Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

158j 8 Pseudomonas aurantiaca SR1: Plant Growth Promoting Traits, Secondary Metabolites
Table 8.1 Effect of coinoculation with P. aurantiaca SR1 on shoot
and root length in an alfalfa cultivar.
Treatment Shoot length (cm) Root length (cm)
Coinoculation 8.2000 0.29 a
P. aurantiaca SR1 6.2000 0.73 b
S. meliloti 3DOh13 7.0800 0.42 a
Control N2 5.3800 0.51 c
Control 3.4000 0.30 c
[50] and soybean [50]. Knight and Langston-Unkefer found that inoculation of
nodulating alfalfa roots by means of a toxin-releasing Pseudomonas syringae pv. tabaci
significantly increased plant growth, nitrogenase activity, nodule number, total nodule
weight and nitrogen yield under controlled growth conditions [51].
In our studies, both P. aurantiaca SR1 and S. meliloti strain 3DOh13 were cultured
on tryptic soy broth (TSB) medium at 28 1 C. The optical cell densities at 600 nm
(OD600) were 0.22 and 0.36, which corresponded to approximately 4.5 · 10 8 and
6.8 · 10 8 CFU ml 1 of P. aurantiaca SR1 and S. meliloti 3DOh13, respectively.
The inoculant was prepared by mixing P. aurantiaca SR1 and S. meliloti strain
3DOh13 in a 1 : 1 ratio (vol/vol). The optical cell density at 600 nm (OD600) was 0.25,
which corresponded to cell numbers of S. meliloti strain 3DOh13 of approximately
6.6 · 10 8 and 6.3 · 10 8 CFU ml 1 of P. aurantiaca SR1. One gram of sterilized seeds
was inoculated with the mixed bacterial suspension, and populations of bacteria on
inoculated seeds were 10 5 CFU/seed.
The greenhouse conditions were as follows: air temperature of 28 2 C and
additional illumination of 220 mEm 2 seg 1 for a photoperiod of 16 : 8 h (day: night).
In bacterial coexistence assays, P. aurantiaca SR1 did not exercise any inhibiting
effect on the growth of S. meliloti. P. aurantiaca SR1 coinoculated with S. meliloti
stimulated the length and root shoot growth at 45 days after sowing (Table 8.1).
Additionally, the coinoculation with S. meliloti strain 3DOh13 and P. aurantiaca
SR1 resulted in a significant increase in the fresh and dry shoots and root weight
(Table 8.2) and in the number of nodules.
Even though there is a possibility of great variability in field results, if a positive
effect of a PGPR is seen on a specific crop in greenhouse studies, there is a strong
likelihood that such effect will carry through to field conditions [52].
8.4
Field Experiments with P. aurantiaca SR1 in Wheat (Triticum aestivum L.)
14.7800 0.47 a
11.8200 0.67 b
13.2800 0.52 b
9.7200 0.43 c
7.5400 0.89 c
Mean SE, means with different letters a,b,c in the same column differ significantly at P < 0.05
(Bonferroni test). Plants were grown for 45 days in the greenhouse.
PGPR strains promote wheat growth because of their ability to transform root
exudates into phytohormones that are absorbed by the roots, thus allowing nitrogen
fertilizer application to reduce [53–55]. This conversion into substances by the root