Australian Herbicide Resistance Initiative (AHRI)

A double EPSPS gene mutation endowing glyphosate resistance shows a remarkably high resistance cost

This paper, ‘A double EPSPS gene mutation endowing glyphosate resistance shows a remarkably high resistance cost’ was published in Plant Cell & Environment, 2017, by Han, Vila-Aiub, Jalaludin, Yu & Powles.

This study was done with an Eleusine indica population originating from Malaysia that evolved resistance to glyphosate.  This population was studied by AHRI PhD research student Adam Jalaludin. He established that this population exhibits extremely high-level glyphosate resistance (Jalaludin et al, 2014, Weed Research).  Subsequent AHRI mechanistic studies found that this high-level glyphosate resistance is conferred by a double mutation within the EPSPS gene, known as TIPS, 102 Ile/106 Ser (Yu et al, 2015, Plant Physiology).  Thus far, this is the only known case of this novel glyphosate resistance endowing double TIPS mutation evolving in weed plants under glyphosate selection.

In studying the Eleusine with this TIPS double mutation it was visually evident that homozygous TIPS resistant plants did not grow as well as the wild-type susceptible plants. This led to detailed studies to establish if the TIPS double mutation imposed an ecological resistance fitness cost.  Much preliminary work was required to minimise genetic diversity, establish homozygosity, and so forth,  so that good fitness studies could be undertaken.  When competition studies were done with Eleusine homozygous for the 106 Serine mutation there was NO resistance cost of this 106 Serine mutation. However, in competition studies with Eleusine homozygous for the TIPS double mutation there is a large resistance cost.  When the competition was high, imposed by a rice crop, the resistance cost for the homozygous TIPS double mutation was even higher.  Intriguingly, this resistance cost did not occur with the compound heterozygous Rr TIPS plants.

Thus there is a clear expression of a significant resistance fitness cost associated with the homozygous TIPS glyphosate resistance-endowing double mutation in this glyphosate-resistant Eleusine population.  This is not due to the homozygous 106 Ser mutation alone, nor is there a resistance fitness cost in the compound heterozygous Rr TIPS plants. Clearly, in this glyphosate-resistant Eleusine population, while the TIPS homozygous double mutation gives high-level glyphosate resistance (high resistance benefit) this is offset by a high resistance cost.  If this large resistance cost is always associated with the TIPS double mutation this will limit the success of this double mutation even though it confers very high-level glyphosate resistance.

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