Brassinosteroid signaling regulates PIN5/10-mediated intracellular auxin homeostasis to promote lateral root development in tomato.

Auxin plays a central role in lateral root (LR) development, with other hormones modulating this process by interacting with auxin. However, the interplay between brassinosteroids (BRs) and auxin in LR development remains poorly understood. Here, we demonstrate that PIN5 and PIN10 are involved in BR-mediated LR development in tomato. LR formation is impaired in the tomato BR biosynthesis mutant dwf and the signaling mutant bzr1, which is defective in the key transcription factor BZR1. Treatment with 28-homobrassinolide (HBR) increases the density of LRs (including emerged and unemerged), as does BZR1 overexpression. Further analysis revealed that BZR1 regulates the auxin response and directly enhances the transcription of PIN5 and PIN10, which are expressed in LR primordia and the root apical meristem. PIN5 and PIN10 regulate the balance of free and conjugated auxin levels, presumably by controlling auxin transport from the endoplasmic reticulum to the nucleus. Mutation in PIN5 or PIN10 reduces nuclear auxin import and impairs auxin signaling, impeding BR-induced LR formation. Molecular docking and isotopic yeast assays confirmed that PIN5 and PIN10 function as bona fide auxin transporters. Collectively, these results show that BR signaling promotes LR development by transcriptionally activating the expression of PIN5 and PIN10, which are essential for maintaining auxin homeostasis. These findings advance our understanding of the regulatory networks that control root development, providing insights for improving agricultural productivity.