Mobility of FLOWERING LOCUS T protein as a systemic signal in trifoliate orange and its low accumulation in grafted juvenile scions

FLOWERING LOCUS T1 and TERMINAL FLOWER1 regulatory networks mediate flowering initiation in apple

Flower bud formation is a critical process that directly determines yield and fruit quality in fruit crops. Floral induction is modulated by the balance between 2 flowering-related proteins, FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1); however, the mechanisms underlying the establishment and maintenance of this dynamic balance remain largely elusive. Here, we showed that in apple (Malus × domestica Borkh.), MdFT1 is predominantly expressed in spur buds and exhibits an increase in expression coinciding with flower induction; in contrast, MdTFL1 exhibited downregulation in apices during flower induction, suggesting that MdTFL1 has a role in floral repression. Interestingly, both the MdFT1 and MdTFL1 transcripts are directly regulated by transcription factor basic HELIX-LOOP-HELIX48 (MdbHLH48), and overexpression of MdbHLH48 in Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) results in accelerated flowering. Binding and activation analyses revealed that MdbHLH48 functions as a positive regulator of MdFT1 and a negative regulator of MdTFL1. Further studies established that both MdFT1 and MdTFL1 interact competitively with MdWRKY6 protein to facilitate and inhibit, respectively, MdWRKY6-mediated transcriptional activation of target gene APPLE FLORICAULA/LFY (AFL1, an apple LEAFY-like gene), ultimately regulating apple flower bud formation. These findings illustrate the fine-tuned regulation of flowering by the MdbHLH48-MdFT1/MdTFL1-MdWRKY6 module and provide insights into flower bud formation in apples.

ACC SYNTHASE4 inhibits gibberellin biosynthesis and FLOWERING LOCUS T expression during citrus flowering

Flowering is an essential process in fruit trees. Flower number and timing have a substantial impact on the yield and maturity of fruit. Ethylene and gibberellin (GA) play vital roles in flowering, but the mechanism of coordinated regulation of flowering in woody plants by GA and ethylene is still unclear. In this study, a lemon (Citrus limon L. Burm) 1-aminocyclopropane-1-carboxylic acid synthase gene (CiACS4) was overexpressed in Nicotiana tabacum and resulted in late flowering and increased flower number. Further transformation of citrus revealed that ethylene and starch content increased, and soluble sugar content decreased in 35S:CiACS4 lemon. Inhibition of CiACS4 in lemon resulted in effects opposite to that of 35S:CiACS4 in transgenic plants. Overexpression of the CiACS4-interacting protein ETHYLENE RESPONSE FACTOR3 (CiERF3) in N. tabacum resulted in delayed flowering and more flowers. Further experiments revealed that the CiACS4-CiERF3 complex can bind the promoters of FLOWERING LOCUS T (CiFT) and GOLDEN2-LIKE (CiFE) and suppress their expression. Moreover, overexpression of CiFE in N. tabacum led to early flowering and decreased flowers, and ethylene, starch, and soluble sugar contents were opposite to those in 35S:CiACS4 transgenic plants. Interestingly, CiFE also bound the promoter of CiFT. Additionally, GA3 and 1-aminocyclopropanecarboxylic acid (ACC) treatments delayed flowering in adult citrus, and treatment with GA and ethylene inhibitors increased flower number. ACC treatment also inhibited the expression of CiFT and CiFE. This study provides a theoretical basis for the application of ethylene to regulate flower number and mitigate the impacts of extreme weather on citrus yield due to delayed flowering.

Regulatory frameworks involved in the floral induction, formation and developmental programming of woody horticultural plants: a case study on blueberries

Flowering represents a crucial stage in the life cycles of plants. Ensuring strong and consistent flowering is vital for maintaining crop production amidst the challenges presented by climate change. In this review, we summarized key recent efforts aimed at unraveling the complexities of plant flowering through genetic, genomic, physiological, and biochemical studies in woody species, with a special focus on the genetic control of floral initiation and activation in woody horticultural species. Key topics covered in the review include major flowering pathway genes in deciduous woody plants, regulation of the phase transition from juvenile to adult stage, the roles of CONSTANS (CO) and CO-like gene and FLOWERING LOCUS T genes in flower induction, the floral regulatory role of GA-DELLA pathway, and the multifunctional roles of MADS-box genes in flowering and dormancy release triggered by chilling. Based on our own research work in blueberries, we highlighted the central roles played by two key flowering pathway genes, FLOWERING LOCUS T and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1, which regulate floral initiation and activation (dormancy release), respectively. Collectively, our survey shows both the conserved and diverse aspects of the flowering pathway in annual and woody plants, providing insights into the potential molecular mechanisms governing woody plants. This paves the way for enhancing the resilience and productivity of fruit-bearing crops in the face of changing climatic conditions, all through the perspective of genetic interventions.

New Advances in the Study of Regulation of Tomato Flowering-Related Genes Using Biotechnological Approaches

The tomato is a convenient object for studying reproductive processes, which has become a classic. Such complex processes as flowering and fruit setting require an understanding of the fundamental principles of molecular interaction, the structures of genes and proteins, the construction of signaling pathways for transcription regulation, including the synchronous actions of cis-regulatory elements (promoter and enhancer), trans-regulatory elements (transcription factors and regulatory RNAs), and transposable elements and epigenetic regulators (DNA methylation and acetylation, chromatin structure). Here, we discuss the current state of research on tomatoes (2017-2023) devoted to studying the function of genes that regulate flowering and signal regulation systems using genome-editing technologies, RNA interference gene silencing, and gene overexpression, including heterologous expression. Although the central candidate genes for these regulatory components have been identified, a complete picture of their relationship has yet to be formed. Therefore, this review summarizes the latest achievements related to studying the processes of flowering and fruit set. This work attempts to display the gene interaction scheme to better understand the events under consideration.

Advancing tree genomics to future proof next generation orchard production

The challenges facing tree orchard production in the coming years will be largely driven by changes in the climate affecting the sustainability of farming practices in specific geographical regions. Identifying key traits that enable tree crops to modify their growth to varying environmental conditions and taking advantage of new crop improvement opportunities and technologies will ensure the tree crop industry remains viable and profitable into the future. In this review article we 1) outline climate and sustainability challenges relevant to horticultural tree crop industries, 2) describe key tree crop traits targeted for improvement in agroecosystem productivity and resilience to environmental change, and 3) discuss existing and emerging genomic technologies that provide opportunities for industries to future proof the next generation of orchards.

Tandem Expression of a Mobile RNA and Its RNA-Binding Protein(s) Enhances Tuber Productivity in Potato

A significant number of discoveries in past two decades have established the importance of long-distance signaling in controlling plant growth, development, and biotic and abiotic stress responses. Numerous mobile signals, such as mRNAs, proteins, including RNA-binding proteins, small RNAs, sugars, and phytohormones, are shown to regulate various agronomic traits such as flowering, fruit, seed development, and tuberization. Potato is a classic model tuber crop, and several mobile signals are known to govern tuber development. However, it is unknown if these mobile signals have any synergistic effects on potato crop improvement. Here, we employed a simple innovative strategy to test the cumulative effects of a key mobile RNA, StBEL5, and its RNA-binding proteins, StPTB1, and -6 on tuber productivity of two potato cultivars, Solanum tuberosum cv. Désirée and subspecies andigena, using a multi-gene stacking approach. In this approach, the coding sequences of StBEL5 and StPTB1/6 are driven by their respective native promoters to efficiently achieve targeted expression in phloem for monitoring tuber productivity. We demonstrate that this strategy resulted in earliness for tuberization and enhanced tuber productivity by 2-4 folds under growth chamber, greenhouse, and field conditions. This multi-gene stacking approach could be adopted to other crops, whose agronomic traits are governed by mobile macromolecules, expanding the possibilities to develop crops with improved traits and enhanced yields.

Variation in floral form of CRISPR knock-outs of the poplar homologs of LEAFY and AGAMOUS after FT heat-induced early flowering

Plant migration and gene flow from genetically modified or exotic trees to nearby lands or by crossing with wild relatives is a major public and regulatory concern. Many genetic strategies exist to mitigate potential gene flow; however, the long delay in onset of flowering is a severe constraint to research progress. We used heat-induced FT overexpression to speed assessment of the expected floral phenotypes after CRISPR knockout of poplar homologs of the key floral genes, LEAFY and AGAMOUS. We selected events with previously characterized CRISPR-Cas9 induced biallelic changes then re-transformed them with the Arabidopsis thaliana FLOWERING LOCUS T (AtFT) gene under control of either a strong constitutive promoter or a heat-inducible promoter. We successfully obtained flowering in both a male and female clones of poplar, observing a wide range of inflorescence and floral forms among flowers, ramets, and insertion events. Overall, flowers obtained from the selected LFY and AG targeted events were consistent with what would be predicted for loss-of-function of these genes. LFY-targeted events showed small catkins with leaf-like organs, AG-targeted events had nested floral organs consistent with reduction in floral determinacy and absence of well-formed carpels or anthers. These findings demonstrate the great developmental plasticity of Populus flowers during genetically accelerated flowering, which may be of horticultural value. They also provide an informative early view of floral phenotypes and apparent sterility from knockouts of both these gene targets.

Citrus ACC synthase CiACS4 regulates plant height by inhibiting gibberellin biosynthesis

Dwarfism is an agronomic trait that has substantial effects on crop yield, lodging resistance, planting density, and a high harvest index. Ethylene plays an important role in plant growth and development, including the determination of plant height. However, the mechanism by which ethylene regulates plant height, especially in woody plants, remains unclear. In this study, a 1-aminocyclopropane-1-carboxylic acid synthase (ACC) gene (ACS), which is involved in ethylene biosynthesis, was isolated from lemon (Citrus limon L. Burm) and named CiACS4. Overexpression of CiACS4 resulted in a dwarf phenotype in Nicotiana tabacum and lemon and increased ethylene release and decreased gibberellin (GA) content in transgenic plants. Inhibition of CiACS4 expression in transgenic citrus significantly increased plant height compared with the controls. Yeast two-hybrid assays revealed that CiACS4 interacted with an ethylene response factor (ERF), CiERF3. Further experiments revealed that the CiACS4-CiERF3 complex can bind to the promoters of 2 citrus GA20-oxidase genes, CiGA20ox1 and CiGA20ox2, and suppress their expression. In addition, another ERF transcription factor, CiERF023, identified using yeast one-hybrid assays, promoted CiACS4 expression by binding to its promoter. Overexpression of CiERF023 in N. tabacum caused a dwarfing phenotype. CiACS4, CiERF3, and CiERF023 expression was inhibited and induced by GA3 and ACC treatments, respectively. These results suggest that the CiACS4-CiERF3 complex may be involved in the regulation of plant height by regulating CiGA20ox1 and CiGA20ox2 expression levels in citrus.

A bZIP transcription factor (CiFD) regulates drought- and low-temperature-induced flowering by alternative splicing in citrus

Drought and low temperature are two key environmental factors that induce adult citrus flowering. However, the underlying regulation mechanism is poorly understood. The bZIP transcription factor FD is a key component of the florigen activation complex (FAC) which is composed of FLOWERING LOCUS T (FT), FD, and 14-3-3 proteins. In this study, isolation and characterization of CiFD in citrus found that there was alternative splicing (AS) of CiFD, forming two different proteins (CiFDα and CiFDβ). Further investigation found that their expression patterns were similar in different tissues of citrus, but the subcellular localization and transcriptional activity were different. Overexpression of the CiFD DNA sequence (CiFD-DNA), CiFDα, or CiFDβ in tobacco and citrus showed early flowering, and CiFD-DNA transgenic plants were the earliest, followed by CiFDβ and CiFDα. Interestingly, CiFDα and CiFDβ were induced by low temperature and drought, respectively. Further analysis showed that CiFDα can form a FAC complex with CiFT, Ci14-3-3, and then bind to the citrus APETALA1 (CiAP1) promoter and promote its expression. However, CiFDβ can directly bind to the CiAP1 promoter independently of CiFT and Ci14-3-3. These results showed that CiFDβ can form a more direct and simplified pathway that is independent of the FAC complex to regulate drought-induced flowering through AS. In addition, a bHLH transcription factor (CibHLH96) binds to CiFD promoter and promotes the expression of CiFD under drought condition. Transgenic analysis found that CibHLH96 can promote flowering in transgenic tobacco. These results suggest that CiFD is involved in drought- and low-temperature-induced citrus flowering through different regulatory patterns.

Quantitative Analysis of Florigen for the Variability of Floral Induction in Cabbage/Radish Inter-generic Grafting

Grafting-induced flowering is a key phenomenon to understand systemic floral induction caused by florigen. It can also be used as a breeding technique enabling rapid seed production of crops with long generation times. However, the degree of floral induction in grafted plants is often variable. Moreover, it is difficult in some crop species. Here, we explored the factors promoting variability in the grafting-induced flowering of cabbage (Brassica oleracea L. var. capitata), an important vegetable crop with a long generation time, via the quantitative analysis of florigen accumulation. Significant variability in the flowering response of grafted cabbage was observed when rootstocks of different genotypes were used. As reported previously, B. oleracea rootstocks did not induce the flowering of grafted cabbage plants, but radish (Raphanus sativus L.) rootstocks unstably did, depending on the accessions used. Immunoblotting analysis of the FLOWERING LOCUS T (FT) protein, a main component of florigen, revealed that floral induction was quantitatively correlated with the level of accumulated FT protein in the grafted scion. To identify rootstock factors that cause variability in the floral induction of the grafted scion, we investigated FT protein accumulation and flowering response in grafted scions when the transcription levels of FT and the leaf area of rootstocks were altered by vernalization, daylength and leaf trimming treatments. We concluded that increasing the total amount of FT protein produced in the rootstock is important for the stable floral induction of the grafted cabbage, and this can be accomplished by increasing FT transcription and the leaf area of the rootstock.

Functional diversification and molecular mechanisms of FLOWERING LOCUS T/TERMINAL FLOWER 1 family genes in horticultural plants

Flowering is an important process in higher plants and is regulated by a variety of factors, including light, temperature, and phytohormones. Flowering restriction has a considerable impact on the commodity value and production cost of many horticultural crops. In Arabidopsis, the FT/TFL1 gene family has been shown to integrate signals from various flowering pathways and to play a key role in the transition from flower production to seed development. Studies in several plant species of the FT/TFL1 gene family have revealed it harbors functional diversity in the regulation of flowering. Here, we review the functional evolution of the FT/TFL1 gene family in horticulture plants and its unique regulatory mechanisms; in addition, the FT/TFL1 family of genes as an important potential breeding target is explored.