Mutation in brassinosteroids biosynthesis or catabolism gene disrupts reactive oxygen species and auxin homeostasis during root growth in tomato

• DWF and CYP734A8 maintain ROS/auxin homeostasis which is essential for root growth. • SHAM restores root cell length in cyp734a8 and root meristem size in dwf mutant. • Kyn alleviates the decrease in root cell length in the cyp734a8 mutant. Brassinosteroids (BRs) regulate cell division and elongation. However, the precise roles of BRs metabolism in root development of horticultural plants remain unclear. Here, we studied the functions of the BRs catabolic gene CYP734A8 and biosynthesis gene DWF in tomato root growth. CYP734A8 and DWF expressed in the root tips. Loss of function of either CYP734A8 or DWF resulted in reduced growth of tomato primary roots. Additionally, mature cell length was decreased in cyp734a8 mutant. BRs biosynthesis inhibitor BRZ exaggerated inhibition of cell elongation in dwf mutant, but mitigated the root growth inhibition in cyp734a8 mutant by reversing the decreases in meristem zone (MZ) size and mature cell length. RNA-seq analysis revealed that the homeostasis of reactive oxygen species (ROS) and auxin were disturbed in cyp734a8 and dwf mutants. Specifically, the expression of auxin response genes were enhanced in cyp734a8 mutant, in association with higher auxin levels in root tips. Furthermore, ROS accumulation pattern was altered in cyp734a8 and dwf mutants. Notably, the inhibitor of peroxidase, SHAM, improved root growth in both mutants through increasing mature cell length in cyp734a8 and MZ size in dwf, respectively. The inhibition of auxin biosynthesis partially rescued the root elongation defects in cyp734a8 mutant. H 2 O 2 treatment restored root elongation in the cyp734a8 mutant in association with suppression of auxin response and transport genes. Collectively, our findings demonstrated that both CYP734A8 and DWF genes are essential for maintaining appropriate spatial-temporal BR signaling, which is critical for ROS and auxin patterning during tomato root growth.