Research Focus
Reduced terpene levels in cottonseed add food to fiber
Belinda J. Townsend
1,2
and Danny J. Llewellyn
1
1
CSIRO Plant Industry, GPO Box 1600, Canberra, ACT, 2601, Australia
2
Current address: Broom’s Barn Research Station, Rothamsted Research, Higham, Bury St Edmunds, Suffolk, IP28 6NP, UK
Using RNA interference (RNAi) technology, the levels of
a toxic phytoprotectant have recently been reduced
specifically in the seeds of cotton to generate a novel
dual-purpose crop. By engineering an endogenous ter-
pene pathway, there is now the exciting potential for an
added-value, genetically modified crop with the cash
value of the fiber supported by the improved nutritional
value and expanded food and feed use for the cotton-
seed, which is normally a low-value by-product.
Cotton as a major fiber crop
Farm management systems are under increasing pressure
to improve their efficiency and environmental effects in
response to global climate change, starvation in developing
countries, and the demands of consumers in developed
countries. Making the most out of every hectare of arable
land in a sustainable fashion is a growing challenge to
agriculture worldwide. Cotton is never far from criticism
owing to the intensive application of agricultural chemicals
and water required to maintain high yields of fiber. The
fiber is a valuable cash crop to millions of farmers world-
wide and a major export earner for many third World
countries.
Cotton fibers develop as specialized trichomes on the
epidermal layer of the developing seed, from which they
are harvested at maturity. Cottonseed is therefore an
important by-product of fiber production. Although high
in protein (23%) and oil (21%), the seed has limited uses
owing to the presence of gossypol, a toxic compound that
accumulates naturally in the plant. Gossypol renders cot-
tonseed unpalatable to humans and other monogastric
animals, although the seed has many other uses, for
example, as an edible oil after refining to remove gossypol,
as an animal feed for ruminants, and in numerous indus-
trial roles, such as the production of soaps and explosives.
A recent report from the laboratory of K. Rathore and
colleagues at Texas A&M University (http://www.tamu.
edu/) described the use of RNA interference (RNAi) in
transformed cotton plants to reduce specifically the final
concentration of gossypol in the seed to below the limits
approved by the World Health Organization (WHO),
thereby producing a new food source [1]. The gossypol-free
seed would require the same farming practices as those
currently used to produce fiber but with the prospect of
providing farmers in developing countries with both a cash
crop and a food source for the same input and equivalent
land area. The gossypol-free seed could also be used as a
feed for a wider range of livestock, such as pigs and poultry,
and would streamline the processing of seed for oil because
the gossypol need not be removed chemically.
Diverse roles for cotton terpenes in plant defense
Plants are sessile organisms that have adapted to produce
a diverse array of natural products to protect themselves
against pests, diseases and environmental pressures. Gos-
sypol is a terpenoid aldehyde present throughout the
cotton plant that acts as a phytoalexin (induced) and as
a phytoanticipin (constitutive) to protect against a variety
of microbial diseases and insect pests. It is the end-product
of the sesquiterpene pathway, which includes a suite of
other related bioactive compounds with similar roles
(Figure 1) [2]. Gossypol is sequestered within visible pig-
mented lysigenous glands unique to the tribe Gossypieae,
which includes cultivated cotton (Gossypium hirsutum).
Gossypol glands are found throughout the plant except in
the seed coat, xylem, stele tissue and young roots [3]. The
terpene composition varies in each plant tissue. However,
in non-green tissues gossypol is the predominant sesqui-
terpene, and in the seed it accumulates to 1.3% of the
seed dry weight [4,5].
Protection of aerial parts of the plant is largely afforded
by the heliocides – condensates of the sesquiterpenes with
monoterpenes – which are effective against caterpillar
pests such as Helicoverpa armigera and Heliothis virescens
[5]. Defense against bacterial and fungal diseases is par-
tially provided by the constitutive sesquiterpenes; how-
ever, pathogen challenge induces the rapid synthesis of
new terpenes [2]. A well-characterized example is the
induction of gossypol and precursors by Verticillium dah-
liae, which causes Verticillium wilt in cotton [5]. Aspergil-
lus flavus [6] and the bacterial blight pathogen,
Xanthomonas campestris pv. malvacearum (Xcm) [7], are
most sensitive to another sub-group of induced terpenes
called the cadinane-type sesquiterpenes. Gossypol and
related terpenes in many parts of the plant are therefore
essential for the survival of cotton under normal agricul-
tural conditions, where it is exposed to a variety of pests
and diseases, although its presence in the seed might be
expendable.
The challenge of producing gossypol-free seed
A glanded-plant–glandless-seed trait does occur naturally
in the native Australian cotton species Gossypium sturtia-
num. Gossypol-filled glands develop as the seeds germi-
nate, to provide the needed protection against pests and
pathogens. Efforts to breed this trait into cultivated cotton
have proven difficult owing to the considerable genome
Update
TRENDS in Biotechnology Vol.25 No.6
Corresponding author: Townsend, B.J. (belinda.townsend@bbsrc.ac.uk).
Available online 12 April 2007.
www.sciencedirect.com 0167-7799/$ – see front matter . Crown Copyright ß 2007 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2007.03.012