By Erika Van Andel
Some
soil environments are harder on
There are many different
microbes that colonize on the roots of plants which help their hosts to grow
called plant growth promoting rhizobacteria. Some fix nitrogen so that it is in
a form the plants can use, others produce hormones, and others help alleviate
stress on the plant.
Of these stress alleviating
bacteria, a group of them have been shown to help their hosts by producing an
enzyme, ACC deaminase, which hydrolyzes ACC, meaning it breaks bonds in ACC by
adding water (Palaniyandi et al., 2014). ACC is a precursor to ethylene and the
production of ACC deaminase reduces stress on the plant by reducing the amount
of ethylene in the cells and keeping the levels of ACC equal to that of the
outside environment. In previous research ACC deaminase producing bacteria have
been shown to help alleviate stress in plants in floods, droughts, heavy
metallic polluted areas, organically polluted areas and soils with high salt
content. This study focused on finding a type of bacteria that would alleviate
stress in soil with a high salinity.
The researchers began with
samples of 54 bacteria isolated from a soil sample, and tested them for their
ACC deaminase activity. They did this by plating them with ACC, with a nitrogen
source, or nothing. Those with ACC deaminase activity were able to use it as a
nitrogen source and grow with ACC at a rate comparable to the growth with a
nitrogen source. This narrowed the search down to 8 types of bacteria. These
were all given to seedlings to test their growth promoting abilities. This is
where a clear leader in the search for a bacteria to help tomato plants
withstand high salt conditions came forward, strain PGPA39 was the only strand
to significantly promote growth in the seedlings, increasing both their overall
biomass and their number of roots. All 8 bacterial strains were tested for
their ability to withstand high concentrations of NaCl. Again strain PGPA39 did
the best by withstanding a 6% concentration while most others only withstood
2%.
 |
The effect of PGPA39 on tomato plant size and roots.
A) after 23 days, B), after 60 days, and C) roots after 60 days.
1 is salt stressed without PGPA39, 2 is salt stressed with PGPA39,
and 3 is a control that is not under stress.
|
Having
decided that PGPA39 showed the most promise, researchers sought to identify
this strain. They compared one section of its DNA that tends to be similar
across species to an online database to find the best match, and found that it
shared 99% of this DNA with the genus Streptomyces.
Moving on to the research that
could really help your garden, tomato plants with the PGPA39 bacteria were
grown in soil with added NaCl over a period of 60 days, as were plants without
the bacteria and without NaCl to serve as a control. The growth of the PGPA39
plants was significantly higher than that of the plants with salt but without
bacteria, and was just lower than the growth rate of the control. In addition
to this, the plants living with the bacteria had significantly higher biomass
than those without, very close to that of the control plants. This means that
even in high salt soil you could grow just as many tomatoes as your grandma
always did in her garden!
So, while it might be difficult
for you to get your hands on this exact strain of bacteria for your backyard
garden, know that the presence of bacteria can help grow heartier plants. These
bacteria could be helping the fruits and vegetables in your supermarket grow no
matter the conditions.
Palaniyandi, S.A., Damodharan, K., Yang, S.H. and Suh, J.W.
(2014), Streptomyces sp. strain PGPA39 alleviates salt stress
and promotes growth of ‘Micro Tom’ tomato plants. J Appl Microbiol, 117: 766–773.
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