Auxin plays an important role to mediates endogenous developmental signals and environmental cues and thus control root growth plasticity.Aluminum stress,which inhibits root growth,is one of the widespread environmental cues in acid (pH<5)soils.Our previous study showed that TAA1 was locally induced in the root-apex transition-zone thus involves in aluminum-induced Arabidopsis root growth inhibition.Here,we report that YUCCA (YUC),which encodes flavin monooxygenase-like proteins,regulates local auxin biosynthesis in the root apex transition zone (TZ) in response to AI stress.AI stress up-regulates the expression of YUCs in the root-apex TZ and YUC-regulated root growth inhibition in an ethylene signalling dependent manner.Ethylene-insensitive3 (EIN3) is involved into the direct regulation of YUC9 transcription in this process.Furthermore,we demonstrated that PHYTOCHROME INTERACTING FACTOR4 (PIF4) functions as a transcriptional activator for YUC5/8/9.PIF4 promotes AI-inhibited primary root growth by regulating the local expression of YUCs and auxin signal in the root-apex TZ.The AI-induced expression of PIF4 in TZ is downstream of ethylene signaling.Taken together,our results highlight a regulatory cascade for YUCs-regulated local auxin biosynthesis in the root-apex TZ mediating root growth inhibition in response to AI stress.