Rhizobacteria able to produce phytotoxic metabolites

Abstract

To contribute for the development of environmental friendly methods for weed control, a selection of rhizobacteria able to produce phytotoxic substances was carried out. Initially, 35 strains previously isolated from plants in the south of Minas Gerais State (Brazil) were grown in tryptic soy broth. After removal of bacterial cells, the resulting liquids were freeze-dried and extracted with methanol/ethyl acetate (1:1). The extracts were concentrated under vacuum and dissolved in water to be submitted to a lettuce (Lactuca sativa L.) seed assay. Metabolites produced by five strains reduced the number of normal seedlings to values statistically bellow the one observed for the negative control, being the most expressive results obtained with Bacillus cereus Frankland and Frankland, isolated from Ricinus communis L., which was able to cause rotted rootlets to 82.4% of seedlings. The bacterium metabolites also avoided germination of 52% Brachiaria decumbens Stapf seeds and the remaining 48% resulted in abnormal seedlings. Metabolites from B. cereus were submitted to a purification process guided by the lettuce seed assay. As a consequence, one substance causing rotted rootlets to all lettuce seedlings during the seed assay at 0.057 g/L was isolated and will be identified in future studies.

Selection of phytotoxin producing rhizobacteria

“…Similarly, K. pneumoniae metabolites presented no effect on lettuce seed germination, but reduced the wheat coleoptile length and mass. This result seems to be in accordance with the previously reported absence of phytotoxic effects on lettuce seeds by the metabolites of such bacterium (Carvalho et al 2007).…”

“…The toxic effects of B. cereus (strain 83-02) metabolites on lettuce seed germination and wheat coleoptile growth seemed reasonable (Table I), since in a previous work with another strain of this bacterium, isolated from Ricinus communis L., the corresponding metabolites affected lettuce seed germination and signalgrass (Brachiaria decumbens Stapf) seed germination to a great extent (Carvalho et al 2007). According to the authors, metabolites from B. cereus (strain 57-02) caused necrosis on 82.6% of lettuce roots and afforded 48% of abnormal signalgrass seedlings (seeds exposed to water presented 20% of abnormal seedlings).…”

“…Since phytotoxin production is one of the mechanisms by which rhizobacteria act against plants (Alstrom andBurns 1989, Loper andSchroth 1986), in a preliminary work crude metabolites from some rhizobacteria were submitted to an assay with lettuce seeds (Lactuca sativa L.) to select those potentially useful for the development of new products to control weeds (Carvalho et al 2007). In order to continue this research, this paper presents the effect of crude metabolites produced by some rhizobacterial strains different from those used in the previous work on wheat coleoptiles (Triticum aestivum L.) and lettuce seeds, which are considered excellent phytotoxin detectors (Stonard and Miller-Wideman 1994).…”

Biological control of Fusarium oxysporum f. sp. phaseoli by Trichoderma harzianum and its use for common bean seed treatment

“…In recent years, biofungicides have been developed and commercialized worldwide (Shali et al, 2010). Due to the negative aspects involved in the use of agrochemicals such as contamination of the environment with toxic residues that may affect humans and animals (Carvalho et al, 2007), the use of biofungicides in seed treatments has been expanded to many crops (Corrêa et al, 2008), which is true especially in Brazil where Trichoderma spp. is the most used micoorganism in bioformulations (Carvalho et al, 2011a).…”

Ação de metabólitos voláteis e não voláteis de Trichoderma harzianum sobre o crescimento de Cladosporium herbarum

“…De acordo com Jayaprakashvel et al (2010), a autoclavagem (121°C por 15 minutos) de metabólito proveniente de um agente de biocontrole pode reduzir em 24% a sua atividade antifúngica quando comparada a não autoclavagem. Já Carvalho et al (2007) postulam que moléculas bioativas, presentes em metabólitos de origem microbiana, podem se decompor em outras moléculas menos ativas ou mesmo inativas, quando submetidas a condições inadequadas que levem à degradação das moléculas. O fato de alguns desses metabólitos se mostrarem estáveis em várias temperaturas suportaria a idéia de que os mesmos possam se manter ativos frente a diferentes fatores, como insolação e temperaturas mais elevadas dos solos de diferentes regiões, conforme mencionado anteriormente.…”