The document summarizes a study that isolated bacteria from soil in the Mojave Desert to identify microbes that could help plants grow under arid conditions. Thirty unique bacterial colonies were characterized through assays testing for plant growth promoting traits like phosphate solubilization, siderophore production, antibiotic production, and cellulase activity. Many isolates exhibited siderophore production while fewer showed phosphate solubilization or cellulase activity. One isolate produced antibiotics. A trapping experiment found that soil samples effectively nodulated legume plants, indicating nitrogen fixation. The results suggest that the desert microbial community can help native plants survive harsh environments.

1. As dryland farming becomes more important for food production, the
goal to find a sustainable method to promote crop growth under arid
conditions is imperative. Thus, we looked to the Mojave Desert for a
solution as it is home to plants living under year-round extreme climate
and soil conditions. We isolated bacteria from the rhizosphere of Larrea
tridentata, commonly found in the Mojave Desert, and studied their plant
growth promoting (PGP) traits as we hypothesized that these plants
depend on the microbial community for their growth and survival. We
isolated a total of 30 unique bacterial colonies and characterized them
through four cultivation-dependent assays: 1) siderophore production; 2)
phosphate solubilization; 3) antibiotic production; and 4) cellulase
activity. Many isolates exhibited siderophore production whereas only a
few demonstrated phosphate-solubilizing and cellulase activity. Only a
single colony exhibited antibiotic production. Following a PCR of the 16S
ribosomal gene and sequencing, 16 of the original 30 isolates were
identified as actinomycete and Bacillus strains. eDNA isolation was also
performed. A trapping experiment in which legume plants are inoculated
with the original soil samples showed effective root nodulation
suggesting that nitrogen fixation occurred. Our results indicate that the
microbial community of the Mojave Desert can help the plants survive
adverse conditions.
Hung Dang2
, Janeva Chung2
, Nakin Bhandari2
, Nhi Nguyen1
Microbial activity found in the soil of the
Mojave desert suggests plant growth promotion
1
Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, CA 90095
2
Department of Molecular Cell, Developmental Biology and Cell Development, University of California, Los Angeles, CA 90095
Abstract
Materials & Methods
Assays for Plant Growth Promoting Activity
Master Chart
Acknowledgements
Discussion
Future DirectionsTrapping Experiment
Because there was presence of Nitrogen-fixing bacteria
that only associated specifically with a certain type of plant
species, it is best to employ Koch’s postulate. Thus, the
isolates should be re-inoculated into Siratro to see if they
can again nodulate the plants. Further, PGPB assays testing
for activities are also needed for those particulate nodule
isolates. Finally, because these strains come from extreme
climate conditions, an experiment to test the durability of
these bacteria to know what optimal conditions for later use
is necessary. Finally, once abilities are finalized, a list of
recommended strains of bacteria should be composed as a
reference for bio-inoculates in dryland farming.
● We would like to thank the following:
○ Dr. Ann Hirsch, Dr. Maskit Maymon, Dr. Pilar
Martinez, and Paul Yang for their tutorage.
○ Dr. Kris Reddi and Anjana Amirapu for assisting us in
the setup of materials for our experiments.
○ The Department of Molecular, Cellular, and
Developmental Biology for funding this project.
Phosphate Solubilizing Activity
Using PVK plates as medium,
we discovered that 8 out of the 30
isolate exhibited signs of phos -
phate solubilizing activity, with
most of the activity not covering
more than 20mm in diameter. The
strain that showed highest level of
activity is G1.
Siderophore Production
Using both LB and TY plates, bacteria
was plated from liquid culture into 5
equidistant spots. Once the bacteria grew,
Cas overlay was poured over each plate
and halo formation was analyzed after
waiting for the Cas overlay to set. 17 out
of the 30 isolates exhibited siderophore
activity. Isolate 21 has the highest
calculated amount of siderophore activity.
Antibiotic Production
We discovered a con
-taminate in 3A liquid
culture that most likely
came from our soil. This
strain exhibited anti-
biotic activity as it killed
of the lawn growth
(evident in the halos).
No other colony
exhibited activity.
16S Identification
Of the assays, the Cas Assay had
the highest amount of positive
results. In addition, Cas plates
were only used to confirm Cas
overlay positives, resulting in
many unplated Cas plates. Straings
G1, G2, G4, 1A, 2A, and 2 were
capable of more than one activity.
Multiple strains of bacteria that were isolated displayed
phosphate solubilizing activity, cellulase activity, and
siderophore production. With G1 and G2 showing very
prominent capabilities. These we believe could be good
candidates for future use as bioinoculates. Two distinct
bacterial colonies were isolated from the nodules of the
siratro plant in the trapping experiment. One of them is
characterized to be a Rhizobium species which is known for
their ability to form endosymbiotic nitrogen fixing
association with root plant. In regards to our contaminate
strain, we believe that the reason why it became so
prominent is due to its antibiotic production which could
have wiped out the native strain.
Out of five plant species that were inoculated with
the Mojave desert, Siratro is the only plant that
nodulated. Even though there is no significant
difference in biomass, the plant’s physical appearance
(greener than control) indicates that the bacteria
associated with the plants through root nodules did
promote plant growth. The squash of the nodules
yielded colonies which were then analyzed. The
following bacteria were characterized:
● Kocuria rhizophila strain DC2201- 100%
● Rhizobium sp. strain X2Ac9- 100%
.
The figure shows phylogenetic relationship of the
closest sequence matches from our cultivable isolates. Of
the 9 bacterial isolated from soil sample, the majority were
proteobacterial species (strain 1, 7,9, 12, 18). The rest
were actinobacteria (strain 17, G2, G3) and firmicutes
(strain 2 and 3A).
Cellulase Production
Bacteria was plated from liquid culture
on TY-CMC plates. Once grown the plates
were then subsequently washed with
Congo Red, NaCl, and then finally HCl.
Cellulase activity was not well defined. 7
of the 30 isolates exhibited cellulase
activity with strains A3 and A4 exhibiting
the highest ratio of halo to plaque.
[Picture: Negative result]