Unravelling the role of proline in glyphosate-mediated toxicity - tolerance mechanism or stress signal?

Glyphosate (GLY), the most widely used herbicide, has been regarded as an emergent environmental contaminant due to its constant and cumulative use, with potential harm to non-target organisms, such as crops, disrupting cells' redox balance. Therefore, plants need to fine-tune their antioxidant (AOX) mechanisms to thrive under GLY-contaminated environments. Proline overaccumulation is a common response in plants exposed to GLY, yet its role in GLY-induced toxicity remains unclear. Thus, this study explores whether Pro overaccumulation in response to GLY is perceived as a downstream tolerance mechanism or an early-warning stress signal. To investigate this, Arabidopsis thaliana T-DNA mutant lines for Pro biosynthetic (P5CS1) and catabolic genes (ProDH) were used and screened for their GLY susceptibility. Upon seedlings' exposure to GLY (0.75 mg L) for 14 days, the herbicide led to reduced biomass in all genotypes, accompanied by Pro overaccumulation. Mutants with heightened Pro levels (prodh) exhibited the greatest biomass reduction, increased lipid peroxidation (LP), and hydrogen peroxide (HO) levels, accompanied by a compromised performance of the AOX system. Conversely, p5cs1-4, mutants with lower Pro levels, demonstrated an enhanced AOX system activation, not only with increased levels of glutathione (GSH) and ascorbate (AsA), but also with increased activity of both ascorbate peroxidase (APX) and catalase (CAT). These findings suggest that Pro overaccumulation under GLY exposure is associated with stress sensitivity rather than tolerance, highlighting its potential as an early-warning signal for GLY toxicity in non-target plants and for detecting weed resistance.