Light Intensity and Floral Transition: Chloroplast Says "Time to Flower!"

A multi-omics approach identifies bHLH71-like as a positive regulator of yellowing leaf pepper mutants exposed to high-intensity light

Light quality and intensity can have a significant impact on plant health and crop productivity. Chlorophylls and carotenoids are classes of plant pigments that are responsible for harvesting light energy and protecting plants from the damaging effects of intense light. Our understanding of the role played by plant pigments in light sensitivity has been aided by light-sensitive mutants that change colors upon exposure to light of variable intensity. In this study, we conducted transcriptomic, metabolomic, and hormone analyses on a novel yellowing mutant of pepper (yl1) to shed light on the molecular mechanism that regulates the transition from green to yellow leaves in this mutant upon exposure to high-intensity light. Our results revealed greater accumulation of the carotenoid precursor phytoene and the carotenoids phytofluene, antheraxanthin, and zeaxanthin in yl1 compared with wild-type plants under high light intensity. A transcriptomic analysis confirmed that enzymes involved in zeaxanthin and antheraxanthin biosynthesis were upregulated in yl1 upon exposure to high-intensity light. We also identified a single basic helix-loop-helix (bHLH) transcription factor, bHLH71-like, that was differentially expressed and positively correlated with light intensity in yl1. Silencing of bHLH71-like in pepper plants suppressed the yellowing phenotype and led to reduced accumulation of zeaxanthin and antheraxanthin. We propose that the yellow phenotype of yl1 induced by high light intensity could be caused by an increase in yellow carotenoid pigments, concurrent with a decrease in chlorophyll accumulation. Our results also suggest that bHLH71-like functions as a positive regulator of carotenoid biosynthesis in pepper.

Chloroplasts prevent precocious flowering through a GOLDEN2-LIKE-B-BOX DOMAIN PROTEIN module

The timing of flowering is tightly controlled by signals that integrate environmental and endogenous cues. Sugars produced by carbon fixation in the chloroplast are a crucial endogenous cue for floral initiation. Chloroplasts also convey information directly to the nucleus through retrograde signaling to control plant growth and development. Here, we show that mutants defective in chlorophyll biosynthesis and chloroplast development flowered early, especially under long-day conditions, although low sugar accumulation was seen in some mutants. Plants treated with the bleaching herbicide norflurazon also flowered early, suggesting that chloroplasts have a role in floral repression. Among retrograde signaling mutants, the golden2-like 1 (glk1) glk2 double mutants showed early flowering under long-day conditions. This early flowering was completely suppressed by constans (co) and flowering locus t (ft) mutations. Leaf vascular-specific knockdown of both GLK1 and GLK2 phenocopied the glk1 glk2 mutants. GLK1 and GLK2 repress flowering by directly activating the expression of B-BOX DOMAIN PROTEIN 14 (BBX14), BBX15, and BBX16 via CCAATC cis-elements in the BBX genes. BBX14/15/16 physically interact with CO in the nucleus, and expression of BBXs hampered CO-mediated FT transcription. Simultaneous knockdown of BBX14/15/16 by artificial miRNA (35S::amiR-BBX14/15/16) caused early flowering with increased FT transcript levels, whereas BBX overexpression caused late flowering. Flowering of glk1/2 and 35S::amiR-BBX14/15/16 plants was insensitive to norflurazon treatment. Taking these observations together, we propose that the GLK1/2-BBX14/15/16 module provides a novel mechanism explaining how the chloroplast represses flowering to balance plant growth and reproductive development.

PEBP Signaling Network in Tubers and Tuberous Root Crops

Tubers and tuberous root crops are essential carbohydrate sources and staple foods for humans, second only to cereals. The developmental phase transition, including floral initiation and underground storage organ formation, is controlled by complex signaling processes involving the integration of environmental and endogenous cues. FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1/CENTRORADIALIS (TFL1/CEN), members of the phosphatidylethanolamine-binding protein (PEBP) gene family, play a central role in this developmental phase transition process. FT and FT-like proteins have a function to promote developmental phase transition, while TFL1/CEN act oppositely. The balance between FT and TFL1/CEN is critical to ensure a successful plant life cycle. Here, we present a summarized review of the role and signaling network of PEBP in floral initiation and underground storage organ formation, specifically in tubers and tuberous root crops. Lastly, we point out several questions that need to be answered in order to have a more complete understanding of the PEBP signaling network, which is crucial for the agronomical improvement of tubers and tuberous crops.

Integrated analysis of miRNAome transcriptome and degradome reveals miRNA-target modules governing floral florescence development and senescence across early- and late-flowering genotypes in tree peony

As a candidate national flower of China, tree peony has extremely high ornamental, medicinal and oil value. However, the short florescence and rarity of early-flowering and late-flowering varieties restrict further improvement of the economic value of tree peony. Specific miRNAs and their target genes engaged in tree peony floral florescence, development and senescence remain unknown. This report presents the integrated analysis of the miRNAome, transcriptome and degradome of tree peony petals collected from blooming, initial flowering, full blooming and decay stages in early-flowering variety Paeonia ostii 'Fengdan', an early-flowering mutant line of Paeonia ostii 'Fengdan' and late-flowering variety Paeonia suffruticosa 'Lianhe'. Transcriptome analysis revealed a transcript ('psu.G.00014095') which was annotated as a xyloglucan endotransglycosylase/hydrolase precursor XTH-25 and found to be differentially expressed across flower developmental stages in Paeonia ostii 'Fengdan' and Paeonia suffruticosa 'Lianhe'. The miRNA-mRNA modules were presented significant enrichment in various pathways such as plant hormone signal transduction, indole alkaloid biosynthesis, arachidonic acid metabolism, folate biosynthesis, fatty acid elongation, and the MAPK signaling pathway. Multiple miRNA-mRNA-TF modules demonstrated the potential functions of MYB-related, bHLH, Trihelix, NAC, GRAS and HD-ZIP TF families in floral florescence, development, and senescence of tree peony. Comparative spatio-temporal expression investigation of eight floral-favored miRNA-target modules suggested that transcript 'psu.T.00024044' and microRNA mtr-miR166g-5p are involved in the floral florescence, development and senescence associated agronomic traits of tree peony. The results might accelerate the understanding of the potential regulation mechanism in regards to floral florescence, development and abscission, and supply guidance for tree peony breeding of varieties with later and longer florescence characteristics.

Molecular mechanisms and evolutionary history of phytomelatonin in flowering

Flowering is a critical stage in plant life history, which is coordinated by environmental signals and endogenous cues. Phytomelatonin is a widely distributed indoleamine present in all living organisms and plays pleiotropic roles in plant growth and development. Recent evidence has established that phytomelatonin could modulate flowering in many species, probably in a concentration-dependent manner. Phytomelatonin seems to associate with floral meristem identification and floral organ formation, and the fluctuation of phytomelatonin might be important for flowering. Regarding the underlying mechanisms, phytomelatonin interacts with the central components of floral gene regulatory networks directly or indirectly, including the MADS-box gene family, phytohormones, and reactive oxygen species (ROS). From an evolutionary point of view, the actions of phytomelatonin in flowering probably evolved during the period of the diversification of flowering plants and could be regarded as a functional extension of its primary activities. The presumed evolutionary history of phytomelatonin-modulated flowering is proposed, presented in the chronological order of the appearance of phytomelatonin and core flowering regulators, namely DELLA proteins, ROS, and phytohormones. Further efforts are needed to address some intriguing aspects, such as the exploration of the association between phytomelatonin and photoperiodic flowering, phytomelatonin-related floral MADS-box genes, the crosstalk between phytomelatonin and phytohormones, as well as its potential applications in agriculture.

Transcriptome Analysis of Light-Regulated Monoterpenes Biosynthesis in Leaves of Mentha canadensis L

Light is a key environmental aspect that regulates secondary metabolic synthesis. The essential oil produced in mint (Mentha canadensis L.) leaves is used widely in the aromatics industry and in medicine. Under low-light treatment, significant reductions in peltate glandular trichome densities were observed. GC-MS analysis showed dramatically reduced essential oil and menthol contents. Light affected the peltate glandular trichomes’ development and essential oil yield production. However, the underlying mechanisms of this regulation were elusive. To identify the critical genes during light-regulated changes in oil content, following a 24 h darkness treatment and a 24 h recovery light treatment, leaves were collected for transcriptome analysis. A total of 95,579 unigenes were obtained, with an average length of 754 bp. About 56.58% of the unigenes were annotated using four public protein databases: 10,977 differentially expressed genes (DEGs) were found to be involved in the light signaling pathway and monoterpene synthesis pathway. Most of the TPs showed a similar expression pattern: downregulation after darkness treatment and upregulation after the return of light. In addition, the genes involved in the light signal transduction pathway were analyzed. A series of responsive transcription factors (TFs) were identified and could be used in metabolic engineering as an effective strategy for increasing essential oil yields.

Comparative transcriptome profiling of multi-ovary wheat under heterogeneous cytoplasm suppression

DUOII is a multi-ovary wheat line with two or three pistils and three stamens in each floret. The multi-ovary trait of DUOII is controlled by a dominant gene, whose expression can be suppressed by the heterogeneous cytoplasm of TeZhiI (TZI), a line with the nucleus of common wheat and the cytoplasm of Aegilops. DUOII (♀) × TZI (♂) shows multi-ovary trait, while TZI (♀) × DUOII (♂) shows mono-ovary. Observing the developmental process, we found that the critical stage of additional pistil primordium development was when the young spikes were 2–6 mm long. To elucidate the molecular mechanisms that are responsible for the heterogeneous cytoplasmic suppression of the multi-ovary gene, we RNA-sequenced the entire transcriptome of 2–6 mm long young spikes obtained from the reciprocal crosses between DUOII and TZI. A total of 600 differentially expressed genes (DEGs) was identified. Functional annotation of these DEGs showed that the heterogeneous cytoplasmic suppression of additional pistil development mainly involved four pathways, i.e., chloroplast metabolism, DNA replication and repair, hormone signal transduction, and trehalose-6-phosphate in the primordium development stage, which cooperated to modulate the multi-ovary gene expression under heterogeneous cytoplasmic suppression.

Comparative proteomic analysis of multi-ovary wheat under heterogeneous cytoplasm suppression

BACKGROUND

DUOII is a multi-ovary wheat (Triticum aestivum L.) line with two or three pistils and three stamens in each floret. The multi-ovary trait of DUOII is controlled by a dominant gene, whose expression can be suppressed by the heterogeneous cytoplasm of TeZhiI (TZI), a line with the nucleus of common wheat and the cytoplasm of Aegilops. Crosses between female DUOII plants and male TZI plants resulted in multi-ovary F₁s; whereas, the reciprocal crosses resulted in mono-ovary F₁s. Although the multi-ovary trait is inherited as single trait controlled by a dominant allele in lines with a Triticum cytoplasm, the mechanism by which the special heterogeneous cytoplasm suppresses the expression of multi-ovary is not well understood.

RESULTS

Observing the developmental process, we found that the critical stage of additional pistil primordium development was when the young spikes were 2-6 mm long. Then, we compared the quantitative proteomic profiles of 2-6 mm long young spikes obtained from the reciprocal crosses between DUOII and TZI. A total of 90 differentially expressed proteins were identified and analyzed based on their biological functions. These proteins had obvious functional pathways mainly implicated in chloroplast metabolism, nuclear and cell division, plant respiration, protein metabolism, and flower development. Importantly, we identified two key proteins, Flowering Locus K Homology Domain and PEPPER, which are known to play an essential role in the specification of pistil organ identity. By drawing relationships between the 90 differentially expressed proteins, we found that these proteins revealed a complex network which is associated with multi-ovary gene expression under heterogeneous cytoplasmic suppression.

CONCLUSIONS

Our proteomic analysis has identified certain differentially expressed proteins in 2-6 mm long young spikes, which was the critical stage of additional primordium development. This paper provided a universal proteomic profiling involved in the cytoplasmic suppression of wheat floral meristems; and our findings have laid a solid foundation for further mechanistic studies on the underlying mechanisms that control the heterogeneous cytoplasm-induced suppression of the nuclear multi-ovary gene in wheat.

Apple tree flowering is mediated by low level of melatonin under the regulation of seasonal light signal

Melatonin regulates the seasonal reproduction in photoperiodic sensitive animals. Its function in plants reproduction has not been extensively studied. In the current study, the effects of melatonin on the apple tree flowering have been systematically investigated. For consecutive 2-year monitoring, it was found that the flowering was always associated with the drop of melatonin level in apple tree. Melatonin application before flowering postponed apple tree flowering with a dose-dependent manner. The increased melatonin levels at a suitable range also resulted in more flowering. The data indicated that similar to the animals, the melatonin also serves as the signal of the environmental light to regulate the plant reproduction. It was mainly the blue and far-red light to regulate the gene expression of melatonin synthetic enzymes and melatonin production in plants. The seasonal alterations of the blue and far-red lights coordinated well with the changes of the melatonin levels and led to decreased melatonin level before flowering. The mechanism studies showed that melatonin per se inhibits all the four flowering pathways in apple. The results not only provide the basic knowledge for melatonin research, but also uncover melatonin as a chemical message of light signal to mediate plant reproduction. This information can be potentially used to control flowering period and prolong the harvest time, helpfully to open a new avenue for increasing crop yield by melatonin application.

OFP1 Interaction with ATH1 Regulates Stem Growth, Flowering Time and Flower Basal Boundary Formation in Arabidopsis

Ovate Family Protein1 (OFP1) is a regulator, and it is suspected to be involved in plant growth and development. Meanwhile, Arabidopsis Thaliana Homeobox (ATH1), a BEL1-like homeodomain (HD) transcription factor, is known to be involved in regulating stem growth, flowering time and flower basal boundary development in Arabidopsis. Previous large-scale yeast two-hybrid studies suggest that ATH1 possibly interact with OFP1, but this interaction is yet unverified. In our study, the interaction of OFP1 with ATH1 was verified using a directional yeast two-hybrid system and bimolecular fluorescence complementation (BiFC). Our results also demonstrated that the OFP1-ATH1 interaction is mainly controlled by the HD domain of ATH1. Meanwhile, we found that ATH1 plays the role of transcriptional repressor to regulate plant development and that OFP1 can enhance ATH1 repression function. Regardless of the mechanism, a putative functional role of ATH1-OFP1 may be to regulate the expression of the both the GA20ox1 gene, which is involved in gibberellin (GA) biosynthesis and control of stem elongation, and the Flowering Locus C (FLC) gene, which inhibits transition to flowering. Ultimately, the regulatory functional mechanism of OFP1-ATH1 may be complicated and diverse according to our results, and this work lays groundwork for further understanding of a unique and important protein⁻protein interaction that influences flowering time, stem development, and flower basal boundary development in plants.