CmRCD1 represses flowering by directly interacting with CmBBX8 in summer chrysanthemum

The B-box protein CmBBX8 recruits chromatin modifiers CmFDM2/CmSWI3B to induce flowering in summer chrysanthemum

The transition from vegetative to reproductive growth is essential for the flowering process of plants. In summer chrysanthemum, CmBBX8 exploits prominence function in floral transition by activating the expression of CmFTL1. However, how CmBBX8 induces CmFTL1 during the photoperiod inductive cycles remains unknown. Here, we show that CmBBX8 interacts with the SGS3-like protein CmFDM2, and the CmFDM2 overexpression strains presented early flowering, while knockdown strains delayed flowering. Additionally, CmFDM2 could bind to the CmFTL1 promoter and activate the expression of CmFTL1, and associate with chromatin remodeling factor CmSWI3B, and CmBBX8 induces flowering dependent on CmFDM2 and CmSWI3B. CmFDM2 also partially depends on CmSWI3B. The CmSWI3B knockdown strains exhibited a significant late flowering phenotype. Interestingly, CmBBX8 also interacts with CmSWI3B. Moreover, the level of H3K27me3 at the CmFTL1 locus was reduced when CmBBX8 and CmFDM2/CmSWI3B occupied the locus to promote chrysanthemum flowering during the photoperiod inductive cycles, which was accompanied by the increasing level of CmFTL1 transcripts. Thus, our work provides novel insights into the gradually increasing level of CmFTL1 for the floral transition through CmBBX8 recruiting chromatin modifiers CmFDM2/CmSWI3B.

The rice microRNA159-SPOROCYTELESS EAR2 module regulates starch biosynthesis during pollen development and maintains male fertility

Starch is an indispensable energy reserve for pollen and failure of starch biosynthesis in pollen leads to male sterility in flowering crops. Nonetheless, the regulatory mechanisms underlying starch biosynthesis in rice (Oryza sativa) pollen remain unclear. Here, we identified a target of the microRNA OsmiR159, SPOROCYTELESS ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR-ASSOCIATED AMPHIPHILIC-REPRESSION 2 (OsSPEAR2). OsSPEAR2 is predominantly expressed in mature pollen and OsSPEAR2 possesses transcriptional repressor activity and localizes in the nucleus. Disruption of OsSPEAR2 results in severely shrunken pollen grains and male sterility. OsSPEAR2 interacts with multiple OsTCPs, including OsTCP14. OsTCP14 is a target of OsmiR319 and a knockout mutation in OsTCP14 partially rescues the defective pollen phenotype of Osspear2. In addition, transcriptome analyses revealed significant downregulation of numerous genes associated with carbohydrate metabolism, specifically in Osspear2 anthers, including several genes critical for starch biosynthesis. Moreover, OsTCP14 directly represses the expression of the essential starch biosynthesis gene OsUGP2; however, this repression could be alleviated by OsSPEAR2. Noteworthily, embryophyte-specific SPEAR2 and SPOROCYTELESS were also identified as miR159 targets involved in regulating plant growth and development in Arabidopsis (Arabidopsis thaliana), indicating that the miR159-SPEAR regulatory module may be conserved among embryophytes. Collectively, our findings reveal OsmiR159-OsSPEAR2-OsTCP14-OsUGP2 as a regulatory cascade that modulates starch biosynthesis during pollen development in rice.

A GDSL motif-containing lipase modulates Sclerotinia sclerotiorum resistance in Brassica napus

Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum (Lib.) De Bary is a devastating disease infecting hundreds of plant species. It also restricts the yield, quality, and safe production of rapeseed (Brassica napus) worldwide. However, the lack of resistance sources and genes to S. sclerotiorum has greatly restricted rapeseed SSR-resistance breeding. In this study, a previously identified GDSL motif-containing lipase gene, B. napus GDSL LIPASE-LIKE 1 (BnaC07.GLIP1), encoding a protein localized to the intercellular space, was characterized as functioning in plant immunity to S. sclerotiorum. The BnaC07.GLIP1 promoter is S. sclerotiorum-inducible and the expression of BnaC07.GLIP1 is substantially enhanced after S. sclerotiorum infection. Arabidopsis (Arabidopsis thaliana) heterologously expressing and rapeseed lines overexpressing BnaC07.GLIP1 showed enhanced resistance to S. sclerotiorum, whereas RNAi suppression and CRISPR/Cas9 knockout B. napus lines were hyper-susceptible to S. sclerotiorum. Moreover, BnaC07.GLIP1 affected the lipid composition and induced the production of phospholipid molecules, such as phosphatidylethanolamine, phosphatidylcholine, and phosphatidic acid, which were correlated with decreased levels of reactive oxygen species (ROS) and enhanced expression of defense-related genes. A B. napus bZIP44 transcription factor specifically binds the CGTCA motif of the BnaC07.GLIP1 promoter to positively regulate its expression. BnbZIP44 responded to S. sclerotiorum infection, and its heterologous expression inhibited ROS accumulation, thereby enhancing S. sclerotiorum resistance in Arabidopsis. Thus, BnaC07.GLIP1 functions downstream of BnbZIP44 and is involved in S. sclerotiorum resistance by modulating the production of phospholipid molecules and ROS homeostasis in B. napus, providing insights into the potential roles and functional mechanisms of BnaC07.GLIP1 in plant immunity and for improving rapeseed SSR disease-resistance breeding.

ChBBX6 and ChBBX18 are positive regulators of anthocyanins biosynthesis and carotenoids degradation in Cerasus humilis

B-box zinc-finger transcription factor (BBX) plays important regulatory roles in plant secondary metabolism. Here, we identified 21 BBXs that could be further categorized into five subfamilies from Cerasus humilis. Two segmentally duplicated Subfamily IV members, ChBBX6 and ChBBX18, were found to share high homology with reported anthocyanin-related BBXs and express highly in fruits with high anthocyanins but low carotenoids contents. Their transient overexpression in apple and C. humilis fruits both led to significantly increased anthocyanins accumulation and significantly upregulated expression of anthocyanins-related genes. However, their overexpression resulted in decreased carotenoids accumulation and greatly upregulated the expression of carotenoids-related genes especially degradation-related genes. Additionally, their overexpression both greatly improved the ABA content in C. humilis fruits. Through yeast one-hybrid and dual-luciferase reporter assays, we found that both ChBBX6 and ChBBX18 could bind to and activate the promoters of chalcone synthase (ChCHS), flavanone 3-hydroxylase (ChF3H), and 9-cis-epoxycarotenoid dioxygenase 5 (ChNCED5). Our study demonstrates that ChBBX6 and ChBBX18 are positive regulators of anthocyanins biosynthesis and carotenoids degradation and can provide basis for understanding the roles of BBX genes in C. humilis.

The OsSRO1c-OsDREB2B complex undergoes protein phase transition to enhance cold tolerance in rice

Cold stress is one of the major abiotic stress factors affecting rice growth and development, leading to significant yield loss in the context of global climate change. Exploring natural variants that confer cold resistance and the underlying molecular mechanism responsible for this is the major strategy to breed cold-tolerant rice varieties. Here, we show that natural variations of a SIMILAR to RCD ONE (SRO) gene, OsSRO1c, confer cold tolerance in rice at both seedling and booting stages. Our in vivo and in vitro experiments demonstrated that OsSRO1c possesses intrinsic liquid-liquid phase-separation ability and recruits OsDREB2B, an AP2/ERF transcription factor that functions as a positive regulator of cold stress, into its biomolecular condensates in the nucleus, resulting in elevated transcriptional activity of OsDREB2B. We found that the OsSRO1c-OsDREB2B complex directly responds to low temperature through dynamic phase transitions and regulates key cold-response genes, including COLD1. Furthermore, we showed that introgression of an elite haplotype of OsSRO1c into a cold-susceptible indica rice could significantly increase its cold resistance. Collectively, our work reveals a novel cold-tolerance regulatory module in rice and provides promising genetic targets for molecular breeding of cold-tolerant rice varieties.

Two B-box proteins orchestrate vegetative and reproductive growth in summer chrysanthemum

Floral transition, the switch from vegetative to reproductive growth, is extremely important for the growth and development of flowering plants. In the summer chrysanthemum, CmBBX8, a member of the subgroup II B-box (BBX) family, positively regulates the transition by physically interacting with CmERF3 to inhibit CmFTL1 expression. In this study, we show that CmBBX5, a B-box subgroup I member comprising two B-boxes and a CCT domain, interacts with CmBBX8. This interaction suppresses the recruitment of CmBBX8 to the CmFTL1 locus without affecting its transcriptional activation activity. CmBBX5 overexpression led to delayed flowering under both LD (long-day) and SD (short-day) conditions, while lines expressing the chimeric repressor gene-silencing (CmBBX5-SRDX) exhibited the opposite phenotype. Subsequent genetic evidence indicated that in regulating flowering, CmBBX5 is partially dependent on CmBBX8. Moreover, during the vegetative growth period, levels of CmBBX5 expression were found to exceed those of CmBBX8. Collectively, our findings indicate that both CmERF3 and CmBBX5 interact with CmBBX8 to dampen the regulation of CmFTL1 via distinct mechanisms, which contribute to preventing the premature flowering of summer chrysanthemum.

GhRCD1 promotes cotton tolerance to cadmium by regulating the GhbHLH12-GhMYB44-GhHMA1 transcriptional cascade

Heavy metal pollution poses a significant risk to human health and wreaks havoc on agricultural productivity. Phytoremediation, a plant-based, environmentally benign, and cost-effective method, is employed to remove heavy metals from contaminated soil, particularly in agricultural or heavy metal-sensitive lands. However, the phytoremediation capacity of various plant species and germplasm resources display significant genetic diversity, and the mechanisms underlying these differences remain hitherto obscure. Given its potential benefits, genetic improvement of plants is essential for enhancing their uptake of heavy metals, tolerance to harmful levels, as well as overall growth and development in contaminated soil. In this study, we uncover a molecular cascade that regulates cadmium (Cd2+) tolerance in cotton, involving GhRCD1, GhbHLH12, GhMYB44, and GhHMA1. We identified a Cd2+-sensitive cotton T-DNA insertion mutant with disrupted GhRCD1 expression. Genetic knockout of GhRCD1 by CRISPR/Cas9 technology resulted in reduced Cd2+ tolerance in cotton seedlings, while GhRCD1 overexpression enhanced Cd2+ tolerance. Through molecular interaction studies, we demonstrated that, in response to Cd2+ presence, GhRCD1 directly interacts with GhbHLH12. This interaction activates GhMYB44, which subsequently activates a heavy metal transporter, GhHMA1, by directly binding to a G-box cis-element in its promoter. These findings provide critical insights into a novel GhRCD1-GhbHLH12-GhMYB44-GhHMA1 regulatory module responsible for Cd2+ tolerance in cotton. Furthermore, our study paves the way for the development of elite Cd2+-tolerant cultivars by elucidating the molecular mechanisms governing the genetic control of Cd2+ tolerance in cotton.

ETHYLENE RESPONSE FACTOR 070 inhibits flowering in Pak-choi by indirectly impairing BcLEAFY expression

APETALA2/ethylene responsive factors respond to ethylene and participate in many biological and physiological processes, such as plant morphogenesis, stress resistance, and hormone signal transduction. Ethylene responsive factor 070 (BcERF070) is important in flowering. However, the underlying molecular mechanisms of BcERF070 in floral transition in response to ethylene signaling have not been fully characterized. Herein, we explored the function of BcERF070 in Pak-choi [Brassica campestris (syn. Brassica rapa) ssp. chinensis]. Ethylene treatment induced BcERF070 expression and delayed flowering in Pak-choi. Silencing of BcERF070 induced flowering in Pak-choi. BcERF070 interacted with major latex protein-like 328 (BcMLP328), which forms a complex with helix-loop-helix protein 30 (BcbHLH30) to enhance the transcriptional activity of BcbHLH30 on LEAFY (BcLFY), ultimately promoting flowering. However, BcERF070 impaired the BcMLP328-BcbHLH30 complex activation of LEAFY (BcLFY), ultimately inhibiting flowering in Pak-choi. BcERF070 directly promoted the expression of the flowering inhibitor gene B-box 29 (BcBBX29) and delayed flowering by reducing FLOWERING LOCUS T (BcFT) expression. These results suggest that BcERF070 mediates ethylene-reduced flowering by impairing the BcMLP328-BcbHLH30 complex activation of BcLFY and by directly promoting the gene expression of the flowering inhibition factor BcBBX29 to repress BcFT expression. The findings contribute to understanding the molecular mechanisms underlying floral transition in response to ethylene in plants.

The RAV transcription factor TEMPRANILLO1 involved in ethylene-mediated delay of chrysanthemum flowering

TEMPRANILLO1 (TEM1) is a transcription factor belonging to related to ABI3 and VP1 family, which is also known as ethylene response DNA-binding factor 1 and functions as a repressor of flowering in Arabidopsis. Here, a putative homolog of AtTEM1 was isolated and characterized from chrysanthemum, designated as CmTEM1. Exogenous application of ethephon leads to an upregulation in the expression of CmTEM1. Knockdown of CmTEM1 promotes floral initiation, while overexpression of CmTEM1 retards floral transition. Further phenotypic observations suggested that CmTEM1 involves in the ethylene-mediated inhibition of flowering. Transcriptomic analysis established that expression of the flowering integrator CmAFL1, a member of the APETALA1/FRUITFULL subfamily, was downregulated significantly in CmTEM1-overexpressing transgenic plants compared with wild-type plants but was verified to be upregulated in amiR-CmTEM1 lines by quantitative RT-PCR. In addition, CmTEM1 is capable of binding to the promoter of the CmAFL1 gene to inhibit its transcription. Moreover, the genetic evidence supported the notion that CmTEM1 partially inhibits floral transition by targeting CmAFL1. In conclusion, these findings demonstrate that CmTEM1 acts as a regulator of ethylene-mediated delayed flowering in chrysanthemum, partly through its interaction with CmAFL1.

An overview of floral regulatory genes in annual and perennial plants

The floral initiation in angiosperms is a complex process influenced by endogenous and exogenous signals. With this approach, we aim to provide a comprehensive review to integrate this complex floral regulatory process and summarize the regulatory genes and their functions in annuals and perennials. Seven primary paths leading to flowering have been discovered in Arabidopsis under several growth condition that include; photoperiod, ambient temperature, vernalization, gibberellins, autonomous, aging and carbohydrates. These pathways involve a series of interlinked signaling pathways that respond to both internal and external signals, such as light, temperature, hormones, and developmental cues, to coordinate the expression of genes that are involved in flower development. Among them, the photoperiodic pathway was the most important and conserved as some of the fundamental loci and mechanisms are shared even by closely related plant species. The activation of floral regulatory genes such as FLC, FT, LFY, and SOC1 that determine floral meristem identity and the transition to the flowering stage result from the merging of these pathways. Recent studies confirmed that alternative splicing, antisense RNA and epigenetic modification play crucial roles by regulating the expression of genes related to blooming. In this review, we documented recent progress in the floral transition time in annuals and perennials, with emphasis on the specific regulatory mechanisms along with the application of various molecular approaches including overexpression studies, RNA interference and Virus-induced flowering. Furthermore, the similarities and differences between annual and perennial flowering will aid significant contributions to the field by elucidating the mechanisms of perennial plant development and floral initiation regulation.

RCD1 Promotes Salt Stress Tolerance in Arabidopsis by Repressing ANAC017 Activity

Plants have evolved diverse strategies to accommodate saline environments. More insights into the knowledge of salt stress regulatory pathways will benefit crop breeding. RADICAL-INDUCED CELL DEATH 1 (RCD1) was previously identified as an essential player in salt stress response. However, the underlying mechanism remains elusive. Here, we unraveled that Arabidopsis NAC domain-containing protein 17 (ANAC017) acts downstream of RCD1 in salt stress response, and its ER-to-nucleus transport is triggered by high salinity. Genetic and biochemical evidence showed that RCD1 interacts with transmembrane motif-truncated ANAC017 in the nucleus and represses its transcriptional activity. Transcriptome analysis revealed that genes associated with oxidation reduction process and response to salt stress are similarly dysregulated in loss-of-function rcd1 and gain-of-function anac017-2 mutants. In addition, we found that ANAC017 plays a negative role in salt stress response by impairing the superoxide dismutase (SOD) enzyme activity. Taken together, our study uncovered that RCD1 promotes salt stress response and maintains ROS homeostasis by inhibiting ANAC017 activity.

BBX7 interacts with BBX8 to accelerate flowering in chrysanthemum

The quantitative control of FLOWERING LOCUS T (FT) activation is important for the floral transition in flowering plants. However, the flowering regulation mechanisms in the day-neutral, summer-flowering chrysanthemum plant remain unclear. In this study, the chrysanthemum BBX7 homolog CmBBX7 was isolated and its flowering function was identified. The expression of CmBBX7 showed a diurnal rhythm and CmBBX7 exhibited higher expression levels than CmBBX8. Overexpression of CmBBX7 in transgenic chrysanthemum accelerated flowering, whereas lines transfected with a chimeric repressor (pSRDX-CmBBX7) exhibited delayed flowering. Yeast single hybridization, luciferase, electrophoretic mobility shift, and chromatin immunoprecipitation assays showed that CmBBX7 directly targets CmFTL1. In addition, we found that CmBBX7 and CmBBX8 interact to positively regulate the expression of CmFTL1 through binding to its promoter. Collectively, these results highlight CmBBX7 as a key cooperator in the BBX8-FT module to control chrysanthemum flowering.

An ethylene-responsive transcription factor and a B-box protein coordinate vegetative growth and photoperiodic flowering in chrysanthemum

Plants employ several endogenous and exogenous signals to guarantee timely floral transitions with floral integrators. To avoid premature flowering, flowering plants must control the balance between vegetative and floral development. As a Group II member of BBX family, CmBBX8 promotes flowering by directly activating CmFTL1 in summer-flowering chrysanthemum. However, the mechanisms underlying this floral transition is yet to be elucidated. Here, we report that the chrysanthemum ERF3 homologue, CmERF3, physically interacts with CmBBX8 through yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BiFC), pull-down, and luciferase complementation (LCI) assays. We found that CmERF3 was highly expressed at the vegetative stage and rarely expressed in the reproductive phase, indicating that CmERF3 may play a critical role in maintaining vegetative growth to prevent premature flowering. Rhythm analysis revealed that CmERF3 had a different response to rhythm compared to CmBBX8. Knockdown of CmERF3 facilitated floral initiation, whereas overexpression of CmERF3 delayed floral transition. We further found that CmERF3 repressed the transactivation activity of CmBBX8 on the downstream CmFTL1 gene. Collectively, our results indicate that the CmERF3-CmBBX8 transcriptional complex is a crucial module that balances the vegetative growth and reproductive development of chrysanthemum.

The chrysanthemum DEAD-box RNA helicase CmRH56 regulates rhizome outgrowth in response to drought stress

Plants have evolved complex mechanisms to reprogram growth in response to drought stress. In herbaceous perennial plant species, the rhizome, which is normally an organ for propagation and food storage, can also support plant growth in stressful environments, and allows the plant to perennate and survive stress damage. However, the mechanisms that regulate rhizome growth in perennial herbs during abiotic stresses are unknown. Here, we identified a chrysanthemum (Chrysanthemum morifolium) DEAD-box RNA helicase gene, CmRH56, that is specifically expressed in the rhizome shoot apex. Knock down of CmRH56 transcript levels decreased the number of rhizomes and enhanced drought stress tolerance. We determined that CmRH56 represses the expression of a putative gibberellin (GA) catabolic gene, GA2 oxidase6 (CmGA2ox6). Exogenous GA treatment and silencing of CmGA2ox6 resulted in more rhizomes. These results demonstrate that CmRH56 suppresses rhizome outgrowth under drought stress conditions by blocking GA biosynthesis.

The BBX gene CmBBX22 negatively regulates drought stress tolerance in chrysanthemum

BBX transcription factors play vital roles in plant growth, development, and stress responses. Although BBX proteins have been studied in great detail in the model plant Arabidopsis, their roles in crop plants such as chrysanthemum are still largely uninvestigated. Here, we cloned CmBBX22 and further determined the function of CmBBX22 in response to drought treatment. Subcellular localization and transactivation assay analyses revealed that CmBBX22 was localized in the nucleus and possessed transactivation activity. Overexpression of CmBBX22 in chrysanthemum was found to reduce plant drought tolerance, whereas expression of the chimeric repressor CmBBX22-SRDX was found to promote a higher drought tolerance than that shown by wild-type plants, indicating that CmBBX22 negatively regulates drought tolerance in chrysanthemum. Transcriptome analysis and physiological measurements indicated the potential involvement of the CmBBX22-mediated ABA response, stomatal conductance, and antioxidant responses in the negative regulation of drought tolerance in chrysanthemum. Based on the findings of this study, we were thus able to establish the mechanisms whereby the transcriptional activator CmBBX22 negatively regulates drought tolerance in chrysanthemum via the regulation of the abscisic acid response, stomatal conductance, and antioxidant responses.

An Evolutionary Analysis of B-Box Transcription Factors in Strawberry Reveals the Role of FaBBx28c1 in the Regulation of Flowering Time

Flowering connects vegetative and generative developmental phases and plays a significant role in strawberry production. The mechanisms that regulate strawberry flowering time are unclear. B-box transcription factors (BBXs) play important roles in the flowering time regulation of plants. Nevertheless, BBXs in octoploid cultivated strawberry (Fragaria ananassa) and their functions in flowering time regulation have not been identified. Here, we identified 51 FaBBXs from cultivated strawberry and 16 FvBBXs from diploid wild strawberry (Fragaria vesca), which were classified into five groups according to phylogenetic analysis. Further evolutionary analysis showed that whole-genome duplication or segmental duplication is a crucial factor that leads to the expansion of the BBX gene family in two strawberry species. Moreover, some loss and acquisition events of FaBBX genes were identified in the genome of cultivated strawberry that could have affected traits of agronomic interest, such as fruit quality. The promoters of FaBBX genes showed an enrichment in light-responsive, cis-regulatory elements, with 16 of these genes showing changes in their transcriptional activity in response to blue light treatment. On the other hand, FaBBX28c1, whose transcriptional activity is reduced in response to blue light, showed a delay in flowering time in Arabidopsis transgenic lines, suggesting its role in the regulation of flowering time in cultivated strawberry. Our results provide new evolutionary insight into the BBX gene family in cultivated strawberry and clues regarding their function in flowering time regulation in plants.