Effect of cytochrome P450s inhibitors on imidazolinone resistance in sunflower

Imidazolinone resistance genes have been introgressed from a sunflower wild population (PUR H. annuus L. collected in Kansas) to elite inbred lines. A digenic model has been proposed as the genetic basis of the inheritance of imidazolinone resistance. This model assumes that both parents must express two resistance genes, the major semidominant gene Imr1 and the modifier Imr2, to achieve complete resistance in the hybrids. Imr1 is an allelic variant of the AHAS1 locus that codes for the acetohydroxyacid synthase catalytic subunit. The effect of the modifier locus, Imr2, remains unknown and it could be related to non-target-site resistance. Cytochrome P450s-mediated herbicide metabolism has been identified as being responsible for the resistance to AHAS inhibitors in several species. The objective of this study was to evaluate the growth response to imazapyr in combination with P450s inhibitors in sunflower plantlets. • Two sunflower inbred lines were used in this study: HA425 and HA89, classified as resistant (Imr1Imr1Imr2Imr2) and susceptible (imr1imr1imr2imr2) respectively. Seeds were placed in plastic pots filled with commercial perlite and watered by capillarity with nutritive solution. After 7 days of incubation under controlled conditions plants were treated with 0 (control), 1, 10 and 100 μM imazapyr alone and in combination with P450s inhibitor. Two P450s inhibitors were assayed in independent experiments: piperonyl butoxide (PBO) and 1-aminobenzotriazole (ABT). After 15 days plantlets were collected and images were obtained by a scanner. Primary root length (RL) and the longest lateral root length (LRL) were measured from scanned images using ImageJ 1.44. Tomato Analyzer 3.0 software was used to measure shoot length (SL), foliar area (FA), and color of the first pair of leaves. The leaf color was estimated through the hue angle parameter. Three replications were done in each experiment and each replication consisted of 10 plants (total number of plants=960). Data were analyzed by two-way analysis of variance. • The reduction of hue angle with increasing doses of herbicide was associated with the appearance of leaf symptoms such as chlorosis and necrosis. The effect of imazapyr was significant (p<0.05) in both lines for all the evaluated variables but the resistant line decreased its growth at higher herbicide doses. The interaction between the effects of imazapyr and P450s inhibitors on LRL was significant (p<0.05) only for the resistant line. Furthermore, a significant interaction between the effects of imazapyr and PBO (p<0.05) was observed on FA in the resistant line. The largest differences in LRL and FA between absence and presence of P450s inhibitor occurred at the intermediate dose of herbicide (10 μM imazapyr). • The increased phytotoxicity of imazapyr in the resistant line when P450s inhibitor was present is aligning with suggestions previously made that P450s mediates a detoxification mechanism and contributes to herbicide resistance. This mechanism could be related to the effect of the modifier gene Imr2. • This study contributes to understanding the mechanisms of imidazolinone resistance in this species. These results encourage further experimentation on the molecular and biochemical levels to asses the role of P450s in endowing herbicide resistance.