Plants coordinate their growth and developmental programs with various endogenous signals and environmental challenges. Phytochrome interacting factor 4 (PIF4) plays a critical positive role in thermoresponsive gene expression and hypocotyl growth in Arabidopsis, whereas early flowering 3 (ELF3) negatively regulates the activity of PIF4 at elevated temperatures. However, it is unknown how ELF3 activity is regulated at warm temperatures. Here, we report the identification of B-box 18 (BBX18) and BBX23 as important thermomorphogenesis regulators in Arabidopsis. BBX18 and BBX23 mutations result in reduced thermoresponsive hypocotyl elongation. In contrast, BBX18 overexpression promotes hypocotyl growth at elevated temperatures, which depends on either PIF4 or constitutive photomorphogenic 1 (COP1). BBX18 and BBX23 interact with ELF3 or COP1. Knocking out BBX18 and BBX23 increases ELF3 abundance under normal and warm temperature conditions. The expression of multiple thermoresponsive genes is impaired in both a PIF4 mutant and a BBX18/BBX23 double mutant. Thus, our findings reveal an important role of B-box proteins during thermomorphogenesis and provide insights into our understanding of how warm temperature signals regulate ELF3 activity and PIF4-dependent genes.

Summary: Xylem development is a process of xylem cell terminal differentiation that includes initial cell division, cell expansion, secondary cell wall formation and programmed cell death (PCD). PCD in plants and apoptosis in animals share many common characteristics. Caspase‐3, which displays Asp‐Glu‐Val‐Asp (DEVD) specificity, is a crucial executioner during animal cells apoptosis. Although a gene orthologous to caspase‐3 is absent in plants, caspase‐3‐like activity is involved in many cases of PCD and developmental processes. However, there is no direct evidence that caspase‐3‐like activity exists in xylem cell death. In this study, we showed that caspase‐3‐like activity is present and is associated with secondary xylem development in Populus tomentosa. The protease responsible for the caspase‐3‐like activity was purified from poplar secondary xylem using hydrophobic interaction chromatography (HIC), Q anion exchange chromatography and gel filtration chromatography. After identification by liquid chromatography‐tandem mass spectrometry (LC‐MS/MS), it was revealed that the 20S proteasome (20SP) was responsible for the caspase‐3‐like activity in secondary xylem development. In poplar 20SP, there are seven α subunits encoded by 12 genes and seven β subunits encoded by 12 genes. Pharmacological assays showed that Ac‐DEVD‐CHO, a caspase‐3 inhibitor, suppressed xylem differentiation in the veins of Arabidopsis cotyledons. Furthermore, clasto‐lactacystin β‐lactone, a proteasome inhibitor, inhibited PCD of tracheary element in a VND6‐induced Arabidopsis xylogenic culture. In conclusion, the 20S proteasome is responsible for caspase‐3‐like activity and is involved in xylem development.

Crop production faces persistent threats from insect-vector-borne viral diseases^1,2. Recent advancements have revealed the intricate immune mechanisms that plants deploy against viral pathogens^3,4,5,6,7,8. However, the molecular mechanisms through which plant hosts recognize viral infections and initiate antiviral defence at disease onset have not been elucidated. Here, through the natural infection of rice by inoculation with insect vectors carrying the natural forms of viruses, we show that viral coat proteins are perceived by the RING1–IBR–RING2-type ubiquitin ligase (RBRL), initiating the first step of the natural antiviral response in rice. RBRL subsequently targets an adaptor protein of the transcriptional repression complex of the jasmonate pathway, NOVEL INTERACTOR OF JAZ 3 (NINJA3), for degradation through the ubiquitination system, inducing jasmonate signalling and activating downstream antiviral defence. We further show that this phenomenon is a universal molecular mechanism used by rice plants to perceive viral infections and initiate antiviral signalling cascades. This approach is important not only for obtaining a deeper understanding of virus–host interactions but also for further disease resistance breeding.