Light Quality-controlled Phytochemicals Biosynthesis in Vegetables and Fruits

Hong WANG, Ying WANG，Shuang XU，Weimin ZHU

Protected Horticultural Research Institute, Shanghai Academy of Agricultural Sciences, Shanghai/Key Laboratory of Protected Horticultural Technology, Shanghai 201403, China

Phytochemicals (beneficial nutrients) constitute one of the most important groups of the bioactive compounds in vegetables and fruits,such as proteins,lipids,carbohydrates, vitamins, minerals, phenolic compounds and so on. These compounds play diverse roles in lowering cardiovascular disease risk,protecting against cancer and promoting human nutrition[1-2].In recent years,the interest in healthy diets has increased.So consumers have attached great importance to the quality of vegetables and fruits, particularly to the nutrient content[3]. However, there is an in creasing evidence suggests that nutrient content in vegetables and fruits have significant reductions as time to harvest has declined and yield has in creased[4-5]. So increasing the nutrient content in vegetables and fruits, except for rapid growth and yield has become an important research issue for many researchers[6-7].

The phytochemicals of vegetables and fruits vary according to species, cultivar, light conditions,temperature, saline stress or fertilization[8-10].So specific treatments,including biotic (genetics) and abiotic (environment and agronomical conditions)can be used to enhance phytochemicals in vegetables and fruits[11-13]. As the only energy source and an important environmental signal, light plays an important role in regulating plant growth and development, plant photosynthesis in particular. There is a growing body of evidence that suggests phytochemical biosynthesis and accumulation are closely related to photosynthates in plants[14-15]. So reasonable light conditions play a significant role in the accumulation of phytochemicals in fruits and vegetables.

Plants can sense the different aspects of light in their growth environment including light intensity, duration(photoperiod), quality (specific wave lengths) and direction[16]. There is no doubt that light affects phytochemicalsaccumulation in vegetables and fruits.Abundant fruit shading and bagging experiments have confirmed the importance of light conditions on the accumulation of phytochemicals[17-19].Several recent reviews have been dealing with the different aspects of plant response to light[20-22]. In the present review, we focus on the role of light quality on the accumulation of phytochemicals in fruits and vegetables.

Light Quality Composition

Sunlight reaching the earth’s surface changes the spectral and energetic profile during the day and all the year round. It is the highest at noon and has higher peak in summer. The spectral irradiance of solar radiation is a mixture of UV radiation which includes UV-B (280-320 nm), UV-A/B(300-400 nm), photosynthetically active radiation (400-700 nm), far red(700-800 nm)and near infrared,ranging from 280 to 1100 nm[23]. The photosynthetically active radiation (PAR)is subdivided in different bands, which are blue (B, 400-500 nm), green (G,500-570 nm),yellow(Y,570-590 nm)and red(R,600-700 nm)light[24-25].

The Role of Photoreceptors

Light signals are perceived by at least four kinds of photoreceptor families to coordinate their response to the ambient light environment, including the phototropins and cryptochromes, which both sense UV-A and blue light, the phytochromes, which absorb red/far-red light, and UV-B photoreceptor (UVR8), which regulate both developmental and UV-protective processes[26].Plants employ these photoreceptors to coordinate their response to the ambient light environment[27-29].

For higher plants, phytochromes(PHYA, PHYB, PHYC, PHYD, PHYE)are the major photoreceptors to perceive the red(R)and far-red(FR)light(600-750 nm), and play a significant role in regulating almost every aspect of plant growth and development,such as seed germination, elongation of hypocotyls, shoot branching, circadian rhythms, and flowering time[30-31].Cryptochromes, which were the first plant blue-light receptors to be recognized at the molecular level[32], are flavin-containing photoreceptors for UV-A, blue and green light. There are three cryptochromes identified in Arabidopsis(CRY1,CRY2,CRY3).Cryptochromes are the major regulators of plant photomorphogenic, regulating various aspects of plant development and growth, including the accumulation of secondary metabolites,such as flavonoids[33-34].Phototropins are bluelight receptors, and named after their roles in adjusting higher plant phototropism[35]. Phototropins are light-activated serine/threonine kinases.There are two phototropins (PHOT1,PHOT2) in Arabidopsis, which regulate a range of photoresponses that serve to optimize photosynthetic efficiency and promote growth particularly under weak light conditions[36].Furthermore recent findings have suggested that phototropins also play a role in the biosynthesis of secondary metabolites[37].

Light Quality-controlled Phytochemicals Biosynthesis

Phytochemicals can be divided into at least two classes:those that are directly influenced by light stimulus,such as carotenoids, phenolics and those that are indirectly affected through metabolic pathways such as ascorbic acid, tocopherols. While it is widely known that light intensity can positively affect phytochemicals accumulation,the effects of light quality are more complex and often reported with mixed results[38].

Ascorbic acid

Ascorbic acid is a molecule that produced in different plant organs. It is considered that ascorbic acid is accumulated in photosynthetic tissues,but high content can also be found in fruit[39]. In plant, ascorbic acid is also involved in growth, defense, and in inducing oxidative stress[40]. Furthermore, AsA is of vital importance in the prevention of scorbutic symptoms in the human body[41]. It was widely understand that ascorbic acid synthesis is stimulated by light[42]and some of the enzymes involved in its synthesis are more active in higher intensity of light[43]. Moreover,light quality is a key factor in influencing the biosynthesis and accumulation of AsA in plants.

It has been reported that in Euglena gracilis only blue light was found to mediate AsA increase[44]. This is consistent with the results of Cheng et al. who found that AsA content was higher grown under blue light and a mixture of red and blue light than that grown under red light[45]. Furthermore,Samuoliene et al. showed that green LED light is more effectively in enhancing vitamin C content in baby leaf lettuce compared with blue LED light[46]. On the other hand, Mastropasqua et al. found that the AsA content increased in blue light and red light with different levels, trends and timings, and the increase in AsA content observed after transfer oat segments from red light to blue light, suggesting that the pathways of red and blue light perception constitute an interactive network that triggers a signal transduction process that involves in gene expression and in controlling ascorbate biosynthesis[47]. It is possible that there is a co-cooperation between phytochrome and BL-photoreceptors[48]. And this is in agreement with other previous findings[49-50].Compared to the mustard seedlings grown in the dark, it has been found that the AsA content increased with illumination from red light[51].However,AsA biosynthesis also seems to be influenced by light quality in a cultivar-specific manner. Li and Kubota found that light quality did not produce a significant difference in AsA content in red leaf lettuce[52]. Furthermore, UV light has a negative effect on AsA biosynthesis[42].Liu and Yang found that AsA content was significantly decreased when pea seedlings were exposed to UV-A (365 nm)for 1 h at night.

Tocopherols

Tocopherols are lipophilic antioxidants and belong to the vitamin-E family.Tocopherols occur as α-,β-,γ-,and δ-forms, and the form is determined by the number and position of methyl groups on the aromatic ring[53].Different tocopherol forms have various roles to the total vitamin E activity.For example, α-tocopherol has the highest vitamin E activity, followed by β-tocopherol, γ-tocopherol, and δ-tocopherol[54-55]. For the human diet,peppers, asparagus, cabbage, broccoli, and eggplant are the richestsources of natural vitamin E. Recent founding has shown that the concentration of tocopherols in plant tissues is light related[56].An increased content of α-tocopherol under high light stress was observed in Arabidopsis[57-59]as well as other plant species.

Although it is known that the light irradiance level affect tocopherols concentration in plants, the knowledge regarding the effect of light quality on tocopherol is still limited. Samuolien(2012) found that supplemental blue and green lights have significantly positive effect on the total tocopherol content in red leaf ‘Multired 4’ baby lettuce and tocopherol contents was in order: 535 ＞505 ＞455 ＞470 nm[60].Furthermore, they found that shortterm irradiation with supplemental red 638 nm LEDs before harvesting in the greenhouse had a significant effect on tocopherols. In addition to visible light,UV light affects the biosynthesis of tocopherols in the certain vegetables and fruits. In tomato, α-tocopherolm was significantly decreased with the UV-C treatment compared to controls[61]. Studies on microgreens found that supplemental higher UV-A intensity(12.4 μmol/(m2·s)) practically all increased the α-tocopherol content.These evidences have been to support the idea that shorter supplemental UVA wavelengths at the higher intensity level were a kind of stress for the microgreens, and α-tocopherol content increased as the antioxidant to protect plant against stress[62].

Nitrate

Nitrate, presented in all plant tissues is considered as one of the quality characteristics in vegetables and fruits.Nitrate is not inherently toxic,but excessive ingested nitrate is known as a serious potential threat to human health, as it could be converted into nitrite,which in human body can cause carcinogenic nitrosamines[63]or methaemoglobinaemia[64]. It has been shown that vegetables are the primary source of nitrate, which contribute about 80% of human dietary intake of nitrate[65]. The large amount of nitrate contained in vegetables has seriously affected the human health[66-68].

Light has been known as one of the major factors affecting nitrate level in vegetables as to regulate the activity of nitrate reductase[69-70]. It was found that nitrate reductase is rapidly decreased in dark so nitrate was accumulated in dark period[71-72]. Among light factors, light quality is an important factor to influence nitrate accumulation[73]. Literature data show that red light can significantly increase the activity of nitrate reductase, however,blue light increased the nitrogen content and was less efficient in decreased the content of nitrate in plants than red light[74], which means that red light can significantly reduce the nitrate content in plants.But it was found that mixed red and blue light is more effective to decrease nitrate content, and this is in agreement with the conclusion that supplementation of blue component to red light promoted assimilation of nitrogen[75]. In accordance with this,nitrate content in lettuce was lowest at R/B ratio of 8[76], and Urbonaviciut et al.(2007) also found lower nitrate accumulation in lettuce was that grown under a combination of 86%red light and 14% blue light[77]. It appears that in the range of blue and red light show the most prominent effect for plant growth and increasing carbohydrate levels in plants[78-79].

The effect of UV radiation on the biosynthesis of nitrate also studied with microgreens.It is found that supplemental UV-A irradiance increased the nitrate content in the microgreens,this phenomenon may be because that UV-A and blue light depend on the same light photoreceptor in plant.Furthermore, Balakumar et al.(1999a, b)found that UV-B light has two target sites in the nitrate assimilation pathway which is catalyzed by the two metalloproteins,NR and NiR.UV-B resulted in the reduction in activities of NiR and NR. Balakumar et al.(1999a,b) revealed that NR is more sensitive to UV-B than NiR[80-81].

Carotenoids

Carotenoids are synthesized by photosynthetic organisms, including plants, algae and some non-photosynthetic bacteria and fungi whereas absent from animals[82].β-carotene and α-carotene present in food products are the primary precursors of vitamin A.In plants,carotenoids act as the antenna pigments to reduce the damage of photosynthetic components caused by the active triplet state of the chlorophyll molecule[83]. The main components of carotenoids such as zeaxanthin, lutein, and lycopene can reduce the probabilities of developing certain forms of cancer, cardiovascular diseases, vascular disease and delaying or curing age-related eye diseases[84-87].

As carotenoids are closely related to photosynthesis, the most important factors influencing carotenoid biosynthesis are light quality and quantity.Carotenoid concentrations in vegetables can be optimized by regulation of light quality. The literature data showed that plant pigments have specific light absorption spectra. Chlorophylls and carotenoids have higher light absorption occurred at 400-500 and 630-680 nm and lower at 530-610 nm[74]. In addition, the absorption peaks of various carotenoids were difference. Generally speaking, maximum absorption of lutein appeared at 448 nm, β-carotene at 454 nm, and xanthophylls at 446 nm[88-89].

Studies on the effects of light quality on carotenoid biosynthesis showed that UV-B light decreases βcarotene content; blue light increases β-carotene and xanthophylls content,and red light improve lutein and lycopene synthesis, whereas far-red light inhibits lycopene and β-carotene synthesis[90]. For example, Ohashi-Kaneko et al. found that the carotenoid content was higher grown under blue fluorescent lamps than that grown under white fluorescent lamps with the same PPFD (300 μmol/m2·s)in spinach[91]. Li et al. further showed that lutein and β-carotene concentrations in spinach were significantly increased when plants grown under blue fluorescent lamps with a PPFD at 300 μmol/(m2·s)[92]. On the other hand,some literature sources found that maximum biosynthesis of lutein and β-carotene occurred at red wavelengths of light[88]. They found that lutein and β-carotene concentrations were highest under red LED light (640 nm) and blue LED light (440 nm) in kale, respectively. Obviously, the effects of red and blue light on carotenoid content vary among species and cultivars.

Flavonoids

Flavonoids such as anthocyanins,flavonols, and proanthocyanidins(PAs) are important factors of quality and economic value of fruits and vegetables, because they influence on aroma, color, antioxidant properties and astringency[93]. Flavonoids compounds in fruits have drawn much attention for their contribution to human health through their antioxidant, anticarcinogenic, antiviral, and antimicrobial characteristics. Flavonols often have the roles in photoprotection and generally acted as free-radical scavengers and ultraviolet (UV) protectants.Anthocyan is the major determinant of plant colors and serves as a signal for pollinators in flowers. PAs can offer protection during the early stages of fruit development against pathogen attack and herbivory[94-95].

The accumulation of flavonoids is affected by light quality in various plants. It was found that blue and UVlight show the most significant effect in the accumulation of flavonoids in fruits and vegetables, often by increasing the expression of flavonoid pathway genes. UV-B, UV-A, and blue light were considered to increase gene expressions of chalcone synthase(CHS), the first committed step in the flavonoid biosynthetic pathway, mediated by specific photoreceptors in Arabidopsis[96-97].In poplar leaves,UV irradiation remarkably influenced the accumulation of flavonoid compounds,such as PA and flavonol[98],whereas in Arabidopsis flavonol was reported to be effective in screening out UV radiation[99-101]. Similarly, in grapevines, UV light induced biosynthesis of flavonol in the leaves and berry skins, which resulted in efficient UV protection[102-103].

Flavonoid biosynthesis increased by blue light has recently been demonstrated by phototropins at molecular level in strawberry, when expression of phototropin 2, FaPHOT2, was shown to increase in anthocyanin content[104]. Furthermore, knockdown of FaPHOT2, resulted in decreased anthocyanin content while overexpression increased accumulation of anthocyanins in strawberry fruit.Also, the overexpression of blue light sensing cryptochrome in tomato resulted in the accumulation of anthocyanins in tomato fruits[105].

The induction of anthocyanin production by visible light has been extensively studied in several plant species, and it was found that the composition of light quality regulated the biosynthesis of anthocyanins in Arabidopsis[106], lettuce (Lactuca sativa L.)[107], grape (Vitis vinifera L.)[108-109],strawberry (Fragaria x ananassa -eston- Duchesne ex Rozier)[110]. Anthocyanin biosynthesis is major associated with blue light through cryptochromes, but the regulation of anthocyanin biosynthesis via cryptochromes requires active phytochrome, and this requirement only needs low levels of either phytochrome A or phytochrome B.For example, in unripe strawberries, blue light remarkably increased the biosynthesis of anthocyanins[104]. In grape berries, anthocyanin concentrations were highest in blue light-treated skin,followed by red light treatment[112]. Interestingly, the accumulation of anthocyanin in response to light has been shown to continue in postharvest fruits.Blue light (40 μmol/(m2·s)) was significantly increased the total anthocyanin content in strawberry fruits after four days treatment at 5 ℃compared to the control fruits[113]. Meanwhile, the treatment also increased the activities of the enzymes of the general flavonoid pathway including phenylalanine ammonialyase (PAL), glucose-6 phosphatase(G6PC),cinnamate-4-hydroxylase (C4H), 4-coumarate,coenzyme A ligase (4CL), CHS, F3H,DFR,ANS,and UFGT.

Conclusions

In conclusion, phytochemicals composition in fruits and vegetables are strongly affected by surrounding light conditions,especially light quality.But variation in response can be different between and even within species. Furthermore, interaction of specific light quality with other environmental factors can also change the response markedly. Although light on the biosynthesis of phytochemicals has been extensively studied, molecular mechanisms of light controlled phytochemicals biosynthesis have only known a little. Understanding the light signaling machinery in fruits and vegetables is important to generate and select fruits and vegetables enriched with phytochemicals compounds to obtain desirable dietary and health-beneficial properties.The same compounds can also affect self-life of the fruits and vegetables. Moreover,several recent studies show that preand post-harvest treatments with selected light conditions have potential roles in commercial applications.

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