ECAP is a key negative regulator mediating different pathways to modulate salt stress-induced anthocyanin biosynthesis in Arabidopsis

Ethylene is the key phytohormone to enhance arsenic resistance in Arabidopsis thaliana

The toxic metalloid arsenic is prevalent in the environment and poses a threat to nearly all organisms. However, the mechanism by which phytohormones modulate arsenic resistance is not well-understood. Therefore, we analyzed multiple phytohormones based on the results of transcriptome sequencing, content changes, and related mutant growth under arsenic stress. We found that ethylene was the key phytohormone in Arabidopsis thaliana response to arsenic. Further investigation showed the ethylene-overproducing mutant eto1-1 generated less malondialdehyde (MDA), H2O2, and O2•- under arsenic stress compared to wild-type, while the ethylene-insensitive mutant ein2-5 displayed opposite patterns. Compared to wild-type, eto1-1 accumulated a smaller amount of arsenic and a larger amount of non-protein thiols. Additionally, the immediate ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), enhanced resistance to arsenic in wide-type, but not in mutants with impaired detoxification capability (i.e., cad1-3, pad2-1, abcc1abcc2), which confirmed that ethylene regulated arsenic detoxification by enhancing arsenic chelation. ACC also upregulated the expression of gene(s) involved in arsenic detoxification, among which ABCC2 was directly transcriptionally activated by the ethylene master transcription factor ethylene-insensitive 3 (EIN3). Overall, our study shows that ethylene is the key phytohormone to enhance arsenic resistance by reducing arsenic accumulation and promoting arsenic detoxification at both physiological and molecular levels.

The adaptor protein ECAP, the corepressor LEUNIG, and the transcription factor BEH3 interact and regulate microsporocyte generation in Arabidopsis

Histospecification and morphogenesis of anthers during development in Arabidopsis (Arabidopsis thaliana) are well understood. However, the regulatory mechanism of microsporocyte generation at the pre-meiotic stage remains unclear, especially how archesporial cells are specified and differentiate into 2 cell lineages with distinct developmental fates. SPOROCYTELESS (SPL) is a key reproductive gene that is activated during early anther development and remains active. In this study, we demonstrated that the EAR motif-containing adaptor protein (ECAP) interacts with the Gro/Tup1 family corepressor LEUNIG (LUG) and the BES1/BZR1 HOMOLOG3 (BEH3) transcription factor to form a transcription activator complex, epigenetically regulating SPL transcription. SPL participates in microsporocyte generation by modulating the specification of archesporial cells and the archesporial cell-derived differentiation of somatic and reproductive cell layers. This study illustrates the regulation of SPL expression by the ECAP-LUG-BEH3 complex, which is essential for the generation of microsporocytes. Moreover, our findings identified ECAP as a key transcription regulator that can combine with different partners to regulate gene expression in distinct ways, thereby facilitating diverse processes in various aspects of plant development.

Jasmonate signaling pathway confers salt tolerance through a NUCLEAR FACTOR-Y trimeric transcription factor complex in Arabidopsis

Jasmonate (JA) is a well-known phytohormone essential for plant response to biotic stress. Recently, a crucial role of JA signaling in salt resistance has been highlighted; however, the specific regulatory mechanism remains largely unknown. In this study, we found that the NUCLEAR FACTOR-Y (NF-Y) subunits NF-YA1, NF-YB2, and NF-YC9 form a trimeric complex that positively regulates the expression of salinity-responsive genes, whereas JASMONATE-ZIM DOMAIN protein 8 (JAZ8) directly interacts with three subunits and acts as the key repressor to suppress both the assembly of the NF-YA1-YB2-YC9 trimeric complex and the transcriptional activation activity of the complex. When plants encounter high salinity, JA levels are elevated and perceived by the CORONATINE INSENSITIVE (COI) 1 receptor, leading to the degradation of JAZ8 via the 26S proteasome pathway, thereby releasing the activity of the NF-YA1-YB2-YC9 complex, initiating the activation of salinity-responsive genes, such as MYB75, and thus enhancing the salinity tolerance of plants.

RHA2b-mediated MYB30 degradation facilitates MYB75-regulated, sucrose-induced anthocyanin biosynthesis in Arabidopsis seedlings

Anthocyanins play diverse roles in plant physiology and stress adaptation. In Arabidopsis, the MYB-bHLH-WD40 (MBW) complex has a crucial role in the regulation of anthocyanin synthesis. Here, we report that the R2R3-MYB transcription factor MYB30 and the ubiquitin E3 ligase RHA2b participate in anthocyanin biosynthesis through regulation of the MBW complex. MYB30 was found to negatively regulate sucrose-induced anthocyanin biosynthesis in Arabidopsis seedlings. Expression of multiple genes involved in flavonoid or anthocyanin biosynthesis was affected in the myb30 mutant, and MYB30 directly repressed the expression of MYB75, which encodes a core component of the MBW complex, by binding to its promoter. Moreover, MYB30 physically interacted with MYB75 to inhibit its activity by repressing MBW complex assembly. In addition, sucrose treatment significantly promoted MYB30 degradation via the action of RHA2b. The ubiquitination and degradation of MYB30 were significantly attenuated in the rha2b mutant under high-sucrose treatment, and further analysis showed that MYB75 directly promoted RHA2b expression in response to high sucrose. Our work thus reveals an anthocyanin biosynthetic regulatory module, RHA2b-MYB30, that controls the function of the MBW complex via MYB75. The repression of MYB75 by MYB30 is released by MYB75-induced RHA2b expression, thus ensuring the self-activation of MYB75 when anthocyanin synthesis is needed.

ABA functions in low phosphate-induced anthocyanin accumulation through the transcription factor ABI5 in Arabidopsis

KEY MESSAGE

ABI5 functions in ABA-mediated anthocyanin accumulation in plant response to low phosphate. Low phosphate (LP)-induced anthocyanin biosynthesis and accumulation play an important role in plant adaptive response to phosphate starvation conditions. However, whether and how the stress phytohormone abscisic acid (ABA) participates in LP-induced anthocyanin accumulation remain elusive. Here, we report that ABA is required for LP-induced anthocyanin accumulation in Arabidopsis thaliana. Disrupting ABA DEFICIENT2 (ABA2), a key ABA-biosynthetic gene, or BETA-GLUCOSIDASE1 (BG1), a major gene implicated in converting conjugated ABA to active ABA, significantly impairs LP-induced anthocyanin accumulation, as LP-induced expression of the anthocyanin-biosynthetic genes Chalcone Synthase (CHS) is dampened in the aba2 and bg1 mutant. In addition, LP-induced anthocyanin accumulation is defective in the mutants of ABA signaling pathway, including ABA receptors, ABA Insensitive2, and the transcription factors ABA Insensitive5 (ABI5), suggesting a role of ABI5 in ABA-mediated upregulation of anthocyanin-biosynthetic genes in plant response to LP. Indeed, LP-induced expression of CHS is repressed in the abi5-7 mutant but further promoted in the ABI5-overexpressing plants compared to the wild-type. Moreover, ABI5 can bind to and transcriptionally activate CHS, and the defectiveness of LP-induced anthocyanin accumulation in abi5-7 can be restored by overexpressing CHS. Collectively, our findings illustrates that ABI5 functions in ABA-mediated LP-induced anthocyanin accumulation in Arabidopsis.

Arabidopsis MYB21 Negatively Regulates KTN1 to Fine-Tune the Filament Elongation

The growth process of the stamen filament is crucial for plant reproduction. However, the molecular mechanisms underlying the regulation of filament growth remain largely unclear. Our study has identified that MYB21 is involved in the regulation of filament growth in Arabidopsis. In comparison to the wild type, the cell length of the filaments is notably reduced in the myb21 mutant. Moreover, we found that KTN1, which encodes a microtubule-severing enzyme, is significantly upregulated in the myb21 mutant. Additionally, yeast one-hybrid assays demonstrated that MYB21 can bind to the promoter region of KTN1, suggesting that MYB21 might directly regulate the expression of KTN1. Finally, transcriptional activity experiments showed that MYB21 is capable of suppressing the driving activity of the KTN1 promoter. This study indicates that the MYB21-KTN1 module may play a precise regulatory role in the growth of Arabidopsis filament cells.

Environmental Stimuli and Phytohormones in Anthocyanin Biosynthesis: A Comprehensive Review

Anthocyanin accumulation in plants plays important roles in plant growth and development, as well as the response to environmental stresses. Anthocyanins have antioxidant properties and play an important role in maintaining the reactive oxygen species (ROS) homeostasis in plant cells. Furthermore, anthocyanins also act as a "sunscreen", reducing the damage caused by ultraviolet radiation under high-light conditions. The biosynthesis of anthocyanin in plants is mainly regulated by an MYB-bHLH-WD40 (MBW) complex. In recent years, many new regulators in different signals involved in anthocyanin biosynthesis were identified. This review focuses on the regulation network mediated by different environmental factors (such as light, salinity, drought, and cold stresses) and phytohormones (such as jasmonate, abscisic acid, salicylic acid, ethylene, brassinosteroid, strigolactone, cytokinin, and auxin). We also discuss the potential application value of anthocyanin in agriculture, horticulture, and the food industry.

The Role of Anthocyanins in Plant Tolerance to Drought and Salt Stresses

Drought and salinity affect various biochemical and physiological processes in plants, inhibit plant growth, and significantly reduce productivity. The anthocyanin biosynthesis system represents one of the plant stress-tolerance mechanisms, activated by surplus reactive oxygen species. Anthocyanins act as ROS scavengers, protecting plants from oxidative damage and enhancing their sustainability. In this review, we focus on molecular and biochemical mechanisms underlying the role of anthocyanins in acquired tolerance to drought and salt stresses. Also, we discuss the role of abscisic acid and the abscisic-acid-miRNA156 regulatory node in the regulation of drought-induced anthocyanin production. Additionally, we summarise the available knowledge on transcription factors involved in anthocyanin biosynthesis and development of salt and drought tolerance. Finally, we discuss recent progress in the application of modern gene manipulation technologies in the development of anthocyanin-enriched plants with enhanced tolerance to drought and salt stresses.

MsMYB741 is involved in alfalfa resistance to aluminum stress by regulating flavonoid biosynthesis

Aluminum (Al) toxicity severely restricts plant growth in acidic soils (pH < 5.0). In this study, an R2R3-MYB transcription factor (TF) gene, MsMYB741, was cloned from alfalfa. Its function and gene regulatory pathways were studied via overexpression and RNA interference of MsMYB741 in alfalfa seedlings. Results showed that root elongation increased as a result of MsMYB741 overexpression (MsMYB741-OE) and decreased with MsMYB741 RNA interference (MsMYB741-RNAi) in alfalfa seedlings compared with the wild-type under Al stress. These were attributed to the reduced Al content in MsMYB741-OE lines, and increased Al content in MsMYB741-RNAi lines. MsMYB741 positively activated the expression of phenylalanine ammonia-lyase 1 (MsPAL1) and chalcone isomerase (MsCHI) by binding to MYB and ABRE elements in their promoters, respectively, which directly affected flavonoid accumulation in roots and secretion from root tips in plants under Al stress, eventually affecting Al accumulation in alfalfa. Additionally, MsABF2 TF directly activated the expression of MsMYB741 by binding to the ABRE element in its promoter. Taken together, our results indicate that MsMYB741 transcriptionally activates MsPAL1 and MsCHI expression to increase flavonoid accumulation in roots and secretion from root tips, leading to increased resistance of alfalfa to Al stress.

Transcription Factor SlAREB1 Is Involved in the Antioxidant Regulation under Saline–Alkaline Stress in Tomato

Basic leucine zipper (bZIP) transcription factors of the ABA-responsive element binding factor/ABA-responsive element binding proteins (ABF/AREB) subfamily have been implicated in abscisic acid (ABA) and abiotic stress responses in plants. However, the specific function of ABF/AREB transcription factors under saline–alkaline stress is unclear. Here, we identified four ABF/AREB transcription factors in tomato and found that SlAREB1 strongly responded to both ABA and saline–alkaline stress. To further explore the function of SlAREB1 under saline–alkaline stress, SlAREB1-overexpressing lines were constructed. Compared with wild-type plants, SlAREB1-overexpressing transgenic tomato plants showed reduced malondialdehyde content, increased the relative water content, and alleviated the degradation of chlorophyll under saline–alkaline stress. Importantly, SlAREB1 directly physically interacted with SlMn-SOD, which improved the activity of antioxidant enzymes and increased the scavenging of excess reactive oxygen species. Overall, the overexpression of SlAREB1 increased the antioxidant capacity of the transgenic tomato under saline–alkaline stress.

Environmental and Genetic Factors Involved in Plant Protection-Associated Secondary Metabolite Biosynthesis Pathways

Plant specialized metabolites (PSMs) play essential roles in the adaptation to harsh environments and function in plant defense responses. PSMs act as key components of defense-related signaling pathways and trigger the extensive expression of defense-related genes. In addition, PSMs serve as antioxidants, participating in the scavenging of rapidly rising reactive oxygen species, and as chelators, participating in the chelation of toxins under stress conditions. PSMs include nitrogen-containing chemical compounds, terpenoids/isoprenoids, and phenolics. Each category of secondary metabolites has a specific biosynthetic pathway, including precursors, intermediates, and end products. The basic biosynthetic pathways of representative PSMs are summarized, providing potential target enzymes of stress-mediated regulation and responses. Multiple metabolic pathways share the same origin, and the common enzymes are frequently to be the targets of metabolic regulation. Most biosynthetic pathways are controlled by different environmental and genetic factors. Here, we summarized the effects of environmental factors, including abiotic and biotic stresses, on PSM biosynthesis in various plants. We also discuss the positive and negative transcription factors involved in various PSM biosynthetic pathways. The potential target genes of the stress-related transcription factors were also summarized. We further found that the downstream targets of these Transcription factors (TFs) are frequently enriched in the synthesis pathway of precursors, suggesting an effective role of precursors in enhancing of terminal products. The present review provides valuable insights regarding screening targets and regulators involved in PSM-mediated plant protection in non-model plants.