Spread of herbicide resistant weedy rice (red rice, Oryza sativa L.) after five years of Clearfield rice cultivation in Italy

The weedy relative of cultivated rice, red rice, can invade and severely infest rice fields, as reported by rice farmers throughout the world. Because of its close genetic relationship to commercial rice, red rice has proven difficult to control. Clearfield (Cl) varieties, which are resistant to the chemical group of AHAS (acetohydroxyacid synthase) inhibiting herbicides, provide a highly efficient opportunity to control red rice infestations; in order to reduce the risk of herbicide resistance spreading from cultivated rice to red rice, stewardship guidelines are regularly released. In Italy, the cultivation of Cl cultivars started in 2006. In the year 2010, surveillance of the possible escape of herbicide resistance was carried out. 168 red rice plants were sampled in 16 fields from 6 locations cultivated in 2010 with Cl and traditional cultivars. A first subsample of 119 plants was analysed by herbicide treatment and the presence of the resistance was found in 62 plants. 78 out of the same 119 plants were randomly selected and analysed at the level of the AHAS gene to search for the Cl mutation determining the resistant genotype: the Cl mutation was present in all the resistant plants. Nuclear and chloroplast microsatellite markers revealed a high correlation between genetic similarity and herbicide resistance. The results clearly show that Cl herbicide-resistant red rice plants are present in the field, showing genetic relationships with the Cl variety. Finding plants homozygous for the mutation suggests that the cross event occurred within the past few years and that these plants are in the F2 generation or further. These observations raise the possibility that Cl red rice is already moving with the cultivated rice seed.