External and Internal Root Colonization of Maize by Two Pseudomonas
Strains: Enumeration by Enzyme-Linked Immunosorbent Assay (ELISA)
E. Benizri, A. Schoeny, C. Picard, A. Courtade, A. Guckert
Laboratoire Agronomie et Environnement ENSAIA-INRA, 2, Avenue de la Foreˆt de Haye, BP 172, F-54505 Vandoeuvre-les-Nancy, France
Received: 26 September 1996 / Accepted: 1 November 1996
Abstract. Maize root colonization by two fluorescent Pseudomonas strains M.3.1. and TR335, isolated
respectively from maize and tomato roots, were studied in hydroponic conditions. Each bacterium was
inoculated separately, and three different colonization areas were studied: nutrient solution, rhizoplane,
and endorhizosphere. The two Pseudomonas strains established large rhizosphere populations, and
rhizoplane colonization of the entire root system was similar for both strains. However, strain M.3.1.
colonized the endorhizosphere more efficiently than strain TR335. Seminal root cuttings from the tip to
the seed allowed the assessment of colonization of three different root areas (i.e., root cap and elongation
area, root-hair zone, and mature zone). Rhizoplane colonizations of all these three areas by M.3.1. were
significantly the same, whereas strain TR335 colonized the rhizoplane of the root cap and elongation area
more actively than the root-hair zone and mature zone. Population size of the strain M.3.1. in the internal
tissue of these areas was greater than that of strain TR335. Co-inoculations of the two strains indicated a
stimulation of the population size of strain M.3.1. regardless of root area studied, whereas population size
of strain TR335 remained unchanged. These results demonstrated that external and internal maize root
tissues were colonized to a greater extent by a strain isolated from a maize rhizosphere than by one
isolated from another rhizosphere.
Inoculation of soil, seeds, or plants with specific Pseudo-
monas sp. strains has been shown to significantly increase
crop yields. This increase is related to the promotion of
plant growth and protection against pathogenic microor-
ganisms [20]. The mechanisms by which Pseudomonas
sp. exert their beneficial effect on plants can be very
diverse. Pseudomonas sp. produce plant hormones, which
promote root growth [4, 16, 20]. Under conditions of
Fe
31
limitation, some Pseudomonas sp. produce sidero-
phores that are able to scavenge the traces of Fe
31
and
thus deprive other microbes. Other Pseudomonas sp.
secrete volatile and/or antibiotic compounds, which re-
duce the density and the deleterious activity of patho-
genic microorganisms [2, 11, 22, 26, 31]. Kloepper and
Schroth [18] introduced the term plant growth-promoting
rhizobacteria (PGPR) to describe these strains that are
capable of stimulating plant growth. The effectiveness of
distinct PGPR strains in controlling soil-borne plant
pathogens is generally assumed to be related to their
efficiency of root colonization. However, little is known
of the mechanisms involved in root colonization by
pseudomonads and of the existence of crop specificity of
Pseudomonas strains.
By studying bacterial colonization of rhizosphere,
rhizoplane, and endorhizosphere, Bahme and Schroth [1]
observed that 41% of a Pseudomonas fluorescens strain
inoculated on potatoes were associated with the rhizo-
sphere soil, whereas 54% adhered loosely to the root
surface and 5% adhered tightly or were internal to the
root surface. In contrast, by using the same approaches,
Miller et al. [25] found that fluorescent pseudomonads
were lower in the rhizoplane than in the rhizosphere and
barely detectable in the endorhizosphere. Several other
authors have studied bacterial colonization all along
roots, from the apex to the seed [1, 23]. They observed
that populations of rhizobacterial samplings of root
segments adjacent to seed parts were more important than
those near the apex. In contrast, after inoculations of
soils, Rovira and Campbell [30], Guckert et al. [13], and
Campbell and Greaves [5] detected more bacteria on the
Correspondence to: E. Benizri
C
URRENT
M
ICROBIOLOGY
Vol. 34 (1997), pp. 297–302
An International Journal
R
Springer-Verlag New York Inc. 1997