BES1 directly binds to the promoter of the ACC oxidase 1 gene to regulate gravitropic response in the roots of Arabidopsis thaliana

Maize ZmBES1/BZR1-1 transcription factor negatively regulates drought tolerance

Drought stress is a common abiotic factor and restricts plant growth and development. Exploring maize stress-related genes and their regulatory mechanisms is crucial for ensuring agricultural productivity and food security. The BRI1-EMS1 suppressor (BES1)/brassinazole-resistant 1 (BZR1) transcription factors (TFs) play important roles in plant growth, development, and stress response. However, maize ZmBES1/BZR1s are rarely reported. In the present study, the ZmBES1/BZR1-1 gene was cloned from maize B73 and functionally characterized in transgenic Arabidopsis and rice in drought stress response. The ZmBES1/BZR1-1 protein possessed a conserved bHLH domain characterized by BES1/BZR1 TFs, localized in the nucleus, and showed transcription activation activity. The expression of ZmBES1/BZR1-1 exhibited no tissue specificity but drought-inhibitory expression in maize. Under drought stress, overexpression of ZmBES1/BZR1-1 resulted in the enhancement of drought sensitivity of transgenic Arabidopsis and rice with a lower survival rate, reactive oxygen species (ROS) level and relative water content (RWC) and a higher stomatal aperture and relative electrolyte leakage (REL). The RNA-seq results showed that 56 differentially expressed genes (DEGs) were regulated by ZmBES1/BZR1-1 by binding to E-box elements in their promoters. The GO analysis showed that the DEGs were significantly annotated with response to oxidative stress and oxygen level. The study suggests that the ZmBES1/BZR1-1 gene negatively regulates drought stress, which provides insights into further underlying molecular mechanisms in the drought stress response mediated by BZR1/BES1s.

24-epibrassinolide improves cadmium tolerance and lateral root growth associated with regulating endogenous auxin and ethylene in Kentucky bluegrass

The application of phytohormones is a viable technique to increase the efficiency of phytoremediation in heavy metal-contaminated soils. The objective of this study was to determine how the application of 24-epibrassinolide (EBR), a brassinosteroid analog, could regulate root growth and tolerance to cadmium (Cd) stress in Kentucky bluegrass. As a result, the number of lateral root primordia and total root length in the Cd-treated seedlings decreased by 33.1 % and 56.5 %, respectively. After the application of EBR, Cd accumulation in roots and leaves, and the negative effect of Cd on root growth were reduced under Cd stress. Additionally, the expression of the brassinosteroid signaling gene PpBRI1 was significantly upregulated by exogenous EBR. Moreover, exogenous EBR upregulated the expression of genes encoding antioxidant enzymes and improved the activity of antioxidant enzymes, thereby reduced oxidative stress in roots. Finally, targeted hormonomics analysis highlighted the utility of the application of EBR to alleviate the effect of Cd on the reduction in auxin (IAA) content and the increase in ethylene (ACC) content. These were known to be associated with the upregulation in the expression of auxin biosynthesis gene PpYUCCA1 and downregulation in the expression of ethylene biosynthesis gene PpACO1 in the roots treated with Cd stress. Overall, the application of EBR alleviated Cd-induced oxidative stress in addition to improving root elongation and lateral root growth crosstalk with auxin and ethylene in Kentucky bluegrass subjected to Cd stress. This study further highlights the potential role of brassinosteroids in improving the efficiency of phytoremediation for Cd-contaminated soils.

Transcription factors DkBZR1/2 regulate cell wall degradation genes and ethylene biosynthesis genes during persimmon fruit ripening

BRASSINAZOLE RESISTANT (BZR) transcription factors are critical components of the brassinosteroid signalling pathway, but their possible roles in fruit ripening have rarely been reported. In this study, four BZR sequences were isolated from persimmon fruit. Among the four BZR genes, DkBZR1/2 were expressed in persimmon fruit; DkBZR1 protein amount decreased and dephosphorylated DkBZR2 gradually accumulated during the storage period. DkBZR1/2 proteins were localized in both the nucleus and cytoplasm and accumulated in the nucleus after 24-epibrassinolide treatment. DkBZR1 suppressed the transcription of Diospyros kaki endo-1,4-betaglucanase 1 (DkEGase1) and 1-aminocyclopropane-1-carboxylate synthase 1 (DkACS1) by binding to the BR response element (BRRE) in their promoters, and DkBZR2 activated the transcription of pectate lyase 1 (DkPL1) and 1-aminocyclopropane-1-carboxylate oxidase 2 (DkACO2) by binding to the E-box motif in their promoters. Transient overexpression of DkBZR2 promoted the conversion of acid-soluble pectin to water-soluble pectin and increased ethylene production in persimmon fruit. Our findings indicate that DkBZR1 and DkBZR2 serve as repressors and activators of persimmon fruit ripening, respectively.

Endogenous level of abscisic acid down-regulated by brassinosteroids signaling via BZR1 to control the growth of Arabidopsis thaliana

The increased level of endogenous abscisic acid (ABA) in brassinosteroid (BR)-deficient mutants, such as det2 and cyp85a1 × cyp85a2, suggests that ABA synthesis is inhibited by endogenous BRs in Arabidopsis thaliana. Expression of the ABA biosynthesis gene ABA-deficient 2 (ABA2) was negatively regulated by exogenously applied BR but up-regulated by the application of brassinazole and in det2 and cyp85a1 × cyp85a2. In addition, ABA2 expression decreased in bzr1-1D, showing that ABA biosynthesis is inhibited by BR signaling via BZR1, intermediated by ABA2, in Arabidopsis. Four cis-element sequences (E-boxes 1-4) in the putative promoter region of ABA2 were identified as BZR1 binding sites. The electrophoretic mobility shift assay and chromatin immune precipitation analysis demonstrated that BZR1 directly binds to overlapped E-boxes (E-box 3/4) in the promoter region of ABA2. The level of endogenous ABA was decreased in bzr1-1D compared to wild-type, indicating that binding of BZR1 to the ABA2 promoter inhibits ABA synthesis in Arabidopsis. Compared to wild-type, aba2-1 exhibited severely reduced growth and development. The abnormalities in aba2-1 were rescued by the application of ABA, suggesting that ABA2 expression and ABA synthesis are necessary for the normal growth and development of A. thaliana. Finally, bzr1-KO × aba2-1 exhibited inhibitory growth of primary roots compared to bzr1-KO, verifying that ABA2 is a downstream target of BZR1 in the plant. Taken together, the level of endogenous ABA is down-regulated by BR signaling via BZR1, controlling the growth of A. thaliana.

BES1 negatively regulates the expression of ACC oxidase 2 to control the endogenous level of ethylene in Arabidopsis thaliana

Quantitative reverse transcription PCR (qRT-PCR) analysis and ProACO2::GUS expression showed that ACO2 was highly expressed in the shoots of Arabidopsis seedlings under light conditions. Exogenously applied aminocyclopropane-1-carboxylic acid (ACC) enhanced the expression of ACO2, whereas Co²⁺ ions suppressed its expression. In comparison with wild-type seedlings, the ACO2 knockdown mutant aco2-1 produced less ethylene, which resulted in the inhibited growth of Arabidopsis seedlings. Exogenously applied brassinolide reduced the expression of ACO2. ACO2 expression was increased in det2, a brassinosteroid (BR)-deficient mutant; however, it was decreased in bes1-D, a brassinosteroid insensitive 1-EMS-suppressor 1 (BES1)-dominant mutant. In the putative promoter region of ACO2, 11 E-box sequences for BES1 binding but not BR regulatory element sequences for brassinazole-resistant 1 (BZR1) binding were found. Chromatin immunoprecipitation assay showed that BES1 could directly bind to the E-boxes located in the putative promoter region of ACO4. Less ethylene was produced in bes1-D seedlings compared with wild-type seedlings, suggesting that the direct binding of BES1 to the ACO2 promoter may negatively regulate ACO2 expression to control the endogenous level of ethylene in Arabidopsis seedlings.

The regulation of ethylene biosynthesis: a complex multilevel control circuitry

The gaseous plant hormone ethylene is produced by a fairly simple two-step biosynthesis route. Despite this pathway's simplicity, recent molecular and genetic studies have revealed that the regulation of ethylene biosynthesis is far more complex and occurs at different layers. Ethylene production is intimately linked with the homeostasis of its general precursor S-adenosyl-l-methionine (SAM), which experiences transcriptional and posttranslational control of its synthesising enzymes (SAM synthetase), as well as the metabolic flux through the adjacent Yang cycle. Ethylene biosynthesis continues from SAM by two dedicated enzymes: 1-aminocyclopropane-1-carboxylic (ACC) synthase (ACS) and ACC oxidase (ACO). Although the transcriptional dynamics of ACS and ACO have been well documented, the first transcription factors that control ACS and ACO expression have only recently been discovered. Both ACS and ACO display a type-specific posttranslational regulation that controls protein stability and activity. The nonproteinogenic amino acid ACC also shows a tight level of control through conjugation and translocation. Different players in ACC conjugation and transport have been identified over the years, however their molecular regulation and biological significance is unclear, yet relevant, as ACC can also signal independently of ethylene. In this review, we bring together historical reports and the latest findings on the complex regulation of the ethylene biosynthesis pathway in plants.

Multifaceted roles of HEAT SHOCK PROTEIN 90 molecular chaperones in plant development

HEAT SHOCK PROTEINS 90 (HSP90s) are molecular chaperones that mediate correct folding and stability of many client proteins. These chaperones act as master molecular hubs involved in multiple aspects of cellular and developmental signalling in diverse organisms. Moreover, environmental and genetic perturbations affect both HSP90s and their clients, leading to alterations of molecular networks determining respectively plant phenotypes and genotypes and contributing to a broad phenotypic plasticity. Although HSP90 interaction networks affecting the genetic basis of phenotypic variation and diversity have been thoroughly studied in animals, such studies are just starting to emerge in plants. Here, we summarize current knowledge and discuss HSP90 network functions in plant development and cellular homeostasis.

Brassinosteroids signaling via BZR1 down-regulates expression of ACC oxidase 4 to control growth of Arabidopsis thaliana seedlings

ProACO4-GUS expression and RT-PCR analysis revealed that ACO4 is predominantly expressed in shoots of Arabidopsis seedlings under light conditions. ACO4-overexpressed mutant 35S-ACO4 produced more ethylene relative to the wild-type, which resulted in reduced growth of Arabidopsis seedlings. The abnormal growth of seedlings recurred after the application of Co²⁺ ions, suggesting that ACO4 is a functional ACO necessary to regulate the growth and development of Arabidopsis seedlings. Exogenously-applied brassinosteroids (BRs) inhibited the expression of ACO4, and an enhanced ACO4 expression was found in det2, a BR-deficient mutant. Additionally, expression of ACO4 was decreased in bzr1-D (a BZR1-dominant mutant), implying that BR signaling negatively regulates ACO4 expression via BZR1 in Arabidopsis. In the intergenic region of ACO4, four E-boxes and a BR regulatory element (BRRE) are found. Electrophoretic mobility shift and chromatin immunoprecipitation assays showed that BZR1 binds directly to the BRRE in the putative promoter region of ACO4. By binding of BZR1 to BRRE, less ethylene was produced, which seems to regulate the growth and development of Arabidopsis seedlings.

Settling for Less: Do Statoliths Modulate Gravity Perception?

Plants orientate their growth either towards (in roots) or away from (in shoots) the Earth's gravitational field. While we are now starting to understand the molecular architecture of these gravity response pathways, the gravity receptor remains elusive. This perspective looks at the biology of statoliths and suggests it is conceivable that their immediate environment may be tuned to modulate the strength of the gravity response. It then suggests how mutant screens could use this hypothesis to identify the gravity receptor.