In vitro control of floral transition in tomato (Lycopersicon esculentum Mill.), the model for autonomously flowering plants, using the late flowering uniflora mutant

SlJMJ14, identified via QTL‑seq and fine mapping, controls flowering time in tomatoes

KEY MESSAGE

A major QTL, qLF2.1, for flowering time in tomatoes, was fine mapped to chromosome 2 within a 51.37-kb interval, and the SlJMJ14 gene was verified as the causal gene by knockout. Tomato flowering time is an important agronomic trait that affects yield, fruit quality, and environmental adaptation. In this study, the high-generation inbred line 19108 with a late-flowering phenotype was selected for the mapping of the gene that causes late flowering. In the F2 population derived from 19108 (late flowering) × MM (early flowering), we identified a major late-flowering time quantitative trait locus (QTL) using QTL-seq, designated qLF2.1. This QTL was fine mapped to a 51.37-kb genomic interval using recombinant analysis. Through functional analysis of homologous genes, Solyc02g082400 (SlJMJ14), encoding a histone demethylase, was determined to be the most promising candidate gene. Knocking out SlJMJ14 in MM resulted in a flowering time approximately 5-6 days later than that in the wild-type plants. These results suggest that mutational SlJMJ14 is the major QTL for the late-flowering phenotype of the 19108 parental line.

EARLY FLOWERING is a dominant gain-of-function allele of FANTASTIC FOUR 1/2c that promotes early flowering in tomato

Flowering time, an important factor in plant adaptability and genetic improvement, is regulated by various genes in tomato (Solanum lycopersicum). In this study, we characterized a tomato mutant, EARLY FLOWERING (EF), that developed flowers much earlier than its parental control. EF is a dominant gain-of-function allele with a T-DNA inserted 139 bp downstream of the stop codon of FANTASTIC FOUR 1/2c (FAF1/2c). The transcript of SlFAF1/2c was at elevated levels in the EF mutant. Overexpressing SlFAF1/2c in tomato plants phenocopied the early flowering trait of the EF mutant. Knocking out SlFAF1/2c in the EF mutant reverted the early flowering phenotype of the mutant to the normal flowering time of the wild-type tomato plants. SlFAF1/2c promoted the floral transition by shortening the vegetative phase rather than by reducing the number of leaves produced before the emergence of the first inflorescence. The COP9 signalosome subunit 5B (CSN5B) was shown to interact with FAF1/2c, and knocking out CSN5B led to an early flowering phenotype in tomato. Interestingly, FAF1/2c was found to reduce the accumulation of the CSN5B protein by reducing its protein stability. These findings imply that FAF1/2c regulates flowering time in tomato by reducing the accumulation and stability of CSN5B, which influences the expression of SINGLE FLOWER TRUSS (SFT), JOINTLESS (J) and UNIFLORA (UF). Thus, a new allele of SlFAF1/2c was discovered and found to regulate flowering time in tomato.

Physiological and Molecular Approaches for Developing Thermotolerance in Vegetable Crops: A Growth, Yield and Sustenance Perspective

Vegetables are a distinct collection of plant-based foods that vary in nutritional diversity and form an important part of the healthy diet of the human being. Besides providing basic nutrition, they have great potential for boosting human health. The balanced consumption of vegetables is highly recommended for supplementing the human body with better nutrition density, dietary fiber, minerals, vitamins, and bioactive compounds. However, the production and quality of fresh vegetables are influenced directly or indirectly by exposure to high temperatures or heat stress (HS). A decline in quality traits and harvestable yield are the most common effects of HS among vegetable crops. Heat-induced morphological damage, such as poor vegetative growth, leaf tip burning, and rib discoloration in leafy vegetables and sunburn, decreased fruit size, fruit/pod abortion, and unfilled fruit/pods in beans, are common, often rendering vegetable cultivation unprofitable. Further studies to trace down the possible physiological and biochemical effects associated with crop failure reveal that the key factors include membrane damage, photosynthetic inhibition, oxidative stress, and damage to reproductive tissues, which may be the key factors governing heat-induced crop failure. The reproductive stage of plants has extensively been studied for HS-induced abnormalities. Plant reproduction is more sensitive to HS than the vegetative stages, and affects various reproductive processes like pollen germination, pollen load, pollen tube growth, stigma receptivity, ovule fertility and, seed filling, resulting in poorer yields. Hence, sound and robust adaptation and mitigation strategies are needed to overcome the adverse impacts of HS at the morphological, physiological, and biochemical levels to ensure the productivity and quality of vegetable crops. Physiological traits such as the stay-green trait, canopy temperature depression, cell membrane thermostability, chlorophyll fluorescence, relative water content, increased reproductive fertility, fruit numbers, and fruit size are important for developing better yielding heat-tolerant varieties/cultivars. Moreover, various molecular approaches such as omics, molecular breeding, and transgenics, have been proved to be useful in enhancing/incorporating tolerance and can be potential tools for developing heat-tolerant varieties/cultivars. Further, these approaches will provide insights into the physiological and molecular mechanisms that govern thermotolerance and pave the way for engineering "designer" vegetable crops for better health and nutritional security. Besides these approaches, agronomic methods are also important for adaptation, escape and mitigation of HS protect and improve yields.

Nitrogen differentially modulates photosynthesis, carbon allocation and yield related traits in two contrasting Capsicum chinense cultivars

Yield-related traits of Capsicum chinense are highly dependent on coordination between vegetative and reproductive growth, since the formation of reproductive tissues occurs iteratively in new sympodial bifurcations. In this study, we used two C. chinense cultivars (Biquinho and Habanero), contrasting for fruit size and fruit set, to investigate the responses of nitrogen (N) deficiency and excess on growth, photosynthesis, carbon (C) and N metabolisms as well as yield-related traits. Both cultivars increased biomass allocation to leaves in conditions of higher N supply and exhibited a parabolic behavior for fruit biomass allocation. Plants growing under N-deficiency produced a lower number of flowers and heavier fruits. Contrarily, plants under high N condition tended to decrease their CO₂ assimilation rate, harvest index and fruit weight. Biquinho, the cultivar with lower fruit size and higher fruit set, was initially less affected by excess of N due to its continuous formation of new reproductive sinks in relation to Habanero (which has lower fruit set and higher fruit size). The results suggest that N amount influences sucrose supply to different organs and can differentially affect yield-related traits between Capsicum cultivars with contrasting source-sink relations.

The Effects of DNA Methylation Inhibition on Flower Development in the Dioecious Plant Salix Viminalis

DNA methylation, an important epigenetic modification, regulates the expression of genes and is therefore involved in the transitions between floral developmental stages in flowering plants. To explore whether DNA methylation plays different roles in the floral development of individual male and female dioecious plants, we injected 5-azacytidine (5-azaC), a DNA methylation inhibitor, into the trunks of female and male basket willow (Salix viminalis L.) trees before flower bud initiation. As expected, 5-azaC decreased the level of DNA methylation in the leaves of both male and female trees during floral development; however, it increased DNA methylation in the leaves of male trees at the flower transition stage. Furthermore, 5-azaC increased the number, length and diameter of flower buds in the female trees but decreased these parameters in the male trees. The 5-azaC treatment also decreased the contents of soluble sugars, starch and reducing sugars in the leaves of the female plants, while increasing them in the male plants at the flower transition stage; however, this situation was largely reversed at the flower development stage. In addition, 5-azaC treatment decreased the contents of auxin indoleacetic acid (IAA) in both male and female trees at the flower transition stage. These results indicate that hypomethylation in leaves at the flower transition stage promotes the initiation of flowering and subsequent floral growth in Salix viminalis, suggesting that DNA methylation plays a similar role in vegetative–reproductive transition and early floral development. Furthermore, methylation changes during the vegetative–reproductive transition and floral development were closely associated with the biosynthesis, metabolism and transportation of carbohydrates and IAA. These results provide insight into the epigenetic regulation of carbohydrate accumulation.

In Vitro Plant Regeneration of Zenia Insignis Chun

Zenia insignis Chun is a large, fast-growing deciduous tree. In this study, we successfully developed a reliable and efficient protocol for the regeneration of fertile plants via callus induction from leaf segments of young Z. insignis seedlings. The best results were obtained with a medium containing 11.00 μM 6-benzyladenine (6-BA), 1.20 μM indole-3-butytric acid (IBA), and 0.45 μM 2,4-dichlorophenoxyacetic acid (2,4-D), which yielded morphogenic callus within 2 weeks at a frequency of 62.23%. We tested the effect of IBA alone and in combination with 6-BA on the bud differentiation response of Z. insignis callus. Shoots differentiated normally when cultured on differentiation medium containing 6.00 μM 6-BA and 1.20 μM IBA. Regenerated buds elongated successfully in medium containing 1.20 μM gibberellic acid (GA3). The elongated shoots were then transferred to Murashige and Skoog basal medium supplemented with various combinations of naphthalene acetic acid (NAA) for root induction; well-developed roots were achieved on MS basal medium supplemented with 0.01 μM NAA at a rooting rate of 89.23%. Rooted plantlets were successfully acclimatised to a greenhouse at a survival rate exceeding 90.00%.

In vitro flowering in Oldenlandia umbellata L

BACKGROUND

Oldenlandia umbellata L. (Indian madder) is an antique Ayurvedic Indian herb and a source of various anthraquinone derivatives. The red dye from its roots has been used in diverse applications since ancient times.

OBJECTIVES

To establish reliable and effective protocols for in vitro flowering of O. umbellata.

MATERIALS AND METHODS

For in vitro flowering, organogenic calli were subcultured onto Murashige and Skoog (MS) medium supplemented with various concentrations of Naphthalene acetic acid (NAA) (0.15-1.0 mg/l) and Benzyladenine(BA) (0.5-1.5 mg/l) with and without 0.4% of coconut milk (CM).

RESULTS

The highest number of in vitro flowers (22.8%) and best response (92.73%) was achieved on MS medium supplemented with 0.7 mg/l NAA + 1.5 mg/l BA with 0.4% CM. It was found that MS medium devoid of BA promoted best root development (47.3 per calli) as well as response (100%). It was also observed that when embryogenic calli grown in depletion of required nutrition transferred to fresh media induced more flowering. In vivo and in vitro floral comparative analysis revealed that in vitro flower induction was required for short time duration (20.67 days) than in vivo flower.

CONCLUSION

To the best of our knowledge, this is the first report on in vitro flowering and this study will help to overcome problems associated with flower development and seed production. As a result, this study may be a potent conservation tool to restore innate population size in its natural habitat.

Manipulation of flowering time and branching by overexpression of the tomato transcription factor SlZFP2

Flowering of higher plants is orchestrated by complex regulatory networks through integration of various environmental signals such as photoperiod, temperature, light quality and developmental cues. In Arabidopsis, transcription of the flowering integrator gene FLOWERING LOCUS T (FT) that several flowering pathways converge to is directly regulated by more than ten transcription factors. However, very little is known about the transcriptional regulation of the FT homolog SINGLE FLOWER TRUESS (SFT) in the day-neutral plant tomato (Solanum lycopersicum). Previously, we showed that the zinc finger transcription factor SlZFP2 plays important roles in regulation of seed germination and fruit ripening in tomato and also found that overexpression of SlZFP2 impacted flowering and branching. Here, we characterized in detail the early flowering and high branching phenotypes by overexpression of this transcription factor. Our data showed that overexpression of SlZFP2 accelerated flowering in an SFT-dependent manner as demonstrated by elevated SFT expression in the leaves and the transcription factor's binding ability to SFT promoter in vitro and in vivo. Furthermore, overexpression of the SlZFP2 gene in the sft plants failed to rescue the mutant's late flowering. Through analysis of grafting phenotype, growth response of branches to auxin application and transcriptome profiling by RNA sequencing, we also showed that overexpression of SlZFP2 affected shoot apical dominance through multiple regulatory pathways. Our results suggest that the transcription factor SlZFP2 has potential applications in genetic modification of plant architecture and flowering time for tomato production and other crops as well.

Role of sugars under abiotic stress

Sugars are the most important regulators that facilitate many physiological processes, such as photosynthesis, seed germination, flowering, senescence, and many more under various abiotic stresses. Exogenous application of sugars in low concentration promote seed germination, up regulates photosynthesis, promotes flowering, delayed senescence under various unfavorable environmental conditions. However, high concentration of sugars reverses all these physiological process in a concentration dependent manner. Thus, this review focuses the correlation between sugars and their protective functions in several physiological processes against various abiotic stresses. Keeping in mind the multifaceted role of sugars, an attempt has been made to cover the role of sugar-regulated genes associated with photosynthesis, seed germination and senescence. The concentration of sugars determines the expression of these sugar-regulated genes. This review also enlightens the interaction of sugars with several phytohormones, such as abscisic acid, ethylene, cytokinins and gibberellins and its effect on their biosynthesis under abiotic stress conditions.

Delayed flowering is associated with lack of photosynthetic acclimation in Pigeon pea (Cajanus cajan L.) grown under elevated CO₂

In the present study, we investigated the likely consequences of future atmospheric CO2 concentrations [CO2] on growth, physiology and reproductive phenology of Pigeonpea. A short duration Pigeonpea cultivar (ICPL 15011) was grown without N fertilizer from emergence to final harvest in CO2 enriched atmosphere (open top chambers; 550μmolmol(-1)) for two seasons. CO2 enrichment improved both net photosynthetic rates (Asat) and foliar carbohydrate content by 36 and 43%, respectively, which further reflected in dry biomass after harvest, showing an increment of 29% over the control plants. Greater carboxylation rates of Rubisco (Vcmax) and photosynthetic electron transport rates (Jmax) in elevated CO2 grown plants measured during different growth periods, clearly demonstrated lack of photosynthetic acclimation. Further, chlorophyll a fluorescence measurements as indicated by Fv/Fm and ΔF/Fm' ratios justified enhanced photosystem II efficiency. Mass and number of root nodules were significantly high in elevated CO2 grown plants showing 58% increase in nodule mass ratio (NMR) which directly correlated with Pn. Growth under high CO2 showed significant ontogenic changes including delayed flowering. In conclusion, our data demonstrate that the lack of photosynthetic acclimation and increased carbohydrate-nitrogen reserves modulate the vegetative and reproductive growth patterns in Pigeonpea grown under elevated CO2.

Regulatory dephosphorylation of CDK at G₂/M in plants: yeast mitotic phosphatase cdc25 induces cytokinin-like effects in transgenic tobacco morphogenesis

BACKGROUND

During the last three decades, the cell cycle and its control by cyclin-dependent kinases (CDKs) have been extensively studied in eukaryotes. This endeavour has produced an overall picture that basic mechanisms seem to be largely conserved among all eukaryotes. The intricate regulation of CDK activities includes, among others, CDK activation by CDC25 phosphatase at G₂/M. In plants, however, studies of this regulation have lagged behind as a plant Cdc25 homologue or other unrelated phosphatase active at G₂/M have not yet been identified.

SCOPE

Failure to identify a plant mitotic CDK activatory phosphatase led to characterization of the effects of alien cdc25 gene expression in plants. Tobacco, expressing the Schizosaccharomyces pombe mitotic activator gene, Spcdc25, exhibited morphological, developmental and biochemical changes when compared with wild type (WT) and, importantly, increased CDK dephosphorylation at G₂/M. Besides changes in leaf shape, internode length and root development, in day-neutral tobacco there was dramatically earlier onset of flowering with a disturbed acropetal floral capacity gradient typical of WT. In vitro, de novo organ formation revealed substantially earlier and more abundant formation of shoot primordia on Spcdc25 tobacco stem segments grown on shoot-inducing media when compared with WT. Moreover, in contrast to WT, stem segments from transgenic plants formed shoots even without application of exogenous growth regulator. Spcdc25-expressing BY-2 cells exhibited a reduced mitotic cell size due to a shortening of the G₂ phase together with high activity of cyclin-dependent kinase, NtCDKB1, in early S-phase, S/G₂ and early M-phase. Spcdc25-expressing tobacco ('Samsun') cell suspension cultures showed a clustered, more circular, cell phenotype compared with chains of elongated WT cells, and increased content of starch and soluble sugars. Taken together, Spcdc25 expression had cytokinin-like effects on the characteristics studied, although determination of endogenous cytokinin levels revealed a dramatic decrease in Spcdc25 transgenics.

CONCLUSIONS

The data gained using the plants expressing yeast mitotic activator, Spcdc25, clearly argue for the existence and importance of activatory dephosphorylation at G₂/M transition and its interaction with cytokinin signalling in plants. The observed cytokinin-like effects of Spcdc25 expression are consistent with the concept of interaction between cell cycle regulators and phytohormones during plant development. The G₂/M control of the plant cell cycle, however, remains an elusive issue as doubts persist about the mode of activatory dephosphorylation, which in other eukaryotes is provided by Cdc25 phosphatase serving as a final all-or-nothing mitosis regulator.

The fission yeast mitotic activator cdc25 and sucrose induce early flowering synergistically in the day-neutral Nicotiana tabacum cv. Samsun

Here, the tobacco (Nicotiana tabacum) day-neutral (DN) cv. Samsun transformed with the Schizosaccharomyces pombe mitotic activator gene Spcdc25 was used to study the onset of flowering. Wild type (WT) and cdc25 plants were grown from seeds in vitro until they were 20 cm high. Apical and basal nodes were then subcultured repeatedly and the regenerated plants were used to document time to flowering and the number of leaves formed before flowering. Three sucrose treatments (3, 5 or 7% (weight/volume)) were used and measurements of leaf endogenous soluble carbohydrates were performed. In the 3% treatment, cdc25 plants flowered but WT plants did not. The higher sucrose treatments enabled WT flowering; two-thirds of the plants flowered at 5%, while all plants flowered at 7% sucrose. However, in all treatments, cdc25 plants exhibited significantly earlier flowering and fewer leaves compared with wild type. Remarkably, a typical acropetal flowering gradient in WT plants did not occur in cdc25 plants. In cdc25 leaves, there were significantly higher amounts of endogenous sugars with a higher proportion of sucrose compared with WT. Our data demonstrate that Spcdc25 expression and sucrose act synergistically to induce precocious flowering.

Nutrient sensing in plant meristems

Plants need nutrient to grow and plant cells need nutrient to divide. The meristems are the factories and cells that are left behind will expand and differentiate. However, meristems are not simple homogenous entities; cells in different parts of the meristem do different things. Positional cues operate that can fate cells into different tissue domains. However, founder/stem cells persist in specific locations within the meristem e.g. the quiescent centre of root apical meristem (RAM) and the lower half of the central zone of the shoot apical meristem (SAM). Given the complexity of meristems, do their cells simply respond to a diffusing gradient of photosynthate? This in turn begs the question, why do stem cell populations tend to have longer cell cycles than their immediate descendants given that like all other cells they are directly in the path of diffusing nutrient? In this review, we have examined the extent to which nutrient sensing might be operating in meristems. The scene is set for sugar sensing, the plant cell cycle, SAMs and RAMs. Special emphasis is given to the metabolic regulator, SnRK1 (SNF1-related protein kinase 1), hexokinase and the trehalose pathway in relation to sugar sensing. The unique plant cell cycle gene, cyclin-dependent kinase B1;1 may have evolved to be particularly responsive to sugar signalling pathways. Also, the homeobox gene, STIMPY, emerges strongly as a link between sugar sensing, plant cell proliferation and development. Flowering can be influenced by sucrose and glucose levels and both meristem identity and organ identity genes could well be differentially sensitive to sucrose and glucose signals. We also describe how meristems deal with extra photosynthate as a result of exposure to elevated CO2. What we review are numerous instances of how developmental processes can be affected by sugars/nutrients. However, given the scarcity of knowledge we are unable to provide uncontested links between nutrient sensing and specific activities in meristems.

Characterization of tomato (Solanum lycopersicum L.) mutants affected in their flowering time and in the morphogenesis of their reproductive structure

The impact of the season on flowering time and the organization and morphogenesis of the reproductive structures are described in three tomato mutants: compound inflorescence (s), single flower truss (sft), and jointless (j), respectively, compared with their wild-type cultivars Ailsa Craig (AC), Platense (Pl), and Heinz (Hz). In all environmental conditions, the sft mutant flowered significantly later than its corresponding Pl cultivar while flowering time in j was only marginally, but consistently, delayed compared with Hz. The SFT gene and, to a lesser extent, the J gene thus appear to be constitutive flowering promoters. Flowering in s was delayed in winter but not in summer compared with the AC cultivar, suggesting the existence of an environmentally regulated pathway for the control of floral transition. The reproductive structure of tomato is a raceme-like inflorescence and genes regulating its morphogenesis may thus be divided into inflorescence and floral meristem identity genes as in Arabidopsis. The s mutant developed highly branched inflorescences bearing up to 200 flowers due to the conversion of floral meristems into inflorescence meristems. The S gene appears to be a floral meristem identity gene. Both sft and j mutants formed reproductive structures containing flowers and leaves and reverting to a vegetative sympodial growth. The SFT gene appears to regulate the identity of the inflorescence meristem of tomato and is also involved, along with the J gene, in the maintenance of this identity, preventing reversion to a vegetative identity. These results are discussed in relation to knowledge accumulated in Arabidopsis and to domestication processes.

Control of plant development and gene expression by sugar signaling

Coordination of development with the availability of nutrients, such as soluble sugars, may help ensure an adequate supply of building materials and energy with which to carry out specific developmental programs. For example, in-vivo and in-vitro experiments suggest that increasing sugar levels delay seed germination and stimulate the induction of flowering and senescence in at least some plant species. Higher sugar concentrations can also increase the number of tubers formed by potatoes and can stimulate the formation of adventitious roots by Arabidopsis. New insights into the mechanisms by which sugar-response pathways interact with other response pathways have been provided by microarray experiments examining sugar-regulated gene expression under different light and nitrogen conditions.

UNIFLORA, a pivotal gene that regulates floral transition and meristem identity in tomato (Lycopersicon esculentum)

•  Flowering of uniflora (uf), a tomato (Lycopersicon esculentum) mutant which consistently produces solitary flowers instead of inflorescences, is late and highly asynchronous in winter. This puzzling behaviour prompted us to further investigate flowering regulation in this mutant to improve our understanding of UNIFLORA gene function. •  Growing plants under different daylengths and light intensities revealed that flowering time in uf is dependent on daily light energy integral. Transferring plants from low to high light energy integrals at different times after sowing showed that the light-conditions effect was stage dependent, suggesting that interactions between light energy integrals and endogenous regulatory pathways affect meristem sensitivity to flowering signals. •  Carbohydrate analyses suggested that one of these signals could be sucrose, but other interacting factors are probably generated by the root system, as indicated by grafting experiments. •  The UNIFLORA gene thus appears to have a dual role in tomato: floral transition regulation and the maintenance of inflorescence meristem identity.

C : N ratio increases in the phloem sap during floral transition of the long-day plants Sinapis alba and Arabidopsis thaliana

In plants of Sinapis alba and Arabidopsis thaliana, leaf exudate (phloem sap) was analysed during and after a single long day inducing flowering and in control short days. The amounts of carbohydrates and amino acids were measured to estimate the organic C : N ratio. In both species, the C : N ratio of the phloem sap increased markedly and early during the inductive treatment, suggesting that an inequality in organic C and N supply to the apical meristem may be important at floral transition.