Evidences for the Involvement of Auxin, Ethylene and ROS Signaling during Allelochemical Benzoic Acid-Mediated Primary Root Inhibition of Arabidopsis.

Allelopathy is mediated by plant-derived secondary metabolites (allelochemicals), which are released by donor plants, and affect the growth and development of receptor plants. The plant root is the first organ sensing soil allelochemicals that produces shorter primary root, but the mechanisms underlying this process remain elusive. Here, we reported that a model allelochemical benzoic acid (BA) inhibited primary root elongation of Arabidopsis seedlings by reducing the sizes of both the meristem and elongation zones and that auxin signaling affected this process. An increase in auxin level in the root tips was associated with increased expression of auxin biosynthesis genes and auxin polar transporter AUX1 and PIN2 under BA stress. Mutant analyses demonstrated that AUX1 and PIN2 rather than PIN1 were required for the inhibition of primary root elongation during BA exposure. Furthermore, BA stimulated ethylene evolution, whereas blocking BA-induced ethylene signaling with ethylene biosynthesis inhibitor (Co2+), ethylene perception antagonist (1-MCP) or ethylene signaling mutant lines etr1-3 and ein3eil1 compromised BA-mediated inhibition of root elongation, up-regulation of auxin biosynthesis-related genes and AUX1 and PIN2, indicating that ethylene signal was involved in auxin-mediated inhibition of primary root elongation during BA stress. Further analysis revealed that BA-induced reactive oxygen species (ROS) burst contributed to BA-mediated root growth inhibition without affecting auxin and ethylene signals. Taken together, our results reveal that allelochemical BA inhibits root elongation by increasing auxin accumulation by stimulating auxin biosynthesis and AUX1/PIN2-mediated auxin transport via stimulating ethylene production and an auxin/ethylene independent ROS burst.