Parasitic
weeds of the genus Striga—also known as witchweeds—are a major threat to upland
rice production in several parts of Africa. The roots of Striga seedlings
penetrate the root cells of host plants, among them, rice, and leave them weak
and unproductive. AfricaRice and its partners are making significant headway in
the struggle to control Striga in rice fields across the continent.
Attractive
as it might seem to have a plethora of pink or orange flowers brightening up a
rice field, Striga is an extremely sinister bedfellow. These parasitic weeds
get their nutrients and water directly from the host plant’s root system and
the host plant’s energy is diverted to support the parasite. Heavy infestation
can result in complete crop failure.
Two
main species of Striga attack rice in Africa. Striga hermonthica is a problem
in Côte d’Ivoire, northern Nigeria, and Uganda. It infests about 40% of all
cereal-producing areas (including sorghum, millet, and maize) of sub-Saharan
Africa, causing US$7 to $13 billion in losses annually, according to figures
cited by Infonet Biovision. Striga asiatica is prevalent in Madagascar, Malawi,
and Tanzania and causes severe damage in rice, particularly in areas with
erratic rainfall and poor soil fertility.
Both
species are difficult to manage because the first 4 to 7 weeks of their life
cycle take place underground, inaccessible for mechanical control. Striga can
be controlled by herbicides, but effective and affordable herbicide
technologies are not yet available for rice farmers in Africa.
Fatal chemical attraction
Each
Striga plant is capable of producing up to 250,000 tiny seeds, which can remain
viable in the soil for many years. Striga seeds germinate only in the presence
of host-derived chemicals such as strigolactones, as this guarantees the
existence of a suitable host to parasitize. Rice roots exude such
strigolactones.
“The
very small seeds of Striga have very small energy reserves,” explains
AfricaRice weed scientist Jonne Rodenburg. “Hence, they have to tap into
host-plant resources very quickly.”
Getting at the root of the problem
As
part of a project funded by the UK Department for International Development and
the Biotechnology and Biological Sciences Research Council, and led by the
University of Sheffield, Dr. Rodenburg’s team field-screened 18 upland NERICA
varieties, their parents, and resistant and local checks for their resistance
to both species.
“From
work on sorghum and maize, it is known that durable resistance is hard to
find,” says Dr. Rodenburg. “Striga species are genetically highly variable, and
so they tend to overcome resistance based on a single mechanism very quickly.
It takes only one or a few plants able to circumvent the resistance to reinfest
a whole field over a few cropping seasons.”
AfricaRice
has partnered with Wageningen University to look at pre-attachment resistance.
Muhammad Jamil, a PhD student with Prof. Harro Bouwmeester, screened upland
NERICA varieties and their parents in the laboratory to identify and quantify
strigolactones.
Varieties
that produced significantly fewer strigolactones showed lower Striga
infestation, whereas those that produced the largest amounts of strigolactones
showed the most severe infestation (see box).
Meanwhile,
Mamadou Cissoko, a PhD student at the University of Sheffield, under the
supervision of Prof. Julie Scholes, was looking for post-attachment resistance
or mechanisms for preventing the development of Striga in rice after it has
germinated and attached to the roots.
A genetic block
“The
work going on in Sheffield is very exciting,” says Dr. Rodenburg. “They have
identified the chromosome carrying Striga resistance genes.” This could lead to
the identification of the first Striga resistance gene in any cereal crop (the only
Striga resistance gene currently known is in the legume cowpea).
“Thirty
years of research on Striga resistance in maize and sorghum have not brought
scientists as close as we seem to be after just a few years,” Dr. Rodenburg
says. “This will pave the way for targeted breeding using molecular markers.”
Marker-assisted
breeding has the potential to insert a single gene—in this case, the gene for
Striga resistance—into an already adapted and popular rice variety. This would
accelerate the process of making Striga-resistant rice available to farmers.
However, this step is still a few years down the road.
Sifting through the gene pool
In
the meantime, Dr. Rodenburg and his partners are excited by the findings of the
pre- and post-attachment resistance screening, and by the fact that some NERICA
varieties exhibit both resistance mechanisms and also show resistance in the
field against both Striga species.
“Rice
varieties (or breeding lines) that exhibit the complete range of pre- and
post-attachment and field resistance are just the sort of thing that we were
looking for,” Dr. Rodenburg says. “The next step in this process will be to
screen more adapted varieties and to test a subset of the resistant NERICA
varieties in participatory varietal selection trials.
“We
will do this in Uganda, where the need for S. hermonthica resistance is urgent,
and in Madagascar and Tanzania, in some of the most important hot spots for S.
asiatica in upland rice,” he adds.
Screening
work in Madagascar, which also includes promising local and advanced varieties,
is carried out in collaboration with FOFIFA (the Madagascan national program)
and the Centre de coopération internationale en recherche agronomique pour le
développement.
Understanding the invisible enemy
In
the future, AfricaRice will work with the University of Sheffield, Makerere
University, Kenyatta University, and CIAT on identifying multiple quantitative
trait loci (QTLs) and candidate resistance genes that underlie rice resistance
to different Striga species and ecotypes, and characterize—for the first
time—Striga loci that enable parasites to overcome specific host resistances.
Combined
with participatory varietal selection trials, this effort should validate and
enhance previous findings and make adapted cultivars with durable broad-based
resistance available to farmers.
More information
Jamil
M, Rodenburg J, Charnikhova T, Bouwmeester HJ. 2011. Pre-attachment Striga
hermonthica resistance of New Rice for Africa (NERICA) cultivars based on low
strigolactone production. New Phytol. 192(4):964-975.
