Jacques Gasquez

The Dominance of the Herbicide Resistance Cost in Several Arabidopsis thaliana Mutant Lines

Resistance evolution depends upon the balance between advantage and disadvantage (cost) conferred in treated and untreated areas. By analyzing morphological characters and simple fitness components, the cost associated with each of eight herbicide resistance alleles (acetolactate synthase, cellulose synthase, and auxin-induced target genes) was studied in the model plant Arabidopsis thaliana. The use of allele-specific PCR to discriminate between heterozygous and homozygous plants was used to provide insights into the dominance of the resistance cost, a parameter rarely described. Morphological characters appear more sensitive than fitness (seed production) because 6 vs. 4 differences between resistant and sensitive homozygous plants were detected, respectively. Dominance levels for the fitness cost ranged from recessivity (csr1-1, ixr1-2, and axr1-3) to dominance (axr2-1) to underdominance (aux1-7). Furthermore, the dominance level of the herbicide resistance trait did not predict the dominance level of the cost of resistance. The relationship of our results to theoretical predictions of dominance and the consequences of fitness cost and its dominance in resistance management are discussed.

Modelling binary mixtures of herbicides in populations resistant to one of the components: evaluation for resistance management

BACKGROUND Herbicide mixtures are commonly proposed to delay the selection of herbicide resistance in susceptible populations (called the SM strategy). However, in practice, herbicide mixtures are often used when resistance to one of the two active ingredients has already been detected in the targeted population (called the RM strategy). It is doubtful whether such a practice can select against resistance, as the corresponding selection pressure is still exerted. As a consequence, the effect of mixtures on the evolution of an already detected resistance to one of the herbicides in the combination remains largely unexplored. In the present work, a simple model was developed to explore further the necessary and sufficient conditions under which a binary RM strategy might stabilise or even reduce resistance frequency. RESULTS Covering the hypothetical largest range of parameters, 39% of 9000 random simulations attest that the RM strategy might theoretically reduce resistance frequency. When strong enough, high genetic cost of resistance, negative cross-resistance between the herbicides associated in the mixture and reduced selection differential between resistant and susceptible plants can counterbalance the resistance advantage to one of the two applied herbicides. However, the required conditions for an RM strategy to ensure resistance containment in natural conditions seldom overlap with experimental parameter estimates given in the literature. CONCLUSION It is concluded that the sufficient conditions for an RM strategy to be effective would rarely be encountered. As a consequence, the strategy of formulating mixtures with herbicides for which resistance has already been detected should be avoided.

A multisite-cooperative research programme on risk assessment of transgenic crops

Genetically modified plants are now being commercialised in several countries as regulatory authorities consider that the balance of risk versus benefit is beneficial. However, numerous questions remain unanswered, especially the impact of these plants when used over large areas and under a range of variable environmental conditions. Some issues need to be re-evaluated [1, 2]. Risk/safety analysis, as well as prospects of transgenic crops depend on the scale which is to be considered. Extrapolation of methods, and laboratory and greenhouse results, to large-scale farmers’ fields, may provide useful preliminary data, but is not a sound approach to the study of the consequences of the commercial release of transgenic crops. Risk/safety analysis involves hazard identification and risk assessment. Hazards are scale-dependent and need to be tested on an appropriate scale. Change to a region’s flora, for instance, is not a matter for a greenhouse test, but must be studied at the regional level. Risks associated with identified hazards depend on local conditions, e.g. non-proportionality of pollen spread with source size, regional variation of crop management practices, interaction between crops, genotype variability of populations of wild relatives, etc. A main concern is to estimate the value of predictability for agriculture and environment of results collected on different scales. In order to answer this question, several institutes (Association Generale des Producteurs de Mais AGPM, Centre Technique Interprofessionnel des Oleagineux Metropolitains CETIOM, Institut National de la Recherche Agronomique INRA, Institut Technique de la Betterave ITB, Institut Technique des Cerelaes et des Fourrages ITCF), seed producers (KWS, Novartis) and agrochemical companies (Agrevo, Monsanto, Rhone-Poulenc) jointly designed an experiment on an agricultural scale. This was a multisite testing of several gene constructs with different crops rotating in farmers’ fields over several years, representing a true field situation.

MUTATION FOR TRIAZINE RESISTANCE WITHIN SUSCEPTIBLE POPULATIONS OF CHENOPODIUM ALBUM L.

The first reported resistant species in France is Chenopodium album which is also the most widespread species all over the world. There is only one genotype within each resistant population and this genotype is the only resistant biotype (R) in the region. But the R genotype may be different according to different regions. In order to understand how such resistant biotypes occur, many plants have been sampled within several susceptible populations which have never been treated with triazines. In almost all the populations, a high frequency (c. 3%) of mutated plants (I) appears among the offspring of some susceptible plants (Sp) grown without any selection pressure. Molecular analysis of ct DNA showed that the psbA gene of I plants carried the same mutation as typical R plants found in the field but when they are treated at seedling stage they are not so resistant as R plants. No heteroplasmy was detected within I plants. When I plants are treated with non-lethal doses at cotyledon stage their resultant progeny are resistant. This change is definitively inherited and is stable in further generations. The appearance of resistance depends on the presence of the particular Sp genotype only. So a R biotype will spread only in a population having this Sp biotype. As it could have been killed by previous treatments used in other crops, some fields or regions may be prone to have resistant populations while others may remain free from this problem for many years.