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Applied and Environmental Microbiology, June 2004, p. 3794, Vol. 70, No. 6
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.6.3794.2004

LETTER TO THE EDITOR

Isolation and Identification of Actinobacteria from Plant Roots


   LETTER
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Coombs and Franco (1) reported their isolation and identification of actinobacteria from surface-sterilized root tissues of wheat plants. They claim that their study reveals a novel plant-microbe interaction. While they recognize that actinobacteria inhabit the rhizospheres of many plant species (2), they failed to recognize that Tokala et al. had previously described the colonization of Pisum sativum root nodules and roots by Streptomyces lydicus WYEC108 (3), a similar plant-microbe interaction that had previously been undescribed. It was shown that P. sativum nodule colonization by Streptomyces is a natural event, based on the finding of actinobacteria colonizing the root nodules of pea plants growing in agricultural fields. It was also shown that S. lydicus WYEC108 hyphal growth penetrated the surface layers of the nodules and that colonization aided nodular assimilation of iron and molybdenum. These findings taken together with those of Coombs and Franco (1) strengthen the argument that plants benefit from harboring actinobacteria within their healthy tissues. While the findings of Coombs and Franco showed culture-dependent evidence of plant-root colonization, previous findings that were not cited in their paper show that colonization of the plant roots of P. sativum is important for the health of the plant in the natural environment.


   REFERENCES
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  1. Coombs, J. T., and C. M. M. Franco. 2003. Isolation and identification of actinobacteria from surface-sterilized wheat roots. Appl. Environ. Microbiol. 69:5603-5608.[Abstract/Free Full Text]
  2. Crawford, D. L., J. M. Lynch, J. M. Whipps, and M. A. Ousley. 1993. Isolation and characterization of actinomycete antagonists of a fungal root pathogen. Appl. Environ. Microbiol. 59:3899-3905.[Abstract/Free Full Text]
  3. Tokala, R. K., J. L. Strap, C. M. Jung, D. L. Crawford, M. H. Salove, L. A. Deobald, J. F. Bailey, and M. J. Morra. 2002. Novel plant-microbe rhizosphere interaction involving Streptomyces lydicus WYEC108 and the pea plant (Pisum sativum). Appl. Environ. Microbiol. 68:2161-2171.[Abstract/Free Full Text]
Don L. Crawford*
Janice L. Strap
Ranjeet K. Tokala
Department of Microbiology, Molecular Biology, and Biochemistry
University of Idaho
Moscow, ID 83844-3052

* Phone: (208) 885-6113, Fax: (208) 885-6518, Email: donc{at}uidaho.edu.


Authors' Reply


   LETTER 
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In our paper (2) that is referred to in the Letter to the Editor, we claimed that it was the first report of nonpathogenic actinobacteria that were isolated as endophytes of wheat plants.

Please note that our manuscript was first sent to Applied and Environmental Microbiology (AEM) on 6 March 2002, prior to the publication of Tokala et al. (4). The AEM referees who reviewed our manuscript requested clear evidence that the sterilization procedures that we used were effective in removing microorganisms from the surface of the plant tissue, in order to justify our claim that the actinobacteria that were isolated were indeed true endophytes.

In our resubmission we included results of the validation studies that proved that the surface sterilization protocol was effective. In addition, we submitted a second manuscript, subsequently published in AEM (1), to show the endophytic colonization of germinating wheat plants by using one of our actinobacterial isolates that was tagged with green fluorescent protein.

Consequently, we considered that we were asked to make a clear distinction between actinobacteria that were isolated as true endophytes and those that were isolated from the external surfaces of the roots of plants or rhizosphere soil. In this regard, we did refer to the publication by Crawford et al. (3) in which the authors reported the isolation of S. lydicus WYEC108 from a rhizosphere soil.

Therefore, as the thrust of our paper was the isolation and characterization of endophytic actinobacteria that were obtained from surface-sterilized tissue of wheat roots, we restricted our discussion to other reports of true endophytic actinobacteria that were also obtained from surface-sterilized plant tissue. Tokala et al. (4) did not report the use of a surface sterilization protocol for the isolation of S. lydicus WYEC108 from pea plants; hence, reference to their publication was not included. Nevertheless, we concur that the colonization of the roots of pea plants by an actinobacterium and the resultant physiological effects, as shown by Tokala et al. (4), constitute an important, beneficial plant-microbe association.


   REFERENCES 
Top
 Letter
 References
 Letter 
 References 
 

  1. Coombs, J. T., and C. M. M. Franco. 2003. Visualization of an endophytic Streptomyces species in wheat seed. Appl. Environ. Microbiol. 69:4260-4262.[Abstract/Free Full Text]
  2. Coombs, J. T., and C. M. M. Franco. 2003. Isolation and identification of actinobacteria isolated from surface-sterilized wheat roots. Appl. Environ. Microbiol. 69:5603-5608.
  3. Crawford, D. L., J. M. Lynch, J. M. Whipps, and M. A. Ousley. 1993. Isolation and characterization of actinomycete antagonists of a fungal root pathogen. Appl. Environ. Microbiol. 59:3899-3905.
  4. Tokala, R. K., J. L. Strap, C. M. Jung, D. L. Crawford, M. H. Salove, L. A. Deobald, J. F. Bailey, and M. J. Morra. 2002. Novel plant-microbe rhizosphere interaction involving Streptomyces lydicus WYEC108 and the pea plant (Pisum sativum). Appl. Environ. Microbiol. 68:2161-2171.
Christopher M. M. Franco*
Justin T. Coombs
Department of Medical Biotechnology
School of Medicine
Flinders University
Bedford Park, South Australia 5042, Australia

* Phone: 618 8204 5764, Fax: 618 8204 5764, Email: Chris.Franco{at}flinders.edu.au.

Applied and Environmental Microbiology, June 2004, p. 3794, Vol. 70, No. 6
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.6.3794.2004





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