Salt-stress induced modulation of chlorophyll biosynthesis during de-etiolation of rice seedlings

Transcriptomic analysis of salt stress responsive genes in Rhazya stricta

Rhazya stricta is an evergreen shrub that is widely distributed across Western and South Asia, and like many other members of the Apocynaceae produces monoterpene indole alkaloids that have anti-cancer properties. This species is adapted to very harsh desert conditions making it an excellent system for studying tolerance to high temperatures and salinity. RNA-Seq analysis was performed on R. stricta exposed to severe salt stress (500 mM NaCl) across four time intervals (0, 2, 12 and 24 h) to examine mechanisms of salt tolerance. A large number of transcripts including genes encoding tetrapyrroles and pentatricopeptide repeat (PPR) proteins were regulated only after 12 h of stress of seedlings grown in controlled greenhouse conditions. Mechanisms of salt tolerance in R. stricta may involve the upregulation of genes encoding chaperone protein Dnaj6, UDP-glucosyl transferase 85a2, protein transparent testa 12 and respiratory burst oxidase homolog protein b. Many of the highly-expressed genes act on protecting protein folding during salt stress and the production of flavonoids, key secondary metabolites in stress tolerance. Other regulated genes encode enzymes in the porphyrin and chlorophyll metabolic pathway with important roles during plant growth, photosynthesis, hormone signaling and abiotic responses. Heme biosynthesis in R. stricta leaves might add to the level of salt stress tolerance by maintaining appropriate levels of photosynthesis and normal plant growth as well as by the participation in reactive oxygen species (ROS) production under stress. We speculate that the high expression levels of PPR genes may be dependent on expression levels of their targeted editing genes. Although the results of PPR gene family indicated regulation of a large number of transcripts under salt stress, PPR actions were independent of the salt stress because their RNA editing patterns were unchanged.

Phenanthrene-triggered Chlorosis is caused by elevated Chlorophyll degradation and leaf moisture

Leaf is an important organ in responding to environmental stresses. To date, chlorophyll metabolism under polycyclic aromatic hydrocarbon (PAH) stress is still unclear. Here we reveal, for the first time, the chlorophyll metabolism of wheat seedling leaves in response to phenanthrene (a model PAH) exposure. In this study, the hydroponic experiment was employed, and the wheat seedlings were exposed to phenanthrene to observe the response at day 1, 3, 5, 7 and 9. Over the exposure time, wheat leaf color turns light. With the accumulation of phenanthrene, the concentrations of glutamate, 5-aminolevulinic acid, uroporphyrinogen III, protoporphyrin IX, Mg-protoporphyrin IX and protochlorophyllide increase while the concentrations of porphobilinogen and Chlorophyll b decrease. Also chlorophyll a content rises initially and then declines. Uroporphyrinogen III synthase and chlorophyllase are activated and porphobilinogen deaminase activity declines in the treatments. Both chlorophyll synthesis and degradation are enhanced, but the degradation rate is faster. Phenanthrene accumulation has significant and positive effects on increase of glutamate, 5-aminolevulinic acid, uroporphyrinogen III, protoporphyrin IX, Mg-protoporphyrin IX and protochlorophyllide concentrations. There is a negative correlation between phenanthrene accumulation and total chlorophyll. Additionally, the leaf moisture increases. Therefore, it is concluded that wheat leaf chlorosis results from a combination of accelerated chlorophyll degradation and elevated leaf moisture under phenanthrene exposure. Our results are helpful not only for better understanding the toxicity of PAHs to plants and crop PAH-adaptive mechanism in the environment, but also for potentially employing the changes of the chlorophyll-synthesizing precursors and enzyme activities in plant leaves as indicators of plant response to PAH pollution.

Melatonin Improves the Photosynthetic Apparatus in Pea Leaves Stressed by Paraquat via Chlorophyll Breakdown Regulation and Its Accelerated de novo Synthesis

The positive effect of melatonin on the function of the photosynthetic apparatus is known, but little is known about the specific mechanisms of melatonin's action in plants. The influence of melatonin on chlorophyll metabolism of 24-day-old Pisum sativum L. seedlings during paraquat (PQ)-induced oxidative stress was investigated in this study. Seeds were hydro-primed with water (H), 50 and 200 μM melatonin/water solutions (H-MEL50, H-MEL200), while non-primed seeds were used as controls (C). Increases in chlorophyllase activity (key enzyme in chlorophyll degradation) and 5-aminolevulinic acid contents (the first compound in the porphyrin synthesis pathway) were observed in H-MEL50 and H-MEL200 leaf disks. This suggests that melatonin may accelerate damaged chlorophyll breakdown and its de novo synthesis during the first hours of PQ treatment. Elevated level of pheophytin in control leaf disks following 24 h of PQ incubation probably was associated with an enhanced rate of chlorophyll degradation through formation of pheophytin as a chlorophyll derivative. This validates the hypothesis that chlorophyllide, considered for many years, as a first intermediate of chlorophyll breakdown is not. This is indicated by the almost unchanged chlorophyll to chlorophyllide ratio after 24 h of PQ treatment. However, prolonged effects of PQ-induced stress (48 h) revealed extensive discolouration of control and water-treated leaf disks, while melatonin treatment alleviated PQ-induced photobleaching. Also the ratio of chlorophyll to chlorophyllide and porphyrin contents were significantly higher in plants treated with melatonin, which may indicate that this indoleamine both retards chlorophyll breakdown and stimulates its de novo synthesis during extended stress. We concluded that melatonin added into the seeds enhances the ability of pea seedlings to accelerate chlorophyll breakdown and its de novo synthesis before stress appeared and for several hours after, while during prolonged PQ incubation melatonin delays chlorophyll degradation.

Towards efficient photosynthesis: overexpression of Zea mays phosphoenolpyruvate carboxylase in Arabidopsis thaliana

Plants with C4 photosynthesis are efficient in carbon assimilation and have an advantage over C3 photosynthesis. In C4 photosynthesis, the primary CO₂ fixation is catalyzed by phosphoenolpyruvate carboxylase (PEPC). Here, we show that overexpression of Zea mays PEPC cDNA, under the control of ³⁵S promoter, in Arabidopsis thaliana resulted in ~7-10 fold higher protein abundance and ~7-10 fold increase in PEPC activity in the transgenic lines than that in the vector control. We suggest that overexpression of PEPC played an anaplerotic role to increase the supply of 4-carbon carboxylic acids, which provided carbon skeletons for increased amino acid and protein synthesis. Higher protein content must have been responsible for increased metabolic processes including chlorophyll biosynthesis, photosynthesis, and respiration. Consequently, the PEPC-overexpressed transgenic plants had higher chlorophyll content, enhanced electron transport rate (ETR), lower non-photochemical quenching (NPQ) of chlorophyll a fluorescence, and a higher performance index (PI) than the vector control. Consistent with these observations, the rate of CO₂ assimilation, the starch content, and the dry weight of PEPC-overexpressed plants increased by 14-18 %, 10-18 %, and 6.5-16 %, respectively. Significantly, transgenics were tolerant to salt stress as they had increased ability to synthesize amino acids, including the osmolyte proline. NaCl (150 mM)-treated transgenic plants had higher variable to maximum Chl a fluorescence (F _v/F _m) ratio, higher PI, higher ETR, and lower NPQ than the salt-treated vector controls. These results suggest that expression of C4 photosynthesis enzyme(s) in a C3 plant can improve its photosynthetic capacity with enhanced tolerance to salinity stress.

Proteome Dynamics and Physiological Responses to Short-Term Salt Stress in Brassica napus Leaves

Salt stress limits plant growth and crop productivity and is an increasing threat to agriculture worldwide. In this study, proteomic and physiological responses of Brassica napus leaves under salt stress were investigated. Seedlings under salt treatment showed growth inhibition and photosynthesis reduction. A comparative proteomic analysis of seedling leaves exposed to 200 mM NaCl for 24 h, 48 h and 72 h was conducted. Forty-four protein spots were differentially accumulated upon NaCl treatment and 42 of them were identified, including several novel salt-responsive proteins. To determine the functional roles of these proteins in salt adaptation, their dynamic changes in abundance were analyzed. The results suggested that the up-accumulated proteins, which were associated with protein metabolism, damage repair and defense response, might contribute to the alleviation of the deleterious effect of salt stress on chlorophyll biosynthesis, photosynthesis, energy synthesis and respiration in Brassica napus leaves. This study will lead to a better understanding of the molecular basis of salt stress adaptation in Brassica napus and provides a basis for genetic engineering of plants with improved salt tolerance in the future.

Genome-wide expression profiles of Pyropia haitanensis in response to osmotic stress by using deep sequencing technology

BACKGROUND

Pyropia haitanensis is an economically important marine crop grown in harsh intertidal habitats of southern China; it is also an excellent model system for studying mechanisms of stress tolerance. To understand the molecular mechanisms underlying osmotic tolerance and adaptation to intertidal environments, a comprehensive analysis of genome-wide gene expression profiles in response to dehydration and rehydration in Py. haitanensis was undertaken using digital gene expression profile (DGE) approaches combined with de novo transcriptome sequencing.

RESULTS

RNA-sequencing of the pooled RNA samples from different developmental phases and stress treatments was performed, which generated a total of 47.7 million clean reads. These reads were de novo assembled into 28,536 unigenes (≥ 200 bp), of which 18,217 unigenes (63.83 %) were annotated in at least one reference database. DGE analysis was performed on four treatments (two biological replicates per treatment), which included moderate dehydration, severe dehydration, rehydration, and normal conditions. The number of raw reads per sample ranged from 12.47 to 15.79 million, with an average of 14.69 million reads per sample. After quality filtering, the number of clean reads per sample ranged from 11.83 to 15.04 million. All distinct sequencing reads were annotated using the transcriptome of Py. haitanensis as reference. A total of 1,681 unigenes showed significant differential expression between moderate dehydration and normal conditions, in which 977 genes were upregulated, and 704 genes were downregulated. Between severe dehydration and normal conditions, 1,993 unigenes showed significantly altered expression, which included both upregulated (1,219) and downregulated genes (774). In addition, 1,086 differentially expressed genes were detected between rehydration and normal conditions, of which 720 genes were upregulated and 366 unigenes were downregulated. Most gene expression patterns in response to dehydration differed from that of rehydration, except for the synthesis of unsaturated fatty acids, several transcription factor families, and molecular chaperones that have been collectively implicated in the processes of dehydration and rehydration in Py. haitanensis.

CONCLUSIONS

Taken together, these data provide a global high-resolution analysis of gene expression changes during osmotic stress that could potentially serve as a key resource for understanding the biology of osmotic acclimation in intertidal red seaweed.