Pengmin Li

Heterogeneous behavior of PSII in soybean (Glycine max) leaves with identical PSII photochemistry efficiency under different high temperature treatments

The purpose of this study is to demonstrate the heterogeneous behavior of PSII in soybean (Glycine max) leaves and identical maximum PSII photochemistry efficiency (F(V)/F(M)) under different high temperature treatments. We observed that, with an identical decrease in F(V)/F(M) in soybean leaves caused by different high temperature treatments, chlorophyll a fluorescence differed significantly, indicating different behaviors in the photosynthetic apparatus. The quantitative analysis showed that, with an identical F(V)/F(M), leaves treated at 48 degrees C showed a higher W(K), an indicator of damage to the oxygen-evolving complex along with a lower O(2) evolution rate compared with leaves treated at 45 degrees C. This demonstrated that the donor side of PSII was damaged more severely at 48 degrees C than at 45 degrees C despite the same decrease in F(V)/F(M) in the two heat-treated leaves. The ratios of Q(A)- and Q(B)-reducing PSII reaction centers to total PSII reaction centers were both lower in leaves treated at 48 degrees C than in leaves treated at 45 degrees C with an identical F(V)/F(M), indicating that the acceptor side of PSII was also more damaged by heat treatment at 48 degrees C than at 45 degrees C. However, when damage to the donor side of PSII was similar in leaves treated at two different temperatures, the acceptor side of PSII was damaged less severely at 48 degrees C, which accounted for higher electron transport rate at the acceptor side of PSII in leaves treated at 48 degrees C than in leaves treated at 45 degrees C.

Primary and secondary metabolism in the sun-exposed peel and the shaded peel of apple fruit.

The metabolism of carbohydrates, organic acids, amino acids and phenolics was compared between the sun-exposed peel and the shaded peel of apple fruit. Contents of sorbitol and glucose were higher in the sun-exposed peel, whereas those of sucrose and fructose were almost the same in the two peel types. This was related to lower sorbitol dehydrogenase activity and higher activities of sorbitol oxidase, neutral invertase and acid invertase in the sun-exposed peel. The lower starch content in the sun-exposed peel was related to lower sucrose synthase activity early in fruit development. Dark respiratory metabolism in the sun-exposed peel was enhanced by the high peel temperature due to high light exposure. Activities of most enzymes in respiratory metabolism were higher in the sun-exposed peel, but the concentrations of most organic acids were relatively stable, except pyruvate and oxaloacetate. Due to the different availability of carbon skeletons from dark respiration in the two peel types, amino acids with higher C/N ratios are accumulated in the sun-exposed peel whereas those with lower C/N ratios are accumulated in the shaded peel. Contents of anthocyanins and flavonols and activities of phenylalanine ammonia-lyase, UDP-galactose:flavonoid 3-O-glucosyltransferase and several other enzymes were higher in the sun-exposed peel than in the shaded peel, indicating the entire phenylpropanoid pathway is upregulated in the sun-exposed peel. Comprehensive analyses of the metabolites and activities of enzymes involved in primary metabolism and secondary metabolism have allowed us to gain a full picture of the metabolic network in the two peel types under natural light exposure.

Two MYB transcription factors regulate flavonoid biosynthesis in pear fruit (Pyrus bretschneideri Rehd.)

Flavonoid compounds play important roles in the modern diet, and pear fruits are an excellent dietary source of these metabolites. However, information on the regulatory network of flavonoid biosynthesis in pear fruits is rare. In this work, 18 putative flavonoid-related MYB transcription factors (TFs) were screened by phylogenetic analysis and four of them were correlated with flavonoid biosynthesis patterns in pear fruits. Among these MYB-like genes, the specific functions of two novel MYB TFs, designated as PbMYB10b and PbMYB9, were further verified by both overexpression and RNAi transient assays. PbMYB10b, a PAP-type MYB TF with atypical motifs in its conserved region, regulated the anthocyanin and proanthocyanidin pathways by inducing the expression of PbDFR, but its function could be complemented by other MYB TFs. PbMYB9, a TT2-type MYB, not only acted as the specific activator of the proanthocyanidin pathway by activating the PbANR promoter, but also induced the synthesis of anthocyanins and flavonols by binding the PbUFGT1 promoter in pear fruits. The MYBCORE-like element has been identified in both the PbUFGT1 promoter and ANR promoters in most species, but it was not found in UFGT promoters isolated from other species. This finding was also supported by a yeast one-hybrid assay and thus enhanced the likelihood of the interaction between PbMYB9 and the PbUFGT1 promoter.

The role of anthocyanin in photoprotection and its relationship with the xanthophyll cycle and the antioxidant system in apple peel depends on the light conditions

The synthesis of anthocyanin, the xanthophyll cycle, the antioxidant system and the production of active oxygen species (AOS) were compared between red and non-red apple cultivars, in response to either long-term sunlight exposure (high light intensity) during fruit development, or to exposure of bagged fruits to lower light intensity late in fruit development. During fruit development of red and non-red apples, the xanthophyll cycle pool size decreased much more in red apple peel late in development. With accumulation of AOS induced by long-term sunlight exposure, enhancement of the antioxidant system was found. However, this change became significantly lower in red apple than non-red apple as fruit developed, which might serve to accelerate the anthocyanin synthesis in red apple peel. When, late in fruit development, bagged fruits were exposed to sunlight, the accumulation of AOS was lower in red apple peel than in non-red peel. This could be due to the higher anthocyanin concentration in the red peels. Meanwhile, compared with that in non-red cultivar, the xanthophyll cycle and the antioxidant system in red apple peel were protected first but then down-regulated by its higher anthocyanin concentration during sunlight exposure. In conclusions, red and non-red apples peel possess different photoprotective mechanisms under high light conditions. The relationship between anthocyanin synthesis and the xanthophyll cycle, and the antioxidant system, depends on the light conditions that fruit undergoes.

Reactive oxygen species produced via plasma membrane NADPH oxidase regulate anthocyanin synthesis in apple peel

AbstractMain conclusionSolar ultraviolet irradiation regulates anthocyanin synthesis in apple peel by modulating the production of reactive oxygen species via plasma membrane NADPH oxidase instead of other pathways. The synthesis of anthocyanin in apple peels is dependent upon solar irradiation. Using 3-mm commercial glass to attenuate solar UV-A and UV-B light, we confirmed that solar UV irradiation regulated anthocyanin synthesis in apple peels after exposing previously bagged fruit to sunlight. During sunlight exposure, UV attenuation did not affect the expression of MdHY5, MdCOP1, or MdCRY2, but significantly lowered plasma membrane NADPH oxidase activity and superoxide anion concentrations. UV attenuation also reduced the expression levels of MdMYB10, MdPAL, MdCHS, MdF3H, MdDFR, MdANS and MdUFGT1, UDP-glycose:flavonoid 3-O-glycosyltransferase (UFGT) activity, and local concentrations of anthocyanin and quercetin-3-glycoside. In contrast, exogenous application of hydrogen peroxide could enhance anthocyanin and quercetin-3-glycoside synthesis. Xanthophyll cycle pool size on a chlorophyll basis was higher but its de-epoxidation was lower under direct sunlight irradiation than that under UV-attenuating conditions. This suggests that reactive oxygen species (ROS) produced in chloroplast are not major contributors to anthocyanin synthesis regulation. Inhibition of plasma membrane NADPH oxidase activity lowered the production of ROS through this mechanism, significantly inhibited the synthesis of anthocyanin, and increased the total production of ROS in apple peel under direct sunlight irradiation, suggesting that ROS produced via plasma membrane NADPH oxidase regulates anthocyanin synthesis. In summary, solar UV irradiation regulated anthocyanin synthesis in apple peels by modulating the production of ROS via plasma membrane NADPH oxidase.

Photoinhibition-Like Damage to the Photosynthetic Apparatus in Plant Leaves Induced by Submergence Treatment in the Dark

Submergence is a common type of environmental stress for plants. It hampers survival and decreases crop yield, mainly by inhibiting plant photosynthesis. The inhibition of photosynthesis and photochemical efficiency by submergence is primarily due to leaf senescence and excess excitation energy, caused by signals from hypoxic roots and inhibition of gas exchange, respectively. However, the influence of mere leaf-submergence on the photosynthetic apparatus is currently unknown. Therefore, we studied the photosynthetic apparatus in detached leaves from four plant species under dark-submergence treatment (DST), without influence from roots and light. Results showed that the donor and acceptor sides, the reaction center of photosystem II (PSII) and photosystem I (PSI) in leaves were significantly damaged after 36 h of DST. This is a photoinhibition-like phenomenon similar to the photoinhibition induced by high light, as further indicated by the degradation of PsaA and D1, the core proteins of PSI and PSII. In contrast to previous research, the chlorophyll content remained unchanged and the H2O2 concentration did not increase in the leaves, implying that the damage to the photosynthetic apparatus was not caused by senescence or over-accumulation of reactive oxygen species (ROS). DST-induced damage to the photosynthetic apparatus was aggravated by increasing treatment temperature. This type of damage also occurred in the anaerobic environment (N2) without water, and could be eliminated or restored by supplying air to the water during or after DST. Our results demonstrate that DST-induced damage was caused by the hypoxic environment. The mechanism by which DST induces the photoinhibition-like damage is discussed below.

Different effects of light irradiation on the photosynthetic electron transport chain during apple tree leaf dehydration.

Effects of light irradiation on the photosynthetic electron transport chain between P680 and P700 in apple tree leaves was probed with chlorophyll a fluorescence transient and 820 nm transmission measurements during dehydration under different light intensities. The results showed that light accelerated the leaf water-loss rate during dehydration. Leaf dehydration lowered the maximum quantum yield of PSII and the far-red light induced maximal transmission change at 820 nm, but increased the relative variable fluorescence intensity at J-step, especially under increasing irradiation conditions. During leaf dehydration, irradiation lowered the relative variable fluorescence intensity at I-step. At the beginning of leaf dehydration, moderate light accelerated the leaf water-loss rate and then lowered the maximal light-trapping efficiency of P₆₈₀. Upon further dehydration under moderate light or dehydration under high light, light accelerated the water-loss rate and also directly decreased the maximal light-trapping efficiency of P680. The more significant decrease in the exchange capacity of plastoquinones at the Q(B) site was mainly attributed to the faster water-loss rate under moderate light than in the dark. Under high light, irradiation also directly lowered the capacity. The reoxidation of PQH₂ in the dehydrated leaves was enhanced by the light irradiation. The rapidly decreased contents of P700 + plastocyanin were mainly attributed to the faster water-loss rate under light conditions in contrast with that in the dark. The different effects of light irradiations on the photosynthetic electron transport chain might be involved in the acclimation of apple tree leaves to dehydration.

The higher resistance to chilling stress in adaxial side of Rumex K-1 leaves is accompanied with higher photochemical and non-photochemical quenching

Responses of two sides of Rumex K-1 leaves to chilling stress (5 °C, photon flux density of 100 µmol m−2 s−1) were studied by using gas exchange, chlorophyll (Chl) fluorescence, and spectrum reflectance techniques. The Chl and carotenoid contents in the two sides were not affected by chilling treatment, and both were higher in the adaxial side. The maximum quantum yield of photosystem (PS) 2 and fraction of functional PS1 in the abaxial side decreased more markedly than those in the adaxial side during the chilling treatment, indicating that the abaxial side was damaged more significantly than the adaxial side. Before chilling, there were no obvious differences in actual photochemical efficiency of PS2, photosynthesis, and photorespiration between two sides of the leaves. Under chilling stress, the actual photochemical efficiency of PS2, photosynthesis, and photorespiration all declined more significantly in the abaxial side, which was partly attributed to lower carboxylation efficiency in the abaxial side than that in the adaxial side. Non-photochemical quenching was higher in the adaxial side, though the de-epoxidation of xanthophyll cycle pigments’ pool on basis of Chl was higher in the abaxial side. Both the slower decrease in the photochemical quenching and the higher non-photochemical quenching may account for the higher resistance to chilling stress in the adaxial side of Rumex K-1 leaves.

CO2 assimilation and chlorophyll fluorescence in green versus red Berberis thunbergii leaves measured with different quality irradiation

Photosynthesis, photorespiration, and chlorophyll (Chl) fluorescence in green and red Berberis thunbergii leaves were studied with two different measuring radiations, red (RR) and “white” (WR). The photosynthetic and photorespiration rates responded differently to the different radiation qualities, which indicate that the carboxylase and oxygenase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) were affected. Differences in photosynthetic rate between the two color leaves were less under RR than under WR. However, this reduced difference in photosynthetic rate was not correlated with the stomatal response to the measuring radiation qualities. Compared with the WR, the RR reduced the differences in dark-adapted minimum and maximum fluorescence, steady-state fluorescence, light-adapted maximum fluorescence, and actual photochemical efficiency (ΦPS2) of photosystem 2 (PS2), but enlarged the difference in non-photochemical quenching between the two color leaves. Differences in both maximum quantum yield of PS2 and ratio of ΦPS2 to quantum yield of CO2 fixation between the two color leaves were similar under the two measuring radiations. To exclude disturbance of radiation attenuation caused by anthocyanins, it is better to use RR to compare the photosynthesis and Chl fluorescence in green versus red leaves.

Anthocyanin concentration depends on the counterbalance between its synthesis and degradation in plum fruit at high temperature

Anthocyanin synthesis and degradation processes were analyzed at transcript, enzyme, and metabolite levels to clarify the effects of high temperature on the concentration of anthocyanin in plum fruit (Prunus salicina Lindl.). The transcript levels of PsPAL, PsCHS, and PsDFR decreased while those of PsANS and PsUFGT were similar at 35 °C compared with 20 °C. The activities of the enzymes encoded by these genes were all increased in fruits at 35 °C. The concentrations of anthocyanins were higher at 35 °C on day 5 but then decreased to lower values on day 9 compared with that at 20 °C. Furthermore, high temperature (35 °C) increased the concentration of hydrogen peroxide and the activity of class III peroxidase in the fruit. The concentration of procatechuic acid, a product of the reaction between anthocyanin and hydrogen peroxide, hardly changed at 20 °C but was significantly increased at 35 °C on day 9, indicating that anthocyanin was degraded by hydrogen peroxide, which was catalyzed by class III peroxidase. Based on mathematical modeling, it was estimated that more than 60–70% was enzymatically degraded on day 9 when the temperature increased from 20 °C to 35 °C. We conclude that at the high temperature, the anthocyanin content in plum fruit depend on the counterbalance between its synthesis and degradation.