BBX21 integrates brassinosteroid biosynthesis and signalling in the inhibition of hypocotyl growth under shade

Abstract

B-Box-containing zinc finger transcription factors (BBX) are involved in light-mediated growth, affecting processes such as hypocotyl elongation in Arabidopsis thaliana. How-ever, the molecular and hormonal framework that regulates plant growth through BBX proteins is incomplete. Here, we demonstrate that BBX21 inhibits the hypocotyl elon-gation through the brassinosteroid (BR) pathway. BBX21 reduces the sensitivity to 24-epiBL, a synthetic active BR, principally at very low concentrations in simulated shade. The biosynthesis profile of BRs showed that two active BR— brassinolide and 28-homobrassinolide—and 8 of 11 inter-mediates can be repressed by BBX21 under white light (WL) or simulated shade. Furthermore, BBX21 represses the expression of CYTOCHROME P450 90B1 (DWF4/CYP90B1), BRASSINOSTEROID-6-OXIDASE 1 (BR6OX1, CYP85A1) and BR6OX2 (CYP85A2) genes involved in the BR biosynthesis in WL while specifically promoting DWF4 and PHYB ACTI-VATION TAGGED SUPPRESSOR 1 (CYP2B1/BAS1) expres-sion in WL supplemented with far-red (WL + FR), a treat-ment that simulates shade. In addition, BBX21 represses BR signaling genes, such as PACLOBUTRAZOL RESISTANCE1 (PRE1), PRE3 and ARABIDOPSIS MYB-LIKE 2 (MYBL2), and auxin-related and expansin genes, such as INDOLE-3-ACETIC ACID INDUCIBLE 1 (IAA1), IAA4 and EXPANSIN 11 in short-term shade. By a genetic approach, we found that BBX21 acts genetically upstream of BRASSINAZOLE-RESISTANT 1 (BZR1) for the promotion of DWF4 and BAS1 gene expres-sion in shade. We propose that BBX21 integrates the BR homeostasis and shade-light signaling, allowing the fine-tuning of hypocotyl elongation in Arabidopsis.