Natural tolerance of Cuscuta campestris to herbicides inhibiting amino acid biosynthesis

The response of Cuscuta campestris Yuncker, a nonspecific above-ground holoparasite, to amino acid biosynthesis inhibitor (AABI) herbicides, was compared with other resistant and sensitive plants in dose response assays carried out in Petri dishes. Cuscuta campestris was found to be much more resistant to all AABI herbicides tested. The I-50 value of C. campestris growth inhibition by glyphosate was eightfold higher than that of transgenic, glyphosate-resistant cotton (RR-cotton). The I-50 value for C. campestris shoot growth inhibition by sulfometuron was above 500 muM, whereas that of sorghum roots was only 0.004 muM. Cuscuta campestris exposed to glyphosate gradually accumulated shikimate, confirming herbicide penetration into the parasite and interaction with an active form of the target enzyme of the herbicide, 5-enolpyruvylshikimate-3- phosphate synthase. More than half of the C. campestris plants associated with transgenic, glyphosate-resistant sugarbeet (RR-sugarbeet) treated with glyphosate or with transgenic, sulfometuron-resistant tomato (SuR-tomato) treated with sulfometuron recovered and resumed regular growth 20 - 30 days after treatment. New healthy stems developed, followed by normal flowering and seed setting. The results of the current study demonstrate the unique capacity of C. campestris to tolerate high rates of AABI. The mechanism of this phenomenon is yet to be elucidated.