Fruit-localized phytochromes regulate plastid biogenesis, starch synthesis, and carotenoid metabolism in tomato

Recent Advances in Studying the Regulation of Fruit Ripening in Tomato Using Genetic Engineering Approaches

Tomato (Solanum lycopersicum L.) is one of the most commercially essential vegetable crops cultivated worldwide. In addition to the nutritional value, tomato is an excellent model for studying climacteric fruits' ripening processes. Despite this, the available natural pool of genes that allows expanding phenotypic diversity is limited, and the difficulties of crossing using classical selection methods when stacking traits increase proportionally with each additional feature. Modern methods of the genetic engineering of tomatoes have extensive potential applications, such as enhancing the expression of existing gene(s), integrating artificial and heterologous gene(s), pointing changes in target gene sequences while keeping allelic combinations characteristic of successful commercial varieties, and many others. However, it is necessary to understand the fundamental principles of the gene molecular regulation involved in tomato fruit ripening for its successful use in creating new varieties. Although the candidate genes mediate ripening have been identified, a complete picture of their relationship has yet to be formed. This review summarizes the latest (2017-2023) achievements related to studying the ripening processes of tomato fruits. This work attempts to systematize the results of various research articles and display the interaction pattern of genes regulating the process of tomato fruit ripening.

Carotenoids and Tocopherol Profiling in Fleshy Fruits

Carotenoids and tocopherols are among the most powerful lipophilic antioxidants accumulated in fruit and vegetable crops. This chapter describes a method for the separation and quantification of carotenoids/chlorophylls and tocopherols based on microextraction followed by reverse- and normal-phase HPLC, respectively. Using this method, high-throughput, accurate analysis of these compounds can be performed in leaf and fruit samples.

Loss of S-nitrosoglutathione reductase disturbs phytohormone homeostasis and regulates shoot side branching and fruit growth in tomato

S-Nitrosoglutathione plays a central role in nitric oxide (NO) homeostasis, and S-nitrosoglutathione reductase (GSNOR) regulates the cellular levels of S-nitrosoglutathione across kingdoms. Here, we investigated the role of endogenous NO in shaping shoot architecture and controlling fruit set and growth in tomato (Solanum lycopersicum). SlGSNOR silencing promoted shoot side branching and led to reduced fruit size, negatively impacting fruit yield. Greatly intensified in slgsnor knockout plants, these phenotypical changes were virtually unaffected by SlGSNOR overexpression. Silencing or knocking out of SlGSNOR intensified protein tyrosine nitration and S-nitrosation and led to aberrant auxin production and signaling in leaf primordia and fruit-setting ovaries, besides restricting the shoot basipetal polar auxin transport stream. SlGSNOR deficiency triggered extensive transcriptional reprogramming at early fruit development, reducing pericarp cell proliferation due to restrictions on auxin, gibberellin, and cytokinin production and signaling. Abnormal chloroplast development and carbon metabolism were also detected in early-developing NO-overaccumulating fruits, possibly limiting energy supply and building blocks for fruit growth. These findings provide new insights into the mechanisms by which endogenous NO fine-tunes the delicate hormonal network controlling shoot architecture, fruit set, and post-anthesis fruit development, emphasizing the relevance of NO-auxin interaction for plant development and productivity.

Girdling promotes tomato fruit enlargement by enhancing fruit sink strength and triggering cytokinin accumulation

Girdling is a horticultural technique that enhances fruit size by allocating more carbohydrates to fruits, yet its underlying mechanisms are not fully understood. In this study, girdling was applied to the main stems of tomato plants 14 days after anthesis. Following girdling, there was a significant increase in fruit volume, dry weight, and starch accumulation. Interestingly, although sucrose transport to the fruit increased, the fruit's sucrose concentration decreased. Girdling also led to an increase in the activities of enzymes involved in sucrose hydrolysis and AGPase, and to an upregulation in the expression of key genes related to sugar transport and utilization. Moreover, the assay of carboxyfluorescein (CF) signal in detached fruit indicated that girdled fruits exhibited a greater ability to take up carbohydrates. These results indicate that girdling improves sucrose unloading and sugar utilization in fruit, thereby enhancing fruit sink strength. In addition, girdling induced cytokinin (CK) accumulation, promoted cell division in the fruit, and upregulated the expression of genes related to CK synthesis and activation. Furthermore, the results of a sucrose injection experiment suggested that increased sucrose import induced CK accumulation in the fruit. This study sheds light on the mechanisms by which girdling promotes fruit enlargement and provides novel insights into the interaction between sugar import and CK accumulation.

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.

Response of photomorphogenesis and photosynthetic properties of sweet pepper seedlings exposed to mixed red and blue light

Various light spectra, especially red (RL) and blue light (BL), have great effects on physiological processes and growth of plants. Previously, we revealed that the plant photomorphogenesis and photosynthesis of sweet pepper was significantly altered under BL or mixed RL and BL. The present study aimed to elucidate how mixed RL and BL influences plant photosynthesis during photomorphogenesis. We examined the growth, plant morphology, photosynthetic response of sweet pepper seedlings under monochromatic RL, BL, different ratios of mixed RL and BL (9R1B, 6R1B, 3R1B, 1R1B, 1R3B) with the same photosynthetic photon flux density of 300 μmol·m^-2·s^-1. White light (WL) were used as a control. The findings showed that the elongation of hypocotyl and first internode as well as leaf expansion were all stimulated by RL, while significantly restrained by BL compared with WL. Conversely, the leaf development, biomass accumulation and photosynthetic properties were inhibited by RL but promoted by BL. Additionally, compared with WL and other treatments, 3R1B could significantly improve the net photosynthetic rate, gas exchange, photosynthetic electron transport capacity, photochemical efficiency, shoot and root biomass accumulation. Furthermore, seedlings grew robustly and exhibited the greatest value of seedling index when exposed to this treatment. Overall, these results suggested that pepper seedlings grown under 3R1B performed better, possibly due to the more balanced light spectrum. It was more conducive to improve the plant photomorphogenesis and photosynthesis of sweet pepper, and a higher biomass accumulation and energy utilization efficiency could be achieved simultaneously under this mixed light spectrum.

Salt-tolerant alternative crops as sources of quality food to mitigate the negative impact of salinity on agricultural production

An increase of abiotic stress tolerance and nutritive value of foods is currently a priority because of climate change and rising world population. Among abiotic stresses, salt stress is one of the main problems in agriculture. Mounting urbanization and industrialization, and increasing global food demand, are pressing farmers to make use of marginal lands affected by salinity and low-quality saline water. In that situation, one of the most promising approaches is searching for new sources of genetic variation like salt-tolerant alternative crops or underexploited crops. They are generally less efficient than cultivated crops in optimal conditions due to lower yield but represent an alternative in stressful growth conditions. In this review, we summarize the advances achieved in research on underexploited species differing in their genetic nature. First, we highlight advances in research on salt tolerance of traditional varieties of tomato or landraces; varieties selected and developed by smallholder farmers for adaptation to their local environments showing specific attractive fruit quality traits. We remark advances attained in screening a collection of tomato traditional varieties gathered in Spanish Southeast, a very productive region which environment is extremely stressing. Second, we explore the opportunities of exploiting the natural variation of halophytes, in particular quinoa and amaranth. The adaptation of both species in stressful growth conditions is becoming an increasingly important issue, especially for their cultivation in arid and semiarid areas prone to be affected by salinity. Here we present a project developed in Spanish Southeast, where quinoa and amaranth varieties are being adapted for their culture under abiotic stress targeting high quality grain.

SlBBX28 positively regulates plant growth and flower number in an auxin-mediated manner in tomato

SlBBX28 is a positive regulator of auxin metabolism and signaling, affecting plant growth and flower number in tomato B-box domain-containing proteins (BBXs) comprise a family of transcription factors that regulate several processes, such as photomorphogenesis, flowering, and stress responses. For this reason, attention is being directed toward the functional characterization of these proteins, although knowledge in species other than Arabidopsis thaliana remains scarce. Particularly in the tomato, Solanum lycopersicum, only three out of 31 SlBBX proteins have been functionally characterized to date. To deepen the understanding of the role of these proteins in tomato plant development and yield, SlBBX28, a light-responsive gene, was constitutively silenced, resulting in plants with smaller leaves and fewer flowers per inflorescence. Moreover, SlBBX28 knockdown reduced hypocotyl elongation in darkness-grown tomato. Analyses of auxin content and responsiveness revealed that SlBBX28 promotes auxin-mediated responses. Altogether, the data revealed that SlBBX28 promotes auxin production and signaling, ultimately leading to proper hypocotyl elongation, leaf expansion, and inflorescence development, which are crucial traits determining tomato yield.

Overexpression of PSY1 increases fruit skin and flesh carotenoid content and reveals associated transcription factors in apple (Malus × domestica)

Knowledge of the transcriptional regulation of the carotenoid metabolic pathway is still emerging and here, we have misexpressed a key biosynthetic gene in apple to highlight potential transcriptional regulators of this pathway. We overexpressed phytoene synthase (PSY1), which controls the key rate-limiting biosynthetic step, in apple and analyzed its effects in transgenic fruit skin and flesh using two approaches. Firstly, the effects of PSY overexpression on carotenoid accumulation and gene expression was assessed in fruit at different development stages. Secondly, the effect of light exclusion on PSY1-induced fruit carotenoid accumulation was examined. PSY1 overexpression increased carotenoid content in transgenic fruit skin and flesh, with beta-carotene being the most prevalent carotenoid compound. Light exclusion by fruit bagging reduced carotenoid content overall, but carotenoid content was still higher in bagged PSY fruit than in bagged controls. In tissues overexpressing PSY1, plastids showed accelerated chloroplast to chromoplast transition as well as high fluorescence intensity, consistent with increased number of chromoplasts and carotenoid accumulation. Surprisingly, the expression of other carotenoid pathway genes was elevated in PSY fruit, suggesting a feed-forward regulation of carotenogenesis when this enzyme step is mis-expressed. Transcriptome profiling of fruit flesh identified differentially expressed transcription factors (TFs) that also were co-expressed with carotenoid pathway genes. A comparison of differentially expressed genes from both the developmental series and light exclusion  treatment revealed six candidate TFs exhibiting strong correlation with carotenoid accumulation. This combination of physiological, transcriptomic and metabolite data sheds new light on plant carotenogenesis and TFs that may play a role in regulating apple carotenoid biosynthesis.

PIFs- and COP1-HY5-mediated temperature signaling in higher plants

Plants have to cope with the surrounding changing environmental stimuli to optimize their physiological and developmental response throughout their entire life cycle. Light and temperature are two critical environmental cues that fluctuate greatly during day-night cycles and seasonal changes. These two external signals coordinately control the plant growth and development. Distinct spectrum of light signals are perceived by a group of wavelength-specific photoreceptors in plants. PIFs and COP1-HY5 are two predominant signaling hubs that control the expression of a large number of light-responsive genes and subsequent light-mediated development in plants. In parallel, plants also transmit low or warm temperature signals to these two regulatory modules that precisely modulate the responsiveness of low or warm temperatures. The core component of circadian clock ELF3 integrates signals from light and warm temperatures to regulate physiological and developmental processes in plants. In this review, we summarize and discuss recent advances and progresses on PIFs-, COP1-HY5- and ELF3-mediated light, low or warm temperature signaling, and highlight emerging insights regarding the interactions between light and low or warm temperature signal transduction pathways in the control of plant growth.

Development and carotenoid synthesis in dark-grown carrot taproots require PHYTOCHROME RAPIDLY REGULATED1

Light stimulates carotenoid synthesis in plants during photomorphogenesis through the expression of PHYTOENE SYNTHASE (PSY), a key gene in carotenoid biosynthesis. The orange carrot (Daucus carota) synthesizes and accumulates high amounts of carotenoids in the taproot that grows underground. Contrary to other organs, light impairs carrot taproot development and represses the expression of carotenogenic genes, such as DcPSY1 and DcPSY2, reducing carotenoid accumulation. By means of RNA sequencing, in a previous analysis, we observed that carrot PHYTOCHROME RAPIDLY REGULATED1 (DcPAR1) is more highly expressed in the underground grown taproot compared with those grown in light. PAR1 is a transcriptional cofactor with a negative role in shade avoidance syndrome regulation in Arabidopsis (Arabidopsis thaliana) through the dimerization with PHYTOCHROME-INTERACTING FACTORs (PIFs), allowing a moderate synthesis of carotenoids. Here, we show that overexpressing AtPAR1 in carrot increases carotenoid production in taproots grown underground as well as DcPSY1 expression. The high expression of AtPAR1 and DcPAR1 led us to hypothesize a functional role of DcPAR1 that was verified through in vivo binding to AtPIF7 and overexpression in Arabidopsis, where AtPSY expression and carotenoid accumulation increased together with a photomorphogenic phenotype. Finally, DcPAR1 antisense carrot lines presented a dramatic decrease in carotenoid levels and in relative expression of key carotenogenic genes as well as impaired taproot development. These results suggest that DcPAR1 is a key factor for secondary root development and carotenoid synthesis in carrot taproot grown underground.

PHYTOCHROME-INTERACTING FACTORS: a promising tool to improve crop productivity

Light is a key determinant for plant growth, development, and ultimately yield. Phytochromes, red/far-red photoreceptors, play an important role in plant architecture, stress tolerance, and productivity. In the model plant Arabidopsis, it has been shown that PHYTOCHROME-INTERACTING FACTORS (PIFs; bHLH transcription factors) act as central hubs in the integration of external stimuli to regulate plant development. Recent studies have unveiled the importance of PIFs in crops. They are involved in the modulation of plant architecture and productivity through the regulation of cell division and elongation in response to different environmental cues. These studies show that different PIFs have overlapping but also distinct functions in the regulation of plant growth. Therefore, understanding the molecular mechanisms by which PIFs regulate plant development is crucial to improve crop productivity under both optimal and adverse environmental conditions. In this review, we discuss current knowledge of PIFs acting as integrators of light and other signals in different crops, with particular focus on the role of PIFs in responding to different environmental conditions and how this can be used to improve crop productivity.

Phytochrome-Mediated Light Perception Affects Fruit Development and Ripening Through Epigenetic Mechanisms

Phytochrome (PHY)-mediated light and temperature perception has been increasingly implicated as important regulator of fruit development, ripening, and nutritional quality. Fruit ripening is also critically regulated by chromatin remodeling via DNA demethylation, though the molecular basis connecting epigenetic modifications in fruits and environmental cues remains largely unknown. Here, to unravel whether the PHY-dependent regulation of fruit development involves epigenetic mechanisms, an integrative analysis of the methylome, transcriptome and sRNAome of tomato fruits from phyA single and phyB1B2 double mutants was performed in immature green (IG) and breaker (BK) stages. The transcriptome analysis showed that PHY-mediated light perception regulates more genes in BK than in the early stages of fruit development (IG) and that PHYB1B2 has a more substantial impact than PHYA in the fruit transcriptome, in both analyzed stages. The global profile of methylated cytosines revealed that both PHYA and PHYB1B2 affect the global methylome, but PHYB1B2 has a greater impact on ripening-associated methylation reprogramming across gene-rich genomic regions in tomato fruits. Remarkably, promoters of master ripening-associated transcription factors (TF) (RIN, NOR, CNR, and AP2a) and key carotenoid biosynthetic genes (PSY1, PDS, ZISO, and ZDS) remained highly methylated in phyB1B2 from the IG to BK stage. The positional distribution and enrichment of TF binding sites were analyzed over the promoter region of the phyB1B2 DEGs, exposing an overrepresentation of binding sites for RIN as well as the PHY-downstream effectors PIFs and HY5/HYH. Moreover, phyA and phyB1B2 mutants showed a positive correlation between the methylation level of sRNA cluster-targeted genome regions in gene bodies and mRNA levels. The experimental evidence indicates that PHYB1B2 signal transduction is mediated by a gene expression network involving chromatin organization factors (DNA methylases/demethylases, histone-modifying enzymes, and remodeling factors) and transcriptional regulators leading to altered mRNA profile of ripening-associated genes. This new level of understanding provides insights into the orchestration of epigenetic mechanisms in response to environmental cues affecting agronomical traits.

Integrated Transcriptomics and Metabolomics Analyses Reveal the Molecular Mechanisms of Red-light on Carotenoids Biosynthesis in Tomato Fruit

Carotenoids are the main pigments responsible of the coloration and account for the major antioxidant activity of tomato (Solanum lycopersicum L.) fruit. Significant increments in total carotenoids and lycopene levels were observed in tomato fruit illuminated by red-light relative to white light in previous studies, but the mechanism of carotenoids biosynthesis regulated by red-light is still unclear. In the present study, the influence of red-light on carotenoids biosynthesis in postharvest tomato fruit was conducted using targeted metabolomics and transcriptomic methods. A total of 25 differentially accumulated carotenoids and 1939 differentially expressed genes were isolated and identified. The results illustrated that the content of phytoene and lycopene were considerably higher in fruit treated with red-light than those with white light at 12 h. These differentially expressed genes are mainly enriched in plant hormone signal transduction, photosynthesis, secondary metabolite biosynthesis, and plant circadian. Moreover, from the results of co-expression network analysis, 15 transcription factors from red-light treated fruit were screened, of these, transcription factors of SlERF4, SlbHLH93 and SlIAA29, which involves in signal transduction of light and hormones, respectively, that may also play important roles in carotenoids biosynthesis regulated by red-light in tomato fruit. It is concluded that red-light enhanced carotenoids biosynthesis in postharvest tomato fruit and the mechanisms of enhanced carotenoids biosynthesis were not only associated with the direct regulation by red-light signaling, but also with the indirect regulation by hormonal signaling.

Flavonoid biosynthesis is differentially altered in detached and attached ripening bilberries in response to spectral light quality

Light spectral quality is known to affect flavonoid biosynthesis during fruit ripening. However, the response of fruits to different light conditions, when ripening autonomously from the parent plant (detached), has been less explored. In this study, we analyzed the effect of light quality on detached and naturally ripening (attached) non-climacteric wild bilberry (Vaccinium myrtillus L.) fruits accumulating high amounts of anthocyanins and flavonols. Our results indicated contrasting responses for the accumulation of phenolic compounds in the berries in response to red and blue light treatments. For detached berries, supplemental blue light resulted in the highest accumulation of anthocyanins, while naturally ripening berries had elevated accumulation under supplemental red light treatment. Both red and blue supplemental light increased the expression levels of all the major structural genes of the flavonoid pathway during ripening. Notably, the key regulatory gene of anthocyanin biosynthesis, VmMYBA1, was found to express fivefold higher under blue light treatment in the detached berries compared to the control. The red light treatment of naturally ripening berries selectively increased the delphinidin branch of anthocyanins, whereas in detached berries, blue light increased other anthocyanin classes along with delphinidins. In addition, red and far-red light had a positive influence on the accumulation of flavonols, especially quercetin and myricetin glycoside derivatives, in both ripening conditions. Our results of differential light effects on attached and detached berries, which lacks signaling from the mother plant, provide new insights in understanding the light-mediated regulatory mechanisms in non-climacteric fruit ripening.

Review: the effect of light on the key pigment compounds of photosensitive etiolated tea plant

BACKGROUND

Light is the ultimate energy source of plant photosynthesis, which has an important impact on the growth, development, physiology and biochemistry of tea plant. Photosensitive etiolated tea plant belongs to a kind of colored leaf plant, which is a physiological response to light intensity. Compared with conventional green bud and leaf of tea plant, the accumulation of pigment compounds (chlorophyll and carotenoids, etc.) closely related to a series of reactions of photosynthesis in photosensitive etiolated tea plant is reduced, resulting in the difference of leaf color of tea. This specific tea resource has high application value, among which high amino acid is one of its advantages. It can be used to process high-quality green tea with delicious taste and attractive aroma, which has been widely attention. The mechanism of the color presentation of the etiolated mutant tea leaves has been given a high topic and attention, especially, what changes have taken place in the pigment compounds of tea leaves caused by light, which makes the leaves so yellow. At present, there have been a lot of research and reports.

PURPOSE OF THE REVIEW

We describe the metabolism and differential accumulation of key pigment compounds affecting the leaf color of photosensitive etiolated tea that are triggered by light, and discuss the different metabolism and key regulatory sites of these pigments in different light environments in order to understand the "discoloration" matrix and mechanism of etiolated tea resources, answer the scientific question between leaf color and light. It provides an important strategy for artificial intervention of discoloration of colored tea plant.

CONCLUSION

The differential accumulation of pigment compounds in tea plant can be induced phytochrome in response to the change of light signal. The synthesis of chlorophyll in photoetiolated tea plants is hindered by strong light, among which, the sites regulated by coproporphyrinogen III oxidase and chlorophyllide a oxidase is sensitive to light and can be inhibited by strong light, resulting in the aggravation of leaf etiolation. The phenomenon can be disappeared or weakened by shading or reducing light intensity, and the leaf color is greenish, but the increase of chlorophyll-b accumulation is more than that of chlorophyll-a. The synthesis of carotenoids is inhibited strong light, and high the accumulation of carotenoids is reduced by shading. Most of the genes regulating carotenoids are up-regulated by moderate shading and down-regulated by excessive shading. Therefore, the accumulation of these two types of pigments in photosensitive etiolated tea plants is closely related to the light environment, and the leaf color phenotype shape of photosensitive etiolated tea plants can be changed by different light conditions, which provides an important strategy for the production and management of tea plant.

ABCG transporters export cutin precursors for the formation of the plant cuticle

The plant cuticle is deposited on the surface of primary plant organs, such as leaves, fruits, and floral organs, forming a diffusion barrier and protecting the plant against various abiotic and biotic stresses. Cutin, the structural polyester of the plant cuticle, is synthesized in the apoplast. Plasma-membrane-localized ATP-binding cassette (ABC) transporters of the G family have been hypothesized to export cutin precursors. Here, we characterize SlABCG42 of tomato representing an ortholog of AtABCG32 in Arabidopsis. SlABCG42 expression in Arabidopsis complements the cuticular deficiencies of the Arabidopsis pec1/abcg32 mutant. RNAi-dependent downregulation of both tomato genes encoding proteins highly homologous to AtABCG32 (SlABCG36 and SlABCG42) leads to reduced cutin deposition and formation of a thinner cuticle in tomato fruits. By using a tobacco (Nicotiana benthamiana) protoplast system, we show that AtABCG32 and SlABCG42 have an export activity for 10,16-dihydroxy hexadecanoyl-2-glycerol, a cutin precursor in vivo. Interestingly, also free ω-hydroxy hexadecanoic acid as well as hexadecanedioic acid were exported, furthering the research on the identification of cutin precursors in vivo and the respective mechanisms of their integration into the cutin polymer.

The new kid on the block: a dominant-negative mutation of phototropin1 enhances carotenoid content in tomato fruits

Phototropins, the UVA-blue light photoreceptors, endow plants to detect the direction of light and optimize photosynthesis by regulating positioning of chloroplasts and stomatal gas exchange. Little is known about their functions in other developmental responses. A tomato Non-phototropic seedling1 (Nps1) mutant, bearing an Arg495His substitution in the vicinity of LOV2 domain in phototropin1, dominant-negatively blocks phototropin1 responses. The fruits of Nps1 mutant were enriched in carotenoids, particularly lycopene, compared with its parent, Ailsa Craig. On the contrary, CRISPR/CAS9-edited loss of function phototropin1 mutants displayed subdued carotenoids compared with the parent. The enrichment of carotenoids in Nps1 fruits is genetically linked with the mutation and exerted in a dominant-negative fashion. Nps1 also altered volatile profiles with high levels of lycopene-derived 6-methyl 5-hepten2-one. The transcript levels of several MEP and carotenogenesis pathway genes were upregulated in Nps1. Nps1 fruits showed altered hormonal profiles with subdued ethylene emission and reduced respiration. Proteome profiles showed a causal link between higher carotenogenesis and increased levels of protein protection machinery, which may stabilize proteins contributing to MEP and carotenogenesis pathways. The enhancement of carotenoid content by Nps1 in a dominant-negative fashion offers a potential tool for high lycopene-bearing hybrid tomatoes.

The conserved brassinosteroid-related transcription factor BIM1a negatively regulates fruit growth in tomato

Brassinosteroids (BRs) are steroid hormones that play key roles in plant development and defense. Our goal is to harness the extensive knowledge of the Arabidopsis BR signaling network to improve productivity in crop species. This first requires identifying components of the conserved network and their function in the target species. Here, we investigated the function of SlBIM1a, the closest tomato homolog of AtBIM1, which is highly expressed in fruit. SlBIM1a-overexpressing lines displayed severe plant and fruit dwarfism, and histological characterization of different transgenic lines revealed that SlBIM1a expression negatively correlated with fruit pericarp cell size, resulting in fruit size modifications. These growth phenotypes were in contrast to those found in Arabidopsis, and this was confirmed by the reciprocal ectopic expression of SlBIM1a/b in Arabidopsis and of AtBIM1 in tomato. These results determined that BIM1 function depends more on the recipient species than on its primary sequence. Yeast two-hybrid interaction studies and transcriptomic analyses of SlBIM1a-overexpressing fruit further suggested that SlBIM1a acts through its interaction with SlBZH1 to govern the transcriptional regulation of growth-related BR target genes. Together, these results suggest that SlBIM1a is a negative regulator of pericarp cell expansion, possibly at the crossroads with auxin and light signaling.

The light and dark sides of nitric oxide: multifaceted roles of nitric oxide in plant responses to light

Light drives photosynthesis and informs plants about their surroundings. Regarded as a multifunctional signaling molecule in plants, nitric oxide (NO) has been repeatedly demonstrated to interact with light signaling cascades to control plant growth, development and metabolism. During early plant development, light-triggered NO accumulation counteracts negative regulators of photomorphogenesis and modulates the abundance of, and sensitivity to, plant hormones to promote seed germination and de-etiolation. In photosynthetically active tissues, NO is generated at distinct rates under light or dark conditions and acts at multiple target sites within chloroplasts to regulate photosynthetic reactions. Moreover, changes in NO concentrations in response to light stress promote plant defenses against oxidative stress under high light or ultraviolet-B radiation. Here we review the literature on the interaction of NO with the complicated light and hormonal signaling cascades controlling plant photomorphogenesis and light stress responses, focusing on the recently identified molecular partners and action mechanisms of NO in these events. We also discuss the versatile role of NO in regulating both photosynthesis and light-dependent stomatal movements, two key determinants of plant carbon gain. The regulation of nitrate reductase (NR) by light is highlighted as vital to adjust NO production in plants living under natural light conditions.

Carotenoid Biosynthesis and Plastid Development in Plants: The Role of Light

Light is an important cue that stimulates both plastid development and biosynthesis of carotenoids in plants. During photomorphogenesis or de-etiolation, photoreceptors are activated and molecular factors for carotenoid and chlorophyll biosynthesis are induced thereof. In fruits, light is absorbed by chloroplasts in the early stages of ripening, which allows a gradual synthesis of carotenoids in the peel and pulp with the onset of chromoplasts' development. In roots, only a fraction of light reaches this tissue, which is not required for carotenoid synthesis, but it is essential for root development. When exposed to light, roots start greening due to chloroplast development. However, the colored taproot of carrot grown underground presents a high carotenoid accumulation together with chromoplast development, similar to citrus fruits during ripening. Interestingly, total carotenoid levels decrease in carrots roots when illuminated and develop chloroplasts, similar to normal roots exposed to light. The recent findings of the effect of light quality upon the induction of molecular factors involved in carotenoid synthesis in leaves, fruit, and roots are discussed, aiming to propose consensus mechanisms in order to contribute to the understanding of carotenoid synthesis regulation by light in plants.

Far-red radiation stimulates dry mass partitioning to fruits by increasing fruit sink strength in tomato

Far-red (FR) light promotes fruit growth by increasing dry mass partitioning to fruits, but the mechanism behind this is unknown. We hypothesise that it is due to an increased fruit sink strength as FR radiation enhances sugar transportation and metabolism. Tomato plants were grown with or without 50-80 μmol m^-2  s^-1 of FR radiation added to a common background 150-170 μmol m^-2  s^-1 red + blue light-emitting diode lighting. Potential fruit growth, achieved by pruning each truss to one remaining fruit, was measured to quantify fruit sink strength. Model simulation was conducted to test whether the measured fruit sink strength quantitatively explained the FR effect on dry mass partitioning. Starch, sucrose, fructose and glucose content were measured. Expression levels of key genes involved in sugar transportation and metabolism were determined. FR radiation increased fruit sink strength by 38%, which, in model simulation, led to an increased dry mass partitioned to fruits that quantitatively agreed very well with measured partitioning. FR radiation increased fruit sugar concentration and upregulated the expression of genes associated with both sugar transportation and metabolism. This is the first study to demonstrate that FR radiation stimulates dry mass partitioning to fruits mainly by increasing fruit sink strength via simultaneous upregulation of sugar transportation and metabolism.

Light and ripening-regulated BBX protein-encoding genes in Solanum lycopersicum

Light controls several aspects of plant development through a complex signalling cascade. Several B-box domain containing proteins (BBX) were identified as regulators of Arabidopsis thaliana seedling photomorphogenesis. However, the knowledge about the role of this protein family in other physiological processes and species remains scarce. To fill this gap, here BBX protein encoding genes in tomato genome were characterised. The robust phylogeny obtained revealed how the domain diversity in this protein family evolved in Viridiplantae and allowed the precise identification of 31 tomato SlBBX proteins. The mRNA profiling in different organs revealed that SlBBX genes are regulated by light and their transcripts accumulation is directly affected by the chloroplast maturation status in both vegetative and fruit tissues. As tomato fruits develops, three SlBBXs were found to be upregulated in the early stages, controlled by the proper chloroplast differentiation and by the PHYTOCHROME (PHY)-dependent light perception. Upon ripening, other three SlBBXs were transcriptionally induced by RIPENING INHIBITOR master transcriptional factor, as well as by PHY-mediated signalling and proper plastid biogenesis. Altogether, the results obtained revealed a conserved role of SlBBX gene family in the light signalling cascade and identified putative members affecting tomato fruit development and ripening.

Beyond the limits of photoperception: constitutively active PHYTOCHROME B2 overexpression as a means of improving fruit nutritional quality in tomato

Photoreceptor engineering has recently emerged as a means for improving agronomically beneficial traits in crop species. Despite the central role played by the red/far-red photoreceptor phytochromes (PHYs) in controlling fruit physiology, the applicability of PHY engineering for increasing fleshy fruit nutritional content remains poorly exploited. In this study, we demonstrated that the fruit-specific overexpression of a constitutively active GAF domain Tyr252 -to-His PHYB2 mutant version (PHYB2Y252H ) significantly enhances the accumulation of multiple health-promoting antioxidants in tomato fruits, without negative collateral consequences on vegetative development. Compared with the native PHYB2 overexpression, PHYB2Y252H -overexpressing lines exhibited more extensive increments in transcript abundance of genes associated with fruit plastid development, chlorophyll biosynthesis and metabolic pathways responsible for the accumulation of antioxidant compounds. Accordingly, PHYB2Y252H -overexpressing fruits developed more chloroplasts containing voluminous grana at the green stage and overaccumulated carotenoids, tocopherols, flavonoids and ascorbate in ripe fruits compared with both wild-type and PHYB2-overexpressing lines. The impacts of PHYB2 or PHYB2Y252H overexpression on fruit primary metabolism were limited to a slight promotion in lipid biosynthesis and reduction in sugar accumulation. Altogether, these findings indicate that mutation-based adjustments in PHY properties represent a valuable photobiotechnological tool for tomato biofortification, highlighting the potential of photoreceptor engineering for improving quality traits in fleshy fruits.

Phytochrome-Dependent Temperature Perception Modulates Isoprenoid Metabolism

Changes in environmental temperature influence many aspects of plant metabolism; however, the underlying regulatory mechanisms remain poorly understood. In addition to their role in light perception, phytochromes (PHYs) have been recently recognized as temperature sensors affecting plant growth. In particular, in Arabidopsis (Arabidopsis thaliana), high temperature reversibly inactivates PHYB, reducing photomorphogenesis-dependent responses. Here, we show the role of phytochrome-dependent temperature perception in modulating the accumulation of isoprenoid-derived compounds in tomato (Solanum lycopersicum) leaves and fruits. The growth of tomato plants under contrasting temperature regimes revealed that high temperatures resulted in coordinated up-regulation of chlorophyll catabolic genes, impairment of chloroplast biogenesis, and reduction of carotenoid synthesis in leaves in a PHYB1B2-dependent manner. Furthermore, by assessing a triple phyAB1B2 mutant and fruit-specific PHYA- or PHYB2-silenced plants, we demonstrated that biosynthesis of the major tomato fruit carotenoid, lycopene, is sensitive to fruit-localized PHY-dependent temperature perception. The collected data provide compelling evidence concerning the impact of PHY-mediated temperature perception on plastid metabolism in both leaves and fruit, specifically on the accumulation of isoprenoid-derived compounds.

Toward the 'golden' era: The status in uncovering the regulatory control of carotenoid accumulation in plants

Carotenoids are essential pigments to plants and important natural products to humans. Carotenoids as both primary and specialized metabolites fulfill multifaceted functions in plants. As such, carotenoid accumulation (a net process of biosynthesis, degradation and sequestration) is subjected to complicated regulation throughout plant life cycle in response to developmental and environmental signals. Investigation of transcriptional regulation of carotenoid metabolic genes remains the focus in understanding the regulatory control of carotenoid accumulation. While discovery of bona fide carotenoid metabolic regulators is still challenging, the recent progress of identification of various transcription factors and regulators helps us to construct hierarchical regulatory network of carotenoid accumulation. The elucidation of carotenoid regulatory mechanisms at protein level and in chromoplast provides some insights into post-translational regulation of carotenogenic enzymes and carotenoid sequestration in plastid sink. This review briefly describes the pathways and main flux-controlling steps for carotenoid accumulation in plants. It highlights our recent understanding of the regulatory mechanisms underlying carotenoid accumulation at both transcriptional and post-translational levels. It also discusses the opportunities to expand toolbox for further shedding light upon the intrinsic regulation of carotenoid accumulation in plants.

Molecular mechanisms underlying phytochrome-controlled morphogenesis in plants

Phytochromes are bilin-binding photosensory receptors which control development over a broad range of environmental conditions and throughout the whole plant life cycle. Light-induced conformational changes enable phytochromes to interact with signaling partners, in particular transcription factors or proteins that regulate them, resulting in large-scale transcriptional reprograming. Phytochromes also regulate promoter usage, mRNA splicing and translation through less defined routes. In this review we summarize our current understanding of plant phytochrome signaling, emphasizing recent work performed in Arabidopsis. We compare and contrast phytochrome responses and signaling mechanisms among land plants and highlight open questions in phytochrome research.

Downregulation of PHYTOCHROME-INTERACTING FACTOR 4 Influences Plant Development and Fruit Production

Plant development is highly dependent on the ability to perceive and cope with environmental changes. In this context, PIF proteins are key players in the cellular hub controlling responses to fluctuating light and temperature conditions. Reports in various plant species show that manipulation of the PIF4 level affects important agronomical traits. In tomato (Solanum lycopersicum), SlPIF1a and SlPIF3 regulate fruit nutraceutical composition. However, the wider role of this protein family, and the potential of their manipulation for the improvement of other traits, has not been explored. Here we report the effects of constitutive silencing of tomato SlPIF4 on whole-plant physiology and development. Ripening anticipation and higher carotenoid levels observed in SlPIF4-silenced fruits revealed a redundant role of SlPIF4 in the accumulation of nutraceutical compounds. Furthermore, silencing triggered a significant reduction in plant size, flowering, fruit yield, and fruit size. This phenotype was most likely caused by reduced auxin levels and altered carbon partitioning. Impaired thermomorphogenesis and delayed leaf senescence were also observed in silenced plants, highlighting the functional conservation of PIF4 homologs in angiosperms. Overall, this work improves our understanding of the role of PIF proteins-and light signaling-in metabolic and developmental processes that affect yield and composition of fleshy fruits.

Functional characterization of a strong promoter of the early light-inducible protein gene from tomato

The tomato ELIP gene promoter is mainly active in the ripening fruit. Considering its high activity, the promoter could be used for molecular breeding of plants in the future. The ability to obtain new varieties of transgenic plants with economically valuable traits relies on a high level of target gene expression, which is largely controlled by a gene promoter. Hence, research aimed at finding and characterizing new tissue-specific promoters that direct gene expression in specific plant tissues or at certain developmental stages has become the most important field of plant biotechnology. Here, we cloned and characterized the promoter of the early light-inducible protein (ELIP) gene from tomato (Solanum lycopersicum cv. Yalf). ELIPs are produced in the presence of light and putatively function in the chloroplast-to-chromoplast conversion, playing a photorepairing role in the photosynthetic system. Analysis of the promoter sequence revealed multiple cis-acting elements related to light responsiveness, and other motifs involved in plant hormone response and circadian control. To determine the functionality of the promoter, seven 5'-deletion variants were fused with the β-glucuronidase (GUS) reporter gene and introduced into tomato. Histochemical analysis of transgenic tomato plants revealed different levels of GUS activity in most analyzed tissues, depending on the promoter fragment used. The intensity of staining was considerably higher in ripening fruits than in unripe and non-fruit tissues. Quantitative analysis indicated that the level of GUS activity with the longest (full-length) version of the ELIP promoter in ripened fruits was comparable to that in plants expressing the constitutive CaMV35S promoter. Further, the location of both negative and positive regulatory motifs was identified. The described ELIP promoter is a potential tool for various applications in plant biotechnology.

PHYTOCHROME-INTERACTING FACTOR 3 mediates light-dependent induction of tocopherol biosynthesis during tomato fruit ripening

Tocopherols are important antioxidants exclusively produced in plastids that protect the photosynthetic apparatus from oxidative stress. These compounds with vitamin E activity are also essential dietary nutrients for humans. Although the tocopherol biosynthetic pathway has been elucidated, the mechanisms that regulate tocopherol production and accumulation remain elusive. Here, we investigated the regulatory mechanism underlying tocopherol biosynthesis during ripening in tomato fruits, which are an important source of vitamin E. Our results show that ripening under light conditions increases tocopherol fruit content in a phytochrome-dependent manner by the transcriptional regulation of biosynthetic genes. Moreover, we show that light-controlled expression of the GERANYLGERANYL DIPHOSPHATE REDUCTASE (SlGGDR) gene, responsible for the synthesis of the central tocopherol precursor phytyl diphosphate, is mediated by PHYTOCHROME-INTERACTING FACTOR 3 (SlPIF3). In the absence of light, SlPIF3 physically interacts with the promoter of SlGGDR, down-regulating its expression. By contrast, light activation of phytochromes prevents the interaction between SlPIF3 and the SlGGDR promoter, leading to transcriptional derepression and higher availability of the PDP precursor for tocopherol biosynthesis. The unraveled mechanism provides a new strategy to manipulate fruit metabolism towards improving tomato nutritional quality.

Solanum lycopersicum GOLDEN 2-LIKE 2 transcription factor affects fruit quality in a light- and auxin-dependent manner

Plastids are organelles responsible for essential aspects of plant development, including carbon fixation and synthesis of several secondary metabolites. Chloroplast differentiation and activity are highly regulated by light, and several proteins involved in these processes have been characterised. Such is the case of the GOLDEN 2-LIKE (GLK) transcription factors, which induces the expression of genes related to chloroplast differentiation and photosynthesis. The tomato (Solanum lycopersicum) genome harbours two copies of this gene, SlGLK1 and SlGLK2, each with distinct expression patterns. While the former predominates in leaves, the latter is mainly expressed in fruits, precisely at the pedicel region. During tomato domestication, the selection of fruits with uniform ripening fixed the mutation Slglk2, nowadays present in most cultivated varieties, what penalised fruit metabolic composition. In this study, we investigated how SlGLK2 is regulated by light, auxin and cytokinin and determined the effect of SlGLK2 on tocopherol (vitamin E) and sugar metabolism, which are components of the fruit nutritional and industrial quality. To achieve this, transcriptional profiling and biochemical analysis were performed throughout fruit development and ripening from SlGLK2, Slglk2, SlGLK2-overexpressing genotypes, as well as from phytochrome and hormonal deficient mutants. The results revealed that SlGLK2 expression is regulated by phytochrome-mediated light perception, yet this gene can induce chloroplast differentiation even in a phytochrome-independent manner. Moreover, auxin was found to be a negative regulator of SlGLK2 expression, while SlGLK2 enhances cytokinin responsiveness. Additionally, SlGLK2 enhanced chlorophyll content in immature green fruits, leading to an increment in tocopherol level in ripe fruits. Finally, SlGLK2 overexpression resulted in higher total soluble solid content, possibly by the regulation of sugar metabolism enzyme-encoding genes. The results obtained here shed light on the regulatory network that interconnects SlGLK2, phytohormones and light signal, promoting the plastidial activity and consequently, influencing the quality of tomato fruit.

Light Regulation of Carotenoid Biosynthesis in the Peel of Mandarin and Sweet Orange Fruits

Carotenoids are the pigments responsible for the coloration of the peel and pulp of Citrus fruits. Light is one of the major environmental factors influencing coloration and carotenoid content and composition of fleshy fruits and therefore their commercial and nutritional quality. Agronomical observations indicate that citrus fruits exposed to sunlight develop a brighter peel coloration than shaded fruit inside the tree canopy. In the present study, the effect of light deprivation on carotenoid profile, and in the expression of genes of carotenoid metabolism and their precursors have been analyzed in fruits of Clemenules mandarin (Citrus clementine) and Navelina orange (Citrus sinensis). Fruit shading accelerated peel degreening, chlorophyll degradation, and reduced chloroplastic-type carotenoids. Time-course shading experiments revealed that the stage of fruit ripening appears to be determinant for the effect of darkness in carotenoid biosynthesis. Fruit shading produced a down-regulation of the expression of key carotenoids biosynthetic genes (PSY, PDS, ZDS1, LCY2a, LCY2b, and CHX). However, expression of MEP pathway genes (DXS, HDR1, and GGPPS1) and the carotenoid cleavage dioxygenase, CCD4b1, responsible of the formation of the apocarotenoid β-citraurin, were not substantially affected by dark-grown conditions. The content of abscisic acid (ABA), an end product of the carotenoid pathway, was not affected by the light regime, suggesting that effect of shading on the precursor's pool is not sufficient to impair ABA synthesis. A moderate increase in total carotenoid and in the expression of biosynthetic genes was observed in mature dark-grown mandarin and orange fruits. Collectively, results suggest that light stimulates carotenoid biosynthesis in the peel of citrus fruits but a light-independent regulation may also operate.

Light, Ethylene and Auxin Signaling Interaction Regulates Carotenoid Biosynthesis During Tomato Fruit Ripening

Light signaling and plant hormones, particularly ethylene and auxins, have been identified as important regulators of carotenoid biosynthesis during tomato fruit ripening. However, whether and how the light and hormonal signaling cascades crosstalk to control this metabolic route remain poorly elucidated. Here, the potential involvement of ethylene and auxins in the light-mediated regulation of tomato fruit carotenogenesis was investigated by comparing the impacts of light treatments and the light-hyperresponsive high pigment-2 (hp2) mutation on both carotenoid synthesis and hormonal signaling. Under either light or dark conditions, the overaccumulation of carotenoids in hp2 ripening fruits was associated with disturbed ethylene production, increased expression of genes encoding master regulators of ripening and higher ethylene sensitivity and signaling output. The increased ethylene sensitivity observed in hp2 fruits was associated with the differential expression of genes encoding ethylene receptors and downstream signaling transduction elements, including the downregulation of the transcription factor ETHYLENE RESPONSE FACTOR.E4, a repressor of carotenoid synthesis. Accordingly, treatments with exogenous ethylene promoted carotenoid biosynthetic genes more intensively in hp2 than in wild-type fruits. Moreover, the loss of HP2 function drastically altered auxin signaling in tomato fruits, resulting in higher activation of the auxin-responsive promoter DR5, severe down-regulation of AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) genes and altered accumulation of AUXIN RESPONSE FACTOR (ARF) transcripts. Both tomato ARF2 paralogues (Sl-ARF2a and SlARF2b) were up-regulated in hp2 fruits, which agrees with the promotive roles played by these ARFs in tomato fruit ripening and carotenoid biosynthesis. Among the genes differentially expressed in hp2 fruits, the additive effect of light treatment and loss of HP2 function was particularly evident for those encoding carotenoid biosynthetic enzymes, ethylene-related transcription factors, Aux/IAAs and ARFs. Altogether, the data uncover the involvement of ethylene and auxin as part of the light signaling cascades controlling tomato fruit metabolism and provide a new link between light signaling, plant hormone sensitivity and carotenoid metabolism in ripening fruits.