LZF1, a HY5-regulated transcriptional factor, functions in Arabidopsis de-etiolation

In silico analysis of the wheat BBX gene family and identification of candidate genes for seed dormancy and germination

BACKGROUND

B-box (BBX) proteins are a type of zinc finger proteins containing one or two B-box domains. They play important roles in development and diverse stress responses of plants, yet their roles in wheat remain unclear.

RESULTS

In this study, 96 BBX genes were identified in the wheat genome and classified into five subfamilies. Subcellular localization prediction results showed that 68 TaBBXs were localized in the nucleus. Protein interaction prediction analysis indicated that interaction was one way that these proteins exerted their functions. Promoter analysis indicated that TaBBXs may play important roles in light signal, hormone, and stress responses. qRT-PCR analysis revealed that 14 TaBBXs were highly expressed in seeds compared with other tissues. These were probably involved in seed dormancy and germination, and their expression patterns were investigated during dormancy acquisition and release in the seeds of wheat varieties Jing 411 and Hongmangchun 21, showing significant differences in seed dormancy and germination phenotypes. Subcellular localization analysis confirmed that the three candidates TaBBX2-2 A, TaBBX4-2 A, and TaBBX11-2D were nuclear proteins. Transcriptional self-activation experiments further demonstrated that TaBBX4-2A was transcriptionally active, but TaBBX2-2A and TaBBX11-2D were not. Protein interaction analysis revealed that TaBBX2-2A, TaBBX4-2A, and TaBBX11-2D had no interaction with each other, while TaBBX2-2A and TaBBX11-2D interacted with each other, indicating that TaBBX4-2A may regulate seed dormancy and germination by transcriptional regulation, and TaBBX2-2A and TaBBX11-2D may regulate seed dormancy and germination by forming a homologous complex.

CONCLUSIONS

In this study, the wheat BBX gene family was identified and characterized at the genomic level by bioinformatics analysis. These observations provide a theoretical basis for future studies on the functions of BBXs in wheat and other species.

The DBB Family in Populus trichocarpa: Identification, Characterization, Evolution and Expression Profiles

The B-box proteins (BBXs) encode a family of zinc-finger transcription factors that regulate the plant circadian rhythm and early light morphogenesis. The double B-box (DBB) family is in the class of the B-box family, which contains two conserved B-box domains and lacks a CCT (CO, CO-like and TOC1) motif. In this study, the identity, classification, structures, conserved motifs, chromosomal location, cis elements, duplication events, and expression profiles of the PtrDBB genes were analyzed in the woody model plant Populus trichocarpa. Here, 12 PtrDBB genes (PtrDBB1-PtrDBB12) were identified and classified into four distinct groups, and all of them were homogeneously spread among eight out of seventeen poplar chromosomes. The collinearity analysis of the DBB family genes from P. trichocarpa and two other species (Z. mays and A. thaliana) indicated that segmental duplication gene pairs and high-level conservation were identified. The analysis of duplication events demonstrates an insight into the evolutionary patterns of DBB genes. The previously published transcriptome data showed that PtrDBB genes represented distinct expression patterns in various tissues at different stages. In addition, it was speculated that several PtrDBBs are involved in the responsive to drought stress, light/dark, and ABA and MeJA treatments, which implied that they might function in abiotic stress and phytohormone responses. In summary, our results contribute to the further understanding of the DBB family and provide a reference for potential functional studies of PtrDBB genes in P. trichocarpa.

GLK transcription factors accompany ELONGATED HYPOCOTYL5 to orchestrate light-induced seedling development in Arabidopsis

Light-induced de-etiolation is an important aspect of seedling photomorphogenesis. GOLDEN2 LIKE (GLK) transcriptional regulators are involved in chloroplast development, but to what extent they participate in photomorphogenesis is not clear. Here, we show that ELONGATED HYPOCOTYL5 (HY5) binds to GLK promoters to activate their expression, and also interacts with GLK proteins in Arabidopsis (Arabidopsis thaliana). The chlorophyll content in the de-etiolating Arabidopsis seedlings of the hy5 glk2 double mutants was lower than that in the hy5 single mutant. GLKs inhibited hypocotyl elongation, and the phenotype could superimpose on the hy5 phenotype. Correspondingly, GLK2 regulated the expression of photosynthesis and cell elongation genes partially independent of HY5. Before exposure to light, DE-ETIOLATED 1 (DET1) affected accumulation of GLK proteins. The enhanced etioplast development and photosystem gene expression observed in the det1 mutant were attenuated in the det1 glk2 double mutant. Our study reveals that GLKs act downstream of HY5, or additive to HY5, and are likely quantitatively adjusted by DET1, to orchestrate multiple developmental traits during the light-induced skotomorphogenesis-to-photomorphogenesis transition in Arabidopsis.

Genome-Wide Identification of B-Box Gene Family and Candidate Light-Related Member Analysis of Tung Tree (Vernicia fordii)

Light is one of the most important environmental factors for plant growth. In the production process of tung oil tree cultivation, due to the inappropriate growth of shading conditions, the lower branches are often dry and dead, which seriously affects the yield of tung oil trees. However, little is known about the key factors of light-induced tree photomorphogenesis. In this study, a total of 22 VfBBX family members were identified to provide a reference for candidate genes in tung tree seedlings. All members of the VfBBX family have different numbers of highly conserved B-box domains or CCT domains. Phylogenetic evolution clustered the VfBBX genes into four categories, and the highest density of members was on chromosome 6. Interspecific collinearity analysis suggested that there were six pairs of duplicate genes in VfBBX members, but the expression levels of all family members in different growth and development stages of the tung tree were significantly divergent. After different degrees of shading treatment and physiological data determination of tung tree seedlings, the differential expression level and chlorophyll synthesis genes correlation analysis revealed that VfBBX9 was a typical candidate nuclear localization transcription factor that was significantly differentially expressed in light response. This study systematically identified the VfBBX gene family and provided a reference for studying its molecular function, enhanced the theoretical basis for tung tree breeding, and identified excellent varieties.

The transcription factor SbHY5 mediates light to promote aluminum tolerance by activating SbMATE and SbSTOP1s expression

Aluminum (Al) toxicity is a major factor limiting crop yields in acid soils. Sweet sorghum (Sorghum bicolor L.) is a high-efficient energy crop widely grown in tropical and subtropical regions of the world, where acid soil is common and Al toxicity is widespread. Here, we characterized a transcription factor SbHY5 in sweet sorghum, which mediated light to promote plant Al stress adaptation. The expression of SbHY5 was induced by Al stress and increasing light intensity. The overexpression of SbHY5 improved Al tolerance in transgenic plants, which was associated with increased citrate secretion and reduced Al content in roots. Meanwhile, SbHY5 was found to localize to the nucleus and displayed transcriptional activity. SbHY5 directly activated the expression of SbMATE, indicating that a HY5-MATE-dependent citrate secretion pathway is involved in Al tolerance in plants. SbSTOP1 was reported as a key transcription factor, regulating several Al tolerance genes. Here, inspiringly, we found that SbHY5 directly promoted the transcription of SbSTOP1, implying the existence of HY5-STOP1-Al tolerance genes-mediated regulatory pathways. Besides, SbHY5 positively regulated its own transcription. Our findings revealed a novel regulatory network in which a light signaling factor, SbHY5, confers Al tolerance in plants by modulating the expression of Al stress response genes.

The Chemical Composition and Transcriptome Analysis Reveal the Mechanism of Color Formation in Tea (Camellia sinensis) Pericarp

To elucidate the molecular mechanisms underlying the differential metabolism of albino (white), green, and purple pericarp coloration, biochemical profiling and transcriptome sequencing analyses were performed on three different tea pericarps, Zhongbaiyihao (Camellia sinensis L. var. Zhongbai), Jinxuan (Camellia sinensis L. var. Jinxuan), and Baitangziya (Camellia sinensis L. var. Baitang). Results of biochemical analysis revealed that low chlorophyll content and low chlorophyll/carotene ratio may be the biochemical basis for albino characteristics in the 'Zhongbaiyihao' pericarp. The differentially expressed genes (DEGs) involved in anthocyanin biosynthesis, including DFR, F3'5'H, CCoAOMT, and 4-coumaroyl-CoA, were highly expressed in the purple 'Baitangziya' pericarp. In the chlorophyll synthesis of white pericarp, GUN5 (Genome Uncoupled 5) and 8-vinyl-reductase both showed high expression levels compared to the green one, which indicated that albino 'Zhongbaiyihao' pericarp had a higher chlorophyll synthesis capacity than 'Jinxuan'. Meanwhile, chlorophyllase (CLH, CSS0004684) was lower in 'Baitang' than in 'Jinxuan' and 'Zhongbaiyihao' pericarp. Among the differentially expressed transcription factors, MYB59, WRKY41-like2 (CS ng17509), bHLH62 like1 (CS ng6804), and bHLH62-like3 (CSS0039948) were downregulated in Jinxuan pericarp, suggesting that transcription factors played a role in regulating tea pericarp coloration. These findings provide a better understanding of the molecular mechanisms and theoretical basis for utilizing functional components of tea pericarp.

Light-Induced TaHY5-7A and TaBBX-3B Physically Interact to Promote PURPLE PERICARP-MYB 1 Expression in Purple-Grained Wheat

Purple-grained wheat (Triticum aestivum L.) is an important germplasm source in crop breeding. Anthocyanin biosynthesis in the pericarps of purple-grained wheat is largely light-dependent; however, the regulatory mechanisms underlying light-induced anthocyanin accumulation in the wheat pericarp remain unknown. Here we determined that anthocyanins rapidly accumulate in the pericarps of the purple-grained wheat cultivar Heixiaomai 76 (H76) at 16 days after pollination under light treatment. Using transcriptome sequencing, differential gene expression analysis, and phylogenetic analysis, we identified two key genes involved in light signaling in wheat: ELONGATED HYPOCOTYL 5-7A (TaHY5-7A) and B-BOX-3B (TaBBX-3B). TaHY5-7A and TaBBX-3B were highly expressed in purple-grained wheat pericarps. The heterologous expression of TaHY5-7A partially restored the phenotype of the Arabidopsis (Arabidopsis thaliana) hy5 mutant, resulting in increased anthocyanin accumulation and a shortened hypocotyl. The heterologous expression of TaBBX-3B in wild-type Arabidopsis had similar effects. TaHY5-7A and TaBBX-3B were nucleus-localized, consistent with a function in transcription regulation. However, TaHY5-7A, which lacks a transactivation domain, was not sufficient to activate the expression of PURPLE PERICARP-MYB 1 (TaPpm1), the key anthocyanin biosynthesis regulator in purple pericarps of wheat. TaHY5-7A physically interacted with TaBBX-3B in yeast two-hybrid and bimolecular fluorescence complementation assays. Additionally, TaHY5-7A, together with TaBBX-3B, greatly enhanced the promoter activity of TaPpm1 in a dual luciferase assay. Overall, our results suggest that TaHY5-7A and TaBBX-3B collaboratively activate TaPpm1 expression to promote light-induced anthocyanin biosynthesis in purple-pericarp wheat.

The ABI3-ERF1 module mediates ABA-auxin crosstalk to regulate lateral root emergence

Abscisic acid (ABA) is involved in lateral root (LR) development, but how ABA signaling interacts with auxin signaling to regulate LR formation is not well understood. Here, we report that ABA-responsive ERF1 mediates the crosstalk between ABA and auxin signaling to regulate Arabidopsis LR emergence. ABI3 is a negative factor in LR emergence and transcriptionally activates ERF1 by binding to its promoter, and reciprocally, ERF1 activates ABI3, which forms a regulatory loop that enables rapid signal amplification. Notably, ABI3 physically interacts with ERF1, reducing the cis element-binding activities of both ERF1 and ABI3 and thus attenuating the expression of ERF1-/ABI3-regulated genes involved in LR emergence and ABA signaling, such as PIN1, AUX1, ARF7, and ABI5, which may provide a molecular rheostat to avoid overamplification of auxin and ABA signaling. Taken together, our findings identify the role of the ABI3-ERF1 module in mediating crosstalk between ABA and auxin signaling in LR emergence.

Two B-box proteins, PavBBX6/9, positively regulate light-induced anthocyanin accumulation in sweet cherry

Anthocyanin production in bicolored sweet cherry (Prunus avium cv. Rainier) fruit is induced by light exposure, leading to red coloration. The phytohormone abscisic acid (ABA) is essential for this process, but the regulatory relationships that link light and ABA with anthocyanin-associated coloration are currently unclear. In this study, we determined that light treatment of bicolored sweet cherry fruit increased anthocyanin accumulation and induced ABA production and that ABA participates in light-modulated anthocyanin accumulation in bicolored sweet cherry. Two B-box (BBX) genes, PavBBX6/9, were highly induced by light and ABA treatments, as was anthocyanin accumulation. The ectopic expression of PavBBX6 or PavBBX9 in Arabidopsis (Arabidopsis thaliana) increased anthocyanin biosynthesis and ABA accumulation. Overexpressing PavBBX6 or PavBBX9 in sweet cherry calli also enhanced light-induced anthocyanin biosynthesis and ABA accumulation. Additionally, transient overexpression of PavBBX6 or PavBBX9 in sweet cherry peel increased anthocyanin and ABA contents, whereas silencing either gene had the opposite effects. PavBBX6 and PavBBX9 directly bound to the G-box elements in the promoter of UDP glucose-flavonoid-3-O-glycosyltransferase (PavUFGT), a key gene for anthocyanin biosynthesis, and 9-cis-epoxycarotenoid dioxygenase 1 (PavNCED1), a key gene for ABA biosynthesis, and enhanced their activities. These results suggest that PavBBX6 and PavBBX9 positively regulate light-induced anthocyanin and ABA biosynthesis by promoting PavUFGT and PavNCED1 expression, respectively. Our study provides insights into the relationship between the light-induced ABA biosynthetic pathway and anthocyanin accumulation in bicolored sweet cherry fruit.

PpZAT5 suppresses the expression of a B-box gene PpBBX18 to inhibit anthocyanin biosynthesis in the fruit peel of red pear

BBX (B-box) proteins play a vital role in light-induced anthocyanin biosynthesis. PpBBX18 was an indispensable regulator for the induction of anthocyanin biosynthesis in the peel of red pear fruit (Pyrus pyrifolia Nakai.). However, the upstream regulation of BBX genes has not been well characterized. In this study, PpZAT5, a cysteine2/histidine2-type transcription factor, was discovered as the upstream negative regulator of PpBBX18. The results showed that PpZAT5 functions as a transcriptional repressor and directly binds to the CAAT motif of PpBBX18 and inhibits its expression. PpZAT5 expression was inhibited by light, which is converse to the expression pattern of anthocyanin-related structural genes. In addition, less anthocyanin accumulated in the PpZAT5-overexpressing pear calli than in the wild-type pear calli; on the contrary, more anthocyanin accumulated in PpZAT5-RNAi pear calli. Moreover, the crucial genes involved in light-induced anthocyanin biosynthesis were markedly down-regulated in the transcriptome of PpZAT5 overexpression pear calli compared to wild-type. In conclusion, our study indicates that PpBBX18 is negatively regulated by a C2H2-type transcriptional repressor, PpZAT5, which reduces anthocyanin content in pear. The present results demonstrate an upstream molecular mechanism of PpBBX18 and provide insights into light-induced anthocyanin biosynthesis.

The StBBX24 protein affects the floral induction and mediates salt tolerance in Solanum tuberosum

The transition from vegetative growth to reproductive development is a critical developmental switch in flowering plants to ensure a successful life cycle. However, while the genes controlling flowering are well-known in model plants, they are less well-understood in crops. In this work, we generated potato lines both silenced and overexpressed for the expression of StBBX24, a clock-controlled gene encoding a B-box protein located in the cytosol and nuclear chromatin fraction. We revealed that Solanum tuberosum lines silenced for StBBX24 expression displayed much earlier flowering than wild-type plants. Conversely, plants overexpressing StBBX24 mostly did not produce flower buds other than wild-type plants. In addition, RT-qPCR analyses of transgenic silenced lines revealed substantial modifications in the expression of genes functioning in flowering. Furthermore, S. tuberosum lines silenced for StBBX24 expression displayed susceptibility to high salinity with a lower capacity of the antioxidant system and strongly decreased expression of genes encoding Na+ transporters that mediate salt tolerance, contrary to the plants with StBBX24 overexpression. Altogether, these data reveal that StBBX24 participates in potato flowering repression and is involved in salt stress response.

The MdBBX22-miR858-MdMYB9/11/12 module regulates proanthocyanidin biosynthesis in apple peel

Proanthocyanidins (PAs) have antioxidant properties and are beneficial to human health. The fruit of apple (Malus × domestica Borkh.), especially the peel, is rich in various flavonoids, such as PAs, and thus is an important source of dietary antioxidants. Previous research on the regulation of PAs in apple has mainly focussed on the transcription level, whereas studies conducted at the post-transcriptional level are relatively rare. In this study, we investigated the function of mdm-miR858, a miRNA with multiple functions in plant development, in the peel of apple fruit. We showed that mdm-miR858 negatively regulated PA accumulation by targeting MdMYB9/11/12 in the peel. During fruit development, mdm-miR858 expression was negatively correlated with MdMYB9/11/12 expression and PA accumulation. A 5'-RACE experiment, GUS staining assays and transient luminescent assays indicated that mdm-miR858 cleaved and inhibited the expression of MdMYB9/11/12. Overexpression of mdm-miR858 in apple calli, tobacco and Arabidopsis reduced the accumulation of PAs induced by overexpression of MdMYB9/11/12. Furthermore, we found that MdBBX22 bound to the mdm-miR858 promoter and induced its expression. Overexpression of MdBBX22 induced the expression of mdm-miR858 to inhibit the accumulation of PAs in apple calli overexpressing MdMYB9/11/12. Under light stress, MdBBX22 induced mdm-miR858 expression to inhibit PA accumulation and thereby indirectly enhanced anthocyanin synthesis in the peel. The present results revealed that the MdBBX22-miR858-MdMYB9/11/12 module regulates PA accumulation in apple. The findings provide a reference for further studies of the regulatory mechanism of PA accumulation and the relationship between PAs and anthocyanins.

Rice and Arabidopsis BBX proteins: toward genetic engineering of abiotic stress resistant crops

Productivity of crop plants are enormously affected by biotic and abiotic stresses. The co-occurrence of several abiotic stresses may lead to death of crop plants. Hence, it is the responsibility of plant scientists to develop crop plants equipped with multistress tolerance pathways. A subgroup of zinc finger transcription factor family, known as B-box (BBX) proteins, play a key role in light and hormonal regulation pathways. In addition, BBX proteins act as key regulatory proteins in many abiotic stress regulatory pathways, including Ultraviolet-B (UV-B), salinity, drought, heat and cold, and heavy metal stresses. Most of the BBX proteins identified in Arabidopsis and rice respond to more than one abiotic stress. Considering the requirement of improving rice for multistress tolerance, this review discusses functionally characterized Arabidopsis and rice BBX proteins in the development of abiotic stress responses. Furthermore, it highlights the participation of BBX proteins in multistress regulation and crop improvement through genetic engineering.

B-Box Transcription Factor FaBBX22 Promotes Light-Induced Anthocyanin Accumulation in Strawberry (Fragaria × ananassa)

B-box transcription factors (TFs) play a vital role in light-induced anthocyanin accumulation. Here, the FaBBX22 gene encoding 287 amino acids B-box TF was isolated from the cultivated strawberry variety ‘Benihoppe’ and characterized functionally. The expression analysis showed that FaBBX22 was expressed in the roots, stems, leaves, flowers and fruits, and its transcription level was upregulated under the red- or blue-light irradiation. FaBBX22 was localized in the nucleus and showed trans-acting activity in yeast cells. Ectopic overexpression of FaBBX22 in Arabidopsis enhanced the accumulation of anthocyanin. Additionally, we obtained transgenic strawberry calli that overexpressed the FaBBX22 gene, and strawberry calli coloration assays showed that FaBBX22 increased anthocyanin accumulation by upregulating the expression of anthocyanin biosynthetic genes (FaPAL, FaANS, FaF3′H, FaUFGT1) and transport gene FaRAP in a light-dependent manner. Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation assays indicated that FaBBX22 interacted with FaHY5. Furthermore, mutation of the 70th Asp residue in FaBBX22 protein to an Ala residue disrupted the interaction between FaBBX22 and FaHY5. Further, a transient expression assay demonstrated that the co-expression of FaBBX22 and FaHY5 could strongly promote anthocyanin accumulation in strawberry fruits. Collectively, these results revealed the positive regulatory role of FaBBX22 in light-induced anthocyanin accumulation.

Arabidopsis B-box transcription factors BBX20-22 promote UVR8 photoreceptor-mediated UV-B responses

Plants undergo photomorphogenic development in the presence of light. Photomorphogenesis is repressed by the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), which binds to substrates through their valine-proline (VP) motifs. The UV RESISTANCE LOCUS 8 (UVR8) photoreceptor senses UV-B and inhibits COP1 through the cooperative binding of its own VP motif and photosensing core to COP1, thereby preventing COP1 binding to substrates, including the basic leucine zipper (bZIP) transcriptional regulator ELONGATED HYPOCOTYL 5 (HY5). As a key promoter of visible light and UV-B photomorphogenesis, HY5 requires coregulators for its function. The B-box family transcription factors BBX20-BBX22 were recently described as HY5 rate-limiting coactivators under red light, but their role in UVR8 signaling was unknown. Here we describe a hypermorphic bbx21-3D mutant with enhanced photomorphogenesis, carrying a proline-to-leucine mutation at position 314 in the VP motif that impairs the interaction with and regulation by COP1. We show that BBX21 and BBX22 are UVR8-dependently stabilized after UV-B exposure, which is counteracted by a repressor induced by HY5/BBX activity. bbx20 bbx21 bbx22 mutants under UV-B are impaired in hypocotyl growth inhibition, photoprotective pigment accumulation and the expression of several HY5-dependent genes under continuous UV-B, but the immediate induction of marker genes after exposure to UV-B remains surprisingly rather unaffected. We conclude that BBX20-BBX22 contribute to HY5 activity in a subset of UV-B responses, but that additional, presently unknown, coactivators for HY5 are functional in early UVR8 signaling.

Genome-Wide Identification of the B-Box Gene Family and Expression Analysis Suggests Their Potential Role in Photoperiod-Mediated β-Carotene Accumulation in the Endocarp of Cucumber (Cucumis sativus L.) Fruit

Carotenoids are indispensable to plants and essential for human nutrition and health. Carotenoid contents are strongly influenced by light through light-responsive genes such as B-Box (BBX) genes. BBX proteins, a class of zinc-finger transcription factors, mediate many light-signaling pathways, leading to the biosynthesis of important metabolites in plants. However, the identification of the BBX gene family and expression analysis in response to photoperiod-mediated carotenoid accumulation in cucumber remains unexplored. We performed a genome-wide study and determined the expression of cucumber BBX genes (hereafter referred to as CsaBBXs genes) in the endocarp of Xishuangbanna cucumber fruit (a special type of cucumber accumulating a high level of β-carotene in the endocarp) using an RNA-seq analysis of plants previously subjected to two photoperiodic conditions. Here, 26 BBX family genes were identified in the cucumber genome and named serially CsaBBX1 through CsaBBX26. We characterized CsaBBX genes in terms of their phylogenetic relationships, exon-intron structures, cis-acting elements, and syntenic relationships with Arabidopsis thaliana (L.) Heynh. RNA-seq analysis revealed a varied expression of CsaBBX genes under photoperiod treatment. The analysis of CsaBBXs genes revealed a strong positive correlation between CsaBBX17 and carotenoid biosynthetic pathway genes (phytoene synthase, ζ-carotene desaturase, lycopene ε-cyclase, β-carotene hydroxylase-1), thus suggesting its involvement in β-carotene biosynthesis. Additionally, nine CsaBBX genes (CsaBBX 4, 5, 7, 9, 11, 13, 15, 17 and 22) showed a significant positive correlation with β-carotene content. The selected CsaBBX genes were verified by qRT-PCR and confirmed the validity of RNA-seq data. The results of this study established the genome-wide analysis of the cucumber BBX family and provide a framework for understanding their biological role in carotenoid accumulation and photoperiodic responses. Further investigations of CsaBBX genes are vital since they are promising candidate genes for the functional analysis of carotenoid biosynthesis and can provide genetic tools for the molecular breeding of carotenoids in plants.

BBX16 mediates the repression of seedling photomorphogenesis downstream of the GUN1/GLK1 module during retrograde signalling

Plastid-to-nucleus retrograde signalling (RS) initiated by dysfunctional chloroplasts impact photomorphogenic development. We have previously shown that the transcription factor GLK1 acts downstream of the RS regulator GUN1 in photodamaging conditions to regulate not only the well established expression of photosynthesis-associated nuclear genes (PhANGs) but also to regulate seedling morphogenesis. Specifically, the GUN1/GLK1 module inhibits the light-induced phytochrome-interacting factor (PIF)-repressed transcriptional network to suppress cotyledon development when chloroplast integrity is compromised, modulating the area exposed to potentially damaging high light. However, how the GUN1/GLK1 module inhibits photomorphogenesis upon chloroplast damage remained undefined. Here, we report the identification of BBX16 as a novel direct target of GLK1. BBX16 is induced and promotes photomorphogenesis in moderate light and is repressed via GUN1/GLK1 after chloroplast damage. Additionally, we showed that BBX16 represents a regulatory branching point downstream of GUN1/GLK1 in the regulation of PhANG expression and seedling development upon RS activation. The gun1 phenotype in lincomycin and the gun1-like phenotype of GLK1OX are markedly suppressed in gun1bbx16 and GLK1OXbbx16. This study identified BBX16 as the first member of the BBX family involved in RS, and defines a molecular bifurcation mechanism operated by GLK1/BBX16 to optimise seedling de-etiolation, and to ensure photoprotection in unfavourable light conditions.

Phytochrome interacting factor MdPIF7 modulates anthocyanin biosynthesis and hypocotyl growth in apple

Light affects many physiological and developmental processes of plants by regulating the expression and activity of light-responsive proteins. Among them, phytochrome interacting factors (PIFs) play pivotal roles in the regulation of anthocyanin accumulation and hypocotyl growth. However, the molecular mechanism is not well understood, especially in woody plants, such as apple (Malus × domestica). In this study, we identified a light-responsive PIF protein, MdPIF7, in apple and investigated the molecular mechanism of its regulation of anthocyanin biosynthesis and hypocotyl growth. We found that overexpression of MdPIF7 decreased anthocyanin accumulation in transgenic apple materials and promoted hypocotyl elongation in ectopically expressed Arabidopsis (Arabidopsis thaliana). Further investigation showed that MdPIF7 functioned by interacting with B-box 23 (MdBBX23), a positive regulator of anthocyanin biosynthesis in apple and hypocotyl growth inhibition in ectopically expressed Arabidopsis, and attenuating the transcriptional activation of MdBBX23 on LONG HYPOCOTYL 5 (MdHY5). In addition, MdPIF7 interacted with basic region leucine zipper 44 (MdbZIP44) and ethylene response factor 38 (MdERF38), two positive regulators of anthocyanin biosynthesis, and it negatively regulated MdbZIP44- and MdERF38-promoted anthocyanin accumulation by interfering with the interaction between MdbZIP44/MdERF38 and MdMYB1. Taken together, our results reveal that MdPIF7 regulates anthocyanin biosynthesis in apple and hypocotyl growth in ectopically expressed Arabidopsis through MdPIF7-MdBBX23-MdHY5 and MdPIF7-MdbZIP44/MdERF38-MdMYB1 modules. Our findings enrich the functional studies of PIF proteins and provide insights into the molecular mechanism of PIF-mediated anthocyanin biosynthesis and hypocotyl growth.

The UV-B-Induced Transcription Factor HY5 Regulated Anthocyanin Biosynthesis in Zanthoxylum bungeanum

Pericarp color is an important economic characteristic of Zanthoxylum bungeanum. Anthocyanins are the main reason for the pericarp’s red appearance in Z. bungeanum. In this study, through the combined analysis of the metabolome and transcriptome, HY5, whose expression is highly correlated to changes in the anthocyanin content, was screened and identified. Under natural ripening conditions, the Z. bungeanum fruit gradually changed in color from green to red, while bagging resulted in the fruit maintaining its green color. After unbagging, the fruit gradually turned red, and the ZbHY5 expression and anthocyanin content increased. In addition, the leaves changed from green to red after exposure to UV-B radiation, and the ZbHY5 expression and anthocyanin content increased. The transient overexpression of ZbHY5 deepened the redness of the Z. bungeanum leaves and promoted the expression of ZbHY5 and ZbMYB113 as well as anthocyanin accumulation. Bimolecular fluorescence complementation (BIFC) showed that there was an interaction between ZbHY5 and ZbMYB113. These results revealed that under UV-B irradiation, ZbHY5 might regulate the expression levels of the structural genes related to anthocyanin biosynthesis through combination with ZbMYB113, thereby affecting anthocyanin accumulation. This finding provides useful insights for further studies focusing on UV-B-induced anthocyanin accumulation in Z. bungeanum.

An improved repertoire of splicing variants and their potential roles in Arabidopsis photomorphogenic development

Background

Light switches on the photomorphogenic development of young plant seedlings, allowing young seedlings to acquire photosynthetic capacities and gain survival fitness. Light regulates gene expression at all levels of the central dogma, including alternative splicing (AS) during the photomorphogenic development. However, accurate determination of full-length (FL) splicing variants has been greatly hampered by short-read RNA sequencing technologies.

Result

In this study, we adopt PacBio isoform sequencing (Iso-seq) to overcome the limitation of the short-read RNA-seq technologies. Normalized cDNA libraries used for Iso-seq allows for comprehensive and effective identification of FL AS variants. Our analyses reveal more than 30,000 splicing variant models from approximately 16,500 gene loci and additionally identify approximately 700 previously unannotated genes. Among the variants, approximately 12,000 represent new gene models. Intron retention (IR) is the most frequently observed form of variants, and many IR-containing AS variants show evidence of engagement in translation. Our study reveals the formation of heterodimers of transcription factors composed of annotated and IR-containing AS variants. Moreover, transgenic plants overexpressing the IR forms of two B-BOX DOMAIN PROTEINs exhibits light-hypersensitive phenotypes, suggesting their regulatory roles in modulating optimal light responses.

Conclusions

This study provides an accurate and comprehensive portrait of full-length transcript isoforms and experimentally confirms the presence of de novo synthesized AS variants that impose regulatory functions in photomorphogenic development in Arabidopsis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02620-2.

B-box containing protein 1 from Malus domestica (MdBBX1) is involved in the abiotic stress response

B-box proteins (BBXs), which act as transcription factors, mainly regulate photomorphogenesis. However, the molecular functions underlying the activity of plant BBXs in response to abiotic stress remain largely unclear. In this investigation, we found that a BBX from Malus domestica (MdBBX1) was involved in the response to various abiotic stresses. The expression of MdBBX1 was significantly upregulated in response to abiotic stresses and abscisic acid (ABA). Recombinant MdBBX1 increased stress tolerance in Escherichia coli cells. In addition, overexpression of MdBBX1 in Arabidopsis decreased sensitivity to exogenous ABA, resulting in a germination rate and root length that were greater and longer, respectively, than those of wild-type (WT) plants. Moreover, the expression of ABI5 was decreased in MdBBX1-overexpressing lines under ABA treatment. After salt and drought treatments, compared with the WT plants, the MdBBX1 transgenic plants displayed enhanced tolerance and had a higher survival rate. Furthermore, under salt stress, increased proline (PRO) contents, decreased levels of malondialdehyde (MDA), increased activity of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX)) and decreased accumulation of reactive oxygen species (ROS) were observed in the MdBBX1-overexpressing plants. Overall, our results provide evidence that MdBBX1 might play a critical role in the regulation of abiotic stress tolerance by reducing the generation of ROS.

HY5: A Pivotal Regulator of Light-Dependent Development in Higher Plants

ELONGATED HYPOCOTYL5 (HY5), a bZIP-type transcription factor, acts as a master regulator that regulates various physiological and biological processes in plants such as photomorphogenesis, root growth, flavonoid biosynthesis and accumulation, nutrient acquisition, and response to abiotic stresses. HY5 is evolutionally conserved in function among various plant species. HY5 acts as a master regulator of light-mediated transcriptional regulatory hub that directly or indirectly controls the transcription of approximately one-third of genes at the whole genome level. The transcription, protein abundance, and activity of HY5 are tightly modulated by a variety of factors through distinct regulatory mechanisms. This review primarily summarizes recent advances on HY5-mediated molecular and physiological processes and regulatory mechanisms on HY5 in the model plant Arabidopsis as well as in crops.

Multilevel regulation of anthocyanin-promoting R2R3-MYB transcription factors in plants

Anthocyanins are common secondary metabolites in plants that confer red, blue, and purple colorations in plants and are highly desired by consumers for their visual appearance and nutritional quality. In the last two decades, the anthocyanin biosynthetic pathway and transcriptional regulation of anthocyanin biosynthetic genes (ABGs) have been well characterized in many plants. From numerous studies on model plants and horticultural crops, many signaling regulators have been found to control anthocyanin accumulation via regulation of anthocyanin-promoting R2R3-MYB transcription factors (so-called R2R3-MYB activators). The regulatory mechanism of R2R3-MYB activators is mediated by multiple environmental factors (e.g., light, temperature) and internal signals (e.g., sugar, ethylene, and JA) in complicated interactions at multiple levels. Here, we summarize the transcriptional control of R2R3-MYB activators as a result of natural variations in the promoter of their encoding genes, upstream transcription factors and epigenetics, and posttranslational modifications of R2R3-MYB that determine color variations of horticultural plants. In addition, we focus on progress in elucidating the integrated regulatory network of anthocyanin biosynthesis mediated by R2R3-MYB activators in response to multiple signals. We also highlight a few gene cascade modules involved in the regulation of anthocyanin-related R2R3-MYB to provide insights into anthocyanin production in horticultural plants.

Utilization of a Sugarcane100K Single Nucleotide Polymorphisms Microarray-Derived High-Density Genetic Map in Quantitative Trait Loci Mapping and Function Role Prediction of Genes Related to Chlorophyll Content in Sugarcane

Chlorophyll is the most important pigment for plant photosynthesis that plays an important role in crop growth and production. In this study, the chlorophyll content trait was explored to improve sugarcane yield. Two hundred and eighty-five F₁ progenies from the cross YT93-159 × ROC22 with significantly different chlorophyll contents were included as test materials. The chlorophyll content of the +1 leaves during elongation phase was measured using a SPAD-502 meter through a three-crop cycle (plant cane, first ratoon, and second ratoon). Linkage analysis was conducted on a high-density genetic map constructed based on the sugarcane 100K SNP chip. In addition, Fv/Fm, plant height, stalk diameter, brix data were collected on plant cane during the elongation and maturation phases. The results showed that the +1 leaf SPAD values, which can be used as an important reference to evaluate the growth potential of sugarcane, were significantly and positively correlated with the Fv/Fm during elongation phase, as well as with plant height, stalk diameter, and brix during maturity phase (P < 0.01). The broad sense heritability (H ²) of the chlorophyll content trait was 0.66 for plant cane crop, 0.67 for first ratoon crop, and 0.73 for second ratoon crop, respectively, indicating that this trait was mainly controlled by genetic factors. Thirty-one quantitative trait loci (QTL) were detected by QTL mapping. Among them, a major QTL, qCC-R1, could account for 12.95% of phenotypic variation explained (PVE), and the other 30 minor QTLs explained 2.37-7.99% PVE. Twenty candidate genes related to chlorophyll content were identified in the QTLs plus a 200-Kb extension region within either sides, of which four were homologous genes involved in the chlorophyll synthesis process and the remaining 16 played a certain role in chlorophyll catabolic pathway, chloroplast organization, or photosynthesis. These results provide a theoretical reference for analyzing the genetic mechanism of chlorophyll synthesis and subsequent improvement of photosynthetic characteristics in sugarcane.

Genome-wide and expression analysis of B-box gene family in pepper

BACKGROUND

BBX transcription factors are a kind of zinc finger transcription factors with one or two B-box domains, which partilant in plant growth, development and response to abiotic or biotic stress. The BBX family has been identified in Arabidopsis, rice, tomato and some other model plant genomes.

RESULTS

Here, 24 CaBBX genes were identified in pepper (Capsicum annuum L.), and the phylogenic analysis, structures, chromosomal location, gene expression patterns and subcellular localizations were also carried out to understand the evolution and function of CaBBX genes. All these CaBBXs were divided into five classes, and 20 of them distributed in 11 of 12 pepper chromosomes unevenly. Most duplication events occurred in subgroup I. Quantitative RT-PCR indicated that several CaBBX genes were induced by abiotic stress and hormones, some had tissue-specific expression profiles or differentially expressed at developmental stages. Most of CaBBX members were predicated to be nucleus-localized in consistent with the transient expression assay by onion inner epidermis of the three tested CaBBX members (CaBBX5, 6 and 20).

CONCLUSION

Several CaBBX genes were induced by abiotic stress and exogenous phytohormones, some expressed tissue-specific and variously at different developmental stage. The detected CaBBXs act as nucleus-localized transcription factors. Our data might be a foundation in the identification of CaBBX genes, and a further understanding of their biological function in future studies.

MdBBX21, a B-Box Protein, Positively Regulates Light-Induced Anthocyanin Accumulation in Apple Peel

The red coloration of apple (Malus × domestica Borkh.) is due to the accumulation of anthocyanins in the fruit peel. Light is essential for anthocyanin biosynthesis in apple. In this study, we performed a transcriptome sequencing (RNA-seq) analysis of apple fruit exposed to light after unbagging. The identified differentially expressed genes included MdBBX21, which is homologous to Arabidopsis BBX21, suggesting it may be involved in light-induced anthocyanin biosynthesis. Additionally, MdBBX21 was localized in the nucleus and its gene was expressed earlier than MdMYB1 in apple peel treated with light. Overexpressing MdBBX21 in Arabidopsis and apple calli under light increased anthocyanin accumulation. Dual-luciferase and yeast one-hybrid assays confirmed that MdBBX21 binds to the MdHY5, MdBBX20, and MdBBX22-1/2 promoters and induces expression. At the same time, MdHY5 can also activate the expression of MdBBX21. Furthermore, bimolecular fluorescence complementation and yeast two-hybrid assays demonstrated that MdBBX21 can interact with MdHY5. This interaction can significantly enhance MdMYB1 promoter activity. These findings clarify the molecular mechanism by which MdBBX21 positively regulates light-induced anthocyanin accumulation in apple.

Anthocyanins: From Mechanisms of Regulation in Plants to Health Benefits in Foods

Anthocyanins represent the major red, purple, and blue pigments in many flowers, fruits, vegetables, and cereals. They are also recognized as important health-promoting components in the human diet with protective effects against many chronic diseases, including cardiovascular diseases, obesity, and cancer. Anthocyanin biosynthesis has been studied extensively, and both biosynthetic and key regulatory genes have been isolated in many plant species. Here, we will provide an overview of recent progress in understanding the anthocyanin biosynthetic pathway in plants, focusing on the transcription factors controlling activation or repression of anthocyanin accumulation in cereals and fruits of different plant species, with special emphasis on the differences in molecular mechanisms between monocot and dicot plants. Recently, new insight into the transcriptional regulation of the anthocyanin biosynthesis, including positive and negative feedback control as well as epigenetic and post-translational regulation of MYB-bHLH-WD40 complexes, has been gained. We will consider how knowledge of regulatory mechanisms has helped to produce anthocyanin-enriched foods through conventional breeding and metabolic engineering. Additionally, we will briefly discuss the biological activities of anthocyanins as components of the human diet and recent findings demonstrating the important health benefits of anthocyanin-rich foods against chronic diseases.

Karrikins control seedling photomorphogenesis and anthocyanin biosynthesis through a HY5-BBX transcriptional module

The butenolide molecule, karrikin (KAR), emerging in smoke of burned plant material, enhances light responses such as germination, inhibition of hypocotyl elongation, and anthocyanin accumulation in Arabidopsis. The KAR signaling pathway consists of KARRIKIN INSENSITIVE 2 (KAI2) and MORE AXILLARY GROWTH 2 (MAX2), which, upon activation, act in an SCF E3 ubiquitin ligase complex to target the downstream signaling components SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE 2 (SMXL2) for degradation. How degradation of SMAX1 and SMXL2 is translated into growth responses remains unknown. Although light clearly influences the activity of KAR, the molecular connection between the two pathways is still poorly understood. Here, we demonstrate that the KAR signaling pathway promotes the activity of a transcriptional module consisting of ELONGATED HYPOCOTYL 5 (HY5), B-BOX DOMAIN PROTEIN 20 (BBX20), and BBX21. The bbx20 bbx21 mutant is largely insensitive to treatment with KAR₂ , similar to a hy5 mutant, with regards to inhibition of hypocotyl elongation and anthocyanin accumulation. Detailed analysis of higher order mutants in combination with RNA-sequencing analysis revealed that anthocyanin accumulation downstream of SMAX1 and SMXL2 is fully dependent on the HY5-BBX module. However, the promotion of hypocotyl elongation by SMAX1 and SMXL2 is, in contrast to KAR₂ treatment, only partially dependent on BBX20, BBX21, and HY5. Taken together, these results suggest that light- and KAR-dependent signaling intersect at the HY5-BBX transcriptional module.

A poplar B-box protein PtrBBX23 modulates the accumulation of anthocyanins and proanthocyanidins in response to high light

Flavonoids, which modulate plant resistance to various stresses, can be induced by high light. B-box (BBX) transcription factors (TFs) play crucial roles in the transcriptional regulation of flavonoids biosynthesis, but limited information is available on the association of BBX proteins with high light. We present a detailed overview of 45 Populus trichocarpa BBX TFs. Phylogenetic relationships, gene structure, tissue-specific expression patterns and expression profiles were determined under 10 stress or phytohormone treatments to screen candidate BBX proteins associated with the flavonoid pathway. Sixteen candidate genes were identified, of which five were expressed predominantly in young leaves and roots, and BBX23 showed the most distinct response to high light. Overexpression of BBX23 in poplar activated expression of MYB TFs and structural genes in the flavonoid pathway, thereby promoting the accumulation of proanthocyanidins and anthocyanins. CRISPR/Cas9-generated knockout of BBX23 resulted in the opposite trend. Furthermore, the phenotype induced by BBX23 overexpression was enhanced under exposure to high light. BBX23 was capable of binding directly to the promoters of proanthocyanidin- and anthocyanin-specific genes, and its interaction with HY5 enhanced activation activity. We identified novel regulators of flavonoid biosynthesis in poplar, thereby enhancing our general understanding of the transcriptional regulatory mechanisms involved.

Genome-wide analysis of BBX gene family in Tartary buckwheat (Fagopyrum tataricum)

BBX (B-box), a zinc finger transcription factor with one or two B-box domains, plays an important role in plant photomorphogenesis, growth, and development as well as response to environmental changes. In this study, 28 Tartary buckwheat BBX (FtBBX) genes were identified and screened using a comparison program. Their physicochemical properties, gene structures, conserved motifs, distribution in chromosomal, and phylogeny of the coding proteins, as well as their expression patterns, were analyzed. In addition, multiple collinearity analysis in three monocots and three dicot species illustrated that the BBX proteins identified from monocots clustered separately from those of dicots. Moreover, the expression of 11 candidate BBX genes with probable involvement in the regulation of anthocyanin biosynthesis was analyzed in the sprouts of Tartary buckwheat during light treatment. The results of gene structure analysis showed that all the 28 BBX genes contained B-box domain, three genes lacked introns, and these genes were unevenly distributed on the other seven chromosomes except for chromosome 6. The 28 proteins contained 10 conserved motifs and could be divided into five subfamilies. BBX genes of Tartary buckwheat showed varying expression under different conditions demonstrating that FtBBXs might play important roles in Tartary buckwheat growth and development. This study lays a foundation for further understanding of Tartary buckwheat BBX genes and their functions in growth and development as well as regulation of pigmentation in Tartary buckwheat.

Multiple responses contribute to the enhanced drought tolerance of the autotetraploid Ziziphus jujuba Mill. var. spinosa

Background

Polyploid plants often exhibit enhanced stress tolerance. The underlying physiological and molecular bases of such mechanisms remain elusive. Here, we characterized the drought tolerance of autotetraploid sour jujube at phenotypic, physiological and molecular levels.

Results

The study findings showed that the autotetraploid sour jujube exhibited a superior drought tolerance and enhanced regrowth potential after dehydration in comparison with the diploid counterpart. Under drought stress, more differentially expressed genes (DEGs) were detected in autotetraploid sour jujube and the physiological responses gradually triggered important functions. Through GO enrichment analysis, many DEGs between the diploid and autotetraploid sour jujube after drought-stress exposure were annotated to the oxidation–reduction process, photosystem, DNA binding transcription factor activity and oxidoreductase activity. Six reactive oxygen species scavenging-related genes were specifically differentially expressed and the larger positive fold-changes of the DEGs involved in glutathione metabolism were detected in autotetraploid. Consistently, the lower O²⁻ level and malonaldehyde (MDA) content and higher antioxidant enzymes activity were detected in the autotetraploid under drought-stress conditions. In addition, DEGs in the autotetraploid after stress exposure were significantly enriched in anthocyanin biosynthesis, DNA replication, photosynthesis and plant hormone, including auxin, abscisic acid and gibberellin signal-transduction pathways. Under osmotic stress conditions, genes associated with the synthesis and transport of osmotic regulators including anthocyanin biosynthesis genes were differentially expressed, and the soluble sugar, soluble protein and proline contents were significantly higher in the autotetraploid. The higher chlorophyll content and DEGs enriched in photosynthesis suggest that the photosynthetic system in the autotetraploid was enhanced compared with diploid during drought stress. Moreover, several genes encoding transcription factors (TFs) including GRAS, Bhlh, MYB, WRKY and NAC were induced specifically or to higher levels in the autotetraploid under drought-stress conditions, and hub genes, LOC107403632, LOC107422279, LOC107434947, LOC107412673 and LOC107432609, related to 18 up-regulated transcription factors in the autotetraploid compared with the diploid were identified.

Conclusion

Taken together, multiple responses contribute to the enhanced drought tolerance of autotetraploid sour jujube. This study could provide an important basis for elucidating the mechanism of tolerance variation after the polyploidization of trees.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00633-1.

Two B-box domain proteins, BBX28 and BBX29, regulate flowering time at low ambient temperature in Arabidopsis

This paper demonstrates that BBX28 and BBX29 proteins in Arabidopsis promote flowering in association with the CO-FT regulatory module at low ambient temperature under LD conditions. Flowering plants integrate internal developmental signals with external environmental stimuli for precise flowering time control. The expression of BBX29 is up-regulated by low temperature treatment, but the biological function of BBX29 in low temperature response is unknown. In the current study, we examined the biological role of BBX29 and its close-related protein BBX28 in flowering time control under long-day conditions. Although neither BBX28 single mutant nor BBX29 single mutant has a flowering-associated phenotype, the bbx28 bbx29 double mutant plants have an obvious delayed flowering phenotype grown at low ambient temperature (16°C) compared to the wild-type (WT) plants. The expression of FT and TSF was lower in bbx28 bbx29 double mutant plants than in wild-type plants at 16°C. Both BBX28 and BBX29 interact with CONSTANS (CO), an important flowering integrator that directly binds to the FLOWERING LOCUS T (FT) promoter. In the effector-reporter assays, transcriptional activation activity of CO on the FT promoter was reduced in bbx28 bbx29 double mutant plants compared to that in WT plants. Taken together, our results reveal that BBX28 and BBX29 are promoters of flowering in Arabidopsis, especially at low ambient temperature.

Genome-wide identification of B-box proteins and VvBBX44 involved in light-induced anthocyanin biosynthesis in grape (Vitis vinifera L.)

Genome-wide identification, analysis and functional characterization of an unreported VvBBX gene showed a response to light and positive correlation with anthocyanin content, but also inhibition of light-induced anthocyanin synthesis. B-box (BBX) proteins are a class of zinc (Zn) finger transcription factors or regulators characterized by the presence of one or two BBX domains and play important roles in plant growth and development. However, the BBX genes' potential functions are insufficiently characterized in grape, a globally popular berry with high economic value. Here, 25 BBX family genes including a novel member (assigned VvBBX44) were identified genome widely in grape. The expression level of these VvBBXs were analyzed in 'Cabernet Sauvignon' (V. vinifera) stem, flower, leaf, tendril, petiole, and developing berries. The expression of VvBBX44 increased in developing 'Cabernet Sauvignon' berries. Its expression was inhibited in 'Jingxiu' and 'Muscat Hamburg' berry skin without sunlight. Furthermore, overexpression of VvBBX44 decreased the expression of LONG HYPOCOTYL 5 (VvHY5) and UDP-glucose flavonoid 3-O-glucosyltransferase (VvUFGT), and reduced the anthocyanin content in grape calli. Our results suggest that VvBBX44 may play an important role in grape berry coloring by directly repressing VvHY5 expression. This study provides new insights into the potential role of VvBBXs in berry development and light response and contributes to the understanding on the regulation mechanism of VvBBX44 in anthocyanin biosynthesis.

Molecular evidence of the avocado defense response to Fusarium kuroshium infection: a deep transcriptome analysis using RNA-Seq

Fusarium kuroshium is a novel member of the Ambrosia Fusarium Clade (AFC) that has been recognized as one of the symbionts of the invasive Kuroshio shot hole borer, an Asian ambrosia beetle. This complex is considered the causal agent of Fusarium dieback, a disease that has severely threatened natural forests, landscape trees, and avocado orchards in the last 8 years. Despite the interest in this species, the molecular responses of both the host and F. kuroshium during the infection process and disease establishment remain unknown. In this work, we established an in vitro pathosystem using Hass avocado stems inoculated with F. kuroshium to investigate differential gene expression at 1, 4, 7 and 14 days post-inoculation. RNA-seq technology allowed us to obtain data from both the plant and the fungus, and the sequences obtained from both organisms were analyzed independently. The pathosystem established was able to mimic Fusarium dieback symptoms, such as carbohydrate exudation, necrosis, and vascular tissue discoloration. The results provide interesting evidence regarding the genes that may play roles in the avocado defense response to Fusarium dieback disease. The avocado data set comprised a coding sequence collection of 51,379 UniGenes, from which 2,403 (4.67%) were identified as differentially expressed. The global expression analysis showed that F. kuroshium responsive UniGenes can be clustered into six groups according to their expression profiles. The biologically relevant functional categories that were identified included photosynthesis as well as responses to stress, hormones, abscisic acid, and water deprivation. Additionally, processes such as oxidation-reduction, organization and biogenesis of the cell wall and polysaccharide metabolism were detected. Moreover, we identified orthologues of nucleotide-binding leucine-rich receptors, and their possible action mode was analyzed. In F. kuroshium, we identified 57 differentially expressed genes. Interestingly, the alcohol metabolic process biological category had the highest number of upregulated genes, and the enzyme group in this category may play an important role in the mechanisms of secondary metabolite detoxification. Hydrolytic enzymes, such as endoglucanases and a pectate lyase, were also identified, as well as some proteases. In conclusion, our research was conducted mainly to explain how the vascular tissue of a recognized host of the ambrosia complex responds during F. kuroshium infection since Fusarium dieback is an ambrosia beetle-vectored disease and many variables facilitate its establishment.

Beyond Arabidopsis: BBX Regulators in Crop Plants

B-box proteins represent diverse zinc finger transcription factors and regulators forming large families in various plants. A unique domain structure defines them—besides the highly conserved B-box domains, some B-box (BBX) proteins also possess CCT domain and VP motif. Based on the presence of these specific domains, they are mostly classified into five structural groups. The particular members widely differ in structure and fulfill distinct functions in regulating plant growth and development, including seedling photomorphogenesis, the anthocyanins biosynthesis, photoperiodic regulation of flowering, and hormonal pathways. Several BBX proteins are additionally involved in biotic and abiotic stress response. Overexpression of some BBX genes stimulates various stress-related genes and enhanced tolerance to different stresses. Moreover, there is evidence of interplay between B-box and the circadian clock mechanism. This review highlights the role of BBX proteins as a part of a broad regulatory network in crop plants, considering their participation in development, physiology, defense, and environmental constraints. A description is also provided of how various BBX regulators involved in stress tolerance were applied in genetic engineering to obtain stress tolerance in transgenic crops.

The SNL-HDA19 histone deacetylase complex antagonizes HY5 activity to repress photomorphogenesis in Arabidopsis

Reprogramming of the transcriptome during photomorphogenesis requires dynamic changes in chromatin and distribution of histone modifications. However, the chromatin-based regulation of this process remains to be elucidated. Here, we identify the conserved SWI-INDEPENDENT3 LIKE (SNL)-HISTONE DEACETYLASE19 (HDA19) deacetylase complex, including HDA19 and SNL1-SNL6, as a negative regulator of the light signaling pathway. Light-repression of HDA19 and SNLs expression is mediated by photoreceptors. HDA19 and SNLs are required for histone deacetylation and chromatin inactivation of PHYA gene. We further examined the interaction between SNL-HDA19 complex and ELONGATED HYPOCOTYL5 (HY5), and their antagonistic regulation on the expressions of target genes. The HDA19 deacetylase complex is recruited by HY5 to the chromatin regions of two positive light signaling genes, HY5 and B-BOX CONTAINING PROTEIN 22 (BBX22), thereby reduces the accessibility and histone acetylation and represses their expression. HDA19, SNL1, and HY5 associate with the same regulatory regions of HY5 and BBX22, and HY5 binding to these loci is enhanced upon SNL-HDA19 dysfunction. Our study reveals a crucial role for the HDA19 deacetylase complex in light signaling and demonstrates that the functional interplay between chromatin regulators and transcription factors regulates photomorphogenetic responses to the changing light environments.

Apple B-box protein BBX37 regulates jasmonic acid mediated cold tolerance through the JAZ-BBX37-ICE1-CBF pathway and undergoes MIEL1-mediated ubiquitination and degradation

The plant hormone jasmonic acid (JA) is involved in the cold stress response, and the inducer of CBF expression 1 (ICE1)- C-repeat binding factor (CBF) regulatory cascade plays a key role in the regulation of cold stress tolerance. In this study, we showed that a novel B-box (BBX) protein MdBBX37 positively regulates JA-mediated cold-stress resistance in apple. We found that MdBBX37 bound to the MdCBF1 and MdCBF4 promoters to activate their transcription, and also interacted with MdICE1 to enhance the transcriptional activity of MdICE1 on MdCBF1, thus promoting its cold tolerance. Two JA signaling repressors, MdJAZ1 and MdJAZ2 (JAZ, JAZMONATE ZIM-DOMAIN), interacted with MdBBX37 to repress the transcriptional activity of MdBBX37 on MdCBF1 and MdCBF4, and also interfered with the interaction between MdBBX37 and MdICE1, thus negatively regulating JA-mediated cold tolerance. E3 ligase MdMIEL1 (MIEL1, MYB30-Interacting E3 Ligase1) reduced MdBBX37-improved cold resistance by mediating ubiquitination and degradation of the MdBBX37 protein. The data reveal that MIEL1 and JAZ proteins co-regulate JA-mediated cold stress tolerance through the BBX37-ICE1-CBF module in apple. These results will aid further examination of the post-translational modification of BBX proteins and the regulatory mechanism of JA-mediated cold stress tolerance.

Genome-Wide Identification of the B-BOX Genes that Respond to Multiple Ripening Related Signals in Sweet Cherry Fruit

B-BOX proteins are zinc finger transcription factors that play important roles in plant growth, development, and abiotic stress responses. In this study, we identified 15 PavBBX genes in the genome database of sweet cherry. We systematically analyzed the gene structures, clustering characteristics, and expression patterns of these genes during fruit development and in response to light and various hormones. The PavBBX genes were divided into five subgroups. The promoter regions of the PavBBX genes contain cis-acting elements related to plant development, hormones, and stress. qRT-PCR revealed five upregulated and eight downregulated PavBBX genes during fruit development. In addition, PavBBX6, PavBBX9, and PavBBX11 were upregulated in response to light induction. We also found that ABA, BR, and GA₃ contents significantly increased in response to light induction. Furthermore, the expression of several PavBBX genes was highly correlated with the expression of anthocyanin biosynthesis genes, light-responsive genes, and genes that function in multiple hormone signaling pathways. Some PavBBX genes were strongly induced by ABA, GA, and BR treatment. Notably, PavBBX6 and PavBBX9 responded to all three hormones. Taken together, BBX proteins likely play major roles in regulating anthocyanin biosynthesis in sweet cherry fruit by integrating light, ABA, GA, and BR signaling pathways.

Developmental phenotypes of Arabidopsis plants expressing phosphovariants of AtMYB75

The Arabidopsis transcription factor Myeloblastosis protein 75 (MYB75, AT1G56650) is a well-established transcriptional activator of genes required for anthocyanin and flavonoid production, and a repressor of lignin and other secondary cell wall biosynthesis genes. MYB75 is itself tightly regulated at the transcriptional, translational and post-translational levels, including protein phosphorylation by Arabidopsis MAP kinases Examination of the behavior of different phosphovariant versions of MYB75 in vitro and in vivo revealed that overexpression of the MYB75^T131E phosphovariant had a particularly marked effect on global changes in gene expression suggesting that phosphorylated MYB75 could be involved in a broader range of functions than previously recognized. Here, we describe a range of distinct developmental phenotypes observed among Arabidopsis lines expressing various phosphovariant forms of MYB75. Expression of either MYB75^T131E or MYB75^T131A phosphovariants, from the endogenous MYB75 promoter, in Arabidopsis myb75^- mutants (Nossen background), resulted in severely impaired germination rates, and developmental arrest at early seedling stages. Arabidopsis plants overexpressing MYB75^T131E from a strong constitutive Cauliflower mosaic virus (CaMV35S) promoter displayed slower development, with delayed bolting, flowering and onset of senescence. Conversely, MYB75^T131A -overexpressing lines flowered and set seed earlier than either Col-0 WT controls or other MYB75-overexpressors (MYB75^WT and MYB75^T131E ). Histochemical analysis of mature stems also revealed ectopic vessel development in plants overexpressing MYB75; this phenotype was particularly prominent in the MYB75^T131E phosphovariant. These data suggest that MYB75 plays a significant role in plant development, and that this aspect of MYB75 function is influenced by its phosphorylation status.

Molecular pathways regulating elongation of aerial plant organs: a focus on light, the circadian clock, and temperature

Organs such as hypocotyls and petioles rapidly elongate in response to shade and temperature cues, contributing to adaptive responses that improve plant fitness. Growth plasticity in these organs is achieved through a complex network of molecular signals. Besides conveying information from the environment, this signaling network also transduces internal signals, such as those associated with the circadian clock. A number of studies performed in Arabidopsis hypocotyls, and to a lesser degree in petioles, have been informative for understanding the signaling networks that regulate elongation of aerial plant organs. In particular, substantial progress has been made towards understanding the molecular mechanisms that regulate responses to light, the circadian clock, and temperature. Signals derived from these three stimuli converge on the BAP module, a set of three different types of transcription factors that interdependently promote gene transcription and growth. Additional key positive regulators of growth that are also affected by environmental cues include the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSOR OF PHYA-105 (SPA) E3 ubiquitin ligase proteins. In this review we summarize the key signaling pathways that regulate the growth of hypocotyls and petioles, focusing specifically on molecular mechanisms important for transducing signals derived from light, the circadian clock, and temperature. While it is clear that similarities abound between the signaling networks at play in these two organs, there are also important differences between the mechanisms regulating growth in hypocotyls and petioles.

Genome-Wide Characterization of B-Box Gene Family and Its Roles in Responses to Light Quality and Cold Stress in Tomato

Perceiving incoming environmental information is critical for optimizing plant growth and development. Multiple B-box proteins (BBXs) play essential roles in light-dependent developmental processes in plants. However, whether BBXs function as a signal integrator between light and temperature in tomato plants remains elusive. In this study, 31 SlBBX genes were identified from the newly released tomato (Solanum lycopersicum) genome sequences and were clustered into five subgroups. Gene structure and protein motif analyses showed relatively high conservation of closely clustered SlBBX genes within each subgroup; however, genome mapping analysis indicated the uneven distribution of the SlBBX genes on tomato chromosomes. Promoter cis-regulatory elements prediction and gene expression indicated that SlBBX genes were highly responsive to light, hormones, and stress conditions. Reverse genetic approaches revealed that disruption of SlBBX7, SlBBX9, and SlBBX20 largely suppressed the cold tolerance of tomato plants. Furthermore, the impairment of SlBBX7, SlBBX9, and SlBBX20 suppressed the photosynthetic response immediately after cold stress. Due to the impairment of non-photochemical quenching (NPQ), the excess photon energy and electron flow excited by low temperature were not consumed in SlBBX7-, SlBBX9-, and SlBBX20- silenced plants, leading to the over reduction of electron carriers and damage of the photosystem. Our study emphasized the positive roles of light signaling transcription factors SlBBXs in cold tolerance in tomato plants, which may improve the current understanding of how plants integrate light and temperature signals to adapt to adverse environments.

BBX28/BBX29, HY5 and BBX30/31 form a feedback loop to fine-tune photomorphogenic development

Light is one of the key environmental cues controlling photomorphogenic development in plants. A group of B-box (BBX) proteins play critical roles in this developmental process through diverse regulatory mechanisms. In this study we report that BBX29 acts as a negative regulator of light signaling. BBX29 interacts with CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and undergoes COP1-mediated degradation in the dark. Mutant seedlings with loss of BBX29 function show shortened hypocotyls, while transgenic plants overexpressing BBX29 display elongated hypocotyls in the light. Both BBX28 and BBX29 interfere with the binding of ELONGATED HYPOCOTYL 5 (HY5) to the promoters of BBX30 and BBX31, consequently leading to the upregulation of their transcript levels. BBX30 and BBX31 associate with the promoter regions of BBX28 and BBX29, which in turn promotes the expression of these genes. Taken together, this study reveals a transcriptional feedback loop consisting of BBX28, BBX29, BBX30, BBX31, and HY5 that serves to fine-tune photomorphogenesis in response to light in plants.

A Positive Feedback Loop of BBX11-BBX21-HY5 Promotes Photomorphogenic Development in Arabidopsis

Light is the most important environmental factor affecting many aspects of plant development. In this study, we report that B-box protein 11 (BBX11) acts as a positive regulator of red light signaling. BBX11 loss-of-function mutant seedlings display significantly elongated hypocotyls under conditions of both red light and long day, whereas BBX11 overexpression causes markedly shortened hypocotyls under various light states. BBX11 binds to the HY5 promoter to activate its transcription, while both BBX21 and HY5 associate with the promoter of BBX11 to positively regulate its expression. Taken together, our results reveal positive feedback regulation of photomorphogenesis consisting of BBX11, BBX21, and HY5, thus substantiating a transcriptional regulatory mechanism in the response of plants to light during normal development.

Transcription factor PyHY5 binds to the promoters of PyWD40 and PyMYB10 and regulates its expression in red pear 'Yunhongli No. 1'

'Yunhongli No. 1' is a rare and well-colored red pear (Pyrus pyrifolia) germplasm resource, and is popular in the market due to its bright red color and high quality. Light induces the expression of transportation factor genes MYB10, WD40, and HY5, which then activate the expression of critical genes in the anthocyanin biosynthesis pathway to promote the synthesis and accumulation of anthocyanin, thus giving the red coloration. Protein HY5 is considered to be a key regulator for induction of anthocyanin biosynthesis. The MYB10 genes physically interact with HY5 to positively regulate anthocyanin biosynthesis in Arabidopsis, apple, and pear by binding to G-box motifs. However, how these transcription factors are regulated by sunlight remains unclear in 'Yunhongli No. 1'. In this study, the transcription factor PyHY5 was cloned, and subcellular localization assay showed that PyHY5 was distributed in the nucleus. The DNA fragments of PyHY5 had a typical BRLZ domain of the bZIP family, and then were aligned against the promoter sequences of PyMYB10 and PyWD40. Electrophoretic mobility shift and transient expression assays showed that PyHY5 could directly recognize and bind to the G-box motifs in the promoters of PyMYB10 and PyWD40, and so boosted transcriptional activation by co-expression. The results demonstrated that PyHY5 binding to G-box motifs of the promoters of PyMYB10 and PyWD40, enhanced its expression, and then promoted accumulation of anthocyanin in red 'Yunhongli No. 1'.

B-box proteins: Pivotal players in light-mediated development in plants

Light signals mediate a number of physiological and developmental processes in plants, such as flowering, photomorphogenesis, and pigment accumulation. Emerging evidence has revealed that a group of B-box proteins (BBXs) function as central players in these light-mediated developmental processes. B-box proteins are a class of zinc-coordinated transcription factors or regulators that not only directly mediate the transcription of target genes but also interact with various other factors to create a complex regulatory network involved in the precise control of plant growth and development. This review summarizes and highlights the recent findings concerning the critical regulatory functions of BBXs in photoperiodic flowering, light signal transduction and light-induced pigment accumulation and their molecular modes of action at the transcriptional and post-translational levels in plants.

Coordinating light responses between the nucleus and the chloroplast, a role for plant cryptochromes and phytochromes

To promote photomorphogenesis, including plastid development and metabolism, the phytochrome (phy) and the cryptochrome (cry) photoreceptors orchestrate genome-wide changes in gene expression in response to Red (R)- and Blue (B)-light cues. While phys and crys have a clear role in modulating photosynthesis, their role in the coordination of the nuclear genome and the plastome, essential for functional chloroplasts, remains underexplored. Using publicly available genome datasets for WT and phyABCDE or cry1cry2 Arabidopsis seedlings, grown, respectively, under R- or B-light, we bioinformatically analyzed the influence of light inputs and photoreceptors in the control of nuclear genes with a function in the chloroplast, and evaluated the role of phyB in the modulation of plastome-encoded genes. We show gene co-induction by R-phys and B-crys for genes with a chloroplastic function, and also apparent photoreceptor-driven preferential responses. Evidence from phyB in Arabidopsis together with published evidence from CRY2 in tomato also supports the participation of both photoreceptor families in the global modulation of the plastome genes. To begin addressing how these light-sensors orchestrate changes in an organellar genome, we evaluated their effect over genes with potential functions in plastid gene-expression regulation based on their TAIR annotation. Results indicate that both crys and phys modulate 'plastome-regulatory genes' with enrichment in the contribution of crys to all processes and of phys to post-transcription and transcription. Furthermore, we identified a new role for HY5 as a relevant light-signaling component in photoreceptor-based anterograde signaling leading to plastome gene regulation.

Identification of BBX proteins as rate-limiting cofactors of HY5

As a source of both energy and environmental information, monitoring of incoming light is crucial for plants to optimize growth throughout development¹. Concordantly, the light signalling pathways in plants are highly integrated with numerous other regulatory pathways^2,3. One of these signal integrators is the basic leucine zipper domain (bZIP) transcription factor LONG HYPOCOTYL 5 (HY5), which has a key role as a positive regulator of light signalling in plants^4,5. Although HY5 is thought to act as a DNA-binding transcriptional regulator^6,7, the lack of any apparent transactivation domain⁸ makes it unclear how HY5 is able to accomplish its many functions. Here we describe the identification of three B-box containing proteins (BBX20, BBX21 and BBX22) as essential partners for HY5-dependent modulation of hypocotyl elongation, anthocyanin accumulation and transcriptional regulation. The bbx20 bbx21 bbx22 (bbx202122) triple mutant mimics the phenotypes of hy5 in the light and its ability to suppress the cop1 mutant phenotype in darkness. Furthermore, 84% of genes that exhibit differential expression in bbx202122 are also regulated by HY5, and we provide evidence that HY5 requires the B-box proteins for transcriptional regulation. Finally, expression of a truncated dark-stable version of HY5 (HY5(ΔN77)) together with BBX21 mutated in its VP motif strongly promoted de-etiolation in dark-grown seedlings, demonstrating the functional interdependence of these factors. In sum, this work clarifies long-standing questions regarding HY5 action and provides an example of how a master regulator might gain both specificity and dynamicity through the obligate dependence of cofactors.

CmBBX8 accelerates flowering by targeting CmFTL1 directly in summer chrysanthemum

For a flowering plant, the transition from vegetative stage to reproductive growth is probably the most critical developmental switch. In the model plant Arabidopsis thaliana, the product of BBX7, group II member of BBX family, acts to delay floral transition. In this study, a presumed chrysanthemum homolog of a second group gene AtBBX8, designated CmBBX8, had been isolated and characterized. The transcription of CmBBX8 followed a diurnal rhythm as the chrysanthemum floral transition regulator. Overexpression of CmBBX8 accelerated flowering, while its (artificial microRNAs) amiR-enabled knockdown delayed flowering in plants grown under both long- and short-day conditions. Global expression analysis revealed that genes associated with photoperiod were down-regulated in amiR-CmBBX8 lines compared with the wild type, which were verified to be up-regulated in overexpressing lines (OX-CmBBX8) by RT-PCR. A number of in vitro assays were used to show that CmBBX8 targets CmFTL1. Furthermore, the function of CmFTL1 as a floral inducer under long-day conditions was confirmed by the behaviour of engineered summer-flowering chrysanthemum plants. The conclusion is that the BBX8-FT regulatory module is an important determinant of reproductive development in summer-flowering chrysanthemum.

Non-intrinsic ATP-binding cassette proteins ABCI19, ABCI20 and ABCI21 modulate cytokinin response at the endoplasmic reticulum in Arabidopsis thaliana

The non-intrinsic ABC proteins ABCI20 and ABCI21 are induced by light under HY5 regulation, localize to the ER, and ameliorate cytokinin-driven growth inhibition in young Arabidopsis thaliana seedlings. The plant ATP-binding cassette (ABC) I subfamily (ABCIs) comprises heterogeneous proteins containing any of the domains found in other ABC proteins. Some ABCIs are known to function in basic metabolism and stress responses, but many remain functionally uncharacterized. ABCI19, ABCI20, and ABCI21 of Arabidopsis thaliana cluster together in a phylogenetic tree, and are suggested to be targets of the transcription factor ELONGATED HYPOCOTYL 5 (HY5). Here, we reveal that these three ABCIs are involved in modulating cytokinin responses during early seedling development. The ABCI19, ABCI20 and ABCI21 promoters harbor HY5-binding motifs, and ABCI20 and ABCI21 expression was induced by light in a HY5-dependent manner. abci19 abci20 abci21 triple and abci20 abci21 double knockout mutants were hypersensitive to cytokinin in seedling growth retardation assays, but did not show phenotypic differences from the wild type in either control medium or auxin-, ABA-, GA-, ACC- or BR-containing media. ABCI19, ABCI20, and ABCI21 were expressed in young seedlings and the three proteins interacted with each other, forming a large protein complex at the endoplasmic reticulum (ER) membrane. These results suggest that ABCI19, ABCI20, and ABCI21 fine-tune the cytokinin response at the ER under the control of HY5 at the young seedling stage.

Genome-wide identification and analysis of B-BOX gene family in grapevine reveal its potential functions in berry development

BACKGROUND

The B-BOX (BBX) proteins are the class of zinc-finger transcription factors and can regulate plant growth, development, and endure stress response. In plants, the BBX gene family has been identified in Arabidopsis, rice, and tomato. However, no systematic analysis of BBX genes has been undertaken in grapevine.

RESULTS

In this study, 24 grapevine BBX (VvBBX) genes were identified by comprehensive bioinformatics analysis. Subsequently, the chromosomal localizations, gene structure, conserved domains, phylogenetic relationship, gene duplication, and cis-acting elements were analyzed. Phylogenetic analysis divided VvBBX genes into five subgroups. Numerous cis-acting elements related to plant development, hormone and/or stress responses were identified in the promoter of the VvBBX genes. The tissue-specific expressional dynamics of VvBBX genes demonstrated that VvBBXs might play important role in plant growth and development. The transcript analysis from transcriptome data and qRT-PCR inferred that 11 VvBBX genes were down-regulated in different fruit developmental stages, while three VvBBX genes were up-regulated. It is also speculated that VvBBX genes might be involved in multiple hormone signaling (ABA, ethylene, GA3, and CPPU) as transcriptional regulators to modulate berry development and ripening. VvBBX22 seems to be responsive to multiple hormone signaling, including ABA, ethylene GA3, and CPPU. Some VvBBX genes were strongly induced by Cu, salt, waterlogging, and drought stress treatment. Furthermore, the expression of VvBBX22 proposed its involvement in multiple functions, including leaf senescence, abiotic stress responses, fruit development, and hormone response.

CONCLUSIONS

Our results will provide the reference for functional studies of BBX gene family, and highlight its functions in grapevine berry development and ripening. The results will help us to better understand the complexity of the BBX gene family in abiotic stress tolerance and provide valuable information for future functional characterization of specific genes in grapevine.