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 CO₂ assimilation (-18 %), stomatal conductance (-21 %), transpiration (-16 %), and the effective quantum yield of PSII (-15 %), while intercellular CO₂ 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 CO₂ assimilation (-76 %), stomatal conductance (-81 %), transpiration (-79 %), intercellular CO₂ 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.