Linchuan Liu

The plant 26S proteasome is itself a regulatory target. Its abundance, composition, activity, and subcellular localisation are dynamically modulated by the very signalling pathways whose components or regulatory proteins it degrades. In this review, we draw on recent advances across immunity, abiotic stress, hormone signalling, organelle communication, and the proteasome's interplay with autophagy, together with the non-proteolytic functions of the 19S regulatory particle and apoplastic proteasomes, to establish the proteasome as a hub of regulatory plasticity. We argue that the proteasome operates not as a static degradation machine but as a proteolytic resource whose allocation across assembly states, subcellular compartments, and degradation pathways is continuously reconfigured. The central regulatory decision is therefore not which substrate is degraded, but how proteolytic capacity itself is distributed. This regulatory plasticity, and the nodes that mediate it, may inform the development of strategies to enhance proteolytic capacity for crop resilience.

The unfolded protein response (UPR) signaling pathway is activated by the accumulation of misfolded proteins in the endoplasmic reticulum (ER) and stimulates the production of ER chaperones to restore ER proteostasis. However, how UPR-induced proteins return to their pre-stress levels upon removal of ER stress remains unknown. TUNICAMYCIN-INDUCED 1 (TIN1) is an Arabidopsis (Arabidopsis thaliana) protein that is normally expressed in pollen but is rapidly induced by ER stress in vegetative tissues. Here, we show that the ER-stress-induced TIN1 is rapidly degraded in the UPR recovery phase. We found that TIN1 degradation depends on its asparagine-linked glycans and requires both EMS-mutagenized bri1 suppressor 5 (EBS5) and EBS6 for its recruitment to the ER-associated degradation (ERAD) complex. Loss-of-function mutations in the core component of this Arabidopsis ERAD complex greatly stabilize TIN1. Interestingly, 2 other UPR-induced proteins that are coexpressed with TIN1 remained stable upon ER-stress removal, suggesting that rapid degradation during the stress-recovery phase likely applies to a subset of UPR-induced proteins. Further investigation is needed to uncover the mechanisms by which the ERAD machinery selectively degrades UPR-induced ER proteins.

Bri1-EMS Suppressor 1 (BES1) and Brassinazole Resistant 1 (BZR1) are two key transcription factors in the brassinosteroid (BR) signaling pathway, serving as crucial integrators that connect various signaling pathways in plants. Extensive genetic and biochemical studies have revealed that BES1 and BZR1, along with other protein factors, form a complex interaction network that governs plant growth, development, and stress tolerance. Among the interactome of BES1 and BZR1, several proteins involved in posttranslational modifications play a key role in modifying the stability, abundance, and transcriptional activity of BES1 and BZR1. This review specifically focuses on the functions and regulatory mechanisms of BES1 and BZR1 protein interactors that are not involved in the posttranslational modifications but are crucial in specific growth and development stages and stress responses. By highlighting the significance of the BZR1 and BES1 interactome, this review sheds light on how it optimizes plant growth, development, and stress responses.

Maintenance of proteome integrity is essential for cell function and survival in changing cellular and environmental conditions. The endoplasmic reticulum (ER) is the major site for the synthesis of secretory and membrane proteins. However, the accumulation of unfolded or misfolded proteins can perturb ER protein homeostasis, leading to ER stress and compromising cellular function. Eukaryotic organisms have evolved sophisticated and conserved protein quality control systems to ensure protein folding fidelity via the unfolded protein response (UPR) and to eliminate potentially harmful proteins via ER-associated degradation (ERAD) and ER-phagy. In this review, we summarize recent advances in our understanding of the mechanisms of ER protein homeostasis in plants and discuss the crosstalk between different quality control systems. Finally, we will address unanswered questions in this field.

The biosynthesis of C_27–29 sterols from their C₃₀ precursor squalene involves C24-alkylation and the removal of three methyl groups, including two at the C4 position. The two C4 demethylation reactions require a bifunctional enzyme known as 3β-hydroxysteroid dehydrogenase/C4-decarboxylase (3βHSD/D), which removes an oxidized methyl (carboxylic) group at C4 while simultaneously catalyzing the 3β-hydroxyl→3-keto oxidation. Its loss-of-function mutations cause ergosterol-dependent growth in yeast and congenital hemidysplasia with ichthyosiform erythroderma and limb defect (CHILD) syndrome in humans. Although plant 3βHSD/D enzymes were well studied enzymatically, their developmental functions remain unknown. Here we employed a CRISPR/Cas9-based genome-editing approach to generate knockout mutants for two Arabidopsis 3βHSD/D genes, HSD1 and HSD2, and discovered the male gametophytic lethality for the hsd1 hsd2 double mutation. Pollen-specific expression of HSD2 in the heterozygous hsd1 hsd2/+ mutant not only rescued the pollen lethality but also revealed the critical roles of the two HSD genes in embryogenesis. Our study thus demonstrated the essential functions of the two Arabidopsis 3βHSD/D genes in male gametogenesis and embryogenesis.

The biosynthesis of C27-29 sterols from their C30 precursor squalene involves C24-alkylation and removal of three methyl groups, including two at the C4 position. The two C4 demethylation reactions require a bifunctional enzyme known as 3β-hydroxysteroid dehydrogenase/C4-decarboxylase (3βHSD/D) that removes an oxidized methyl (carboxylic) group at C4 while simultaneously catalyzing the 3β-hydroxyl3-keto oxidation. Its loss-of-function mutations cause ergosterol-dependent growth in yeast and congenital hemidysplasia with ichthyosiform erythroderma and limb defect (CHILD) syndrome in humans. Although plant 3βHSD/D enzymes were well studied enzymatically, their developmental functions remain unknown. Here we employed a CRISPR/Cas9-based genome-editing approach to generate knockout mutants for two Arabidopsis 3βHSD/D genes, HSD1 and HSD2, and discovered the male gametophytic lethality for the hsd1 hsd2 double mutation. Pollen-specific expression of HSD2 in the heterozygous hsd1 hsd2/+ mutant not only rescued the pollen lethality but also revealed critical roles of the two HSD genes in embryogenesis. Our study thus demonstrated essential functions of the two Arabidopsis 3βHSD/D genes in male gametogenesis and embryogenesis.

Brassinosteroids (BRs) play vital roles in the plant life cycle and synthetic BRs are widely used to increase crop yield and plant stress tolerance. Among them are 24R-methyl-epibrassinolide (24-EBL) and 24S-ethyl-28-homobrassinolide (28-HBL), which differ from brassinolide (BL, the most active BR) at the C-24 position. Although it is well known that 24-EBL is 10% active as BL, there is no consensus on the bioactivity of 28-HBL. A recent outpouring of research interest in 28-HBL on major crops accompanied with a surge of industrial-scale synthesis that produces mixtures of active (22R,23R)-28-HBL and inactive (22S,23S)-28HBL, demands a standardized assay system capable of analyzing different synthetic “28-HBL” products. In this study, the relative bioactivity of 28-HBL to BL and 24-EBL, including its capacity to induce the well-established BR responses at molecular, biochemical, and physiological levels, was systematically analyzed using the whole seedlings of the wild-type and BR-deficient mutant of Arabidopsis thaliana. These multi-level bioassays consistently showed that 28-HBL exhibits a much stronger bioactivity than 24-EBL and is almost as active as BL in rescuing the short hypocotyl phenotype of the dark-grown det2 mutant. These results are consistent with the previously established structure–activity relationship of BRs, proving that this multi-level whole seedling bioassay system could be used to analyze different batches of industrially produced 28-HBL or other BL analogs to ensure the full potential of BRs in modern agriculture.