Cysteine proteases are activated in sensitive Amaranthus palmeri populations upon treatment with herbicides inhibiting amino acid biosynthesis

The herbicides glyphosate and pyrithiobac inhibit the enzyme 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS) in the aromatic amino acid biosynthetic pathway and acetolactate synthase (ALS) in the branched‐chain amino acid biosynthetic pathway, respectively. Here we characterise the protease activity profiles of a sensitive (S), a glyphosate‐resistant (GR) and a multiple‐resistant (MR) population of Amaranthus palmeri in response to glyphosate and pyrithiobac. Amino acid accumulation and cysteine protease activities were induced with both herbicides in the S population and with pyrithiobac in the GR population, suggesting that the increase in cysteine proteases is responsible for the increased degradation of the available proteins and the observed increase in free amino acids. Herbicides did not induce any changes in the proteolytic activities in the populations with target‐site resistance, indicating that this effect was only induced in sensitive plants.