Natural Variation in ZmASB1 Links Tryptophan Metabolism to Root Growth and Drought Tolerance in Maize

ABSTRACT Root amino acid metabolism is important for maize adaptation to water deficit, but the genetic mechanisms underlying this relationship remain poorly understood. We profiled 18 free amino acids in the roots of 192 diverse maize inbred lines and integrated genome‐wide association analyses with root transcriptome data. This approach identified 118 metabolite QTLs and prioritized ZmASB1 , which encodes the β‐subunit of anthranilate synthase, as a candidate regulator of tryptophan (Trp) accumulation. A cis‐regulatory locus in the ZmASB1 promoter was associated with variation in ZmASB1 expression and root Trp abundance. Functional analyses indicated that ZmASB1 promotes Trp accumulation and root growth through Trp‐dependent auxin biosynthesis. ZmASB1 ‐overexpressing lines showed enhanced root development and drought tolerance, whereas the mutant exhibited the opposite phenotypes. Exogenous Trp or auxin partially rescued the root‐growth and drought‐sensitive phenotypes of the mutant. We identified a 5‐bp insertion/deletion (InDel‐178) in the ZmASB1 promoter that affects promoter activity by altering the binding of the transcription factor ZmDOF17. The favorable ZmASB1 InDel+ allele was enriched in modern maize inbred lines relative to teosinte and landraces. Together, these findings identify a natural cis‐regulatory mechanism controlling ZmASB1 expression and support a ZmASB1 ‐tryptophan‐auxin module that contributes to root development and drought responses in maize seedlings.