The
mechanism of action of the phosphate solubilizing PGPR Paenibacillus polymyxa
Dept. of Cell and Molecular
Biology ,Microbiology,Uppsala University Biomedical Center (BMC) Box 596, S-751
24 Uppsala SWEDEN,
Tel. +46/18/471 45 28 FAX +46/18/673392,
e-mail:
salme.timmusk@icm.uu.se
Paenibacillus
polymyxa,
a common soil bacterium, belongs to the group of phosphate solubilizing plant
growth promoting rhizobacteria. Activities that have been found to be
associated with P. polymyxa-treatment of plants in field experiments
include nitrogen fixation, soil phosphorus solubilization, production of
antibiotics, auxin, cytokinin, chitinase, and hydrolytic enzymes as well as
promotion of increased soil porosity. All these activities might be of importance
for plant growth promotion. Yet, the mechanism by which P. polymyxa exerts its
beneficial effect is not quite understood.
We
have studied changes in plant gene expression induced by inoculation with P.
polymyxa.
A gnotobiotic system was established using Arabidopsis thaliana as model
plant. Subsequent challenge by either the pathogen Erwinia carotovora (biotic
stress) or induction of drought (abiotic stress) indicated that plants that
were previously inoculated by P. polymyxa showed more resistance to
these stresses than control plants. Using RNA-differential display on parallel
RNA preparations from P. polymyxa-treated or untreated
plants, changes in gene expression were investigated. From a small number of
candidate sequences obtained by this approach, one mRNA segment showed a strong
inoculation-dependent increase in abundance. The corresponding gene was
identified as ERD15, previously identified to be drought
stress-responsive. Quantification of mRNA levels of several stress-responsive
genes indicated that P. polymyxa induced mild biotic
stress. This suggests that genes and/or gene classes associated with plant
defenses against abiotic and biotic stress may be co-regulated. Since the
expression on mRNA level is not necessarily connected with protein expression
we are in process of studying the differential expression of proteins induced
by P.polymyxa infection.
Green
fluorescent protein GFP has been found useful for working various bacterial
species. GFP doesn't require specific substrate and makes possible real time
detection and measurement of marked bacteria. There are several cassettes
available for Gram negative bacteria. The cassette we are tying to insert to P.polymyxa, to follow
the infection process and action of the bacterium in the rhizosphere of plants,
is the first cassette applicable for genus Bacillus.