Co-inoculated micro-organisms, however, have to be compatible in the rhizosphere. For example, co-inoculation of Pseudomonas strain WCS374 that can induce hostresistance, with strain WCS358 that strongly competes for ferric iron, causes problems in this respect: Population development of WCS374 is suppressed in the rhizosphere by WCS358, because WCS374 is unable to utilize the only iron chelating siderophore (pseudobactin 358) produced by WCS358, as it lacks the PupA-receptor for this particular pseudobactin (3). However, introduction into strain WCS374 of the genes of WC358 coding for the PupA-receptor, overcomes this problem (J.M. Raaijmakers, P.A.H.M. Bakker, P.J. Weisbeek and B. Schippers, unpublished). The combination, by gene manipulation, of several mechanisms of suppression in one microbial strain is another promising, as yet hardly explored approach. A good example is the introduction of genes from the bacterium Serratia marcescens coding for fungal cell-wall-degrading chitinase, into Pseudomonas strains or into the fungus Trichoderma harzianum, a strong mycoparasite ofRhizoctonia solani (4). As was stated by Cook and Baker (5), "biological control seeks a solution in restoring the biological balance, which in agriculture has been replaced by yield and quality." It may be difficult to achieve this by trying to simulate the complete set of plant/microbial interactions, as they act in nature as the result of a long evolutionary adjustment. However, carefully selected co-inoculation of suppressive micro-organisms, or an association of mechanisms genetically engineered in one strain, could be a successful short cut using evolutionary tools to bring us close to our goal. They even may be made effective against several pathogens prevailing on one crop. Despite the challenging possibilities for practical application, the commercial exploitation of microbial suppression of plant diseases seems to have reached an impasse. There are several reasons for this. Plant health-promoting micro-organisms have erroneously been advertised as biopesticides or biocontrol agents, thereby feeding the

Phytoparasitica 21:4, 1993

277

misconception that they are similar to pesticides. On the contrary, they are helping to restore a natural balance via a multitude of natural mechanisms affecting the plant and its biotic and abiotic environment, thereby promoting plant health and growth. Therefore the term 'plant growth-promoting (micro)biota' (PGPB), which is widely used in the literature, is to be preferred. This may prevent these organisms from being subjected to regulations specially devised for chemical pesticides. An unjustified fear, based largely on ignorance, exists for the use of genetically modified organisms. This urgently needs to be eliminated by education of the public. The uncertainty, at national and international levels, of directions on the use of genetically modified (micro)-organisms, should be resolved. This uncertainty is inconsistent with the large scale research programs, e.g. of the European Community, stimulating the genetic engineering of plants and microorganisms. So far, the financial risks and the risks to a reputation of making the PGPBs available for practical application, are taken largely by industry. This is inconsistent with the aims of governments and the public to reduce drastically the application of chemical pesticides for the benefit of the environment and public health. The chances of a breakthrough in the exploitation of microbial mechanisms to promote plant health thus seem to be hampered in different ways. We have to be alert that these chances will not be missed.

Prof. Bob Schippers Section of Plant Pathology, Dept. of Plant Ecology and Evolutionary Biology, Utrecht University, P.O. Box 800.84, 3508 TB Utrecht, the Netherlands [Telefax: 31-30-518366]

REFERENCES 1. Anon. (1991) Multi-Year Crop Protection Plan, Ministry of Agriculture, Nature Management and Fisheries, the Hague, the Netherlands. 2. Baker, K.F. and Cook, R.J. (1974) Biological Control of Plant Pathogens. W.H. Freeman, San Francisco, CA. Reprinted by the American Phytopathological Society, St. Paul, MN, USA, 1982. 3. Bakker, P.A.H.M., Raaijmakers, J.M. and Schippers, B. (1993) In: Barton, L.L. and Hemming, B.C. [Eds.] Iron Chelation in Plants and Soil Microorganisms. Academic Press, London, UK. pp. 269-281.

278

B. Schippers

4. Chet, I., Ordentlich, A., Shapira, R. and Oppenheim, A. (1991) In: Keister, D.L. and Cregan, P.B. [Eds.] The Rhizosphere and Plant Growth. Kluwer Academic Publ., Dordrecht, the Netherlands. pp. 229-236. 5. Cook, R.J. and Baker, K.F. (1983) The Nature and Practice of Biological Control of Plant Pathogens. The American Phytopathological Sot., St. Paul, MN, USA. 6. D6fago, G. and Haas, D. (1990) In: Bollag, J.M. and Stotzky, G. [Eds.] Soil Biochemistry. Marcel Dekker, Inc., New York, NY. pp. 249-291. 7. De Wit, P.J.G.M. (1992) Annu. Rev. Phytopathol. 30:391-418. 8. Lemanceau, P., Bakker, P.A.H.M., De Kogel, W.J., Alabouvette, C. and Schippers, B. (1992) Appl. Environ. Microbiol. 58:2978-2982. 9. Schippers, B. (1992) In: Tjamos, E.C., Papavizas, G.C. and Cook, R.J. [Eds.] Biological Control of Plant Diseases - Progress and Challenges for the Future. NATO ASI Series A, Life Sciences. Vol. 230. Plenum Press, New York, NY. pp. 21-34. 10. Schippers, B., Bakker, A.W. and Bakker, P.A.H.M. (1987) Annu. Rev. Phytopathol. 25: 339-358. 11. Thomashow, L.S. and Weller, D.M. (1990) In: Hornby, D., Cook, R.J., HeNs, Y., Ko, W.H., Rovira, A.D., Schippers, B. and Scott, P.R. [Eds.) Biological Control of Soil-Borne Plant Pathogens. CAB International, Wallingford, UK. pp. 109-122. 12. Tjamos, E.C., Papavizas, G.C. and Cook, R.J. [Eds.] (1992) Biological Control of Plant Diseases - Progress and Challenges for the Future. NATO ASI Series A, Life Sciences. Vol. 230. Plenum Press, New York, NY. 13. Van Peer, R. and Schippers, B. (1992) Neth. J. Plant Pathol. 98:129-139.

Phytoparasitica 21:4, 1993

279