Acclimation of shade-tolerant and light-resistant Tradescantia species to growth light: chlorophyll a fluorescence, electron transport, and xanthophyll content

Transcriptomic and Metabolomic Analyses Reveal the Key Genes Related to Shade Tolerance in Soybean

Soybean (Glycine max) is an important crop, rich in proteins, vegetable oils and several other phytochemicals, which is often affected by light during growth. However, the specific regulatory mechanisms of leaf development under shade conditions have yet to be understood. In this study, the transcriptome and metabolome sequencing of leaves from the shade-tolerant soybean 'Nanxiadou 25' under natural light (ND1) and 50% shade rate (SHND1) were carried out, respectively. A total of 265 differentially expressed genes (DEGs) were identified, including 144 down-regulated and 121 up-regulated genes. Meanwhile, KEGG enrichment analysis of DEGs was performed and 22 DEGs were significantly enriched in the top five pathways, including histidine metabolism, riboflavin metabolism, vitamin B6 metabolism, glycerolipid metabolism and cutin, suberine and wax biosynthesis. Among all the enrichment pathways, the most DEGs were enriched in plant hormone signaling pathways with 19 DEGs being enriched. Transcription factors were screened out and 34 differentially expressed TFs (DETFs) were identified. Weighted gene co-expression network analysis (WGCNA) was performed and identified 10 core hub genes. Combined analysis of transcriptome and metabolome screened out 36 DEGs, and 12 potential candidate genes were screened out and validated by quantitative real-time polymerase chain reaction (qRT-PCR) assay, which may be related to the mechanism of shade tolerance in soybean, such as ATP phosphoribosyl transferase (ATP-PRT2), phosphocholine phosphatase (PEPC), AUXIN-RESPONSIVE PROTEIN (IAA17), PURPLE ACID PHOSPHATASE (PAP), etc. Our results provide new knowledge for the identification and function of candidate genes regulating soybean shade tolerance and provide valuable resources for the genetic dissection of soybean shade tolerance molecular breeding.

Responses of photosystem to long-term light stress in a typically shade-tolerant species Panax notoginseng

Photosynthetic adaptive strategies vary with the growth irradiance. The potential photosynthetic adaptive strategies of shade-tolerant species Panax notoginseng (Burkill) F. H. Chen to long-term high light and low light remains unclear. Photosynthetic performance, photosynthesis-related pigments, leaves anatomical characteristics and antioxidant enzyme activities were comparatively determined in P. notoginseng grown under different light regimes. The thickness of the upper epidermis, palisade tissue, and lower epidermis were declined with increasing growth irradiance. Low-light-grown leaves were declined in transpiration rate (Tr) and stomatal conductance (Cond), but intercellular CO₂ concentration (C _i) and net photosynthesis rate (P _n) had opposite trends. The maximum photo-oxidation P 700 + (P _m) was greatly reduced in 29.8% full sunlight (FL) plants; The maximum quantum yield of photosystem II (F _v/F _m) in 0.2% FL plants was significantly lowest. Electron transport, thermal dissipation, and the effective quantum yield of PSI [Y(I)] and PSII [Y(II)] were declined in low-light-grown plants compared with high-light-grown P. notoginseng. The minimum value of non-regulated energy dissipation of PSII [Y(NO)] was recorded in 0.2% FL P. notoginseng. OJIP kinetic curve showed that relative variable fluorescence at J-phase (V _J) and the ratio of variable fluorescent F _K occupying the F _J-F _O amplitude (W _k) were significantly increased in 0.2% FL plants. However, the increase in W _k was lower than the increase in V _J. In conclusion, PSI photoinhibition is the underlying sensitivity of the typically shade-tolerant species P. notoginseng to high light, and the photodamage to PSII acceptor side might cause the typically shade-tolerant plants to be unsuitable for long-term low light stress.

Transcriptome analysis reveals the accelerated expression of genes related to photosynthesis and chlorophyll biosynthesis contribution to shade-tolerant in Phoebe bournei

BACKGROUND

Phoebe bournei (P. bournei) is an important and endemic wood species in China. However, the plantation, nursing, and preservation of P. bournei are often affected by light. To investigate its physiological changes and molecular mechanism of low light tolerance, two-year-old P. bournei seedlings were subjected to different shading conditions. With the increase of light intensity in the shade, the leaf color of P. bournei seedlings became darkened, the aboveground/underground biomass significantly increased, the content of chlorophyll increased and the net photosynthetic rate significantly increased.

RESULTS

de novo transcriptome analysis showed that 724 and 3,248 genes were differentially expressed due to low light intensity at T1 (35% light exposure) and T2 (10% light exposure), respectively, when compared to the controls. Furthermore, the differentially expressed genes (DEGs) were implicated in photosynthesis, nitrogen metabolism, plant hormone signal transduction, biosynthesis of secondary metabolites, and protein processing in the endoplasmic reticulum by functional enrichment analysis. Moreover, the expression of HSP, CAB, HEMA1, GSA, DVR, MYB, bHLH, PORA, CAO, GLK, and photosystem I and II complex-related genes significantly increased after low light exposure at T2 and T1.

CONCLUSIONS

The present study suggests that the rapid growth of P. bournei seedlings under shading conditions may be the result of the accelerated expression of genes related to photosynthesis and chlorophyll biosynthesis, which enable plants to maintain a high photosynthesis rate even under low light conditions.

A Comparison of Photoprotective Mechanism in Different Light-Demanding Plants Under Dynamic Light Conditions

Light intensity is highly heterogeneous in nature, and plants have evolved a series of strategies to acclimate to dynamic light due to their immobile lifestyles. However, it is still unknown whether there are differences in photoprotective mechanisms among different light-demanding plants in response to dynamic light, and thus the role of non-photochemical quenching (NPQ), electron transport, and light energy allocation of photosystems in photoprotection needs to be further understood in different light-demanding plants. The activities of photosystem II (PSII) and photosystem I (PSI) in shade-tolerant species Panax notoginseng, intermediate species Polygonatum kingianum, and sun-demanding species Erigeron breviscapus were comparatively measured to elucidate photoprotection mechanisms in different light-demanding plants under dynamic light. The results showed that the NPQ and PSII maximum efficiency (F v'/F m') of E. breviscapus were higher than the other two species under dynamic high light. Meanwhile, cyclic electron flow (CEF) of sun plants is larger under transient high light conditions since the slope of post-illumination, P700 dark reduction rate, and plastoquinone (PQ) pool were greater. NPQ was more active and CEF was initiated more readily in shade plants than the two other species under transient light. Moreover, sun plants processed higher quantum yield of PSII photochemistry (ΦPSII), quantum yield of photochemical energy conversion [Y(I)], and quantum yield of non-photochemical energy dissipation due to acceptor side limitation (Y(NA), while the constitutive thermal dissipation and fluorescence (Φf,d) and quantum yield of non-photochemical energy dissipation due to donor side limitation [Y(ND)] of PSI were higher in shade plants. These results suggest that sun plants had higher NPQ and CEF for photoprotection under transient high light and mainly allocated light energy through ΦPSII and ΦNPQ, while shade plants had a higher Φf,d and a larger heat dissipation efficiency of PSI donor. Overall, it has been demonstrated that the photochemical efficiency and photoprotective capacity are greater in sun plants under transient dynamic light, while shade plants are more sensitive to transient dynamic light.

Electron transport in Tradescantia leaves acclimated to high and low light: thermoluminescence, PAM-fluorometry, and EPR studies

Using thermoluminescence, PAM-fluorometry, and electron paramagnetic resonance (EPR) for assaying electron transport processes in chloroplasts in situ, we have compared photosynthetic characteristics in Tradescantia fluminensis leaves grown under low light (LL, 50-125 µmol photons m^-2 s^-1) or high light (HL, 875-1000 µmol photons m^-2 s^-1) condition. We found differences in the thermoluminescence (TL) spectra of LL- and HL-acclimated leaves. The LL and HL leaves show different proportions of the Q (~ 0 °C) and B (~ 25-30 °C) bands in their TL spectra; the ratios of the "light sums" of the Q and B bands being S_Q/S_B ≈ 1/1 (LL) and S_Q/S_B ≈ 1/3 (HL). This suggests the existence of different redox states of electron carriers on the acceptor side of PSII in LL and HL leaves, which may be affected, in particular, by different capacities of their photo-reducible PQ pools. Enhanced content of PQ in chloroplasts of LL leaves may be the reason for an efficient performance of photosynthesis at low irradiance. Kinetic studies of slow induction of Chl a fluorescence and measurements of P₇₀₀ photooxidation by EPR demonstrate that HL leaves have faster (about 2 times) response to switching on actinic light as compared to LL leaves grown at moderate irradiation. HL leaves also show higher non-photochemical quenching (NPQ) of Chl a fluorescence. These properties of HL leaves (faster response to light and generation of enhanced NPQ) reflect the flexibility of their photosynthetic apparatus, providing sustainability and rapid response to fluctuations of environmental light intensity and solar stress resistance. Analysis of time-courses of the EPR signals of [Formula: see text] induced by far-red (λ_max = 707 nm), exciting predominantly PSI, and white light, exciting both PSI and PSII, suggests that there is a contribution of cyclic electron flow around PSI to electron flow through PSI in HL leaves. The data obtained are discussed in terms of photosynthetic apparatus sustainability of HL and LL leaves under variable irradiation conditions.

Slow induction of chlorophyll a fluorescence excited by blue and red light in Tradescantia leaves acclimated to high and low light

Tradescantia is a good model for assaying induction events in higher plant leaves. Chlorophyll (Chl) fluorescence serves as a sensitive reporter of the functional state of photosynthetic apparatus in chloroplasts. The fluorescence time-course depends on the leaf growth conditions and actinic light quality. In this work, we investigated slow induction of Chl a fluorescence (SIF) excited by blue light (BL, λ_max = 455 nm) or red light (RL, λ_max = 630 nm) in dark-adapted leaves of Tradescantia fluminensis acclimated to high light (~ 1000 µmol photons m^-2 s^-1; HL) or low light (~ 100 µmol photons m^-2 s^-1; LL). Our special interest was focused on the contribution of the avoidance response to SIF kinetics. Bearing in mind that BL and RL have different impacts on photoreceptors that initiate chloroplast movements within the cell (accumulation/avoidance responses), we have compared the SIF patterns during the action of BL and RL. The time-courses of SIF and kinetics of non-photochemical quenching (NPQ) of Chl a fluorescence revealed a certain difference when leaves were illuminated by BL or RL. In both cases, the yield of fluorescence rose to the maximal level P and then, after the lag-phase P-S-M₁, the fluorescence level decreased toward the steady state T (via the intermediate phases M₁-M₂ and M₂-T). In LL-acclimated leaves, the duration of the P-S-M₁ phase was almost two times longer that in HL-grown plants. In the case of BL, the fluorescence decay included the transient phase M₁-M₂. This phase was obscure during the RL illumination. Non-photochemical quenching of Chl a fluorescence has been quantified as [Formula: see text], where [Formula: see text] and [Formula: see text] stand for the fluorescence response to saturating pulses of light applied to dark-adapted and illuminated samples, respectively. The time-courses of such a formally determined NPQ value were markedly different during the action of RL and BL. In LL-grown leaves, BL induced higher NPQ as compared to the action of RL. In HL-grown plants, the difference between the NPQ responses to BL and RL illumination was insignificant. Comparing the peculiarities of Chl a fluorescence induced by BL and RL, we conclude that the avoidance response can provide a marked contribution to SIF and NPQ generation. The dependence of NPQ on the quality of actinic light suggests that chloroplast movements within the cell have a noticeable impact on the formally determined NPQ value. Analyzing kinetics of post-illumination decay of NPQ in the context of solar stress resistance, we have found that LL-acclimated Tradescantia leaves are more vulnerable to strong light than the HL-grown leaves.

Tradescantia-based models: a powerful looking glass for investigation of photoacclimation and photoadaptation in plants

Here, we summarize diverse evidence from species that belong to the genus Tradescantia, which we propose as handy and versatile models for studies of the ecology of photosynthesis and the mechanisms of photoacclimation in higher plants. A valuable feature of this genus is the amazingly broad range of ecological niches occupied by its species: from shady understory of tropical rainforest to deserts and semideserts. The former habitats demand shade tolerance (e.g. that featured by Tradescantia fluminensis), whereas the latter requires succulence and/or high light stress tolerance (evident in e.g. Tradescantia navicularis). At the same time, the acclimative traits of Tradescantia species seem quite moderate at first glance. Certainly, their basic principles of acclimation seem to differ in some aspects from the ones typical for most of other higher plants. This review presents a systematic analysis of irradiance responses of Tradescantia species studied on different timescales. The specifics of Tradescantia responses to irradiance make the plants of this genus a 'multitool' for studies in this field. Similarity of irradiance acclimation patterns is a characteristic feature in the ecologically contrasting Tradescantia species, which may inspire further insights into physiology and evolution of plants.

Photo-reducible plastoquinone pools in chloroplasts of Tradescentia plants acclimated to high and low light

In photosynthetic systems of oxygenic type, plastoquinone (PQ) molecules are reduced by photosystem II (PSII). The turnover of PQ determines the rate of PSII operation. PQ molecules are present in surplus with respect to PSII. In this work, using the pulse amplitude modulation-fluorometry technique, we quantified photo-reducible PQ pools in chloroplasts of two contrasting ecotypes of Tradescantia, acclimated either to low light (~ 100 μmol photons·m^-2 ·s^-1 , LL) or to high light (~ 1000 μmol photons·m^-2 ·s^-1 , HL). The LL-grown plants are characterized by higher capacity of rapidly reducible PQ pool ([PQ]₀ /[PSII] ≈ 8) as compared to HL-grown plants of both species ([PQ]₀ /[PSII] ≈ 4). The elevated content of PQ in LL plants favours photosynthetic electron flow at low-solar irradiance.

Light acclimation of shade-tolerant and sun-resistant Tradescantia species: photochemical activity of PSII and its sensitivity to heat treatment

In this work, we have compared photosynthetic characteristics of photosystem II (PSII) in Tradescantia leaves of two contrasting ecotypes grown under the low light (LL) and high light (HL) regimes during their entire growth period. Plants of the same genus, T. fluminensis (shade-tolerant) and T. sillamontana (sun-resistant), were cultivated at 50-125 µmol photons m^-2 s^-1 (LL) or at 875-1000 µmol photons m^-2 s^-1 (HL). Analyses of intrinsic PSII efficiency was based on measurements of fast chlorophyll (Chl) a fluorescence kinetics (the OJIP test). The fluorescence parameters F_v/F_m (variable fluorescence) and F₀ (the initial level of fluorescence) in dark-adapted leaves were used to quantify the photochemical properties of PSII. Plants of different ecotypes showed different sustainability with respect to changes in the environmental light intensity and temperature treatment. The sun-resistant species T. sillamontana revealed the tolerance to variations in irradiation intensity, demonstrating constancy of maximum quantum efficiency of PSII upon variations of the growth light. In contrast to T. sillamontana, facultative shade species T. fluminensis demonstrated variability of PSII photochemical activity, depending on the growth light intensity. The susceptibility of T. fluminensis to solar stress was documented by a decrease in F_v/F_m and a rise of F₀ during the long-term exposition of T. fluminensis to HL, indicating the loss of photochemical activity of PSII. The short-term (10 min) heat treatment of leaf cuttings caused inactivation of PSII. The temperature-dependent heating effects were different in T. fluminensis and T. sillamontana. Sun-resistant plants T. sillamontana acclimated to LL and HL displayed the same plots of F_v/F_m versus the treatment temperature (t), demonstrating a decrease in F_v/F_m at t ≥ 45 °C. The leaves of shadow-tolerant species T. fluminensis grown under the LL and HL conditions revealed different sensitivities to heat treatment. Plants grown under the solar stress conditions (HL) demonstrated a gradual decline of F_v/F_m at lower heating temperatures (t ≥ 25 °C), indicating the "fragility" of their PSII as compared to T. fluminensis grown at LL. Different responses of sun and shadow species of Tradescantia to growth light and heat treatment are discussed in the context of their biochemical and ecophysiological properties.

Three phases of energy-dependent induction of [Formula: see text] and Chl a fluorescence in Tradescantia fluminensis leaves

In plants, the short-term regulation (STR, seconds to minute time scale) of photosynthetic apparatus is associated with the energy-dependent control in the chloroplast electron transport, the distribution of light energy between photosystems (PS) II and I, activation/deactivation of the Calvin-Benson cycle (CBC) enzymes, and relocation of chloroplasts within the plant cell. In this work, using a dual-PAM technique for measuring the time-courses of P₇₀₀ photooxidation and Chl a fluorescence, we have investigated the STR events in Tradescantia fluminensis leaves. The comparison of Chl a fluorescence and [Formula: see text] induction allowed us to investigate the contribution of the trans-thylakoid pH difference (ΔpH) to the STR events. Two parameters were used as the indicators of ΔpH generation: pH-dependent component of non-photochemical quenching of Chl a fluorescence, and pH_in-dependent rate of electron transfer from plastoquinol (PQH₂) to [Formula: see text] (via the Cyt b₆f complex and plastocyanin). In dark-adapted leaves, kinetics of [Formula: see text] induction revealed three phases. Initial phase is characterized by rapid electron flow to [Formula: see text] (τ_1/2 ~ 5-10 ms), which is likely related to cyclic electron flow around PSI, while the outflow of electrons from PSI is restricted by slow consumption of NADPH in the CBC. The light-induced generation of ΔpH and activation of the CBC promote photooxidation of P₇₀₀ and concomitant retardation of [Formula: see text] reduction (τ_1/2 ~ 20 ms). Prolonged illumination induces additional slowing down of electron transfer to [Formula: see text] (τ_1/2 ≥ 30-35 ms). The latter effect is not accompanied by changes in the Chl a fluorescence parameters which are sensitive to ΔpH generation. We suggest the tentative explanation of the latter results by the reversal of Q-cycle, which causes the deceleration of PQH₂ oxidation due to the back pressure of stromal reductants.

Connectivity between electron transport complexes and modulation of photosystem II activity in chloroplasts

In chloroplasts, photosynthetic electron transport complexes interact with each other via the mobile electron carriers (plastoquinone and plastocyanin) which are in surplus amounts with respect to photosystem I and photosystem II (PSI and PSII), and the cytochrome b ₆ f complex. In this work, we analyze experimental data on the light-induced redox transients of photoreaction center P₇₀₀ in chloroplasts within the framework of our mathematical model. This analysis suggests that during the action of a strong actinic light, even significant attenuation of PSII [for instance, in the result of inhibition of a part of PSII complexes by DCMU or due to non-photochemical quenching (NPQ)] will not cause drastic shortage of electron flow through PSI. This can be explained by "electronic" and/or "excitonic" connectivity between different PSII units. At strong AL, the overall flux of electrons between PSII and PSI will maintain at a high level even with the attenuation of PSII activity, provided the rate-limiting step of electron transfer is beyond the stage of PQH₂ formation. Results of our study are briefly discussed in the context of NPQ-dependent mechanism of chloroplast protection against light stress.