Protection of the photosynthetic apparatus from extreme dehydration and oxidative stress in seedlings of transgenic tobacco

Genome-wide transcriptional profiling and physiological investigation elucidating the molecular mechanism of multiple abiotic stress response in Stevia rebaudiana Bertoni

Considering the major source of plant-derived low/non-calorie steviol glycosides (SGs), comprehensive physiological, biochemical, and deep transcriptional investigations were conducted to explicit deeper insight into multiple abiotic stress responses in Stevia rebaudiana. The physiological indicators including photosynthesis, chlorophyll, relative water content, shoot growth, electrolyte leakage, and SG biosynthesis were negatively impacted under drought (DS), followed by salinity (SS) and waterlogging (WS). Global transcriptional analysis revealed significant upregulated expression of the genes encoding for ROS detoxification (GST, SOD, APX, glutathione peroxidase), osmotic adjustment (alpha-trehalose-phosphate and S-adenosylmethionine decarboxylase), ion transporters (CAX, NHX, CNGS, VPPase, VATPase), water channel (PIP1, TIP) and abiotic stress-responsive candidate genes (LEA, HSPs, and Dehydrins) regulating abiotic stress response in S. rebaudiana. These inferences were complemented with predicted interactome network that revealed regulation of energy metabolism by key stress-responsive genes (GST, HKT1, MAPKs, P5CSs, PIP), transcription factors (HSFA2, DREB1A, DREB2A), and abiotic stress responsive pathways (ABA, ethylene, ion stress). This is the first detailed study to comprehend the molecular regulation of stress response and their interplay under DS, SS, and WS. The key genes and regulators can be functionally validated, and will facilitate targeted gene editing for genetic improvement of crop sustainability under changing environmental conditions in S. rebaudiana.

A seed-specific transcription factor, HSFA9, anticipates UV-B light responses by mimicking the activation of the UV-B receptor in tobacco

Sunflower heat shock factor A9 (HSFA9, hereafter A9) is a transcription factor involved in seed desiccation tolerance and longevity. A9 also links the regulation of seed maturation with that of seedling photomorphogenesis through visible light receptors. Analyses in transgenic Nicotiana tabacum (tobacco) indicated that A9 also affects responses mediated by NtUVR8, the receptor of ultraviolet light B (UV-B). We compared the effects of A9 and UV-B illumination on the nuclear localization of GFP-NtUVR8 in Nicotiana benthamiana leaves. We also used co-immunoprecipitation and limited proteolysis for analyzing the interaction between A9 and NtUVR8. We found that A9, by binding to NtUVR8, induced structural changes that resulted in enhancing the nuclear localization of NtUVR8 by hindering its nuclear export. The localization of UVR8 is crucial for receptor activation and function in Arabidopsis, where UV-B-activated nuclear UVR8 binds the E3 ubiquitin ligase COP1, leading to enhanced UV-B responses and photoprotection. A9 similarly activated NtUVR8 by enhancing COP1 binding without UV-B light. Seedlings and dark-germinated seeds that overexpress A9 showed primed UV-B light stress protection. Our results unveil a UV-B-independent activation mechanism and a role for UVR8 in plant seeds that might contribute to early stress protection, facilitating seedling establishment.

Transcription Factor DOF4.1 Regulates Seed Longevity in Arabidopsis via Seed Permeability and Modulation of Seed Storage Protein Accumulation

Seed longevity is modulated by multiple genetic factors in Arabidopsis thaliana. A previous genome-wide association study using the Elevated Partial Pressure of Oxygen (EPPO) aging assay pinpointed a genetic locus associated with this trait. Reverse genetics identified the transcription factor DOF4.1 as a novel seed longevity factor. dof4.1 loss-of-function plants generate seeds exhibiting higher germination after accelerated aging assays. DOF4.1 is expressed during seed development and RNAseq data show several putative factors that could contribute to the dof4.1 seed longevity phenotype. dof4.1 has reduced seed permeability and a higher levels of seed storage proteins mRNAs (cruciferins and napins) in developing seeds, as compared to wild-type seeds. It has been reported that mutant lines defective in cruciferins or napins present reduced seed longevity. The improved longevity of dof4.1 is totally lost in the quadruple mutant dof4.1 cra crb crc, but not in a dof4.1 line depleted of napins, suggesting a prominent role for cruciferins in this process. Moreover, a negative regulation of DOF4.1 expression by the transcription factor DOF1.8 is suggested by co-inoculation assays in Nicotiana benthamiana. Indeed, DOF1.8 expression anticorrelates with that of DOF4.1 during seed development. In summary, modulation of DOF4.1 levels during seed development contributes to regulate seed longevity.

Here comes the sun: How optimization of photosynthetic light reactions can boost crop yields

Photosynthesis started to evolve some 3.5 billion years ago CO₂ is the substrate for photosynthesis and in the past 200-250 years, atmospheric levels have approximately doubled due to human industrial activities. However, this time span is not sufficient for adaptation mechanisms of photosynthesis to be evolutionarily manifested. Steep increases in human population, shortage of arable land and food, and climate change call for actions, now. Thanks to substantial research efforts and advances in the last century, basic knowledge of photosynthetic and primary metabolic processes can now be translated into strategies to optimize photosynthesis to its full potential in order to improve crop yields and food supply for the future. Many different approaches have been proposed in recent years, some of which have already proven successful in different crop species. Here, we summarize recent advances on modifications of the complex network of photosynthetic light reactions. These are the starting point of all biomass production and supply the energy equivalents necessary for downstream processes as well as the oxygen we breathe.

Diversity of plant heat shock factors: regulation, interactions, and functions

Plants heat shock factors (HSFs) are encoded by large gene families with variable structure, expression, and function. HSFs are components of complex signaling systems that control responses not only to high temperatures but also to a number of abiotic stresses such as cold, drought, hypoxic conditions, soil salinity, toxic minerals, strong irradiation, and to pathogen threats. Here we provide an overview of the diverse world of plant HSFs through compilation and analysis of their functional versatility, diverse regulation, and interactions. Bioinformatic data on gene expression profiles of Arabidopsis HSF genes were re-analyzed to reveal their characteristic transcript patterns. While HSFs are regulated primarily at the transcript level, alternative splicing and post-translational modifications such as phosphorylation and sumoylation provides further variability. Plant HSFs are involved in an intricate web of protein-protein interactions which adds considerable complexity to their biological function. A list of such interactions was compiled from public databases and published data, and discussed to pinpoint their relevance in transcription control. Although most fundamental studies of plant HSFs have been conducted in the model plant, Arabidopsis, information on HSFs is accumulating in other plants such as tomato, rice, wheat, and sunflower. Understanding the function, interactions, and regulation of HSFs will facilitate the design of novel strategies to use engineered proteins to improve tolerance and adaptation of crops to adverse environmental conditions.

The seed-specific heat shock factor A9 regulates the depth of dormancy in Medicago truncatula seeds via ABA signalling

During the later stages of seed maturation, two key adaptive traits are acquired that contribute to seed lifespan and dispersal, longevity and dormancy. The seed-specific heat shock transcription factor A9 is an important hub gene in the transcriptional network of late seed maturation. Here, we demonstrate that HSFA9 plays a role in thermotolerance rather than in ex situ seed conservation. Storage of hsfa9 seeds of Medicago truncatula and Arabidopsis had comparable lifespan at moderate storage relative humidity (RH), whereas at high RH, hsfa9 seeds lost their viability much faster than wild type seeds. Furthermore, we show that in M. truncatula, Mthsfa9 seeds acquired more dormancy during late maturation than wild type. Transient expression of MtHSFA9 in hairy roots and transcriptome analysis of Mthsfa9 Tnt1 insertion mutants identified a deregulation of genes involved in ABA biosynthesis, catabolism and signalling. Consistent with these results, Mthsfa9 seeds exhibited increased ABA levels and higher sensitivity to ABA. These data suggest that in legumes, HSFA9 acts as a negative regulator of the depth of seed dormancy during seed development via the modulation of hormonal balance.

Heat Stress Factors Expressed during Seed Maturation Differentially Regulate Seed Longevity and Seedling Greening

Heat Stress Factor A9 (A9), a seed-specific transcription factor contributing to seed longevity, also enhances phytochrome-dependent seedling greening. The RNA-seq analyses of imbibed-seed transcripts here reported indicated potential additional effects of A9 on cryptochrome-mediated blue-light responses. These analyses also suggested that in contrast to the A9 effects on longevity, which require coactivation by additional factors as A4a, A9 alone might suffice for the enhancement of photomorphogenesis at the seedling stage. We found that upon its seed-specific overexpression, A9 indeed enhanced the expected blue-light responses. Comparative loss-of-function analyses of longevity and greening, performed by similar expression of dominant-negative and inactive forms of A9, not only confirmed the additional greening effects of A9, but also were consistent with A9 not requiring A4a (or additional factors) for the greening effects. Our results strongly indicate that A9 would differentially regulate seed longevity and photomorphogenesis at the seedling stage, A9 alone sufficing for both the phytochrome- and cryptochrome-dependent greening enhancement effects.

Effects of drought stress and water recovery on physiological responses and gene expression in maize seedlings

BACKGROUND

Drought is one of the major factors limiting global maize production. Exposure to long-term drought conditions inhibits growth and leads to yield losses. Although several drought-responsive genes have been identified and functionally analyzed, the mechanisms underlying responses to drought and water recovery treatments have not been fully elucidated. To characterize how maize seedling respond to drought stress at the transcriptional level, we analyzed physiological responses and differentially expressed genes (DEGs) in the inbred line B73 under water deficit and recovery conditions.

RESULTS

The data for relative leaf water content, leaf size, and photosynthesis-related parameters indicated that drought stress significantly repressed maize seedling growth. Further RNA sequencing analysis revealed that 6107 DEGs were responsive to drought stress and water recovery, with more down-regulated than up-regulated genes. Among the DEGs, the photosynthesis- and hormone-related genes were enriched in responses to drought stress and re-watering. Additionally, transcription factor genes from 37 families were differentially expressed among the three analyzed time-points. Gene ontology enrichment analyses of the DEGs indicated that 50 GO terms, including those related to photosynthesis, carbohydrate metabolism, oxidoreductase activities, nutrient metabolism and other drought-responsive pathways, were over-represented in the drought-treated seedlings. The content of gibberellin in drought treatment seedlings was decreased compared to that of control seedlings, while abscisic acid showed accumulated in the drought treated plants. The deep analysis of DEGs related to cell wall development indicated that these genes were prone to be down-regulated at drought treatment stage.

CONCLUSIONS

Many genes that are differentially expressed in responses to drought stress and water recovery conditions affect photosynthetic systems and hormone biosynthesis. The identified DEGs, especially those encoding transcription factors, represent potential targets for developing drought-tolerant maize lines.

Seed-specific transcription factor HSFA9 links late embryogenesis and early photomorphogenesis

HSFA9 is a seed-specific transcription factor that in sunflower (Helianthus annuus) is involved in desiccation tolerance and longevity. Here we show that the constitutive overexpression of HSFA9 in tobacco (Nicotiana tabacum) seedlings attenuated hypocotyl growth under darkness and accelerated the initial photosynthetic development. Plants overexpressing HSFA9 increased accumulation of carotenoids, chlorophyllide, and chlorophyll, and displayed earlier unfolding of the cotyledons. HSFA9 enhanced phytochrome-dependent light responses, as shown by an intensified hypocotyl length reduction after treatments with continuous far-red or red light. This observation indicated the involvement of at least two phytochromes: PHYA and PHYB. Reduced hypocotyl length under darkness did not depend on phytochrome photo-activation; this was inferred from the lack of effect observed using far-red light pulses applied before the dark treatment. HSFA9 increased the expression of genes that activate photomorphogenesis, including PHYA, PHYB, and HY5. HSFA9 might directly upregulate PHYA and indirectly affect PHYB transcription, as suggested by transient expression assays. Converse effects on gene expression, greening, and cotyledon unfolding were observed using a dominant-negative form of HSFA9, which was overexpressed under a seed-specific promoter. This work uncovers a novel transcriptional link, through HSFA9, between seed maturation and early photomorphogenesis. In all, our data suggest that HSFA9 enhances photomorphogenesis via early transcriptional effects that start in seeds under darkness.

Involvement of Abscisic Acid in PSII Photodamage and D1 Protein Turnover for Light-Induced Premature Senescence of Rice Flag Leaves

D1 protein in the PSII reaction center is the major target of photodamage, and it exhibits the highest turnover rate among all the thylakoid proteins. In this paper, rice psf (premature senescence of flag leaves) mutant and its wild type were used to investigate the genotype-dependent alteration in PSII photo-damage and D1 protein turnover during leaf senescence and its relation to ABA accumulation in senescent leaves. The symptom and extent of leaf senescence of the psf mutant appeared to be sunlight-dependent under natural field condition. The psf also displayed significantly higher levels of ABA accumulation in senescent leaves than the wild type. However, the premature senescence lesion of psf leaves could be alleviated by shaded treatment, concomitantly with the strikingly suppressed ABA level in the shaded areas of flag leaves. The change in ABA concentration contributed to the regulation of shade-delayed leaf senescence. The participation of ABA in the timing of senescence initiation and in the subsequent rate of leaf senescence was closely associated with PSII photodamage and D1 protein turnover during leaf senescence, in which the transcriptional expression of several key genes (psbA, psbB, psbC and OsFtsH2) involved in D1 protein biosynthesis and PSII repair cycle was seriously suppressed by the significantly increased ABA level. This response resulted in the low rate of D1 protein synthesis and impaired repair recovery in the presence of ABA. The psf showed evidently decreased D1 protein amount in the senescent leaves. Both the inhibition of de novo synthesized D1 protein and the slow rate of proteolytic removal for the photodamaged D1 protein was among the most crucial steps for the linkage between light-dependent leaf senescence and the varying ABA concentration in psf mutant leaves. OsFtsH2 transcriptional expression possibly played an important role in the regulation of D1 protein turnover and PSII repair cycle in relation to ABA mediated leaf senescence.

Prospects of engineering thermotolerance in crops through modulation of heat stress transcription factor and heat shock protein networks

Cell survival under high temperature conditions involves the activation of heat stress response (HSR), which in principle is highly conserved among different organisms, but shows remarkable complexity and unique features in plant systems. The transcriptional reprogramming at higher temperatures is controlled by the activity of the heat stress transcription factors (Hsfs). Hsfs allow the transcriptional activation of HSR genes, among which heat shock proteins (Hsps) are best characterized. Hsps belong to multigene families encoding for molecular chaperones involved in various processes including maintenance of protein homeostasis as a requisite for optimal development and survival under stress conditions. Hsfs form complex networks to activate downstream responses, but are concomitantly subjected to cell-type-dependent feedback regulation through factor-specific physical and functional interactions with chaperones belonging to Hsp90, Hsp70 and small Hsp families. There is increasing evidence that the originally assumed specialized function of Hsf/chaperone networks in the HSR turns out to be a complex central stress response system that is involved in the regulation of a broad variety of other stress responses and may also have substantial impact on various developmental processes. Understanding in detail the function of such regulatory networks is prerequisite for sustained improvement of thermotolerance in important agricultural crops.

Heat shock transcription factors involved in seed desiccation tolerance and longevity retard vegetative senescence in transgenic tobacco

MAIN CONCLUSION

Transcription factors normally expressed in sunflower seeds delayed vegetative senescence induced by severe stress in transgenic tobacco. This revealed a novel connection between seed heat shock factors, desiccation tolerance and vegetative longevity. HaHSFA9 and HaHSFA4a coactivate a genetic program that, in sunflower (Helianthus annuus L.), contributes to seed longevity and desiccation tolerance. We have shown that overexpression of HaHSFA9 in transgenic tobacco seedlings resulted in tolerance to drastic dehydration and oxidative stress. Overexpression of HaHSFA9 alone was linked to a remarkable protection of the photosynthetic apparatus. In addition, the combined overexpression of HaHSFA9 and HaHSFA4a enhanced all these stress-resistance phenotypes. Here, we find that HaHSFA9 confers protection against damage induced by different stress conditions that accelerate vegetative senescence during different stages of development. Seedlings and plants that overexpress HaHSFA9 survived lethal treatments of dark-induced senescence. HaHSFA9 overexpression induced resistance to effects of culture under darkness for several weeks. Only some homoiochlorophyllous resurrection plants are able to withstand this experimental severe stress condition. The combined overexpression of HaHSFA9 and HaHSFA4a did not result in further slowing of dark-induced seedling senescence. However, combined expression of the two transcription factors caused improved recovery of the photosynthetic organs of seedlings after lethal dark treatments. At later stages of vegetative development, HaHSFA9 delayed the appearance of senescence symptoms in leaves of plants grown under normal illumination. This delay was observed under either control or stress treatments. Thus, HaHSFA9 delayed both natural and stress-induced leaf senesce. These novel observations connect transcription factors involved in desiccation tolerance with leaf longevity.

Overexpression of the Maize psbA Gene Enhances Drought Tolerance Through Regulating Antioxidant System, Photosynthetic Capability, and Stress Defense Gene Expression in Tobacco

The psbA (encoding D1 protein) plays an important role in protecting photosystem II (PSII) from oxidative damage in higher plants. In our previous study, the role of the psbA from maize (Zea mays. L) in response to SO2 stress was characterized. To date, information about the involvement of the psbA gene in drought response is scarce. Here we found that overexpression (OE) of ZmpsbA showed increased D1 protein abundance and enhanced drought stress tolerance in tobacco. The drought-tolerant phenotypes of the OE lines were accompanied by increases of key antioxidant enzymes SOD, CAT, and POD activities, but decreases of hydrogen peroxide, malondialdehyde, and ion leakage. Further investigation showed that the OE plants had much less reductions than the wild-type in the net photosynthesis rate (Pn), stomatal conductance (Gs), and the maximal photochemical efficiency of PSII (Fv/Fm) during drought stress; indicating that OE of ZmpsbA may alleviate photosynthesis inhibition during drought. qRT-PCR analysis revealed that there was significantly increased expression of NtLEA5, NtERD10C, NtAREB, and NtCDPK2 in ZmpsbA-OE lines. Together, our results indicate that ZmpsbA improves drought tolerance in tobacco possibly by alleviating photosynthesis reduction, reducing reactive oxygen species accumulation and membrane damage, and modulating stress defense gene expression. ZmpsbA could be exploited for engineering drought-tolerant plants in molecular breeding of crops.

Co-overexpression of two Heat Shock Factors results in enhanced seed longevity and in synergistic effects on seedling tolerance to severe dehydration and oxidative stress

BACKGROUND

We have previously reported that the seed-specific overexpression of sunflower (Helianthus annuus L.) Heat Shock Factor A9 (HaHSFA9) enhanced seed longevity in transgenic tobacco (Nicotiana tabacum L.). In addition, the overexpression of HaHSFA9 in vegetative organs conferred tolerance to drastic levels of dehydration and oxidative stress.

RESULTS

Here we found that the combined overexpression of sunflower Heat Shock Factor A4a (HaHSFA4a) and HaHSFA9 enhanced all the previously reported phenotypes described for the overexpression of HaHSFA9 alone. The improved phenotypes occurred in coincidence with only subtle changes in the accumulation of small Heat Shock Proteins (sHSP) that are encoded by genes activated by HaHSFA9. The single overexpression of HaHSFA4a in vegetative organs (which lack endogenous HSFA9 proteins) did not induce sHSP accumulation under control growth conditions; neither it conferred thermotolerance. The overexpression of HaHSFA4a alone also failed to induce tolerance to severe abiotic stress. Thus, a synergistic functional effect of both factors was evident in seedlings.

CONCLUSIONS

Our study revealed that HaHSFA4a requires HaHSFA9 for in planta function. Our results strongly support the involvement of HaHSFA4a and HaHSFA9 in transcriptional co-activation of a genetic program of longevity and desiccation tolerance in sunflower seeds. These results would also have potential application for improving seed longevity and tolerance to severe stress in vegetative organs.