Saponins are natural surfactants, or detergents (natural
with distinct foaming characteristics.
They are found in
many plants and are the plant's "immune system".
Natural soaps are an important weapon in the armory that
plants deploy to protect against disease attack, but a report
today, in the international journal Nature, describes how
disease-causing microbes can turn these plant defenses to their
own advantage. Scientists at the Sainsbury Laboratory (SL)
Norwich, UK, have discovered that fungi that attack tomatoes
break down the natural soaps that help protect the plant against
infection. Even worse for the plant, these breakdown products
then interfere with the internal communication system that the
plant relies on to actively fight off invaders.
“Our discovery shows that plants face a double whammy from
attacks by fungi”, said Dr Anne Osbourn (leader of the research
team at the SL). “The first line of defense is transformed into
a weapon that makes the plant unable to protect itself against
Many plants produce natural soaps (called saponins). These
chemicals are toxic to bacteria and fungi and so form part of
the plant’s protection against disease . Researchers have
known for some years that microbes that can successfully infect
saponin-containing plants often produce enzymes that break down
the saponins into less toxic chemicals. This enables the invader
to breach the plant’s first line of defense.
Working with tomato  and tobacco the SL scientists have shown
that saponin breakdown products interfere with essential
communication processes in the plant. Signaling pathways that
would normally set off the alarm system leading to the
activation of defense responses are disabled. So in overcoming
one line of defense the microbe also disrupts the plant’s
ability to trigger its other defense systems.
“A better understanding of how plants and their diseases
interact will eventually help scientists and breeders who are
trying to breed plants with improved natural disease resistance.
The next challenge for us is to find out how the saponin
breakdown products interfere with internal plant signaling
systems, and to establish how common this phenomenon is”,
concludes Dr Osbourn.
 The Sainsbury Laboratory has a worldwide reputation for
research on molecular plant-microbe interactions. The major aim
of the Laboratory is to pursue the fundamental processes
involved in the interactions of plants and their microbial
pathogens and symbionts. Funding for the Laboratory is primarily
through grants from a charitable foundation. In addition grants
are obtained from research councils, the European Union and
other organizations. The laboratory is located at the John Innes
Centre, Norwich, UK, which is an independent, world-leading
research centre in plant and microbial science.
 Many plants produce saponins that have anti-fungal and
anti-bacterial activity. The Solanacae typically contain
glycosylated steroidal and/or steroidal glycoalkaloid saponins.
The main saponin in tomato (Lycopersicon esculentum) is a-tomatine,
which has very strong anti-fungal activity.
The tomato leaf spot fungus (Septoria lycoperscii) will also
infect tobacco (Nicotiana benthamiana). S. lycoperscii produces
tomatinase, an extracellular enzyme, which hydrolyses glucose
from a-tomatine to form b2-tomatine. b2-tomatine has
significantly less anti-fungal activity than a-tomatine.
 In the early stages of infection tomato leaf spot (Septoria
lycoperscii) hyphae enter the plant leaf through the stomata and
grow among the mesophyll cells on the inside of the leaf.
However, mutants of S. lycoperscii, which were unable to produce
tomatinase, were found to be unable to invade tobacco leaves.
Attempted infections with the mutant lines induced severe cell
death around the infection site and molecular analysis
demonstrated that several genes known to be involved in defense
against disease were active. Neither of these responses was seen
in infections with the wild type fungus.
When researchers used gene-silencing technology to switch off
the STG1 gene in tobacco (which is involved in the signal
transmission pathway that leads to disease resistance) they
found these plants were susceptible to the mutant lines of S.
lycoperscii. This demonstrated that the effect of tomatinase is
not simply through de-toxification of a-tomatine, but that there
is interference with internal signaling associated with disease
In subsequent experiments a-tomatine, b2-tomatine or tomatinase
were infiltrated into normal tobacco leaves, which were then
challenge inoculated with wild type or mutant S. lycopersii. The
mutant lines (tomatinase-deficient) were only able to infect
leaves that had been infiltrated with b2-tomatine or tomatinase,
not a-tomatine. This demonstrated that the interference with
host defense is a result of the action of tomatinase and the
presence of b2-tomatine.
Source: John Innes Centre