Junie 2014
64
Molecular genotyping of resistantweeds
reveals status of ALS target site resistance
I
napreviousarticle, theconcept ofmakinguseofDNAmarkers to
identify thepresenceof specific target sitemutations inherbicide
resistantweedshasbeenexplained (
SAGraan/Grain
, September
2013, page 93). Making use of these DNA markers to identify
resistance has proven tobe a quickmethod for in-season screening
for target site resistance.
Sincemolecular genotypingmakes use of the plants’ DNA, it is also
diagnosticallyveryaccurate.Additionally, thesemarkerscan identify
cross mutation patterns and distinguish between homozygous
susceptible (wild type), heterozygous resistant (mutant) and
homozygous resistant (mutant) plants through specific targeted
codon mutations. Uniquely, this molecular genotyping allows for
the detection of target site resistance to different herbicide groups/
classeswithinasinglebulkedsampleor singleplant, simultaneously.
Throughout the pilot phase of this weed resistance allele profiling
(WRAP) service, theARC-SmallGrain Institute (SGI) askedproducers
to send in their bulked representative grass weed samples from
problematic fields for testingat nocost. Importantly, freshgreen leaf
material is essential for extracting high quality DNA, emphasising
theneed for us to receive thematerial as quickly as possible.
During this phase of marker optimisation, a few samples received
couldnot beuseddue to theirpoorwilted/diseased/damagedstatus.
The producers who sent these samples were asked to resend fresh
material for testing. Fully detailed diagnostic reports regarding
results and recommendations in support of herbicidemanagement
actions for specific fields were sent out to each producer. The
majorityof thesamples receivedwere ryegrass, followedby
Bromus
diandrus
(bromegrass) and
Avena fatua
(wildoats).
Results
Herbicides that inhibit Acetolactate Synthase (ALS) prevent the
normal production of amino acids. Amino acids are the “building
blocks” for proteins needed for optimal plant growth and
development. In the Herbicide Resistance Action Committee Chart,
Acetolactate Synthase (ALS) or AHAS inhibitors are classified as
Group B herbicides. This group includes the imidazolinones (IMI’s),
sulfonylureas (SU’s) and trialzolopyrimidine (TP’s) herbicides.
Currently five different target site markers, namely 122, 197, 205,
574 and 653 are optimised to identify mutations towards the ALS
herbicides. The target site mutation that was most common in
the received samples was ALS marker 197 (
Figure 1
). This marker
indicates the following resistance levels; Low to IMI’s, High to SU’s
and Low toTP’s.
In some samples, target sitemutations205 (high levels to IMI’s, SU’s
and TP’s), 574 (high levels of resistance to IMI’s, high to SU’s and
low to TP’s) and 653 (high levels only to IMI’s) were also observed
(
Table 1
). Target site mutation 122 which confers high levels of
resistance to IMI’s was not detected. The frequency of thesemuta-
tions detected in the total samples tested is indicated in
Graph 1
. A
summary of the results of all tested samples to date can be seen in
Table 1.
How to get your ryegrass tested for
resistance?
Producers are welcome to send ryegrass seedlings or seeds to
the ARC-SGI, Bethlehem, in preparation for the coming season.
Ideally, this service should be incorporated into herbicide spraying
schedules during the coming season. Specific diagnostic package
Herbicide resistantweeds /AcetolactateSynthase
Pest control
HESTIANIENABER
and
SCOTT SYDENHAM,
ARC-Small Grain Institute
ON FARM LEVEL
Graph1: Percentageof samples testing sensitive (wild type), heterozygous resistant andhomozygous resistant forALSmutations122, 197,
205, 574 and 653.
