CHLOROPLAST PIGMENTS and THEIR BIOSYNTHESIS IN RELATION TO LIGHT INTENSITY*

Effects of different light intensity on leaf color changes in a Chinese cabbage yellow cotyledon mutant

Leaf color is one of the most important phenotypic features in horticultural crops and directly related to the contents of photosynthetic pigments. Most leaf color mutants are determined by the altered chlorophyll or carotenoid, which can be affected by light quality and intensity. Our previous study obtained a Chinese cabbage yellow cotyledon mutant that exhibited obvious yellow phenotypes in the cotyledons and the new leaves. However, the underlying mechanisms in the formation of yellow cotyledons and leaves remain unclear. In this study, the Chinese cabbage yellow cotyledon mutant 19YC-2 exhibited obvious difference in leaf color and abnormal chloroplast ultrastructure compared to the normal green cotyledon line 19GC-2. Remarkably, low-intensity light treatment caused turn-green leaves and a significant decrease in carotenoid content in 19YC-2. RNA-seq analysis revealed that the pathways of photosynthesis antenna proteins and carotenoid biosynthesis were significantly enriched during the process of leaf color changes, and many differentially expressed genes related to the two pathways were identified to respond to different light intensities. Remarkably, BrPDS and BrLCYE genes related to carotenoid biosynthesis showed significantly higher expression in 19YC-2 than that in 19GC-2, which was positively related to the higher carotenoid content in 19YC-2. In addition, several differentially expressed transcription factors were also identified and highly correlated to the changes in carotenoid content, suggesting that they may participate in the regulatory pathway of carotenoid biosynthesis. These findings provide insights into the molecular mechanisms of leaf color changes in yellow cotyledon mutant 19YC-2 of Chinese cabbage.

Chemistry and biotechnology of carotenoids

Carotenoids are one of the most widespread groups of pigments in nature and more than 600 of these have been identified. Beside provitamin A activity, carotenoids are important as antioxidants and protective agents against various diseases. They are isoprenoids with a long polyene chain containing 3 to 15 conjugated double bonds, which determines their absorption spectrum. Cyclization at one or both ends occurs in hydrocarbon carotene, while xanthophylls are formed by the introduction of oxygen. In addition, modifications involving chain elongation, isomerization, or degradation are also found. The composition of carotenoids in food may vary depending upon production practices, post-harvest handling, processing, and storage. In higher plants they are synthesized in the plastid. Both mevalonate dependent and independent pathway for the formation of isopentenyl diphosphate are known. Isopentenyl diphosphate undergoes a series of addition and condensation reactions to form phytoene, which gets converted to lycopene. Cyclization of lycopene either leads to the formation of β-carotene and its derivative xanthophylls, β-cryptoxanthin, zeaxanthin, antheraxanthin, and violaxanthin or α-carotene and lutein. Even though most of the carotenoid biosynthetic genes have been cloned and identified, some aspects of carotenoid formation and manipulation in higher plants especially remain poorly understood. In order to enhance the carotenoid content of crop plants to a level that will be required for the prevention of diseases, there is a need for research in both the basic and the applied aspects.

Understanding carotenoid metabolism as a necessity for genetic engineering of crop plants

As a proof of concept, the qualitative and quantitative engineering of carotenoid formation has been achieved in crop plants. Successful reports in tomato, potato, rice, and canola all describe the enhancement of carotenoid with nutritional value, while in model systems such as tobacco and Arabidopsis the engineering of carotenoid to confer abiotic stress has been described. For all the successful applications there have been many examples of unintended/unpredicted phenotypes and results. Typically this has resided from our lack of understanding of carotenoid formation and its regulation. In the present article, we will review advances in carotenoid formation and its regulation to illustrate how metabolic engineering experiments have shed light on regulatory mechanisms.

Biochemical traits of lichens differing in relative desiccation tolerance

• Oxidative stress arises when desiccation restricts photosynthesis and light energy is transferred from photo-excited pigments onto ground state oxygen. We tested whether a highly desiccation tolerant lichen, Pseudevernia furfuracea, displays better protection against oxidative stress than more sensitive species, Lobaria pulmonaria and Peltigera polydactyla. • We rehydrated lichens after desiccation periods of 2, 7 and 9 weeks and assessed their viability by measuring CO₂ exchange using IRGA. During desiccation and rehydration, photosynthetic pigments and the antioxidant α-tocopherol were analysed by HPLC, and peroxidases by spectrophotometry. •  Pseudevernia furfuracea contained considerably lower chlorophyll, α-tocopherol and β-carotene concentrations and peroxidase activity than the two other lichens. However, it recovered photosynthesis rapidly, even after remaining in the desiccated state for 2 months while there was a significant delay in the onset of photosynthesis in L. pulmonaria and P. polydactyla. • We conclude that high antioxidant concentrations do not necessarily indicate better adaptation to desiccation. Rather, the ability to rapidly re-establish the species-specific normal antioxidant concentrations during rehydration, even after longer desiccation times, is a characteristic of well-adapted species.

Light-dark regulation of carotenoid biosynthesis in pepper (Capsicum annuum) leaves

The carotenoid content in photosynthetic plant tissue reflects a steady state value resulting from permanent biosynthesis and concurrent photo-oxidation. The contributions of both reactions were determined in illuminated pepper leaves. The amount of carotenoids provided by biosynthesis were quantified by the accumulation of the colourless carotenoid phytoene in the presence of the inhibitor norflurazon. When applied, substantial amounts of this rather photo-stable intermediate were formed in the light. However, carotenoid biosynthesis was completely stalled in darkness. This switch off in the absence of light is related to the presence of very low messenger levels of the phytoene synthase gene, psy and the phytoene desaturase gene, pds. Other carotenogenic genes, such as zds, ptox and Icy-b also were shown to be down-regulated to some extent. By comparison of the carotenoid concentration before and after transfer of plants to increasing light intensities and accounting for the contribution of biosynthesis, the rate of photo-oxidation was estimated for pepper leaves. It could be demonstrated that light-independent degradation or conversion of carotenoids e.g. to abscisic acid is a minor process.

Effect of light intensity on pigments and main acyl lipids during 'natural' chloroplast development in wheat seedlings

The content and composition of pigments and acyl lipids (monogalactosyl diacylglycerol, digalactosyl diacylglycerol and phosphatidyl glycerol) have been investigated in developing chloroplasts isolated from successive 2-cm sections along the leaves of wheat seedlings grown either under 100, 30 or 3 W·m(-2). In all examined stages of plastid development chlorophyll a/b and chlorophyll/carotenoid ratios were higher with increasing irradiance, whereas chlorophyll content expressed on fresh weight basis gradually decreased.Concentrations of monogalactosyl diacylglycerol, digalactosyl diacylglycerol and phosphatidyl glycerol decreased per chlorophyll unit with increasing plastid maturity. The higher was the light intensity applied during plant growth, the higher were galactolipid and phosphatidyl glycerol contents in developing chloroplasts. During plastid development the percentage of α-linolenic acid markedly increased in total and individual acyl lipids. Under high light conditions, the accumulation of this fatty acid proceeded more rapidly. Significantly higher proportion of α-linolenic acid was found in acyl lipid fraction of chloroplasts differentiating in high light grown plants, than in those from plants exposed to lower light intensities. The differences in the double bond index may indicate higher fluidity of thylakoid membranes in sun-type chloroplasts.Trans-3Δ-hexadecenoic acid, virtually absent in the youngest plastids, was found in much higher concentration (per chlorophyll unit and as mol % of phosphatidyl glycerol fatty acids) in chloroplasts developing at high light conditions.

Ribulose bisphosphate carboxylase capacity and chlorophyll content in developing seedlings of Chenopodium rubrum L. growing under light of different qualities and fluence rates

In order to evaluate the aclimation of Chenopodium seedlings to different quantum fluence rates of R and BL, kinetics of Rubisco capacity, Chl content and chloroplast structure were studied. Under monochromatic light photoreceptors are stimulated selectively and their influence on biosynthetis capacities during chloroplast development can be studied.R irradiations saturate Rubisco capacity even at the lowest quantum fluence rates applied, whereas Chl a+b synthesis depends strongly upon fluence rate of R. Under BL irradiations, both Rubisco capacity and Chl content are fluence rate dependent. R irradiations favour Chl b synthesis relative to Chl a, whereas under BL Chl a content is high relative to Chl b. Under R irradiation Pfr is the main photoreceptor involved in regulation of Rubisco capacity whereas under BL a specific BL absorbing photoreceptor may control the response. From the fluence rate dependency under BL irradiations it is concluded that the blue region of the day light spectrum may be the sensor for monitoring fluence rate and causing the characteristic changes in shade and high/low WL adaptation with respect to Rubisco levels in Chenopodium.

Photosynthetic adaptation of pea plants grown at different light intensities: State 1 - State 2 transitions and associated chlorophyll fluorescence changes

A study of pea plants grown at different light intensities has been made. Using a leaf oxygen electrode, it was shown that plants grown under low light intensities had lower saturated rates of photosynthesis than high-light-grown plants however, at low light intensities the photosynthetic rates were similar for both types of plants. State 1- State 2 transitions have been monitored with attached leaves using a modulated fluorescence technique. It is shown that peas grown under low light intensities (20 W m(-2)) had a faster State 1 to State 2 transition when compared with medium-(50 W m(-2)) and high-(70 W m(-2)) light-grown plants. Measurement of fast-fluorescence-induction curves in the absence of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) have shown that low-light plants are, when in State 1, more effective at using Photosystem-two (PSII) light to reduce their plastoquinone pool than high-light plants. Transition from State 1 to State 2 for all plants led to a decrease in the reduction level of the plastoquinone pool inidcating that the transition had increased electron flow through Photosystem one (PSI) relative to PSII. Analyses of fast fluorescence induction in the presence of DCMU indicate that low-light-grown plants have a higher PSII-α/PSII-β ratio than high-light-grown plants. Such a difference is in line with the increase in the PSII/PSI ratio of low-light plants and is reflected in their high chlorophyll b/chlorophyll a ratio and their larger appressed to non-appressed thylakoid-membrane areas. It is suggested that these two latter factors give rise to the faster State 1 - State 2 transitions in low-light plants.