Colonization of rice and Spodoptera frugiperda J.E. Smith (Lepidoptera: Noctuidae) larvae by genetically modified endophytic Methylobacterium mesophilicum

SCIENTIFIC NOTE

Colonization of rice and Spodoptera frugiperda J.E. Smith (Lepidoptera: Noctuidae) larvae by genetically modified endophytic Methylobacterium mesophilicum

Fátima T Rampelotti-Ferreira^I; Anderson Ferreira^II; José D Vendramim^I; Paulo T Lacava^II; João L Azevedo^II; Welington L Araújo^II,III

^IDepto de Entomologia e Acarologia, ESALQ, USP, CP 09, 13.418-900 Piracicaba SP, Brasil

^IIDepto de Genética, ESALQ, USP, CP 83, 13.400-970 Piracicaba, SP, Brasil

^IIINúcleo Integrado de Biotecnologia, Univ de Mogi das Cruzes, Av Dr. Cândido Xavier de Almeida Souza 200, 08.780-911, Mogi das Cruzes, SP, Brasil

ABSTRACT

The colonization of Spodoptera frugiperda J.E. Smith larvae and rice seedlings by genetically modified endophytic bacterium Methylobacterium mesophilicum, and also the possible transfer of this bacterium to inside the larva's body during seedlings consumption were studied. The data obtained by bacterial reisolation and fluorescence microscopy showed that the bacterium colonized the rice seedlings, the larva's body and that the endophytic bacteria present in seedlings could be acquired by the larvae. In that way, the transference of endophytic bacterium from plants to insect can be a new and important strategy to insect control using engineered microorganisms.

Key words: Insecta, Oryza sativa, endophytic microorganism

The biotechnological progresses are allowing the identification of endophytic microorganisms promising for insect biological control (Azevedo et al 2000). Azevedo & Araújo (2007) defined endophytes as all the microorganisms that inhabit the interior of the host plant, without causing apparent damages or visible external structure. The presence of endophytic microorganisms inside the host plant may increase the plant fitness by protecting it against pest and pathogens, improving plant growth and increasing resistance in stressful environments (Azevedo et al 2000, Scherwinski et al 2007). Besides the natural occurrence of endophytes, many studies are being carried out with genetically modified microorganisms (GMM) to evaluate host colonization (Germaine et al 2004, Ferreira et al 2008). Methylobacterium spp. have been described as related with plant systemic resistance (Madhaiyan et al 2004), plant growth and root formation (Senthilkumar et al 2009). In this context, the aim of this work was to study the endophytic colonization of rice seedlings and Spodoptera frugiperda J.E. Smith larvae by the genetically modified Methylobacterium mesophilicum in in vitro conditions.

The endophyte M. mesophilicum strain SR1.6/6 used in this work was previously isolated from Citrus sinensis (Araújo et al 2002) and labeled with Green Fluorescent Protein (gfp) (Gai et al 2009). The M. mesophilicum was introduced in rice seeds to evaluate the plant colonization (Experiment 1), using rice seeds (SCS BRS 112) germinated on MS culture medium (Murashige & Skoog 1962). Seven days after germination, the seedlings roots were submerged in a bacterial cells suspension (10⁸ cells ml^-1) for 1h and transplanted onto MS medium. Bacterial reisolation was carried out five and ten days after inoculation. Five seedlings were submitted to surface disinfection using serial washing in 70% ethanol for 1 min, 2% sodium hypochlorite solution for 2 min, 70% ethanol for 1 min, and two washes in sterilized distilled water. After surface disinfection, the aerial part of these seedlings was triturated in sterile phosphate buffered saline (PBS) and appropriated dilutions were plated onto CHOI 3 (Toyama et al 1998) culture medium amended with 50 µg ml^-1 of tetracycline antibiotic. Plates were incubated at 28ºC for 10 days, upon which the numbers of colony-forming units (CFU) were counted. The disinfection process was checked by plating aliquots of the sterile distilled water used in the final wash onto CHOI 3 and incubated in the same conditions. Also, seedling colonization was evaluated by fluorescence microscopy.

Later, the M. mesophilicum interaction with S. frugiperda was studied (Experiment 2), where third instars of S. frugiperda were maintained in laboratory under artificial diet (Parra 2001, Ferreira Filho pers com). For this, M. mesophilicum cells were inoculated onto the diet by aspersion of 50 µl of bacterial suspension (10⁸ cells ml^-1) and one larva was transferred to each tube, and maintained in laboratory conditions. The control was done using non-inoculated diet tubes. The experiment was done twice, using five replicates per experiment. The analysis to detect the presence of the M. mesophilicum inside the entire larva's body were carried out after 24, 48 e 96h of feeding. Larvae were surface disinfected and bacterial isolation was carried out as described for the rice seedlings. However, the larvae heads were separate from the body, and triturated in PBS. The fecal pellets were also diluted in PBS and plated onto CHOI 3 medium to evaluate the presence of gfp expressing M. mesophilicum. Afterwards, we studied the possible transference of M. mesophilicum from rice seedlings to S. frugiperda during the larva feeding (Experiment 3). Third instars of S. frugiperda were used in this bioassay. Rice seedlings were inoculated with M. mesophilicum as early described in experiment 1 and were maintained in laboratory conditions to one week, when the aerial part of these seedlings was introduced into the tubes containing one S. frugiperda larva. The presence of endophytic M. mesophilicum associated to inner larvae parts, such as heads, body and feces were carried out after 96h.

The gfp tagged endophytic M. mesophilicum colonized rice seedlings after root inoculation. This colonization was observed by reisolation and fluorescence microscopy analysis. The bacterial density ranged from 5 x 10¹ to 8.2 x 10¹ CFU.g^-1 of rice tissue five and ten days after inoculation, respectively.

Also, 96h after inoculation of the gfp tagged M. mesophilicum onto diet, the bacterium was found inside the larvae, suggesting that the larvae fed the diet with the tagged bacterium, which survived inside the insect. The bacterial density inside the larval body ranged from 2 x 10² to 8 x 10² CFU.g^-1. Methylobacterium mesophilicum was not observed in feces samples.

The bacterium inoculated by the roots was able to colonize rice seedlings and be further transferred from the plant to the larvae during insect feeding, reaching 10 x 10¹ CFU.g^-1 of surface disinfected larval tissues. Methylobacterium mesophilicum was not observed in the heads and feces samples, suggesting that although this bacterium could settle inside the larvae, it was not release with the feces. Also, this result indicates that this strategy could be used for biological control of insect that feed on important economical crops, since the gfp gene could be changed by other genes, such as Btcry or protease inhibitors. Similar model was previous proposed by Fahey et al (1991) and Tomasino et al (1995), which used endophytic bacteria carrying the Bt cry genes to protect plants against insect attack.

However, more studies on the mechanisms involved in insect, plant and bacterium interaction are necessary to define a better control strategy. Although, some authors have described M. mesophilicum as a cause of opportunistic infections in immunocompromised hosts (Sanders et al 2000, Imbert et al 2005), many others described the biotechnological importance of this bacterium (Holland 1997, Madhayan et al 2004, Senthilkumar et al 2009), suggesting that diversity and applied studies should be carried out together, to choose some strains able to improve plant protection with low risks to human healthy. The transference of endophytic bacterium from plants to insect can be a new and important strategy to insect control using engineered microorganisms able to express toxins inside the plant and/or insect.

Acknowledgments

We thank Mateus Mondin and Dr Margarida L R de Aguiar-Perecin of the Laboratory of Molecular Cytogenetics for the use of the fluorescent microscope facilities and the support of the CNPq (National Council of Research, Brazil).

Received 31/VII/08.

Accepted 15/IX/09.

Edited by Fernando L Cônsoli - ESALQ/USP

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Publication Dates

• Publication in this collection
  14 May 2010
• Date of issue
  Apr 2010

History

• Accepted
  15 Sept 2009
• Received
  31 July 2008