Effect of the cyanobacteria Synechococcus leopoliensis and Nostoc sp. on plant pathogens and on the interactions sorghum (Sorghum bicolor) - Colletotrichum sublineolum and tobacco (Nicotiana tabacum) - tobacco mosaic virus (TMV)

Author(s)

Robson Marcelo Di Piero

• Document type: Master's Dissertation
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 1999-10-28
• Advisor: Sergio Florentino Pascholati

Abstract

The cyanobacteria are prokaryotic microorganisms which are able to carry out photosynthesis and reproduce by binary fission or by random trichome breakage (in the case of the filamentous ones). Although some species are prejudicial, by producing dangerous substances to animals or leading to lake eutrophization, other cyanobacteria are useful. The most advanced species can fix atmospheric nitrogen due to the presence of heterocysts and enzymes necessary to the process. These cyanobacteria have the potential of improving soil structure and fertility, for example, in rice paddies. There are many reports showing that cyanobacteria are able to produce biologically active compounds against fungi, viruses and bacteria that cause human diseases and some articles also show the antagonistic effect on plant insects and pathogens. Besides that, cyanobacteria are versatile, surviving and developing under unfavorable conditions for life. Thus, the present work was carried out to study the influence of two cyanobacteria, Synechococcus leopoliensis and Nostoc sp., on plant pathogens and to verify the effect of both of them on the expression of diseases caused by some of these pathogens. Results showed that filtrates from cyanobacteria cultures with different ages inhibited infection by TMV and the mycelial growth of Exserohilum turcicum (sorghum pathogen), but they did not exhibit in vitro antagonistic action against Colletotrichum graminicola (maize pathogen), against C. sublineolum (sorghum pathogen) and against Xanthomonas axonopodis pv. phaseoli (bean pathogen). On the other hand, in some cases, the filtrates improved growth of the last two pathogens. When cell suspensions from the cyanobacteria were applied on sorghum and tobacco leaves 48 hours before the plant inoculation with the pathogens, the suspensions reduced the symptoms caused by C. sublineolum and TMV, respectively. The effect was low and only observed on pre-treated leaves, suggesting that it may have occurred by direct action of the cyanobacteria on the pathogens and/or by the response of the plant defense mechanisms. ln one of the bioassays, S. leopoliensis was able to induce accumulation of phytoalexins in soybean tissue, showing the possibility of cyanobacteria to activate plant defense mechanisms. Systemic acquired resistance was not observed in the plants. (AU)