Dicotyledons lacking the multisubunit form of the herbicide-target enzyme acetyl coenzyme A carboxylase may be restricted to the family Geraniaceae

This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999 The aryloxyphenoxypropionate herbicide haloxyfop is transported in the phloem to the sink tissue where, in certain species, it disrupts the production of lipids that are essential for the functioning of membranes and organelles involved in the assimilation, partitioning and transport of carbon. Haloxyfop inhibits a key regulatory enzyme of lipid synthesis, acetyl coenzyme A carboxylase (ACCase), in species that lack a herbicide-insensitive multisubunit (MS) form of ACCase found in most plants. The absence of MS-ACCase, and sensitivity to haloxyfop, was considered to be restricted to monocotyledons in the family Gramineae but has recently been demonstrated for the dicotyledon Erodium moschatum (Geraniaceae). Species related to E. moschatum were examined to determine how widespread this phenomenon is among dicotyledons. In the two families most closely related to the Geraniaceae, four species in the Oxalidaceae and one species in the Tropaeolaceae respectively retained MS-ACCase. Within the family Geraniaceae, certain species in the genera Erodium and Pelargonium, but not those in the genus Geranium, have lost MS-ACCase, indicating that this phenomenon may be restricted to Erodium and Pelargonium. When treated with 104 g ai ha–1 haloxyfop-ethoxyethyl, plants of all 15 species retaining MS-ACCase were resistant while 8 out of 13 species lacking MS-ACCase were susceptible. It is noteworthy that five species lacking MS-ACCase were nonetheless resistant. The mechanism(s) of resistance in such species remains to be determined.