Inhibition of O-acetylserine (thiol) lyase by S-benzyl-L-cysteine: Differential effects on sulfate assimilation and photosynthesis in maize (Zea mays) and sourgrass (Digitaria insularis)

The sulfate assimilation pathway is essential for plant metabolism and represents a promising herbicide target. O -Acetylserine (thiol) lyase, the final enzyme in this pathway, plays a key regulatory role. This study investigated the selective effects of S -benzyl-L-cysteine, a novel O -acetylserine (thiol) lyase inhibitor, on maize ( Zea mays ) and sourgrass ( Digitaria insularis ), a glyphosate-resistant weed. Seeds and seedlings were treated with S -benzyl-L-cysteine (0.0–5.0 mM), and germination, growth, O -acetylserine (thiol) lyase activity, and photosynthetic parameters were evaluated. Sourgrass was highly sensitive to S -benzyl-L-cysteine, with germination reduced by 53% at 2.0 mM and nearly abolished at 5.0 mM. Maize showed tolerance up to 2.0 mM, with growth suppression only at 5.0 mM. S -Benzyl-L-cysteine inhibited O -acetylserine (thiol) lyase activity in roots of both species, more intensely in sourgrass. In leaves, O -acetylserine (thiol) lyase activity increased only in maize, suggesting a compensatory response. Photosynthetic analyses revealed mild effects in maize but pronounced impairment in sourgrass. At 2.0 mM S -benzyl-L-cysteine, maize exhibited moderate reductions in CO 2 assimilation (−18%), stomatal conductance (−21%), transpiration (−16%), and the effective quantum yield of PSII (−15%), while intercellular CO 2 concentration, maximum assimilation rate, maximum carboxylation rate, maximum electron transport rate, and electron transport rate remained unaffected. In sourgrass, S -benzyl-L-cysteine caused sharp declines in CO 2 assimilation (−76%), stomatal conductance (−81%), transpiration (−79%), intercellular CO 2 concentration (−63%), maximum assimilation rate (−62%), maximum carboxylation rate (−72%), maximum electron transport rate (−67%), the effective quantum yield of PSII (−24%), electron transport rate (−46%), accompanied by a 25% increase in non-photochemical quenching. This is the first study to reveal species-specific responses to O -acetylserine (thiol) lyase inhibition in two C4 plants, uncovering sulfur metabolic plasticity as a basis for herbicide selectivity. These findings demonstrate that S -benzyl-L-cysteine selectively impairs sourgrass photosynthesis while sparing maize, likely by differentially affecting sulfur metabolism and photoprotection. O -Acetylserine (thiol) lyase thus emerges as a promising herbicide target. Further research is needed to validate S -benzyl-L-cysteine’s efficacy under field conditions. • S -Benzyl-L-cysteine alters sulfur metabolism in a species-specific manner. • Maize sustains growth and photosynthesis under S -benzyl-L-cysteine stress. • Sourgrass shows strong declines in growth and photosynthetic performance. • S -Benzyl-L-cysteine targets sourgrass, showing promise as a selective herbicide.